Dissertations / Theses on the topic 'Continental magmatism'

Isostasy, flexure and dynamic processes all influence the shape of the Earth’s surface. While the first two processes are well understood, dynamic topography remains controversial. On the continents, dynamic uplift is often expressed by positive long-wavelength gravity anomalies, radial drainage patterns, and slow seismic velocity anomalies within the upper mantle. Volcanic activity and elevated heat flow are also often observed. The aim of this study is to investigate the link between geochemical compositions of intracontinental magmatism and geophysical, geomorphological and geodetic observations of dynamic uplift. Three volcanic regions are considered in detail: western North America, northeast Brazil and Madagascar. The combined database includes 348 new whole-rock geochemical analyses. Rare earth element concentrations of mafic, asthenospheric-derived volcanic samples are exploited to calculate the depth and temperature of melt generation by inverse modelling. A sensitivity test of this modelling scheme is carried out. Lithospheric thickness and mantle temperature are independently determined from shear wave velocity models. Beneath western North America, a negative correlation between shear wave velocities at depths of 70–150 km and degree of melting is observed. Temperatures obtained from igneous compositions and from shear wave velocity profiles beneath volcanic fields closely agree. Melts are produced within, or close to, the spinel-garnet transition zone at depths shallower than $\sim$70 km, yielding mantle potential temperatures of up to 1380$^{\circ}$C. Calculated uplift and heat flow based upon these results match observed surface elevation and heat flow measurements. In northeast Brazil, Jurassic, Cretaceous and Cenozoic phases of mafic igneous activity are recognised. Jurassic magmatic activity probably resulted from spinel-field melting at potential temperatures of $\sim$1380$^{\circ}$C. This episode is associated with regional magmatism during break-up of the Central Atlantic Ocean. Cretaceous compositions record melting at potential temperatures of 1330–70$^{\circ}$C at similar depths. This activity is linked to extension at the time of break-up of the equatorial and South Atlantic Ocean. Cenozoic volcanism comprises low-degree melts within the spinel-garnet transition zone at ambient potential temperature. Shear wave velocity models support these results. Cenozoic volcanism in Madagascar is predominantly alkaline and records small-degree melting with minor temperature anomalies within the spinel-garnet transition zone. Rare tholeiitic basalts record temperatures up to 1360$^{\circ}$C. Analysis of global and regional shear wave velocity models closely matches these results. The principal control on continental magmatism appears to be temperature anomalies within the upper mantle beneath thin lithosphere. Highest mantle potential temperatures correlate with largest dynamic uplift. Mantle potential temperatures $ < $1350$^{\circ}$C are matched with minimal or negative dynamic topography.

Monogenetic volcanism is characterized by a large diversity of eruptive styles, morphologies and deposits. Monogenetic landforms are the result of a complex merging of internal (magma composition, vesiculation) and external (geological setting, fracturation, hydrogeology, substrate stratigraphy, etc) parameters that govern the physics of the eruptions. Changes in these parameters may cause variations in the eruption style several times during the course of such short-lived volcanoes. Monogenetic volcanoes may form in any type of geological environment with scoria cones being the most common volcano type and hydrovolcanic tuff rings, tuff cones, and maars as the second in abundance. These small-volume volcanoes are generally the result of short-lived eruptions but the activity in a monogenetic volcanic field might exceed the total life of composite volcanoes. The attention of this work was focused on the relation between monogenetic volcanic landforms and the external variables that influenced the dynamics of the eruptions (i.e. magmatism vs phreatomagmatism) through a multidisciplinary perspective, in marine and continental geological settings under which monogenetic volcanism may develop. Different case studies representative of this type of activity and of these different environments have been considered. The first one corresponds to the La Crosa De Sant Dalmai volcano (Garrotxa Volcanic Field, southern sector of the Catalan Volcanic Zone), a roughly circular asymmetrical maar-diatreme volcano, which is one of the most characteristic volcanic edifices of this continental monogenetic volcanic field and the largest Quaternary volcanic crater on the Iberian Peninsula. This edifice is an example of monogenetic landform, mostly composed of phreatomagmatic deposits with subordinate Strombolian phases, constructed on a mixed basement made of hard Paleozoic granites and schists rocks and soft Plio-Quaternary deposits. Here, I reconstructed the hydrogeological conditions of the substrate and the implication for the eruptive dynamics. As a second case study, I carried out detailed stratigraphic and sedimentological studies of the succession of El Golfo tuff cone (Lanzarote, Canary Islands). The main objective of the work was to describe in detail the structure and association of facies of this edifice and use this information to infer changes in eruption style and depositional processes. Another type of eruption was studied in the same archipelago at El Hierro, an island essentially characterized by basaltic volcanism with both Strombolian and Hawaiian activity. Here I reported the stratigraphic, lithological, sedimentological and petrographic characteristics of a felsic hydrovolcanic episode in order to discuss, transport/depositional mechanisms, dynamics, relative age and implications for hazard assessment on the island. Finally, the same type of methodology was applied at Deception Island (Southern Shetland Archipelago, Antarctica), determining the lithological and sedimentological characteristics, and clasts distribution (isopach and isopleth maps) of the eruption of 1970. This information was, then, used to determine depositional processes, eruption style and physical parameters (i.e. plume height, erupted volume, VEI) of the eruption in order to compare this episode with the previous 1967 episode, and to deduce their implications to conduct hazard assessment at the island. Each work represents a diverse aspect of hydrovolcanism and the results obtained helped to better understand the eruptive behavior of this type of volcanoes, which is a fundamental task in order to understand the possible future hazards associated with this type of volcanism. The results obtained can be applied to monogenetic volcanic fields worldwide and are, therefore, useful to reconstruct the evolution of a certain volcanic fields, through the study of single monogenetic volcanoes, and to evaluate the possible volcanic hazards, as similar eruptions represent a serious threat, which is often underestimated. A more systematic study is, thus, needed in order to understand the role of shallow-level conditions in the formation of specific volcano types in such complex volcanic fields.
El vulcanismo monogenético se caracteriza por una gran diversidad de estilos eruptivos, morfologías y depósitos. Los tipos de edificios que se forman son el resultado de una compleja combinación de parámetros que rigen la física de la erupción. La atención de este trabajo se centra en la relación entre los edificios volcánicos monogenéticos y las variables externas que influyen en la dinámica de las erupciones (es decir, magmatismo vs freatomagmatismo) a través de un punto de vista multidisciplinar, en ambientes continentales y marinos en los que el vulcanismo puede desarrollar. Diferentes estudios, representativos de este tipo de actividad en diferentes entornos geográficos y geológicos, se han llevado a cabo. El primer ejemplo corresponde al volcán de La Crosa de Sant Dalmai (Campo Volcánico de La Garrotxa) donde se han reconstruido las condiciones hidrogeológicas del sustrato y la implicación para la dinámica eruptiva. Como segundo caso de estudio, se ha realizado una estratigrafía de detalle del cono de toba de El Golfo (Lanzarote, Islas Canarias), donde se han estudiado los mecanismos de emplazamiento de los depósitos para inferir cambios en la interacción magma/agua. Otro tipo de erupción se ha investigado en el mismo archipiélago, en la Isla de El Hierro, determinando las características físicas de un episodio félsico de origen hidrovolcánico ocurrido en una isla que se caracteriza esencialmente por el vulcanismo basáltico tanto Estromboliano como Hawaiiano. Por último, este mismo tipo de metodología se ha aplicado a la Isla Decepción (archipiélago de las Shetland del Sur, Antártida), estableciendo los parámetros físicos de la erupción del 1970 con el fin de comparar este episodio con el evento anterior del 1967, y deducir sus consecuencias para llevar a cabo la evaluación de peligrosidad en la isla. Los resultados obtenidos pueden ser aplicados a campos volcánicos monogenéticos en todo el mundo y, por tanto, son útiles para reconstruir la evolución de ciertos campos volcánicos, a través del estudio de volcanes monogenéticos individuales, para evaluar los posibles riesgos volcánicos, teniendo en cuenta como erupciones similares representan una grave amenaza, que es a menudo subestimada.

Cette étude vise à mieux comprendre le rôle respectif et l’interaction entre les différents mécanismes qui contrôlent l’initiation et le développement des rifts (magmatisme, faille, fabrique lithosphérique, rhéologie). Pour ce faire, le premier réseau local de 35 stations sismologiques a été déployé dans la Divergence Nord-Tanzanienne, la portion la plus jeune du Rift Est-Africain, pendant une période de six mois. Les signaux des séismes proches et lointains sont utilisés comme indicateurs de l’histogénèse, de la résistance crustale, de champs de déformation et de contraintes, et renseignent aussi sur la structure et l’anisotropie lithosphérique. Une crise sismique enregistrée au Nord de la Tanzanie (Gelai) illustre la co-existence de processus magmatiques et tectoniques, avec mise en place d’un dyke et e comportement à la fois asismique (glissement lent) et sismique (séisme de magnitude Mw 5. 9) de failles normales. La crise de Gelai indique le rôle majeur et auparavant Insoupçonné des processus asismiques dans l’accommodation de la déformation. Des séismes crustaux remarquablement profonds ont été observés plus au sud, dans la région du Lac Manyara. La sismicité de Manyara est associée à des glissements en faille normale et en décrochement probablement déclenchés par des fluides en base de croûte. L’ensemble de la sismicité enregistrée dans la DNT indique l’influence de l’héritage structural, ce qui s’observe aussi à l’échelle lithosphérique, par le biais de l’anisotropie sismique
The objective of this study is to better understand the role and the interaction between the different mechanisms that control rift initiation and development (magmatism, fauting, lithospheric fabric, rheology). To this end, a local seismic network has been deployed for the first time in the youngest part of the East African Rift, the North Tanzanian Divergence, for six months. Seismic signal was analyzed to characterize earthquake triggering mechanisms, crustal strength, strain and stress field, and seismic anisotropy. A seismic crisis occurred in North Tanzania (Gelai) involving dyking, seismic and aseismic slip. The Gelai crisis highlights the potential major role of aseismic processes in strain accommodation. Deep crustal earthquakes were recorded in the Lake Manyara region, Manyara seismicity is associated with strike-slip and normal faulting most Iikely triggered by fluids at depth. The influence of structural inheritance on rifting is indicated at lithospheric scale by seismicity and seismic anisotropy

Continental extension causes rifting and thinning of the lithosphere that may result in breakup and eventually the initiation of seafloor spreading and passive continental margin development. Ambiguity exists regarding the roles of magmatism and structural inheritance during rifting and continental breakup during this process. This study focuses on the importance of these controls on the Mesozoic-Cenozoic separation between West Greenland and Eastern Canada. It is important to improve our knowledge of the processes that influenced breakup as the current understanding of these processes is limited and also to reduce hydrocarbon exploration risk in this tectonic setting. During this study passive margin processes were investigated using a variety of methodologies at a range of scales from that of conjugate margin pairs (Chapters 4 and 5), through margin and basin scale studies (Chapter 6) to the smallest scale on individual igneous intrusions (Chapter 7). At the largest scale an assessment of the magmatic and structural asymmetry between the conjugate margins of the Labrador Sea based primarily on field data and subsequent analysis near Makkovik, Labrador, but also other large-scale geophysical datasets demonstrated that early rifting was dominated by simple shear rather than pure shear. In such a scenario Labrador was have been the lower plate margin to the upper plate southwest Greenland margin. Further analysis of field observations indicated that rifting of the Labrador Sea region may have been aided by a favourably orientated basement metamorphic fabric and that observable onshore brittle deformation structures may be related to Mesozoic rifting. Further north in the Davis Strait, seismic interpretation at the margin and basin scale allowed a series of seismic surfaces, isochrons and a new offshore fault map to be produced. The results of this analysis demonstrated that the geometry of rift basins was primarily controlled by pre-existing structures, an assertion supported by observations of reactivation onshore in West Greenland. Finally, at the smallest scale, results of numerical modelling offshore Newfoundland demonstrated that even on non- volcanic passive margins, intrusive magmatism can influence thermal evolution. In addition, the presence of widespread igneous rocks on passive margins may be indicative of regional-scale thermal perturbations that should be considered in source rock maturation studies. Overall, the conclusion of this project is that both magmatism and structural inheritance have profoundly influenced the continental breakup between West Greenland and Eastern Canada, and that interplay between these two complex groups of mechanisms may have also contributed to the geological evolution of this area.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), February 2000.
Includes bibliographical references (p. 255-270).
This thesis studies the dynamics of mantle melting during continental breakups by geophysical, geochemical, and numerical analyses. The first part focuses on the mantle melting and crustal accretion processes during the formation of the Southeast Greenland margin, on the basis of deep-crustal seismic data. A new seismic tomographic method is developed to jointly invert refraction and reflection travel times for a compressional velocity structure, and a long-wavelength structure with strong lateral heterogeneity is successfully recovered, including 30- to 15-km-thick igneous crust within a 150-km-wide continent-ocean transition zone. A nonlinear Monte Carlo analysis is also conducted to establish the absolute uncertainty of model parameters. The derived crustal structure is first used to resolve the origin of a margin gravity high, with new inversion schemes using both seismic and gravity constraints. Density anomalies producing the gravity high seem to be confined within the upper crust, not in the lower crust as suggested for other volcanic margins. A new robust framework is then developed for the petrological interpretation of the velocity structure of igneous crust, and the thick igneous crust formed at the continentocean transition zone is suggested to have resulted from vigorous active upwelling of mantle with only somewhat elevated potential temperature. In the second part, the nature of mantle melting during the formation of the North Atlantic igneous province is studied on the basis of the major element chemistry of erupted lavas. A new fractionation correction scheme based on the Ni concentrations of mantle olivine is used to estimate primary melt compositions, which suggest that this province is characterized by a large degree of major element source heterogeneity. In the third part, the nature of preexisting sublithospheric convection is investigated by a series of finite element analyses, because the strength of such convection is important to define the "normal" state of mantle, the understanding of which is essential to identify any anomalous behavior of mantle such as a mantle plume. The results suggest that small-scale convection is likely in normal asthenosphere, and that the upwelling velocity in such convection is on the order of 1- 10 cm/yr.
by Jun Korenaga.
Ph.D.

Thesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2012.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 53-56).
Elevated concentrations of CO₂ have been proposed as the reason that the Cretaceous climate was 6-14°C warmer than the present, however the source of Cretaceous CO₂ is unknown [Barron, 1983]. This study examines the possibility of continental arc magmatism as a mechanism for CO2 release, specifically as a volatile produced during crustal assimilation and contact metamorphism of carbonates around plutons. Bedrock maps of the North American Cordillera (a region of active continental arc magmatism during the Cretaceous), the relative locations of the carbonates, the Cretaceous plutons, and the calculated "decarbonation zones"around the plutons. These measurements were then input in a thermal and petrologoical model in order to estimate the quantity of CO₂ released by continental arc magmatism. Testing a number of cases with varying parameters, the model found the arc-magmatism-induced temperature difference between the present and Cretaceous global climates to have a lower limit of [Delta]T < 1°C and an upper limit of 5.1 < [Delta]T < 12.3°C. Decarbonation from continental arc magmatism is shown to be a possible mechanism of paleoclimatic warming, and more work is required to either confirm or refute the hypothesis.
by Anna Elizabeth Brunner.
S.B.

The Frontal Cordillera Composite Batholith (FCCB) is made up of a string of granitic plutons thought to be Permo-Triassic in age, that stretch with a N/S strike for the entire length of the Chilean and Argentine Frontal Cordillera (FC). Associated spatially and temporally with the batholith is a series of volcanic rocks called the Choiyoi Formation. Seven FC stocks were studied: the Boca del Rio, Cacheuta, Cerro Médanos, Cerro Arenales, Cerro Bayo, Punta Negra and Punta Blanca. They are high-K, I-type metaluminous, calc-alkaline granitic plutons, which range in composition from granodiorites to monzogranites to syenogranites. Contact relationships between the granitic stocks and the Choiyoi Formation, long thought to be their effusive equivalents, are intrusive, suggesting that the volcanic rocks are older. Chemically, the Choiyoi Formation rocks are also high K, alkalic and silica undersaturated, and similar to the granites. The stocks contain typical metaluminous modal mineralogies of plagioclase, quartz and alkali feldspar felsic phases, with amphibole and biotite as mafics, and magnetite, zircon, titanite, apatite and allanite as accessories. Field studies and P-T data from amphiboles suggest depths of emplacement of less than 10km. All the stocks have been affected by hydrothermal alteration, the main effects being sericitisation and/or argillisation of plagioclase cores, chloritisation of biotite and amphibole and oxidation of magnetite to haematite. The source of the hydrothermal fluids is thought to be the granites themselves. Four of the stocks were modelled with major, trace and rare earth elements. It was concluded that the Cacheuta stock evolved to aplitic compositions by ~35% fractionation of mainly plagioclase and alkali feldspar; the Cerro Arenales stock by ~30% fractionation of plagioclase, biotite and k-spar; and the Punta Negra stock by ~25% fractionation of plagioclase, alkali feldspar and quartz. The Punta Blanca stock was interpreted as consisting of more than one batch of magma. The granites from this segment of the FCCB differ from typical I-type granites in some crucial respects: 1) they are mainly high K, 2) alkali feldspar crystallised early and fractionated in the more evolved stocks, 3) the stocks contain high Ba and intermediate Sr, with U and Th enrichment in the leucocratic stocks, and 4) the stocks form part of a batholith in which granites predominate over granodiorites and tonalites. Typical I-type Cordilleran batholiths like the Coastal Batholith of Peru, are predominantly tonalitic to granodioritic. Three stocks from the study area were dated by the U-Pb zircon method. These are the first such dates for any plutons of the FCCB. The Punta Blanca stock was found to be 276 ± 1 Ma old, the Cerro Médanos stock 263 ± 1 Ma old, and the Cerro Bayo stock 262 ± 3 Ma old. All of these ages are in the Lower Permian. Sr-Nd isotopes suggest that the Choiyoi Formation and FCCB stocks were derived from different lower crustal sources which were variably enriched with [sup]87 Sr and unradiogenic Nd, extracted from the mantle at ~1.3 Ga and ~1.25 Ga, respectively. AFC processes are not thought to be important. The geodynamic circumstances which lead to the generation of the Choiyoi Formation and the FCCB are still not clear. The evidence collected during this study suggests that the Choiyoi phase of volcanism subsequent to the movements of the San Rafael Orogenic Phase at the end of the Carboniferous was possibly the result of extension, which caused extensive melting of a lower crustal source extracted from the mantle at ~1.3 Ga. This was followed by a switch in tectonic style in the Early Permian, possibly due to the intiation of oblique subduction, which lead to the sampling of a younger lower crustal source and the formation of the FCCB. This scenario is analogous to that recorded by Rapela et al., (1996) further to the south in the 'Gondwana' magmatism of Patagonia between the Late Triassic to Jurassic.

The Afar region of Ethiopia is slowly being torn apart by the Red Sea, Gulf of Aden and Main Ethiopian rifts which all meet at this remote, barren corner of Africa. Prior to rifting, volcanism probably started here some 30 million years ago, marked by the arrival of the Afar mantle plume and subsequent eruption of kilometres thick flood basalts. To the north and east the Red Sea and Gulf of Aden rifts have already progressed to become sea-floor spreading centres where new oceanic crust is produced. Active spreading on the Red Sea rift takes a landward step west into Eritrean Afar at approximately 15oN, after which divergence between the Nubian and Arabian tectonic plates is localised into 60 km long, 20 km wide magmatic segments that undergo periodic rifting cycles. This part of Afar is a unique natural laboratory where the process of transition from continental rifting to sea floor spreading can be studied. In September 2005 a dramatic rifting episode began on one such segment of the Red Sea rift in Afar (the Dabbahu magmatic segment), whereby a 60 km long dyke containing an estimated 2.5 km3 magma was intruded in just two weeks, allowing opening of up to 8 m. Since then a further 13 smaller dykes have been intruded, some with fissural eruptions of basaltic lava. Subsidence observed via geodetic observations can only account for a small fraction of the magma supply required to in ate the dykes, suggesting a deep crustal or upper mantle source must exist. The magnetotelluric (MT) method is a passive geophysical technique, used to probe the Earth to reveal subsurface conductivity. The presence of fluids can dramatically increase conductivity by orders of magnitude making the MT method ideally suited to detecting them. MT data collected from 22 sites on profiles near to and crossing the active rift are analysed and interpreted in conjunction with seismic and petrological constraints. They reveal for the first time, the existence of both a mid to lower-crustal magma chamber directly below the rift, and an o -axis zone of partial melt well within the mantle. The volume of melt contained within the crust and upper mantle below the Dabbahu segment is estimated to be at least 350 km3; enough to supply the rift at current spreading rates for almost 30 thousand years, assuming that both melt containing regions supply the rift. Vast amounts of highly conductive material, suggesting the existence of pure melt in places, are also required in the shallow crust close to Dabbahu volcano which lies at the northern end of the segment. Further data collected on the currently inactive Hararo segment which is the next one to the south of Dabbahu, show a smaller zone of partial melt that appears to be pooling at the Moho, inferred seismically to be at about 22 km, but little or no melt is required within the mid-crust. The minimum amount of melt estimated to be contained here is just 21 km3; an order of magnitude less than on the Dabbahu segment, but similar to estimates for melt within the crust found below the rift axis in the continental Main Ethiopian rift. This, along with other morphological evidence, suggests that this rift segment is less mature than the Dabbahu segment to the north, rather than it simply being at a different stage of a rifting cycle. A wide spread layer of highly conductive sediments up to 2 km thick has been imaged at most locations. This was unexpected on the Dabbahu segment where the surface of the Earth is dominated by heavily faulted basalts erupted from fissures, which are seen as a resistive uppermost layer several hundred metres thick. The high conductivity of the sediments is attributed to high heat flow and the presence of brines.

This dissertation investigates the evolution of continental orogenic systems in Central Asia during and between pre-collisional plate convergence (Cordilleran-style orogenesis), syn-collisional plate convergence (collisional orogenesis), and post-collisional tectonic processes within the scope of closing Paleo-Asian and Tethyan ocean basins. A brief introductory chapter outlines the scope and context of the research. Appendix A focuses on the Late Paleozoic closure of the Turkestan ocean basin and subsequent collision between the Karakum–Tarim and Kazakh–Kyrgyz terranes in the South Tian Shan, within the scope of the final amalgamation of the Mesoproterozoic–Permian Central Asian Orogenic Belt. Appendix B focuses on late Cenozoic syn-collisional exhumation of gneiss domes in the India–Asia collision, which is a component of the Triassic–recent Alpine–Himalayan orogenic belt. Abstracts of the results are provided in the respective appendices.

Subduction zones, such as the Andean convergent margin, are the sites at which new continental crust is generated, and where subducting material is either recycled to the crust via arc magmatism or transferred to the deep mantle. The composition of arc magmas and associated new continental crust reflects variable contributions from mantle, crustal and subducted reservoirs. Insights into crustal growth and recycling processes in the southern Central Andes, specifically in the Pampean flat-slab segment, have been gained by utilising a range of petrological, geochronological and geochemical techniques. These techniques have been applied to a suite of Late Cretaceous (~73 Ma) to Late Miocene (~6 Ma) intrusive (granitoids) and extrusive (basalts to rhyolites) arc rocks collected from an east - west transect across the Andean Cordillera. The oxygen and hafnium isotopic composition of the accessory mineral zircon allows mantle-derived melts contaminated with older, upper continental crustal to be identified. Boron isotopic compositions of melt inclusions, combined with concentrations of certain incompatible trace elements, can be used to assess the source and influence of fluids derived from subducting material on the melt source region. The southern Central Andes provides a particularly interesting area to study these processes as the thickness of the continental crust has increased significantly over the course of the Cenozoic (from ~35 km to >50 km) and the angle of the subducting Nazca plate has shallowed since ~18 Ma, causing the position of the volcanic arc to migrate to the east. In order to unravel the complexities involved with constraining the contributions to arc magmas at an active continental margin, a range of geochronological, geochemical, and geothermobarometric techniques, including high resolution, micro-analysis of mineral phases and melt inclusions, have been applied. High resolution, U-Pb dating of magmatic zircon has improved regional stratigraphy in the Pampean flat-slab segment (between ~29 and 32 °S) and provided an accurate temporal constraint for geochemical and geothermobarometric data. The results of in-situ O and Lu-Hf isotope analysis of zircon show both distinct temporal and spatial variations across the Andean arc. The observed isotopic variability is attributed to variable contamination of mantle-derived melts with distinct Andean basement terranes, which vary east – west in composition and age. ‘Mantle-like’ δ18O(zircon) values, juvenile initial ƐHf(zircon) values and a lack of inherited, xenocrystic zircon cores, suggests the Late Cretaceous (~73 Ma) to Eocene (~39 Ma) plutons located in the Principal Cordillera of Chile, experienced very little interaction with the upper continental crust. Amphibole – plagioclase geothermobarometry indicates these calc-alkaline granitoids, which form extensive north – south trending belts, were emplaced at shallow depths in the crust (~4 – 5 km). Therefore the Late Cretaceous to Late Eocene is interpreted as a period of significant upper crustal growth. The isotopic variability in the Late Oligocene (~26 Ma) to Late Miocene (~6 Ma) arc magmatic rocks demonstrates that during thickening of the continental crust and migration of the Andean arc to the east, arc magmas assimilated Late Paleozoic to Early Mesozoic basement. In addition, arc magmas erupted/emplaced in the Argentinean Precordillera (i.e. farthest east from the trench) assimilated a Grenville-aged (~ 1330 – 1030 Ma) basement. The youngest arc magmas (~6 Ma) erupted in the Frontal Cordillera also show evidence for the assimilation of this ancient basement terrane, potentially signalling under-thrusting beneath the Frontal Cordillera. Overall, the later part of the Cenozoic represents a period of crustal reworking. Boron concentrations and isotope ratios measured in pyroxene hosted melt inclusions and for the first time in zircon hosted melt inclusions, are higher than the values expected for the mantle wedge and show significant variations with time. The source of the Paleocene (~61 Ma) arc magmas were influenced by fluids primarily derived from altered oceanic crust. Lower δ11B values and boron concentrations obtained for Oligocene (25 – 23 Ma) arc magmatic rocks reflects a diminished influence of slab-derived fluids reflecting a greater depth to the top of the slab. Fluids derived from serpentinite influenced the source of the arc magmas after ~19.5 Ma. This has been linked with the intersection of the Juan Fernández Ridge, a volcanic seamount chain associated with hydrated and serpentinised oceanic lithosphere.

At the end of the Triassic and during Jurassic times, the Central Atlantic Magmatic Province (CAMP) with a dominant mafic composition, and the mainly felsic Patagonian Province (PA) were formed in the context of Pangea break-up. CAMP basaltic to basaltic-andesitic dykes and sills from Hank and Hodh, in Mauritania, and Kaarta, in Mali, were emplaced at ca. 197 Ma yielding a signature defined by low to high TiO2-values (0.39 - 2.29 wt.%), Nb-Ta troughs, Pb spikes and enriched Nd-Sr-Pb compositions (143Nd/144Nd201Ma: 0.51224 - 0.51241, 87Sr/86Sr201Ma: 0.70572 - 0.70795, 207Pb/204Pb201Ma: 15.61 – 15.68 208Pb/204Pb201Ma: 38.44 – 38.07 and 206Pb/204Pb201Ma: 18.11 – 18.48). Nb/U ratios (= 12 – 15), tracers of recycled material, combined with enriched isotopic signatures suggest the contribution of continental crust in the origin of these magmas. Differences in the ratios of La/Yb of the whole set suggest the existence of an enriched group in Kaarta (La/YbN>4.3) coupled to increasing ratios of Nb/Zr and Th/Y, probably reflecting lower degrees of partial melting in these rocks. Geochemical compositions revealed in this region indicate a strong affinity to the Taoudenni and Marocco CAMP rocks. As a first approach, a possible distribution of the geochemical signatures in regional districts is here suggested, but should be explored by further studies. A thick sequence of Jurassic magmatic rocks of bimodal composition represents the volcanism of the Patagonia Province (PA). It consists of rhyolitic lava flows and ignimbrites of the Marifil (ca. 189-177 Ma) and Chon Aike (ca. 170 – 153 Ma) Formations (Fm), with a minor andesitic component represented by the Bajo Pobre (BP) (ca. 166 – 154 Ma), Cañadón Asfalto (ca. 179 – 176 Ma) and Lonco Trapial Fm, which in total record a period of magmatic activity of at least 38 Ma. New U-Pb ages on zircons for the Marifil Fm confirm that the first felsic pulse partially coincided with the Karoo-Ferrar magmatic event (ca. 184-174Ma). However, observed differences in the life span of the two Provinces (Karoo 10 Ma, PA: 38Ma) and the geochemical contrast of their products (PA: intermediate-felsic, mainly, Karoo: mafic, mainly) argue against a genetic relation between the two provinces, and thus with a common mantle plume origin. Mafic to intermediate Jurassic volcanism represented by the Cañadon Asfalto (CA) and Bajo Pobre Fm are synchronous to the felsic PA events and display the closest evidence of little evolved mantle-derived melts of the Province. Analysis of Cr-spinel inclusions in Opx-rich rocks with occasional anhedral olivine crystals, show compositions (Mg# 62 - 36; Cr# 46 - 32) in the most primitive Cr-spinel grains, which in terms of Al2O3 (22 – 31 wt.%) and TiO2 (1.3 – 1.3 wt.%) are similar to Cr-spinel from MORBs. Geochemical evidences of the whole set of PA indicate a signature typical of arc magmatism characterized by calc-alkaline affinity, Nb-Ta troughs, LILE enrichment and peraluminous character. Particularly, a group of mafic rocks display high Al2O3 (>16 wt.%) accompanied by high Sr (>400ppm), absence of Eu anomaly, high Sr/Y (86-30) and Gd/Yb (3-1.8) which resemble adakitic magma compositions. Geocheochronological evidences strongly suggest that continuous processes should have ruled the origin of PA more than by sporadic events, whereas geochemical data imply the possible contribution of the oceanic subducted crust and continental material in the production of large amounts of silicic magma. Here we propose an alternative model in which the origin of the PA could be the result of a hybrid tectonic setting derived from the subduction of the young and hot Phoenix plate coupled with extensional regime and continental rift formation caused by the break up of Gondwana.
Alla fine del Triassico e durante il Giurassico si sono formate le grandi province magmatiche (LIP) della Central Atlantic Magmatic Province (CAMP) e della Patagonia (PA) contemporanee con la rottura della Pangea. Dichi e sills di composizione basaltica e basaltica-andesitica d’età e composizione affine alla CAMP, sono stati riportati nelle regioni di Hank e Hodh, in Mauritania, e di Kaarta in Mali, che sono oggetto della prima parte di questa tesi. I dati geochimici indicano che i campioni sono caratterizzati da variabili contenuti di TiO2 (0.39 - 2.29 wt.%) e da anomalie negative di Nb-Ta e anomalie positive di Pb, accompagnati da una firma arricchita negli isotopi di Nd-Sr-Pb (143Nd/144Nd201Ma: 0.51224 - 0.51241, 87Sr/86Sr201Ma: 0.70572 - 0.70795, 207Pb/204Pb201Ma: 15.61 – 15.68 208Pb/204Pb201Ma: 38.44 – 38.07 e 206Pb/204Pb201Ma: 18.11 – 18.48). I bassi rapporti di Nb/U (= 12 – 15) sono indicatori dell’apporto di materiale crostale, possibilmente riciclato. Questi valori di Nb/U suggeriscono, insieme alle composizioni isotopiche arricchite, un contributo della crosta continentale nella origine dei magmi CAMP in Mauritania e Mali. Variazioni nei rapporti di La/Yb mostrano l’esistenza di un gruppo arricchito tra le rocce di Kaarta (La/YbN> 4.3). Questa osservazione è anche supportata dalle correlazione positive con Nb/Zr e Th/Y e probabilmente riflette minori gradi di fusione parziale del mantello. Le composizione geochimiche osservate in Hank, Hodh e Kaarta, mostrano una forte affinità con le rocce intrusive del bacino di Taoudenni e le colate CAMP del Marocco. La correlazione fra queste regioni, fa pensare a una possibile distribuzione geografica oppure alla presenza di domini geochimici all’interno della CAMP. Comunque, quest’osservazione è basata su una stima qualitativa e per verificarla son necessari ulteriori studi. Il vulcanismo della Provincia è rappresentato da un’importante sequenza di età Giurassica ed è a composizione bimodale. Comprende infatti lave e ignimbriti riolitiche denominate come Formazione Marifil (ca. 189-177 Ma) e Chon Aike (ca. 170 – 153 Ma). Altre formazioni meno voluminose ma non meno importanti sono rappresentate dall’andesite della Formazione Bajo Pobre (BP) (ca. 166 – 154 Ma), e dai i basalti di Cañadón Asfalto (ca. 179 – 176 Ma) e Lonco Trapial. L’attività magmatica registrata per questa Provincia dura al meno per 38 Ma. Nuove datazioni U-Pb ottenute in questo progetto, confermano l’età delle rioliti di Marifil, che rappresentano quindi l’attività più antica della Provincia, comparabile solo parzialmente con le età conosciute per la LIP del Karoo (ca. 184 – 174 Ma). Le differenze geochimiche e di durata fra queste due LIP (Karoo: ca. 10 Ma e un magmatismo mafico, PA: 38 Ma e un magmatismo dominante acido) non appoggiano l’ipotesi d’un origine comune eventualmente da un mantle-plume unico. Lo studio del vulcanismo mafico – intermedio, rappresentato dalle Formazione Cañadon Asfalto (CA) e Bajo Pobre (BP), è stato sviluppato in questa tesi con l’obiettivo di conoscere le caratteristiche dei magmi meno evoluti che potrebbero portare informazioni sull’ambiente tettonico in cui si è formata la provincia. Con questo fine sono stati realizzati analisi di cromiti incluse in cristali di ortopirosseno e olivina. Le composizioni trovate nei cristali più primitivi (Mg# 62 – 36 e Cr# 46 – 32) di queste formazioni sono quasi identiche anche se sono geograficamente distanti. Il loro contenuto di Al2O3 (22 – 31 wt.%) e TiO2 (1.3 – 1.3 wt.%) è simile alla composizione di Cr-spinelli nei MORB. I dati geochimici in queste rocce, evidenziano una firma d’arco magmatico caratterizzata da una composizione calco-alcalina, anomalie negative di Nb-Ta, arricchimento di LILE e un’affinità peraluminosa. In particolare, alcune rocce mafiche mostrano alti valori di Al2O3 (>16 wt.%) e di Sr (>400ppm), assenza dell’anomalia di Eu e alti rapporti di Sr/Y (86-30) e Gd/Yb (3 - 1.8) comparabili alle composizione di magmi adakitici. La continua attività magmatica registrata dalla PA suggerisce che l’origine di queste rocce sia stato relazionata a processi costanti per almeno 38 Ma, più che ad eventi sporadici e di corta durata. Le evidenze geochimiche qui sposte suggeriscono il contributo di crosta oceanica e materiale continentale nell’origine di questa LIP. Perciò si propone un modello alternativo per l’origine della PA chesarebbe il risultato di un ambiente tettonico ibrido tra la subduzione di una placca giovane e calda che fonde (la placca Pheonix) e un regime distensivo relazionato alla disgregazione della Gondwana.

Este estudo foca no Complexo Várzea do Capivarita (CVC), localizado no sul da Província da Mantiqueira (PM), Brasil. A fim de investigar a evolução geológica do CVC, uma abordagem multi-disciplinar foi utilizada, incluindo geologia de campo, geologia estrutural, petrografia, geoquímica de elementos maiores e traços, isótopos de Sr-Nd e geocronologia U-Pb em zircão (LA-MC-ICP-MS e SHRIMP). O complexo compreende uma variedade de orto- e paragnaisses de composição e idade diversa. Volumes subordinados de sienitos sintectônicos também perfazem o CVC. A deformação é particionada em zonas de cisalhamento do tipo thrust (D1) e transcorrentes (D2), o que sugere tectônica transpressiva. O arcabouço estrutural descrito é possivelmente relacionado a um evento colisional oblíquo. Os estudos petrológicos e geocronológicos enfatizaram os ortognaisses do CVC a fim de avaliar as fontes magmáticas e paleo-ambientes envolvidos. Idades de cristalização obtidas nos domínios de zircão com zonação tipicamente ígnea variaram entre 780 e 790 Ma. Por sua vez, idades entre 640 - 650 Ma foram obtidas em sobrecrescimentos de zircão, sendo interpretadas como o registro da idade do metamorfismo de alto grau e fusão parcial associada. Os dados geocronológicos apresentados também indicaram que ambos os regimes cinemáticos foram contemporâneos, oferecendo, dessa forma, evidencia adicional para a hipótese de colisão oblíqua. Os ortognaisses do Complexo têm composição tonalítica a granítica e são rochas calcioalcalinas meta- a peraluminosas, com razões elevadas de 87Sr/86Sr (i) variando de 0.71628 a 0.72509 e valores εNd (790) entre -7.19 a -10.06. Sua composição e padrões de elementos traços sugerem que representem um magmatismo de arco maduro continental. O magmatismo registrado no CVC é compatível com outras sequências de arco de ca. 800 Ma, incluindo parte das metavulcânicas ácidas do Complexo Metamórfico Porongos (CMP) e os ortognaisses do Cerro Bori, Uruguai. Todas essas associações têm assinatura típica de orógenos acrescionários, contendo idade TDM Meso a Paleoproterozóica, além de forte evidência da participação de proscessos de assimilação crustal/ contaminação. Desta forma, o conjunto de dados apresentados permite interpretar essas associações como parte do mesmo magmatismo, ou pelo menos como fragmentos de arcos magmáticos similares. As assinaturas Sr-Nd e geoquímica sugere que ao menos parte das metavulcânicas do CMP represente os protólitos dos ortognaisses de alto grau inclusos no CVC. Adicionalmente, as evidencias isotópicas também apontam similaridade entre as rochas sedimentares de ambas as unidades, sugerindo que o CVC e o PMC são, ao menos em parte, expressões do mesmo contexto, onde a atividade magmática e sedimentar ocorreu em um mesmo ambiente de arco continental. A corroboração desta premissa é o objetivo principal de estudos de proveniência em andamento, cujos resultados prévios apontam para o caráter vulcano-sedimentar dos metapelitos do CVC e sua relação co-genética com os ortognaisses do CVC. Os dados isotópico Sr-Nd sugerem que os protólitos dos ortognaisses foram gerados por processos de assimilação crustal associados à cristalização fracionada. O modelamento binário (binary mixing model) realizado indica que o magmatismo estudado teria se originado de fontes mantélicas do tipo EM II. Uma seqüência paleoproterozóica de rochas TTG pertencente ao Complexo Arroio dos Ratos (CAR) é possivelmente o principal contaminante crustal assimilado. Em conjunto com as idades de herança descritas no CVC em ca. 2.0 Ga é sugerido que a fusão crustal que gerou o magmatismo do CVC em ca. 790-780 Ma foi predominantemente similar ao CAR.
This study focuses in the Várzea do Capivarita Complex (VCC), exposed in the southern part of the Neoproterozoic Mantiqueira Province (PM), Brazil. To investigate the evolutionary processes that lead the VCC construction, a multidisciplinary approach is taken, which includes field and structural geology, petrography, major and trace-element geochemistry, Sr-Nd isotope and U-Pb zircon geochronology by LA-MC-ICP-MS and SHRIMP. The complex comprises a compositional and age variety of ortho- and paragneisses tectonically interleaved during a high grade event. Subordinate volumes of syntectonic syenites are also part of CVC. The VCC deformation is partitioned into thrusting (D1) and transcurrent (D2) shear zones, suggestive of transpressive tectonics. This structural framework is possibly related to an oblique collision event. Petrological and goechronological studies emphasize the VCC orthogneisses in order to evaluate magmatic sources and related paleo-environments. Igneous crystallization ages obtained in the typical magmatic domains presenting oscillatory zoning in zircons vary between 780 and 790 Ma. Zircon overgrowths have ages mostly in the 650 – 640 Ma range and are interpreted to record the timing of high-grade metamorphism and associated partial melting. Geochronological data presented also indicates that boths kinematic regimes are contemporaneous, offering, therefore, further evidence for the oblique collisional event hypothesis. The VCC ortogneisses comprise tonalitic to granitic compositions and are metaluminous to peraluminous, calc-alkaline rocks, with high 87Sr/86Sr (i) ratios from 0.71628 to 0.72509 and εNd (790) values from -7.19 to -10.06. Their geochemical composition and trace-element patterns are compatible with a continental mature arc. VCC magmatism is correlated with other ca. 800 Ma arc sequences from southern PM, including part of the Porongos Metamorphic Complex (PMC) metavolcanic rocks and the orthogneisses from Cerro Bori, Uruguay. All these associations show signatures typical of accretionary orogens, TDM and Meso to Paleoproteroic inheritance ages, and present strong evidences of crustal assimilation/contamination. Thus, these sequences may be interpreted as part of the same magmatism, or at least as fragments of similar magmatic arcs. Geochemical and Sr-Nd signatures suggest that at least part of the PMC metavolcanic rocks may represent the protoliths of the VCC high grade orthogneisses. This, together with the isotope evidence of similarity between the sedimentary fractions of both unities, suggest that the VCC and PMC are, at least in part, expressions of the same context, wherein the magmatic and sedimentary activity occurred in a single continental arc environment. The corroboration of this premise is the main goal of provenience studies in prep, which previous results points to the volcano-sedimentary character of part of the VCC metapelites and its co-genetic relation with the VCC orthogneisses. Sr-Nd isotope data suggest that the orthogneiss protoliths were generated by crustal assimilation processes associated with fractional crystallization. Binary mixing models indicate that the VCC magmatism originates from evolved EM II mantle sources. A Paleoproterozoic TTG association (ca. 2.0 Ga) from the Arroio dos Ratos Complex (ARC) seems to be the main crustal contaminant assimilated. Together with the small inheritance contribution at ca. 2.0, this suggests that the melted crust at ca. 790-800 Ma was predominantly like ARC.

Dans cette thèse nous quantifions les interactions entre phénomènes tectoniques et magmatiques au niveau d'un rift proche de la rupture continentale: celui du Dabbahu / Manda Hararo en Afar (Ethiopie), qui a été le siège d'un épisode de rifting majeur en 2005. Nous apportons ici des contraintes temporelles sur ces interactions à l'échelle du millier à la dizaine de millier d'années, afin de mieux comprendre l'évolution des processus responsables de l'acquisition de la topographie. Pour cela nous avons produits des âges d'expositions de surfaces de coulées de laves et d'escarpement de failles par la technique des isotopes cosmogéniques. Ces datations ont été combinées à des travaux de cartographie et de géochimie (éléments majeurs et traces). Nous montrons que nord du segment est sous l'influence de la présence d'un volcan terminal qui impacte le développent de la topographie. Le milieu du segment permet d'évaluer l'impact des variations spatiales et temporelles de la localisation des réservoirs sur la topographie. Le long du segment, il apparaît que la topographie se développe quand les réservoirs magmatiques locaux ont une activité modérée à faible, alors que la topographie est inhibée lors des phases de magmatisme intense. Nos observations ont permis de monter que l'épisode de rifting de 2005 n'est pas représentatif de l'évolution globale de la morphologie du rift, et que ce type d'intrusion ne représente sans doute qu'un dixième des injections totales. Enfin, l'accrétion magmatique est soutenue à l'axe par différents réservoirs magmatiques qui semblent se relayer dans le temps, et dont la position peut varier de 15 kilomètres de part et d'autre de l'axe
In this thesis we quantify the interactions between tectonic and magmatic processes in the active Dabbahu rift segment, which experienced in 2005 a major rifting event. Here, we provide accurate constraints at this time scale by producing cosmogenic exposure ages of lavas surfaces and faults scarps, in order to decipher the morphological evolution of a rift depression, and to identify the processes that are responsible for the topography acquisition. The study is based on samples taken on two separate areas of the rift. The field areas were systematically mapped, and all lavas were chemically analysed in major and trace elements. The first studied focused the northern portion of the rift segment, where the lavas of the segment terminating volcano meets the neo-volcanic zone. This area allowed the influence of the presence of such volcanic edifice to be assessed. The second studied area is located at the middle of the segment, and covers the rift western margin up to the rift axis. The topography in this area is sensitive to the evolution in space and time of the location of the different axial magma reservoirs. Our results show that along the rift segment, topography develops during phases of low magmatic activity, and is inhibited during phases of lower magmatic activity. The 2005 appears to be an unusual event that represents about a tenth of the total intrusions necessary to build the depression. The maintaining of magmatic accretion at the axis seems to be the result of a relay between different magma reservoirs. The position of these reservoirs can vary between 0 and 15 km from both sides of the rift axis

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Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico
The Borborema Province (BP) is a geologic domain located in Northeastern Brazil. The BP is limited at the south by the S?o Francisco craton, at the west by the Parna?ba basin, and both at the north and east by coastal sedimentary basins. Nonetheless the BP surface geology is well known, several key aspects of its evolution are still open, notably: i)its tectonic compartmentalization established after the Brasiliano orogenesis, ii) the architecture of its cretaceous continental margin, iii) the elastic properties of its lithosphere, and iv) the causes of magmatism and uplifting which occurred in the Cenozoic. In this thesis, a regional coverage of geophysical data (elevation, gravity, magnetic, geoid height, and surface wave global tomography) were integrated with surface geologic information aiming to attain a better understanding of the above questions. In the Riacho do Pontal belt and in the western sector of the Sergipano belt, the neoproterozoic suture of the collision of the Sul domain of the BP with the Sanfranciscana plate (SFP) is correlated with an expressive dipolar gravity anomaly. The positive lobule of this anomaly is due to the BP lower continental crust uplifting whilst the negative lobule is due to the supracrustal nappes overthrusting the SFP. In the eastern sector of the Sergipano belt, this dipolar gravity anomaly does not exist. However the suture still can be identified at the southern sector of the Maranc? complex arc, alongside of the Porto da Folha shear zone, where the SFP N-S geophysical alignments are truncated. The boundary associated to the collision of the Cear? domain of the BP with the West African craton is also correlated with a dipolar gravity anomaly. The positive lobule of this anomaly coincides with the Sobral-Pedro II shear zone whilst the negative lobule is associated with the Santa Quit?ria magmatic arc. Judging by their geophysical signatures, the major BP internal boundaries are: i)the western sector of the Pernambuco shear zone and the eastern continuation of this shear zone as the Congo shear zone, ii) the Patos shear zone, and iii) the Jaguaribe shear zone and its southwestern continuation as the Tatajuba shear zone. These boundaries divide the BP in five tectonic domains in the geophysical criteria: Sul, Transversal, Rio Grande do Norte, Cear?, and M?dio Corea?. The Sul domain is characterized by geophysical signatures associated with the BP and SFP collision. The fact that Congo shear zone is now proposed as part of the Transversal domain boundary implies an important change in the original definition of this domain. The Rio Grande do Norte domain presents a highly magnetized crust resulted from the superposition of precambrian and phanerozoic events. The Cear? domain is divided by the Senador Pompeu shear zone in two subdomains: the eastern one corresponds to the Or?s-Jaguaribe belt and the western one to the Cear?-Central subdomain. The latter subdomain exhibits a positive ENE-W SW gravity anomaly which was associated to a crustal discontinuity. This discontinuity would have acted as a rampart against to the N-S Brasiliano orogenic nappes. The M?dio Corea? domain also presents a dipolar gravity anomaly. Its positive lobule is due to granulitic rocks whereas the negative one is caused by supracrustal rocks. The boundary between M?dio Corea? and Cear? domains can be traced below the Parna?ba basin sediments by its geophysical signature. The joint analysis of free air anomalies, free air admittances, and effective elastic thickness estimates (Te) revealed that the Brazilian East and Equatorial continental margins have quite different elastic properties. In the first one 10 km < Te < 20 km whereas in the second one Te ? 10 km. The weakness of the Equatorial margin lithosphere was caused by the cenozoic magmatism. The BP continental margin presents segmentations; some of them have inheritance from precambrian structures and domains. The segmentations conform markedly with some sedimentary basin features which are below described from south to north. The limit between Sergipe and Alagoas subbasins coincides with the suture between BP and SFP. Te estimates indicates concordantly that in Sergipe subbasin Te is around 20 km while Alagoas subbasin has Te around 10 km, thus revealing that the lithosphere in the Sergipe subbasin has a greater rigidity than the lithosphere in the Alagoas subbasin. Additionally inside the crust beneath Sergipe subbasin occurs a very dense body (underplating or crustal heritage?) which is not present in the crust beneath Alagoas subbasin. The continental margin of the Pernambuco basin (15 < Te < 25 km) presents a very distinct free air edge effect displaying two anomalies. This fact indicates the existence in the Pernambuco plateau of a relatively thick crust. In the Para?ba basin the free air edge effect is quite uniform, Te ? 15 km, and the lower crust is abnormally dense probably due to its alteration by a magmatic underplating in the Cenozoic. The Potiguar basin segmentation in three parts was corroborated by the Te estimates: in the Potiguar rift Te ? 5 km, in the Aracati platform Te ? 25 km, and in the Touros platform Te ? 10 km. The observed weakness of the lithosphere in the Potiguar rift segment is due to the high heat flux while the relatively high strength of the lithosphere in the Touros platform may be due to the existence of an archaean crust. The Cear? basin, in the region of Munda? and Icara? subbasins, presents a quite uniform free air edge effect and Te ranges from 10 to 15 km. The analysis of the Bouguer admittance revealed that isostasy in BP can be explained with an isostatic model where combined surface and buried loadings are present. The estimated ratio of the buried loading relative to the surface loading is equal to 15. In addition, the lower crust in BP is abnormally dense. These affirmations are particularly adequate to the northern portion of BP where adherence of the observed data to the isostatic model is quite good. Using the same above described isostatic model to calculate the coherence function, it was obtained that a single Te estimate for the entire BP must be lower than 60 km; in addition, the BP north portion has Te around 20 km. Using the conventional elastic flexural model to isostasy, an inversion of crust thickness was performed. It was identified two regions in BP where the crust is thickened: one below the Borborema plateau (associated to an uplifting in the Cenozoic) and the other one in the Cear? domain beneath the Santa Quit?ria magmatic arc (a residue associated to the Brasiliano orogenesis). On the other hand, along the Cariri-Potiguar trend, the crust is thinned due to an aborted rifting in the Cretaceous. Based on the interpretation of free air anomalies, it was inferred the existence of a large magmatism in the oceanic crust surrounding the BP, in contrast with the incipient magmatism in the continent as shown by surface geology. In BP a quite important positive geoid anomaly exists. This anomaly is spatially correlated with the Borborema plateau and the Maca?-Queimadas volcanic lineament. The integrated interpretation of geoid height anomaly data, global shear velocity model, and geologic data allow to propose that and Edge Driven Convection (EDC) may have caused the Cenozoic magmatism. The EDC is an instability that presumably occurs at the boundary between thick stable lithosphere and oceanic thin lithosphere. In the BP lithosphere, the EDC mechanism would have dragged the cold lithospheric mantle into the hot asthenospheric mantle thus causing a positive density contrast that would have generated the main component of the geoid height anomaly. In addition, the compatibility of the gravity data with the isostatic model, where combined surface and buried loadings are present, together with the temporal correlation between the Cenozoic magmatism and the Borborema plateau uplifting allow to propose that this uplifting would have been caused by the buoyancy effect of a crustal root generated by a magmatic underplating in the Cenozoic
A Prov?ncia Borborema (PB) ? um dom?nio geol?gico-estrutural localizado no Nordeste do Brasil, limitado a sul pelo Cr?ton do S?o Francisco, a oeste pela Bacia do Parna?ba e a norte e leste pelas bacias costeiras. Embora bastante estudada por geologia de superf?cie, na PB ainda est?o em aberto aspectos importantes de sua evolu??o, notadamente: i) a sua compartimentagem tect?nica ap?s a Orog?nese Brasiliana, ii) a arquitetura da margem continental implantada no Cret?ceo, iii) as propriedades el?sticas de sua litosfera, e iv) as causas do magmatismo e do soerguimento no Cenoz?ico. Esta Tese empregou dados geof?sicos de cobertura regional (eleva??o, gravimetria, magnetometria, altura geoidal e tomografia), para aportar informa??es de geologia profunda aos problemas acima colocados. A sutura gerada pela colis?o neoproteroz?ica entre o Dom?nio Sul da PB e a Placa Sanfranciscana (PSF) ? marcada, na Faixa Riacho do Pontal e no oeste da Faixa Sergipana, por uma forte anomalia gravim?trica dipolar, cujo pico positivo corresponde ao al?amento da crosta inferior da PB e o negativo corresponde ?s nappes de supracrustais empurradas sobre a PSF. Na regi?o leste da Faixa Sergipana n?o h? assinaturas gravim?tricas que indiquem cavalgamento e flexura de placas, mas a interpreta??o de truncamentos de assinaturas geof?sicas de dire??o N-S da PSF permite localizar a sutura na margem sul do complexo de arco Maranc?, ao longo da Z. C. Porto da Folha. Por sua vez, o limite colisional do Dom?nio Cear? da PB com o Cr?ton Oeste-Africano, ao longo da Z. C. Sobral-Pedro II, ? tamb?m marcado por uma anomalia gravim?trica dipolar, cujo pico positivo coincide com a Z. C. Sobral-Pedro II, e o negativo coincide com o arco magm?tico de Santa Quit?ria. A julgar pela express?o geof?sica, os limites internos mais importantes da PB s?o: i) a Z. C. Pernambuco Oeste e sua continua??o na Z. C. Congo, ii) a Z. C. Patos e iii) a Z. C. Jaguaribe e sua continua??o na Z. C. Tatajuba. Estes limites dividem a PB em cinco grandes dom?nios geof?sicos-tect?nicos: Sul (ou Externo), Transversal, Rio Grande do Norte, Cear? e M?dio Corea?. O Dom?nio Sul ? marcado por assinaturas geof?sicas associadas ? colis?o da PB com a PSF. O Dom?nio Transversal teve a sua concep??o original de limites modificada porque a parte leste do seu limite sul foi associada com a Z. C. Congo. O Dom?nio Rio Grande do Norte apresenta a crosta mais magn?tica da PB, com superposi??o de fontes pr?-cambrianas e faneroz?icas. No Dom?nio Cear?, a Z. C. Senador Pompeu ? o divisor de dois subdom?nios: o leste corresponde ? Faixa Or?s-Jaguaribe e o oeste corresponde ao Cear?-Central, onde ocorre uma assinatura gravim?trica interpretada como uma descontinuidade crustal de dire??o ENE-WSW, que funcionou como um anteparo para as nappes brasilanas, com sentido de deslocamento para sul. O Dom?nio M?dio Corea? apresenta uma anomalia gravim?trica dipolar, cujo pico positivo est? associado com rochas granul?ticas, e o negativo com rochas supracrustais. A assinatura geof?sica do seu limite com o Dom?nio Cear? ? evidente, apesar dos sedimentos da Bacia do Parna?ba. A an?lise conjunta da anomalia ar-livre, admit?ncia ar-livre e estimativas da espessura el?stica efetiva (Te) evidenciou que as margens Leste e Equatorial possuem propriedades el?sticas bastante diferentes: enquanto a primeira tem Te entre 10 e 20 km, a segunda tem Te em torno ou inferior a 10 km. Essa diferen?a ? devida ao enfraquecimento da litosfera da Margem Equatorial produzida pelo magmatismo cenoz?ico. A margem continental da PB apresenta segmenta??es que incorporaram heran?as das estruturas e dos dom?nios pr?-cambrianos, que se correlacionam com os limites conhecidos das bacias. Descrevendo de sul para norte, o limite da separa??o da Bacia Sergipe - Alagoas em duas sub-bacias coincide com a sutura entre o Dom?nio Sul da PB e a PSF; as estimativas de Te indicam, concordantemente, que a Sub-bacia Sergipe (Te ? 20 km) se instalou em uma litosfera mais resistente do que a da Sub-bacia Alagoas (Te ? 10 km). Adicionalmente, no interior da crosta da Sub-bacia Sergipe ocorre um grande corpo denso (underplating ou heran?a crustal?) que n?o continua na Sub-bacia Alagoas. A margem da Bacia de Pernambuco (15 < Te < 25 km) apresenta caracter?sticas diferentes das outras bacias costeiras, porque no Plat? de Pernambuco h? duas anomalias do efeito de borda , o que indica a exist?ncia no plat? de uma crosta continental afinada, contudo ainda relativamente espessa. A Bacia da Para?ba se apresenta bastante uniforme, com Te em torno de 15 km, e possui uma crosta inferior relativamente densa, que foi interpretada como uma modifica??o por underplating magm?tico relacionado com o magmatismo cenoz?ico. A segmenta??o da Bacia Potiguar em tr?s partes ? corroborada pelas estimativas de Te: Rifte Potiguar (Te ? 5 km), Plataforma de Aracati (Te ? 25 km) e Plataforma de Touros (Te ? 10 km). A fragilidade da litosfera na regi?o do Rifte Potiguar est? associada com fluxo t?rmico atual alto, e a resist?ncia relativamente maior da Plataforma de Touros pode ser devida a uma crosta arqueana. A margem da Bacia do Cear?, no trecho das sub-bacias Munda? e Icara?, possui anomalia ar-livre uniforme, com Te entre 10 e 15 km. A an?lise da admit?ncia Bouguer revelou que a condi??o isost?tica da PB ? compat?vel com um modelo em que ocorrem carregamentos combinados na superf?cie e na base da crosta, com a carga da base 15 vezes maior que a do topo. Em adi??o, a PB possui uma crosta inferior anormalmente densa. Estas afirma??es s?o especialmente adequadas para a parte norte da PB porque a? a ader?ncia dos dados observados ao modelo ? maior. Para o mesmo modelo isost?tico e usando a fun??o coer?ncia, estimou-se que a Te da PB deve ser inferior a 60 km, embora sua por??o norte tenha Te de apenas 20 km. A invers?o de espessura de crosta, usando o modelo isost?tico com carga apenas na superf?cie, indicou que existem na PB duas regi?es de espessamento: uma abaixo do Planalto da Borborema (de origem cenoz?ica) e a outra no Dom?nio Cear?, sob o arco magm?tico de Santa Quit?ria (vestigial do Pr?-cambriano). Por outro lado, ocorre um afinamento ao longo do Trend Cariri-Potiguar, que representa o registro no interior do continente de um rifteamento cret?ceo abortado. A interpreta??o das anomalias ar-livre de fontes oce?nicas levou ? proposi??o de que ocorreu um volumoso magmatismo na ?rea oce?nica adjacente ? PB, ao contr?rio da ?rea continental, como indicam as informa??es de geologia de superf?cie. A PB apresenta uma expressiva anomalia positiva de ge?ide, com correla??o espacial com o Planalto da Borborema e o Alinhamento Macau-Queimadas. A integra??o de dados de tomografia de ondas superficiais e de anomalias residuais de ge?ide permitiu interpretar que uma convec??o em pequena escala (Edge Driven Convection-EDC), gerada na interface entre a raiz da litosfera continental fria e o manto quente da ?rea oce?nica, pode ter sido a causa do magmatismo cenoz?ico. O mecanismo de EDC teria causado o arrasto do manto litosf?rico continental frio para dentro do manto astenosf?rico quente, ocasionando assim contraste positivo de densidade, que seria uma componente importante da origem da anomalia de ge?ide. A compatibilidade dos dados gravim?tricos da PB com o modelo isost?tico que combina carregamentos no topo e na base da crosta, e a correla??o temporal entre o magmatismo cenoz?ico e o soerguimento do planalto, permite propor que o soerguimento deste ocorreu por causa do empuxo provocado pela raiz da crosta, produzida por um underplating magm?tico no Cenoz?ico

La formation de la croute continentale est une des conséquences majeures de la différenciation de la Terre. Les avancées récentes dans la compréhension de ce phénomène résultent de l’amélioration des techniques analytiques permettant la mesure in situ des compositions isotopiques en U-Pb-Hf-O de grains de zircon, minéral abondant dans les roches crustales. Cette étude reconstitue l’histoire du segment de croute affleurant dans l’est du Massif Central français (MCF), portion de la chaine Varisque d’Europe de l’Ouest, dans le but d’évaluer les limites d’utilisation des zircons pour retracer l’évolution crustale. L’origine et la signification géodynamique des principales unités lithologiques du MCF ont été étudiées en combinant les approches classiques de la pétrologie avec des données isotopiques U-Pb-Hf-O acquises sur zircon. Deux incohérences majeures existent entre nos résultats et les conclusions tirées de l’étude des zircons considérés hors de leur contexte pétrologique, approche généralement suivie pour analyser l’évolution crustale. Les âges modèles calculés à partir des données Hf suggèrent une importante croissance crustale au Mésoproterozoique dans le MCF, en contradiction avec le fait que 60% de la croute locale soit d’âge Néoproterozoïque. De plus, 5 à 10% de la croute du MCF a été formée durant l’orogènese Varisque sans que cela ne soit enregistré par le zircon. Dans les deux cas, ces incohérences résultent du caractère hybride des signatures isotopiques portées par les zircons. Celles-ci ne peuvent être correctement détectées et interprétées qu’en disposant de données pétrologiques complémentaires sur les roches contenant les grains analysés
The formation of the continental crust is a major consequence of Earth differentiation. Understanding how the crust formed and evolved through time is paramount to locate the vast mineral deposits hosted therein and address its influence on the global climate, ultimately affecting the development of terrestrial life. Recent advances on the topic of continental crust evolution benefited from improvements of analytical techniques enabling in situ measurements of U-Pb- Hf-O isotope compositions in zircon, a widespread accessory mineral of continental igneous rocks. The time constrains derived from the U-Pb chronometer coupled with the petrogenetic information retrieved from Hf-O isotope signatures are currently used to unravel the diversity and succession of magmatic events affecting the continental crust at the regional and global scales. This study reconstructs the evolutionary path followed by the crust segment today exposed in the eastern part of the French Massif Central (FMC), a portion of the Variscan belt of Western Europe, with the aim to investigate the potential flaws of the zircon record of crust evolution. In this scope, the origin and geodynamic significance of the constituent FMC lithological units are tackled by combining conventional petrological observations with zircon U-Pb-Hf-O isotope data. The results obtained following this integrated approach are then confronted to the conclusions that would have been drawn solely from zircon isotopic signatures, taken out of their petrological context, as is commonly performed in studies investigating crust evolution. The oldest rocks of the FMC correspond to Ediacaran (590_550 Ma) meta-sediments deposited in back-arc basins along the northern Gondwana margin. Such basins were fed by a mixed detritus originating from the adjacent Cadomian magmatic arc and a distal Gondwana source, presumably the Sahara Metacraton. Partial melting of these meta-sediments at the Ediacaran/Cambrian boundary led to voluminous S-type granitic magmatism, pinpointing a first major crust reworking event in the FMC. The origin of anatexis likely stems from the transient thickening of the hot, back-arc crust caused by the flattening of the Cadomian subduction. Subordinate melting of the depleted backarc mantle at that time is also documented. During the Lower Paleozoic, rifting of the northern Gondwana provoked coeval crust and (limited) mantle melting. Mantle-derived igneous rocks show markedly diverse trace element and isotopic signatures, consistent with a very heterogeneous mantle source pervasively modi_ed by the Cadomian subduction. Finally, the Variscan collision resulted in crustal melting as evidenced by the emplacement of S-type granites and the formation of migmatite domes, the spatial distribution of which being partly controlled by the crustal architecture inherited from pre-orogenic events. Synchronous intrusion of mafic mantle-derived magmas and their differentiates testify for Variscan post-collisional new continental crust production in the FMC. Two major inconsistencies exist between these results and the zircon record. First, zircon Hf model ages would point to substantial Mesoproterozoic crust formation in the FMC whereas more than 60% of the crust is actually Neoproterozoic in age. Second, new additions to the continental crust volume during the Variscan orogeny are not recorded even though 5 to 10% of the exposed crust formed at that time. The origin of both discrepancies inherently lies in the mixed isotopic signature carried by many zircon grains. Such equivocal information can only be detected when additional petrological constrains on the zircon host rocks are available and provide guidance in interpreting the zircon record of crust evolution

2009/2010
Questa tesi di dottorato è focalizzata sui meccanismi dei sistemi magmatici che possono causare super-eruzioni, e ha come oggetto una sezione fossile che comprende le zone Ivrea-Verbano e la zona Serie dei Laghi (Italia, Nord-Ovest). Questa ricerca presenta evidenze di una caldera di età Permiana che espone il sistema magmatico fino alla profondità di 25 km. La correlazione delle età delle rocce vulcaniche e plutoniche della crosta media ed inferiore, mette in luce come queste costituiscano l’esposizione di un sistema magmatico che ha alimentato una caldera, e evidenzia la relazione cause-effetto tra l’intrusione di fusi basaltici derivanti dal mantello e il vulcanesimo acido. L’eruzione associata al collasso della caldera è stata di VEI > a 8 (Newhall and Self, 1982) e caratterizzata da una unica unità eruttiva a composizione riolitica. Il collasso della caldera è avvenuto in un campo vulcanico maturo, probabilmente tagliando il bordo di una caldera precedente. Il maggior volume di lave eruttate è composto da dacite alte in silice, i termini meno evoluti sono andesi-basalti. Le caratteristiche della caldera e del campo vulcanico sono simili a caldere formatesi durante la transizione da un regime tettonico compressionale ad uno estensionale-transpressivo. Il lavoro documenta una struttura interna della caldera simile a quella descritta per la caldera Grizzly Peak Colorado, USA (Fridrich et al., 1991) dove le frane escono come cunei dal “caldera wall” con una geometria simile ad un “albero di natale rovesciato” insieme ad una zonazione tra zone ricche di litici a zone di ignimbrite con pochi litici. Dopo il collasso, il riempimento della caldera è stato intruso direttamente da granito senza alcune evidenze di “caldera floor”. La composizione delle rocce della caldera del Sesia è compatibile con una ibridazione tra fusi basaltici derivanti dal mantello e una o più componenti anatettiche. La comparazione dell’eruzione che ha causato il collasso della caldera con le rocce del plutone sottostante non mette in luce una parentela con la zona superiore (Upper Valle Mosso); si aprono quindi nuove problematiche che richiedono ulteriori studi isotopici. Abbiamo documentato due stadi di alterazione idrotermale nel riempimento della caldera del Sesia, uno a più alta temperatura ed uno seguente a più bassa temperatura. Si può osservare che la circolazione idrotermale ha causato impoverimento di silice e un inizio di metasomatismo della roccia. La disposizione areale delle vene di quarzo e delle zone di silicificazione indicano che la deposizione ha interessato i confini tra materiali a differente porosità, in particolare tra la porosa ignimbrite intracaldera e materiali meno porosi come le rocce del “caldera wall” (grandi frane intracaldera ed il granito che ha intruso la caldera). Non abbiamo osservato alcuni dei fenomeni associati alla circolazione idrotermale nel granito che intrude il riempimento della caldera, perciò riteniamo che il contatto sia stato un importante confine alla circolazione dei fluidi idrotermali che circolavano principalmente nel riempimento della caldera.
XXIII Ciclo
1976

Knowledge of ocean continent transition (OCT) structure, continent ocean boundary (COB) location and magmatic type are of critical importance for understanding rifted continental margin formation and evolution, and in evaluating petroleum systems in deep-water frontier oil and gas exploration. A suite of quantitative analytical techniques have been developed in order to determine the structure of the OCT, the location of the COB and magmatic type at rifted continental margins; these techniques include the use of gravity anomaly inversion, residual depth anomaly (RDA) and subsidence analysis. Gravity anomaly inversion, incorporating a lithosphere thermal gravity anomaly correction, has been used to determine Moho depth, crustal basement thickness and continental lithosphere thinning. RDA analysis has been used to investigate OCT bathymetric anomalies with respect to expected oceanic bathymetries, and subsidence analysis has been used to determine the distribution of continental lithosphere thinning. These techniques have been applied to the Iberian, Gulf of Aden, northern Angolan, and south-eastern Brazilian rifted continental margins. Integrated quantitative analysis results have been validated using ODP well data and magnetic anomalies along the Iberian rifted continental margin. Results identify a clearly defined zone of exhumed mantle on the Iberian profiles, which is also observed in the ODP well data. In addition to the quantitative analysis techniques, a methodology for the joint inversion of deep long offset seismic data and gravity anomaly data has been developed in order to further constrain the OCT structure and COB location. The joint inversion method solves for coincident seismic and gravity Moho in the time domain and calculates the lateral variations in crustal basement densities and velocities along profile. Integrated quantitative analysis along the northern Angolan profile suggests that exhumed mantle, corresponding to a magma-poor margin, is absent beneath the allochthonous salt and that the thickness of earliest oceanic crust is approximately 7km. Integrated quantitative analysis along the south-eastern Brazilian profile predicts that the earliest oceanic crust is between 7km and 8km thick and that there is no evidence of exhumed mantle as suggested by Zalán et al. (2011). The results from the integrated quantitative analysis have also been used together with post-breakup subsidence modelling to determine the palaeo-bathymetry of the base Loeme salt along northern Angolan profiles. The proximal autochthonous base salt restores to near sea level, but not the distal allochthonous salt. Our interpretation is that the distal salt formed during the late syn-rift while the crust under it was being actively thinned, resulting in additional tectonic subsidence. Residual topography, which we attribute to mantle dynamic topography, has been determined using RDA analysis, with corrections for sediment loading and crustal basement thickness variations. Residual topography of -650m is measured along the Iberian Abyssal Plain; +400m in the Gulf of Aden, and +700m along the northern Angolan margin.

La jonction triple des Afars relie la mer Rouge, le golfe d'Aden et le rift éthiopien. Il est possible d'y étudier les différents stades d'extension depuis le rifting actif jusqu'aux marges continentales passives. Si l'extension et l'activité magmatique sont généralement considérées comme étant restreintes aux segments actifs des rifts ou aux dorsales océaniques, il existe pourtant une activité magmatique sur les flancs des marges continentales passives de l'Afar, du sud de la mer Rouge et du golfe d'Aden. La tomographie de bruit et la tomographie télésismique sont des méthodes complémentaires qui permettent d'imager la croûte ou le manteau supérieur et de mettre en évidence des systèmes magmatiques. Nos études de tomographie ont mis en évidence la présence de systèmes magmatiques complexes sous les flancs du rift Afar, en lien avec des sources hydrothermales en surface. La mise en place de matériel magmatique dense sous forme d'intrusions ou sous-plaqué au niveau de la croûte inférieure de l'Afar accommode l'extension en dehors des segments magmatiques actifs. De plus, les flancs de la marge de la mer Rouge au Yémen sont affectés par un magmatisme provoqué par des remontées localisées de matériel mantellique depuis une large zone de manteau anormalement chaud. Nous suggérons également, sous les épaules des marges passives de l'est du golfe d'Aden, la présence de systèmes magmatiques avec ou sans expression volcanique à la surface. Ces systèmes pourraient être alimentés par des cellules de convection petite échelle se mettant en place à la faveur de variations d'épaisseur de la lithosphère, en bordure de marge et à proximité des grandes zones de fracture du golfe d'Aden
The Afar triple junction is a unique region that links the Red Sea, the Gulf of Aden and the Ethiopian rift, and where it is possible to study the different stages of continental breakup, from early rifting to seafloor spreading and development of continental passive margins. Extension and magmatic activity are generally thought to be restricted to the active volcanic segments of the rift or to oceanic ridges. However, magmatic activity is also present on the continental margins flanks of the Afar rift, southern Red Sea and Gulf of Aden. The ambient noise tomography and teleseismic body-wave tomography are complementary methods that allow imaging the crust and upper mantle, and help to reveal magmatic systems. Our tomographic studies of the Afar region provide evidence for the presence of complex magmatic systems under the Afar rift flanks, currently modifying the crust and linked with hydrothermal systems near the surface. The emplacement of intruded or under-plated dense magmatic material in the Afar lower crust accommodates the extension outside of the active magmatic segments. In addition, the Red Sea margin flanks in Yemen are affected by magmatism caused by localized small-scale upwellings of mantle material, from a broad abnormally hot mantle zone located beneath Afar and southwestern Yemen. We also suggest the presence of magmatic systems with or without a volcanic expression at the surface, beneath the passive margins shoulders of the eastern Gulf of Aden. We propose that these systems could be fed by small-scale convection caused by lithosphere thickness variations at the edge of the Arabian plate and near the Gulf of Aden main fracture zones

The Andean Southern Volcanic Zone (SVZ) is a vast and complex continental arc that has been studied extensively to provide an understanding of arc-magma genesis, the origin and chemical evolution of the continental crust, and geochemical compositions of volcanic products. The present study focuses on distinguishing the magma/sub-arc crustal interaction of eruptive products from the Azufre-Planchon-Peteroa (APP 35°15’S) volcanic center and other major centers in the Central SVZ (CSVZ 37°S - 42°S), Transitional SVZ (TSVZ 34.3-37.0°S), and Northern SVZ (NSVZ 33°S - 34°30’S). New Hf and Nd isotopic and trace element data for SVZ centers are consistent with former studies that these magmas experienced variable depths of crystal fractionation, and that crustal assimilation is restricted to the lower crustal depths with an apparent role of garnet. Thermobarometric calculations applied to magma compositions constrain the depth of magma separation from mantle sources in all segments of the SVZ to(70-90 km). Magmatic separation at the APP complex occurs at an average depth of ~50 km which is confined to the mantle lithosphere and the base of the crust suggesting localized thermal abrasion both reservoirs. Thermobarometric calculations indicate that CSVZ primary magmas arise from a similar average depth of (~54 km) which confines magma separation to the asthenospheric mantle. The northwards along-arc Sr-Nd-Hf isotopic data and LREE enrichment accompanied with HREE depletion of SVZ mafic magmas correlates well with northward increasing crustal thickness and decreasing primary melt separation from mantle source regions indicating an increased involvement of lower crustal components in SVZ magma petrogenesis. The study concludes that the development of mature subduction zones over millions of years of continuous magmatism requires that mafic arc derived melts stagnate at lower crustal levels due to density similarities and emplace at lower crustal depths. Basaltic underplating creates localized hot zone environments below major magmatic centers. These regions of high temperature/partial melting, and equilibration with underplated mafic rocks provides the mechanism that controls trace element and isotopic variability of primary magmas of the TSVZ and NSVZ from their baseline CSVZ-like precursors.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2004.
Includes bibliographical references.
Continental magmatic arcs are among the most dynamic. geologic systems, and documentation of the magmatic, thermal, and tectonic evolution of arcs is essential for understanding the processes of magma generation, ascent and crustal growth. The primary goal of this research is to determine rates of tectonic and magmatic processes in the mid to deep crustal levels of the crystalline core of the Cretaceous North Cascades arc. This region was selected for study because it preserves a -10-40 km depth-section through the arc, which allows an assessment of magmatic and structural processes over a range of crustal levels. The relatively young age of the arc (ca. 100-45 Ma) and the inherent high-precision of U-Pb zircon dates permit absolute uncertainties of <100 ky. Meta-supracrustal rocks of the Cascades core record some of the highest pressures obtained in the North American Cordillera. The timing of deposition and metamorphism of the 9-12 kbar Swakane Gneiss constrain tectonic burial models and the timescales of large crustal displacements within an arc setting. These models involve rapid burial (-7 mm/yr) of a fore- or back-arc basin from ca. 73-68 Ma. Nd isotopic signatures of all meta-clastic terranes of the Cascades core reflect mixing of arc- and craton-derived sediment, and the Swakane Gneiss has the most isotopically-evolved signature of these terranes. Nd isotopic signatures of plutons that intrude the core lack evidence of melting of this isotopically-evolved unit. 40Ar/39Ar and U-Pb thermochronologic data define regional cooling patterns that suggest mid- to Late Cretaceous exhumation coincident with contraction and crustal thickening at the deepest levels of the core, followed by Early Tertiary extension.
(cont.) High-precision U-Pb geochronology reveals internal complexities inherent in the construction of an intrusive magmatic system. The Mount Stuart batholith was constructed over a ca. 5.6 Myr time period with four punctuated intervals of magma emplacement, whereas the Tenpeak intrusion was emplaced in a more continuous process over ca. 2.7 Myr time period. U-Pb zircon dates from two elongate intrusions, the Seven- Fingered Jack and Entiat suites, suggest that they were constructed from multiple magmatic sheets that were partially homogenized at the level of emplacement.
by Jennifer E. Piontek Matzel.
Ph.D.

Les laves alcalines continentales dérivent de magmas mantelliques, riches en éléments volatils, et se sont mises en place, soit dans un environnement orogénique, à proximité des zones de subduction, soit dans un environnement intracontinental éloigné de toute zone de subduction active. Leur genèse implique la fusion partielle des sources mantelliques soit à des profondeurs relativement faibles (<150 km), soit à plus grandes profondeurs (> 150 km) pouvant même atteindre le sommet de la zone de transition du manteau. Au niveau du plateau turco-iranien, les laves alcalines d'âge inférieur à 80 Ma présentent des signatures géochimiques pointant vers deux types de sources, c'est-à-dire des sources influencées par la subduction et d'autres intra-plaques (" OIB-like basalts "). Sur la base de la teneur en SiO2 et de l'indice d'alcalinité, les laves peuvent être regroupées en trois types principaux, les groupes I, II et II. Les groupes I et II sont des laves potassiques/ultra-potassiques et alcalines sodiques avec des compositions chimiques soit influencés par la subduction soit de type OIB, alors que les laves du groupe III ont une composition transitoire entre les séries fortement potassiques calco-alcalines à alcalines dont la composition est influencée par les processus de subduction. En particulier, la chaîne de Salavat, située au nord-ouest de l'Iran, est caractérisée par des laves (coulées, "pillow-lavas") présentant une population abondante de macrocristaux de clinopyroxène zonés. La comparaison des variations de la composition chimiques de ces cristaux zonés avec les éléments majeurs et traces sur roche totale montre que les différentes populations de clinopyroxènes zonés sont représentatives des magmas du groupe I et / ou II. Notre synthèse des images les plus récentes de tomographie sismique montre la présence d'un grand ensemble de corps elliptiques à faible vitesse sismique, d'une taille d'environ 100 km, que nous avons appelés " Poche de compaction ", dispersés entre ~ 400 km et 100 km de profondeur. Un modèle est développé montrant que la faible vitesse sismique des " Poche de compaction " résulte de la percolation et de la concentration dans ces grandes poches, de magmas riches en éléments volatils libérées à environ 410 km de profondeur dans le "Big Mantle Wedge". Ces magmas riches en éléments volatils interagissent avec le manteau environnant à une température inférieure de ~ 100 à 200 ° C à celle de l'adiabat mantellique subcontinental habituel, ce qui entraîne finalement la précipitation de phases minérales hydratées et carbonatées à ~ 8 -6,5 GPa, la fusion partielle du manteau métasomatisé, entre 6,5 GPa et 4 GPa, et enfin la précipitation de phases minérales hydratées / carbonées en dessous de 4 GPa. L'extraction de ces magmas s'effectue par l'intermédiaire d'un essaim de dykes qui se déclenche lorsque le sommet du Poche de compaction traverse successivement ces profondeurs critiques: ~ 240 km (8 GPa), ~ 165 km (5,5 GPa) et ~ 135 km (4,5 GPa). Il est montré que les trois groupes de laves (groupe I, II et III) peuvent être liés à ces trois événements successifs d'extraction à l'état fondu. Il est frappant de constater que la zonation des cristaux de clinopyroxène dans les laves de Salavat peut être représentative d'un magma du groupe I transporté par un dyke généré à 240 km de profondeur, qui se met en place près de la transition spinelle-grenat dans le manteau supérieur où il cristallise partiellement. Environ 0,3 Ma plus tard, lorsqu'un filon produit à 135 km de profondeur transportant un magma du groupe II traverse cette interface, il piège un assemblage partiellement cristallisé du magma du groupe I. Par la suite, ces deux magmas mélangés peuvent se mettre en place dans les niveaux superficiels froids de la croute où ils se solidifient simultanément. Ceci explique pourquoi les cristaux zonés de clinopyroxènes sont en équilibre avec différents types de magmas alcalins
Continental alkaline lavas are volatile-rich mantle derived magmas erupted either in orogenic setting, close to subduction zones, or in intracontinental setting far away from any active subduction zone. Their genesis involves partial melting of mantle sources either at shallow depths (< 150 km) or either at greater depths (> 150 km) down to the top of the Mantle Transition Zone. In the Turkish-Iranian Plateau, alkaline lavas younger than 80 Ma have geochemical signatures pointing to both types, i.e. subduction-influenced and intraplate (Oceanic Island Basalt-like). Based on SiO2 content and alkalinity index, the lavas can be grouped into three main types, group I, II and II. Groups I and II are ultrapotassic-potassic and sodic alkaline lavas with subduction-influenced and Oceanic Island Basalt-like chemical patterns, respectively whereas group III lavas have a transient composition between high-K calc-alkaline to calc-alkaline series with subduction-influenced chemical patterns. Particularly the Salavat Range, located in NW Iran, is characterised by lavas (flow, pillow structures dykes) displaying an abundant population of zoned clinopyroxene macrocrysts. A combination of mineral chemistry of the zoning patterns with the bulk rock major and trace element features evidence that the different populations of zoned clinopyroxene are representative of melts of group I and/or II. Our synthesis of the most recent available seismic tomography images show the presence of a large set of elliptic, ~100 km size, low velocities bodies, we named "Compaction pockets", scattered from ~400 km up to ~ 100 km depth. A model is developed showing that the low velocity of the compaction pockets result from the percolation and concentration into large pockets of volatile-rich melt liberated at ~ 410 km depth in the "Big Mantle Wedge". It is shown to result from interaction of volatile-rich melts with the surrounding mantle at a temperature ~100-200 °C lower than the usual sub-continental mantle adiabat, which eventually results in the precipitation of hydrated and carbonated mineral phases at ~8-6.5 GPa, partial melting of metasomatized mantle at 6.5 down to 4 GPa and finally in the precipitation of hydrated/carbonated phases below 4 GPa. Melt extraction occurs via swarm of dykes triggered when the top of the compaction pocket successively crosses these critical depths: ~240 km (8GPa), ~165 km (5.5 GPa) and ~135 km (4.5 GPa). It is shown that the three groups of lavas (group I, II and III) may be linked to these three successive melt extraction events. Strikingly, the clinopyroxene zonings recorded in the Salavat Range lavas may be representative of group I melt transported by a dyke generated at 240 km depth which pond close to the spinel-garnet mantle transition and eventually partially crystalized there. About 0.3 Ma later, when a 135 km deep dyke transporting a group II melt crosses that last interface, it traps partially crystalized assemblage of group I melt. Thereafter, the couple of mantle-derived melts may pound in a cold horizon where they simultaneously solidify and can explain the formation of clinopyroxene zonings in equilibrium with different alkaline melts

L’activité magmatique miocène en Petite Kabylie s’exprime par la mise en place de roches plutoniques et volcaniques de composition majoritairement riche en K ainsi que moyennement riches en K. Ces roches forment des pointements dispersés sur près de 130 km le long de la marge méditerranéenne de l’Algérie. Dans les deux secteurs d’étude : la Kabylie de Collo et l’Ouest Edough-Cap de Fer, elles recoupent les empilements de nappes de socle et de flyschs crétacés et numidiens. De nouvelles datations U-Pb sur zircons et K-Ar sur roche totale et minéraux séparés ont permis de fixer à 17 Ma le début de l’activité magmatique post-collisionnelle à affinité calco-alcaline riche en K2O. Ces âges obtenus sur le batholithe granitique de Bougaroun (200 km2) sont les plus anciens jamais obtenus dans toute la Marge Méditerranéenne du Maghreb. L’activité magmatique s’étend vers l’Est et atteint la zone ouest-Edough-Cap de Fer vers ~16 Ma puis se poursuit de façon intermittente dans les deux secteurs d’étude à ~15 Ma, 14-13 Ma jusqu’à 11 Ma avec la mise en place de corps filoniens mafiques et felsiques en Kabylie de Collo. En outre, un âge oligocène supérieur (27.0 ± 3.0 Ma et 23.3 ± 3.2 Ma) a été mesuré par la méthode Ar/Ar sur amphiboles des gabbros à caractère océanique du Cap Bougaroun s.s (Kabylie de Collo). Les nouvelles données géochimiques et isotopiques ont permis de mettre en évidence deux sources pour le magmatisme dans les deux secteurs étudiés. Une première source mantellique appauvrie, non modifiée par un composant de subduction qui est à l’origine des gabbros à caractère océanique du Cap Bougaroun s.s et de Bou Maïza au Sud de l’Edough. Ceux-ci pourraient représenter des reliques du stade de rifting d’âge oligocène supérieur en prélude à l’ouverture en position arrière-arc du bassin algérien. Une deuxième source enrichie en terres rares légères et en éléments mobiles est représentée par le manteau lithosphérique subcontinental kabyle précédemment métasomatisé durant la subduction à vergence nord de la lithosphère océanique téthysienne au Paléogène. Les magmas mafiques enrichis en LREE issues de cette source ont ensuite évolué par cristallisation fractionnée et contamination crustale pour former les roches intermédiaires et felsiques de la marge est-algérienne. Nous proposons un modèle tectono-magmatique de rupture de slab téthysien associée à une délamination crustale au niveau des bordures des deux lithosphères continentales africaine et kabyle. A 17 Ma, le flux thermique d’origine asthénosphérique ascendant à travers la déchirure du slab téthysien induit la fusion du manteau téthysien. Les magmas mafiques calco-calcalins moyennement potassiques subissent des échanges chimiques avec le socle africain durant leur ascension à travers celui-ci, générant les magmas intermédiaires et felsiques calco-alcalins riches en K caractérisés par une importante signature crustale
The Miocene igneous activity in Lesser Kabylia includes a ~130 km-long EW-trending lineament that extends along the eastern Algerian margin from Kabylie de Collo to Ouest-Edough-Cap de Fer area. It includes mostly medium-K to High-K calc-alkaline plutonic and volcanic rocks. In the studied area, these magmatic rocks crosscut and/or overlie the inner zones of the Maghrebides represented by basement and Kabylian cretaceous and Numidian flyschs nappes. New U-Pb dating on zircons and K-Ar ages on whole rocks and separated minerals document a 17 Ma onset for the post-collisional K-rich calc-alkaline magmatism. These Upper Burdigalian ages obtained on the Bougaroun pluton are the oldest presently identified for Krich calc-alkaline rocks in the whole 1200 km-long EW trending magmatic belt located along the Mediterranean coast of Maghreb. However, according to new K-Ar ages, magmatic activity started in Ouest Edough zone at ~16 then persisted intermittently in the two studied areas at ~15.5 Ma, 14-13 Ma and stopped at ~11 Ma, with the emplacement of mafic and felsic dykes in Kabylie de Collo. In addition, we measured older (Upper Oligocene) Ar-Ar hornblende ages of 27.0 ± 3.0 Ma and 23.3 ± 3.2 Ma on LREE-depleted gabbros outcropping at Cap Bougaroun sensu stricto. According to our new geochemical and isotopic data, we distinguish two sources for magmatic rocks in the studied area: a depleted mantle source which could represent the ambient asthenosphere still not modified by the subduction processes at the time of emplacement of the Upper Oligocene LREE-depleted gabbros. The latter could be related to the Upper-Oligocene rifting before the back-arc crust formation in Algerian basin or to dyke systems or gabbroic intrusions crosscutting the stretched Kabylian continental crust. An enriched mantle source modified by a subduction component (melt or fluid) escaping from a northward-dipping subducted Tethyan oceanic lithosphere. The enriched mafic magmas are believed to come from this metasomatized mantle and are genetically related to the differentiated rocks through crystal fractionation and assimilation of large amounts of crustal lithologies, during their ascent through the African continental crust. We propose a tectono-magmatic model involving an Early Miocene Tethyan slab breakoff combined with delamination of the edges of the African and Kabylian continental lithospheres. At 17 Ma, the asthenospheric thermal flux upwelling through the slab tear induced the thermal erosion of the Kabylian lithospheric mantle metasomatized during the previous subduction event and triggered its partial melting. We attribute the strong trace element and isotopic crustal signature of Bougaroun felsic rocks to extensive interactions between ascending mafic melts and the African crust underthrust beneath the Kabylie de Collo basement

The processes and fluxes that produce the distinct compositional structure of Earth's continental crust by subduction remain controversial. The rates of oceanic crust production, in contrast, are well quantified and are generally believed to be faster than those responsible for building magmatic systems in subduction settings. Here we show that a recently recognized crustal section, the 30-km-thick Ordovician Sierra Valle Fertil-Sierra Famatina complex in Argentina, was built magmatically within only similar to 4 m.y. More than half of the crustal section represents additions from the mantle, and is preserved as mafic igneous rocks and maficultramafic cumulates; the remainder is tonalite to granodiorite with evidence for widespread assimilation from highly melted metasedimentary units. U-Pb zircon geochronology reveals that the construction of the arc was not a simple bottom-up construction process. This continuous exposure of the arc crust allows the quantification of field constrained magmatic addition rates of 300-400 km(3) km(-1) m.y.(-1). These rates are similar to those determined for modern slow-spreading mid-ocean ridges and are of the same magnitude as magmatic addition rates required to build certain large segments of the continental masses such as the Arabian-Nubian shield, among others. The implication is that significant convective removal of arc roots is required over time in order to build the modern continental crust via subduction-related magmatism.

xviii, 177 p. : ill., maps. A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number.
This dissertation explores several important consequences of H 2 O-rich fluids in magmatic and ore-forming systems within continental arc crust. North Sister, a stratovolcano in the Oregon High Cascades, provides a window into magma generation processes in the deep crust. Eruption of a remarkably limited basaltic andesite composition over the lifespan of this volcano may reflect last equilibration of mantle derived magma within a deep crustal hot zone. High pressure, water-undersaturated phase equilibrium experiments show that an anhydrous, augite-rich gabbro at ∼12 kbar (40 km depth) and ∼ 1175°C is the most probable lithology with which North Sister basaltic andesite with ∼3.5 wt% H 2 O last equilibrated within the deep crust before erupting. While magma often erupts at the planets surface as at North Sister, a greater volume never reaches the surface and solidifies within the upper crust. Exsolution of magmatic fluids is an inevitable consequence of crystallization of hydrous crustal magmas. The fate of these fluids is the focus of the remainder of this dissertation. Modeling of CO 2 and H 2 O variations during crystallization of granitic magma reveals that exsolution of a large mass of fluid occurs only after CO 2 is largely degassed, creating ideal conditions for hydrofracturing and formation of porphyry copper deposits. CO 2 and H 2 O solubility relations suggest that H 2 O-rich magma was required to produce the porphyry-Cu-Mo deposit at Butte, Montana, which may explain its distinctively deep generation. Electron microprobe analyses of Ti in quartz and Zr in rutile in samples from Butte yield porphyry magma temperatures (630-770°C) that overlap substantially with hydrothermal vein temperatures (<430-750°C). Veins display large temperature ranges (50-250°C) that signify variable degrees of cooling of hot magmatic fluids upon contact with cooler wall rock during vein growth. Modeling of Ti diffusion in quartz suggests that individual dikes and veins likely cooled over short timescales (10s-1000s years), indicating that porphyry systems may evolve by episodic magmatic fluid injections with discrete thermal spikes. Modeling of Ti diffusion in quartz combined with electron backscatter diffraction maps show that small hydrothermal quartz veins likely formed by epitaxial growth. This dissertation includes co-authored material both previously published and in preparation for submission.
Committee in charge: A. Dana Johnston, Chairperson, Geological Sciences; Mark Reed, Member, Geological Sciences; Paul Wallace, Member, Geological Sciences; Richard P. Taylor, Outside Member, Physics

Though continental magmatic arcs are factories for new continental crust, a significant proportion of continental arc magmas are recycled from supracrustal material. To evaluate the relative contributions of retroarc underthrusting and trench side partial sediment subduction for introducing supracrustal rocks to the middle and lower crust of continental magmatic arcs, we present results from the deeply exposed country rocks of the Coast Mountains batholith of western British Columbia. Prior work demonstrates that these rocks underwent widespread partial melting that contributed to the Coast Mountains batholith. We utilize U-Pb zircon geochronology, Sm-Nd thermochronology, and field-based studies to document the protoliths and early burial history of amphibolite and granulite-facies metasedimentary rocks in the Central Gneiss Complex. U-Pb detrital zircon data from the structurally highest sample localities yielded similar to 190Ma unimodal age peaks and suggest that retroarc rocks of the Stikine terrane constitute a substantial portion of the Central Gneiss Complex. These supracrustal rocks underwent thrust-related burial and metamorphism at >25km depths prior to similar to 80Ma. These rocks may also be underlain at the deepest exposed structural levels by Upper Cretaceous metasedimentary rocks, which may have been emplaced as a result of trench side underplating or intraarc burial. These results further our understanding of the mechanisms of material transport within the continental lithosphere along Cordilleran subduction margins.

L'etude porte sur les variations dans le temps - de l'albien au pliocene - et dans l'espace - le long d'une transversale des andes du perou central vers 11 de latitude sud - des caracteres petrographiques et geochimiques (elements majeurs et traces, isotopes de sr, nd et pb) des roches magmatiques intrusives et effusives, essentiellement calco-alcalines, associees a la subduction des plaques oceaniques farallon puis nazca sous la marge occidentale du continent sud-americain, et sur les relations entre ce magmatisme, l'evolution des modalites de la convergence ocean-continent, l'evolution orogenique de la marge continentale et la metallogenese a echelle regionale. Les modifications successives des modalites de la subduction permettent de rendre compte de maniere precise de la distribution dans le temps et l'espace du magmatisme, qui est elle-meme independante des crises tectoniques. Les roches plutoniques de type i et le volcanisme calco-alcalin de l'arc derivent des memes sources mantelliques (manteau non deprime modifie par des fluides extraits de la plaque subduite). Des differences de chimisme indiquent que les histoires crustales des magmas qui donneront roches plutoniques et volcaniques divergent tot et que les plutons ne correspondent pas aux chambres magmatiques ayant alimente les volcans. Depuis l'albien les compositions chimiques et isotopiques des roches magmatiques ont evolue essentiellement (pour un meme stade de differentiation) par une succession plus ou moins monotone de sauts qui sont contemporains des crises tectoniques et son interpretes comme resultant d'une augmentation de la contamination crustale et d'une evolution sous pression de plus en plus forte au stade precoce de differentiation des magmas dans des chambres profondes. La distribution dans le temps et l'espace des gisements metalliques associes a ce magmatisme est interpretee en termes d'interaction entre magmas d'origine mantellique et segments particuliers de la croute continentale

Ce mémoire de thèse comporte deux parties: l'une sur les ophiolites et l'autre sur les minéralisations de divers complexes basiques-ultrabasiques, dont les ophiolites, les komatiites et les sills archéens et protérozoïques. Les travaux sur les ophiolites ont abouti à deux résultats majeurs : 1) la reconstitution du puzzle ophiolitique corse ; 2) la diversité des ophiolites avec une classification en deux grands types, dont le magmatisme de l'un s'apparente à celui des rides médio-océaniques, tandis que le magmatisme de l'autre ressemble à celui des arcs immatures. Des études sur la minéralisation des corps basiques-ultrabasiques, il ressort que la nature des magmas parentaux est importante pour juger de leur potentialité métallogénique (Cr, Ni, éléments du groupe du platine EGP). Le caractère saturé à sursaturé des magmas basiques a été noté dans les ophiolites porteuses de gisements de chrome, dans les lopolites stratifiés enrichis en Cr, EGP. Des magmas de type boninitiques seraient à l'origine de certains de ces complexes. En conclusion de la thèse est présentée une classification des complexes basiques-ultrabasiques basées sur l'ordre de cristallisation des minéraux qui permet de discuter de l'origine des gisements des EGP par rapport à celle des autres minéralisations (chromite, concentration sulfurées).

La composition évoluée de la croûte continentale suggère qu'une partie mafique a été perdue au cours du temps géologique. Cependant, certaines intrusions mafiques et ultramafiques, telles que le complexe du Bushveld, Afrique du Sud, et le Great Dyke, Zimbabwe, ont été conservés dans la croûte pendant des milliards d'années. La question est alors de savoir comment les cumulats mafiques sont perdus et, plus précisément, quelle est l'évolution à long terme d'un réservoir de magma y compris éventuellement les processus de post-emplacement et post-cristallisation. Ce travail vise à ce question avec l'approche suivante. Dans un premier temps, nous effectuons des expériences de laboratoire avec des fluides visqueux pour enquêter sur l'instabilité associée à une inversion de flottabilité et d'individuer des lois d'échelle simples régissant les différents régimes dynamiques. Nombreuses intrusions mafiques préservent structures d'écoulement prouvant qu'elles ont été affectées par des instabilités gravitationelles compatibles avec les régimes observés au laboratoire. Dans une deuxième partie, nous étudions les conditions physiques dans lesquelles les intrusions mafiques deviennent instables à l'aide des simulations numériques qui reproduisent les écoulements dans croûte terrestre. La conclusion la plus importante est que le paramètre principal contrôlant l'instabilité est la profondeur d'emplacement. Donc, le collapse gravitationnel des parties mafiques dans la croûte est un processus très commun dans les régions volcaniques. Ce mécanisme devrait faire la lumière sur les processus qui régissent la formation de la croûte terrestre
The evolved bulk composition of the continental crust suggests that a large mafic portion has been lost during the geological time. However, mafic and ultramafic bodies, such as the Bushveld complex, South Africa, and the Great Dyke Zimbabwe, have been preserved in the crust for billions of years. The implied question, then, is how mafic cumulates are lost and, more specifically, what is the long-term evolution of a magma reservoir, possibly including post-emplacement and post-crystallization processes. This work aims at this question with the following approach. First , we perform laboratory experiments with viscous fluids to investigate the instability associated to a buoyancy reversai and derive simple scaling laws governing the different dynamical regimes. Many mafic intrusions preserve flow structures, proving that they were affected by gravitational instabilities consistent with the regimes observed in the laboratory. In a second part, we investigate the physical conditions under which mafic intrusions become unstable using extensive numerical simulations which reproduce crustal flows. The mort important finding is that the main control on the instability is the emplacement depth. The results of this work thus suggests that many of the mafic intrusions we obsei:ve4today at the outcrop are the vestiges of much larger systems that became unstable. Consequently foundering and sinking of mafic cumulates through the crust may be a very common process in volcanic regions. This mechanism should shed light on the processes governing the formation and differentiation of the Barth crust

The Central Atlantic Magmatic Province (CAMP) is the large igneous province (LIP) that coincides with the breakup of the supercontinent Pangea. Major and trace element data, Sr-Nd-Pb radiogenic isotopes, and high-precision olivine chemistry were collected on primitive CAMP dikes from Virginia (VA). These new samples were used in conjunction with a global CAMP data set to elucidate different mechanisms for supercontinent breakup and LIP formation. On the Eastern North American Margin, CAMP flows are found primarily in rift basins that can be divided into northern or southern groups based on differences in tectonic evolution, rifting history, and supercontinental inheritance. Geochemical signatures of CAMP suggest an upper mantle source modified by subduction processes. We propose that the greater number of accretionary events, or metasomatism by sediment melts as opposed to fluids on the northern versus the southern Laurentian margin during the formation of Pangea led to different subduction-related signatures in the mantle source of the northern versus southern CAMP lavas. CAMP samples have elevated Ni and low Ca in olivine phenocrysts indicating a significant pyroxenite component in the source, interpreted here as a result of subduction metasomatism. Different collisional styles during the Alleghanian orogeny in the North and South may have led to the diachroneity of the rifting of Pangea. Furthermore, due to a low angle of subduction, the Rheic Plate may have underplated the lithosphere then delaminated, triggering both the breakup of Pangea and the formation of CAMP.
Master of Science

Dans le cadre du projet YOCMAL ANR, nous avons cartographié les variations de l’épaisseur crustale, la structure et les variations de vitesses sismiques sous le golfe d’Aden occidental, le sud de la mer Rouge et l’île de Socotra (marge sud-est du golfe d’Aden). Nous analysons les fonctions-récepteurs des ondes P dans le but d’aborder le rôle du magmatisme, de l’étirement et de l’amincissement crustal dans un contexte de marges volcaniques et non-volcaniques. Les données de 46 stations sismiques large-bande ont été analysées provenant de réseaux temporaires et permanents (GFZ-Yemen) au le Yémen continental et sur l’île de Socotra. Nous montrons : (1) pour le Yémen occidental, une épaisseur de la croute non étirée de ~35 km d’épaisseur. Elle s’amincit jusqu’à ~22 km dans les régions côtières pour atteindre moins de 14 km sur la côte de la mer Rouge, avec la présence d’un corps à haute vitesse en base de croûte inférieure (HVLC). Le rapport moyen Vp/Vs pour les plateaux du Yémen occidental est de 1. 79, augmentant jusqu’à 1. 92 près des côtes de la mer Rouge et diminuant jusque 1. 68 sur ou à proximité des roches granitiques. (2) Pour l’île de Socotra, l’épaisseur de la croûte non étirée est de ~28 km et de 21km vers l’ouest de l’île, avec un amincissement crustal jusqu’à ~16 km en allant vers la côte située le plus au nord-ouest de l’île. De faibles rapports Vp/Vs (1. 65-1. 71) sont observés près des zones granitiques, des valeurs intermédiaires (1. 75-1. 84) près des bassins sédimentaires et des valeurs plus fortes (supérieure à 1. 89) sous l’extrémité nord-ouest de l’île de Socotra. A l’axe actif de la dorsale d’Aden, à l’ouest du golfe, un important événement magmato-sismique s’est se produit. Nous utilisons les réseaux regions sismologiques de Djibouti et du Yémen pour localiser les 1134 séismes de taille modérée qui se produisent de Nov. 2010 à Mar. 2011, le long d’un segment de direction N115°E de la vallée axiale centré sur la longitude 44°E et la latitude 12°N. Le moment sismique cumulé associé à cette séquence atteint 6. 8x1017 N. M et avec un très faible moment sismique versus le moment géodésique, nous estimons une ouverture horizontal d’environ 0. 4 à 2. 2 m. Nous interprétons cet événement, confiné à un seul segment de la ride océanique, causé par l’activité magmatique suivie par de intrusions rapides dans la croûte. Des mécanismes décrochants sont cohérents avec la formation d’une zone transformante à la limite ouest du segment actif et au passage vers la zone ou la lithosphère est transitionnelle
In the framework of YOCMAL ANR project, we map crustal thickness, structure and bulk seismic velocity variations across the western Gulf of Aden, southern Red Sea and Socotra (southeastern margin of the Gulf of Aden). We analyse P-wave receiver functions in order to address the role of magmatism, faulting and crustal thinning in both volcanic and nonvolcanic margins setting. We analyse teleseismic data from 46 stations of the temporary networks with permanent (GFZ-Yemen) in Yemen mainland and Socotra Island. We show that: (1) for western Yemen the thickness of unextended crust on the Yemen plateau is ~35 km and thins to ~22 km in coastal areas reaching less than 14 km on the Red Sea coast, with the evidence of the presence of a high velocity lower crust (HVLC). The average Vp/Vs ratio for the western Yemen Plateau is 1. 79, increasing to ~1. 92 near the Red Sea coast and decreasing to 1. 68 for stations located on or near the granitic rocks; and (2) for Socotra Island the thickness of unextended crust for the central Socotra is ~28 km thins to ~21 km to the west with a trend of crustal thinning to ~16 km on the west-northernmost coast of the Island. Low Vp/Vs ratio of 1. 65-1. 71 is observed for the stations located on or near granitic bodies, intermediate values of 1. 75-1. 84 are observed for the sedimentary basins and higher value more than 1. 89 is observed below the northern coast. In the spreading axis of the western Gulf of Aden, a significant seismo-volcanic event occurred. We used regional seismic networks in Djibouti and Yemen to locate 1134 moderate size earthquakes occurred from Nov. 2010 to Mar. 2011, along N115°E segment of the axial valley centered at 44. 024°E and 12. 021°N. The cumulative seismic moment associated with this sequence 6. 8x1017N. M, and taking into account a very low seismic versus geodetic moment, we estimate anhorizontal opening of ~0. 4 to 2. 2m. We interpreted this event to be mainly caused by magmatic activity focused along a single spreading segment of the Aden ridge, followed by strike-slip focal mechanisms at the locus of the change of trend and limit of the oceanic lithosphere

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Os maciços do Estreito (MES), Canabrava (MCB) e Mato Verde (MMV) localizam-se no extremo sudoeste do Estado da Bahia e nordeste do Estado de Minas Gerais. Estes maciços possuem forma alongada, controlada por sistemas de falhas regionais, estão intrudidos nas rochas arqueanas do Complexo Santa Izabel (CSI), e são constituídos essencialmente por rochas monzoníticas, sieníticas e graníticas, com grandes cristais de feldspato alcalino, tendo a biotita como mineral máfico principal. Os dados geocronológicos (U-Pb e 207Pb/206Pb) existentes para o Maciço do Estreito possibilitam estabelecer que 2,05 Ga, representa sua idade de colocação e cristalização. As similaridades entre os maciços estudados permitem sugerir que foi nesta época que se colocaram os corpos Canabrava e Mato Verde. Esta idade confirma que este magmatismo faz parte não só do Batólito Guanambi, mas, também, do magmatismo alcalino-potássico Transamazônico no Estado da Bahia. Os maciços em estudo são compostos predominantemente por rochas hololeucocráticas a leucocráticas, levemente anisotrópicas. A mineralogia e história de cristalização destas rochas são muito semelhantes, e apontam para condições de evolução e de cristalização similares. Os dados químicos mostram que as rochas do MES, MCB e MMV são: saturadas a super-saturadas em sílica; alcalinas, com alcalinidade média; potássicas a fortemente potássicas; metaluminosas a peraluminosas; enriquecidas em Ba, Sr, K, P2O5, ETR e empobrecidas em Y, Nb e Ti. Os padrões dos ETR mostram um forte enriquecimento dos ETRL em relação aos ETRP. Os espectros obtidos nos diagramas multielementares são caracterizados por: acentuados vales em Nb, P e Ti; vales ocasionais em Ba e Sr; picos em Rb; e anomalias positivas e negativas de Th. As relações entre os elementos maiores, elementos incompatíveis e compatíveis, bem como o modelamento petrogenético, indicam que a cristalização fracionada foi o principal processo atuante na evolução das rochas destas intrusões. Os diversos dados químicos e petrográficos evidenciam uma afinidade shoshonítica para as rochas dos maciços em estudo. As assinaturas geoquímicas identificadas são compatíveis com a de magmas associados a ambiente orogênico do tipo arco continental.
The Estreito, Canabrava and Mato Verde massifs are located in the southwester of Bahia State and northeast of Minas Gerais State. Theses massifs have elongated shape, are controlled by faults regional systems and intrude Archean rocks from Santa Izabel Complex. They are composed by monzonitic, syenitic and granitic rocks with porphires of alkali feldspar and biotite as the main mafic mineral. The geochronological data (U-Pb and 207Pb/206Pb) for the Estreito Massif demonstrate that it was emplaced 2.05 Ga ago, and the similarity with the others studied massifs suggest that they were emplaced at the same time. This age also allow us to relate this magmatism with the Guanambi Batholith and the Transamazonic alkaline-potassic magmatism in the Bahia State. These massifs are composed by hololeucocratic to leucocratic and anisotropic rocks. The mineralogy and crystallization history are very similar and point out to the same conditions of crystallization and evolution. Geochemical data show that all the rocks of these massifs are Si-satured to – oversatured; alkaline, with medium alcalinity; potassic; metaluminous to peraluminous; Ba, Sr, K, P2O2 and REE enriched; low Ti, Nb and Y content. Chondrite-normalized REE patterns show strongly fractionated LREE. The multielementar diagrams are characterized by Nb, P and Ti depletions, occasionally with Ba and Sr; peak in Rb; and negative and positive anomalies in Th. The relationship between major, incompatible and compatible elements, together with the petrogenetic modeling show that fractionated crystallization was the main controlled process during the massifs rocks evolution. Chemical and petrographic data reveal a shoshonitic affinity for these massifs. The geochemical signature point to a magma associated with orogenic environment as continental arc.

Les rifts continentaux résultent de l'action de contraintes extensives dont la magnitude est suffisante pour déformer un continent (forces aux limites des plaques, mouvements asthénosphériques). Cette déformation, contrôlée notamment par la rhéologie ou encore l'héritage structural lithosphérique, se réalise par des processus magmatiques (“dyking”) et tectoniques (rupture sur faille et étirement ductile) dont l'importance relative est mal connue et variable d'un segment de rift à l'autre. Afin de mieux comprendre comment ces différents facteurs et processus interagissent, la Divergence Nord-Tanzanienne (DNT) apparaît comme une cible privilégiée du Rift Est-Africain : elle représente un stade précoce du rift et montre une transition abrupte dans le style morphotectonique et l'expression du volcanisme. Cette étude a consisté à déployer un réseau sismologique local dans la DNT et à exploiter les données issues de l'enregistrement continu de l'activité sismique pendant 6 mois (campagne SEISMO-TANZ 2007). Les signaux des séismes proches et lointains sont utilisés comme indicateurs de la sismogénèse, de la résistance crustale, des champs de déformation et de contraintes, et renseignent aussi sur la structure et la fabrique (anisotropie) lithosphérique. La sismicité est essentiellement localisée dans la branche centrale de la DNT, au sud des lacs Natron (sud du volcan Gelaï) et Manyara. La crise sismique observée à Gelaï illustre la co-existence de processus magmatiques et tectoniques, avec la mise en place d'un dyke et le comportement à la fois asismique (glissement lent) et sismique (séisme de magnitude Mw 5.9) de failles normales. Les structures géologiques mises en jeu dans cette crise sont orientées NE-SW, obliques à l'axe ~N-S du rift dans la zone. Cette direction est parallèle à la fabrique tectonique antérieure qui est ré-empruntée par le rift Cénozoïque (faille Eyasi). A Manyara, les séismes sont remarquablement profonds (~20-35 km) et révèlent un décrochement sénestre sur un plan NE-SW. Ils illustrent le développement du rift vers le sud/sud-ouest sur la branche centrale Natron-Manyara-Balangida, au contact du craton tanzanien en profondeur. La contrainte principale minimum calculée dans la zone est orientée WNW-ESE et le régime tectonique local associé est transtensif. Il est fort probable que des fluides soient associés au déclenchement de cette séquence sismique profonde et de longue durée. L'influence de l'héritage structural dans l'expression des processus magmatique et tectonique accommodant la déformation s'observe aussi à l'échelle lithosphérique, par le biais de l'anisotropie sismique. Nos résultats soulignent 3 points majeurs: (1) les structures lithosphériques héritées (contrastes rhéologiques, fabriques crustale et mantellique) exercent un contrôle majeur sur la localisation et l'expression précoce du rifting continental; (2) la distribution des séismes en profondeur apparaît être un bon révélateur des propriétés rhéologiques de la croûte (transition fragile-ductile); et (3) dès le stade du rift immature (où la croûte est peu étirée), les processus magmatiques semblent jouer un rôle prépondérant dans l'accommodation de la déformation, en étroite interaction avec les processus tectoniques.

Les résultats acquis au cours de cette thèse apportent de nouvelles contraintes sur la cinématique récente du rift de Tadjoura (rt) et des zones marginales du Triangle Afar. Le rift de Tadjoura est interprété en termes d’hémi-graben accommodant environ 25-30% d’extension et concentrant une partie importante de la déformation dans sa partie axiale. L’analyse géométrique et statistique des populations de failles exposées sur ses deux flancs montre un gradient de déformation en direction de la zone transverse d’Arta qui est interprétée comme une discontinuité préexistante, de premier ordre, provocant (1) le blocage mécanique de la propagation du rift et (2) son transfert, par saut de rift, vers la zone Asal-Ghoubbet. L’étude structurale et géochimique/géochronologique des séries volcaniques synrift miocènes (et leur substratum mésozoïque), des reliefs d’Ali Sabieh nous amène à attribuer leur structure antiformale à la mise en place d’un laccolite basique infra-miocène, antérieur aux traps Dalha-Somalis. Les reliefs Danikil résultent, quant à eux, de mouvements verticaux plus récents, probablement d’origine tectonique et en liaison avec le rifting Tadjoura/Asal. Ces résultats originaux permettent de proposer de nouvelles hypothèses à l’échelle de l’ensemble du rifting Afar, et plus particulièrement concernant (i) le sens de propagation des axes de rifts récents, (ii) la distribution spatiale du magmatisme initial (de type trap) à 30 ma et (iii) l’évolution cinématique du micro-bloc Danakil et la nature de sa limite orientale avec le domaine « Mer Rouge »
The results of our study supply new constraints about the recent kinematical history of the Tadjoura Gulf and the Ali Sabieh range in the eastern part of the Afar Triangle. The Tadjoura rift is interpreted as a typical half-graben, accommodating 25-30% of bulk extension, and showing a progressive focussing of strain along its axial part with times. The geometrical and statistical analysis of fault networks indicates a gradient of extensional strain that we ascribe to the lateral locking of faut tip propagation against the Arta submeridian inherited faut zone. That mechanical disturbance is assumed to have caused the transfer of extension from the Tadjoura rift into the Ghoubbet-Asal trough, via a rift jump process. From structural and geochemical-radiometrical studies of older (Miocene) synrift volcanics in the Ali Sabieh range, their regional-scale antiformal structure is assigned to the emplacement of a shallow-level mafic intrusion (laccolith-type) during Miocene times. By contrast, the edification of the Danakil range is related to more recent Tadjoura/Asal rift tectonics. From our results, new asumptions are teatatively proposed to a number of key-issues dealing with the overall kinematic development of the Afar rift system, such as : the sens of active rift axes propagation, the spatial distribution of initial trap volcanism (30 Ma), the evolution of the Danakil microblock, as well as the nature of the eastern margin with the Red Sea fault-basinal domain to the E

The structural geology of the Coastal CordiIIera of northern Chile (25°15'-27°15'S) is dominated by three fracture systems: (1) the margin-parallel, trench-linked Atacama Fault Zone (AFZ), (2) a northwest¬trending network of faults, and (3) northeast-trending discrete fracture zones. The margin-oblique fracture sets appear to relate to magnetic fabrics imaged deep in the lithosphere and suggests that these fault zones utilise long-lived zones of weakness. The interaction of these fractures was an important structural control on the focusing of melts and ore fluids' in the leading edge of the Andean margin. Magnetite-dominated deposits comprise one major style of genetically-related mineral deposits within the study area. Primary' fluids from a magnetite-apatite deposit (Carmen) are moderate to highly saline (c. 17 to 37wt% NaCl eq.), Na-Ca-Fe±Mg-Cl-H20 brines. Magnetite from the main-stage of mineralisation was precipitated from a fluid with a high alSo content of circa lO%lIvs SMOW, at temperatures of -300 to 500°C and a mean depth of -2.5km, indicating a magmatic provenance of these magnetite deposits. Comb and dendritic textures indicate precipitation from a low viscosity ore fluid, consistent with fluid densities of 0.9 to 1.lg/cm3 and high contents of Cl, F and H20. These data point to a submagmatic origin of the ores. The second major type of fracture-controlled mineralisation in the Coastal CordiIIera are fault-hosted veins and breccia-pipes of specular haematite-cha1copyrite ore. They are found in close spatial relationship with magnetite-dominated styles of mineralisation and are thought to have formed by cooling and oxidation of an originally magnetite-bearing ore fluid, by mixing with a carbonate-bearing, extraneously sourced fluid. This explains the intergrowth of ore minerals with calcite and/or siderite, and the geological setting of these deposits, under a carbonate-precipitating, shallow-marine marginal basin, which is tentatively suggested to source the extraneous fluid. Further investigation is needed to confirm this. A first-order structural control on the distribution of magnetite-dominated deposits is imposed by the Atacama Fault Zone, due to its role in the emplacement of dioritic and granodioritic magmas, which are the likely sources of the magnetite-mineralisation. Magnetite-dominated deposits were emplaced along broadly north-south-trending brittle and mylonitic segments of the sinistral AFZ. The structural controls on the mineralisation imposed by brittle faults are not fully understood. In several of the larger deposits, east¬northeast-fractures are important, although their precise role remains obscure. Magnetite-dominated mineralisation along the mylonitic shear zones of the AFZ and Chivato Fault Zone, appear associated with cyclic deformation. This is linked to pulses of hydrothermal fluids expelled from an intrusion, or episodic fluid flow accompanying high coseismic strain rates, experienced by the mylonites due to their high crustal level. The structural control on the haematite-cha1copyrite breccias is imposed by northwest trending faults. Breccia-pipes and veins are located in dilationaI sites along these faults, such as jogs and/or fault bends. Their textures may also interpreted to relate to the earthquake cycle. Along the AFZ belt of brittle and mylonitic shear zones, haematite-cha1copyrite-bearing, northwest-trending fractures commonly cross-cut the north-south oriented, magnetite bodies. This represents a change in the predominant structures which focused hydrothermal fluids throughout the evolution of a mineralising system. A model is proposed in which, 'locking-up' of the mylonite occurs as the temperature passes through the quartz plastic-brittle transition temperature (-300°C). This causes deformation to switch from localised displacements along the mylonitic shear zones of the AFZ, to more distributed deformation along 'basement-controlled' northwest faults. These faults are more favourable to accommodate displacements due to their orientation and that they are probably nucleated upon inherited zones of crustal weakness. The temperature of the brittle-ductile transition is approximately intermediate between the formation temperatures of the two classes of Fe-oxide deposit, hence switching of the fracture kinematics is broadly contemporaneous, with the change in mineralisation style.

Le systeme radiometrique sm-nd a ete utilise pour caracteriser les processus magmatiques qui se produisent dans la croute continentale en contexte anorogenique. Les exemples etudies proviennent de la province anorogenique fini-paleozoique du se de la france (corse et esterel). Associe au rb-sr, le systeme radiometrique sm-nd a permis de mettre en evidence l'importance des processus de reinjections pour l'evolution des chambres magmatiques basiques. Des donnees quantitatives sur la dynamique de ces chambres ont egalement pu etre obtenues grace a ces resultats isotopiques. La comparaison des evolutions isotopiques de la petite chambre magmatique etudiee (fozzano) avec d'autres de plus grande taille (kalka, australie; skaergaard, groenland; kiglapait, labrador) indique qu'il existe une relation etroite entre les modalites de l'assimilation crustale et le volume des chambres. L'evaluation de l'homogeneite des compositions isotopiques du nd dans trois types petrographiques differents de granites anorogeniques a ete realisee, a l'echelle de l'intrusion et a l'echelle de la province corse. Les resultats contrastes obtenus demontrent qu'au sein d'une meme province, des granites anorogenqiues peuvent avoir des geneses tres differentes. Les magmas parentaux possibles de ces granites vont depuis des liquides d'anatexie de croute inferieure basique, jusqu'a des magmas mantelliques sous satures en silice qui se melangent avec la croute inferieure basique. Le modele propose pour la corse est egalement valable pour des magmatismes equivalents situes dans les white mountains (new hampshire) et dans le fosse d'oslo (norvege). Les grands volumes de rhyolites de l'esterel ont des compositions isotopiques du nd extremement constantes, probablement acquises dans la croute profonde lors d'un processus d'homogeneisation. La chambre magmatique superficielle n'a ete le siege que de processus de differenciation, sans assimilation crustale significative. Il est montre que le sm-nd sur mineraux est un geochronometre fiable et relativement precis pour les granites. Ces nouvelles determinations geochronologiques indiquent que le debut du magmatisme anorogenique de corse est precoce (environ 330 ma), puisqu'il s'est produit avant la fin de la mise en place du batholite calco-alcalin. Cette etude isotopique sur mineraux a egalement mis en evidence que les phases majeures et accessoires des granites ont des reponses constrastees, et parfois meme inattendues, vis-a-vis du systeme sm-nd au cours des interactions avec des fluides. Cette etude de la remobilisation hydrothermale du sm-nd a l'echelle des mineraux et de la roche totale a permis d'estimer des distances minimales de transport des terres rares dans la croute continentale granitique par des fluides. Un episode hydrothermal a egalement ete date en corse a environ 200 ma par sm-nd sur mineraux. Ce travail montre le grand interet du systeme radiometrique sm-nd pour les etudes d'interactions fluides-roches lorsque les lanthanides sont remobilises

O presente trabalho tem como objetivo estudar a evolução térmica de corpos intrusivos em diferentes níveis da crosta formados por múltiplas injeções de magma em forma de sils, levando em consideração mudança de fase e liberação de calor latente em um intervalo finito de temperaturas. A análise proposta inclui o estudo dos modelos de transferência de calor puramente condutivos utilizando a equação da evolução da entalpia que integra a mudança composicional dos magmas. A evolução térmica do corpo ígneo gerado por introdução do magma basáltico através de sils na crosta inferior obtém-se usando o método das diferenças finitas em uma dimensão, além disso se desenvolvem duas formas de construção do corpo ígneo neste nível: acréscimo por cima e acréscimo por baixo, sendo o primeiro caso o mais eficiente em manter o sistema a temperaturas elevadas durante mais tempo. Na crosta superior a evolução térmica do corpo ígneo de composição próxima ao tonalito introduzido por meio de sils é resolvida em duas dimensões com o método dos elementos finitos utilizando uma geometria com simetria axial no eixo z. O desenvolvimento destes corpos depende do ritmo de acréscimo do magma e das propriedades térmicas tanto da crosta como do magma injetado, podendo desenvolver-se como uma câmara magmática, um repositório de fusão ou um corpo altamente cristalino. A segunda fase do trabalho consistiu na solução da equação de condução de calor com mudança de estado em câmaras magmáticas esféricas, esferóides oblatos e esferóides prolatos com o método de elementos finitos para analisar evolução do sistema crosta-câmara magmática e introduzir o estudo da variação da viscosidade em função da temperatura. A geometria da câmara magmática influencia fortemente os fluxos de calor à crosta e portanto a criação de um halo dúctil que pode afetar a estabilidade dela mesma.
The purpose of this work is to study the thermal evolution of intrusive bodies formed by successive sills injection in different levels of the crust, taking into account the phase change and latent heat release in a finite temperatures interval. Our numerical models involve heat transfer only by conduction. Thermal evolution of the system is found with the energy balance equation of enthalpy that includes phase and composition change of magmas. The thermal evolution of the basaltic igneus body intruded into the lower crust is evaluated using finite difference scheme. At this level of the crust we develop two different ways of igneus body construction: over- accretion and under-accretion. Over-accretion is the more efficient way for keeping high temperatures in the time. In the upper crust, the thermal evolution of a tonalite igneus body formed by successive sils injection is computed with finite elements method in a geometry with rotational symmetry about z axis. Thermal and geological development of this igneus bodys strongly depends on magma emplacement rate and magma and crust thermal proprerties. These features defines if the igneus body develops in a magmatic chamber, a melt reservior or a high crystalline body. In the second part of this work we examine heat transfere from magamatic chambers of different geometries into the country rock, solving the entalphy equation with latent heat release effects with finite element method. The magmatic chambers geometries constructed are oblate spheroid, prolate spheroid and spherical. The viscosity of the rock is also obteined from temperature field.

Le fossé de la Bénoué est un trait tectonique majeur (1000 km) à la surface du globe. Différencié au cours du Crétacé au sein du socle africain, en bordure du golfe de Guinée. Cette structure de premier ordre ébauchée dès le Jurassique terminal, se compose de plusieurs dépôt-centres de 6000 m de profondeur qui évoluent à partir du Crétacé inférieur lors d'une période de rifting dans un régime d'extension, la composante principale minimale étant orientée ENE-WSW. Une sédimentation continentale typique de bassin d'effondrement, avec dépôts de cônes alluviaux sub-aériens. Faciès palustres et lacustres de bassin peu profond (Groupe inférieur de Bima, groupe de Keana) s'établit avec l'installation des grands lacs africains crétacés. A l'amont du fossé, ces lacs sont. Rapidement comblés par un fort influx détritique. Et recouverts par les dépôts post-rift de plaine fluviatile en tresse (Formation supérieure de Bima) du fleuve Paléo-Bénoué. A l'aval du fossé, une sédimentation turbiditique (Groupe d'Asu river) alimente à l'Albien des bassins marins profonds. Au Crétacé supérieur, la sédimentation argilo-détritique et ses épisodes carbonates sont placés sous l'influence des variations eustatiques globales des niveaux marins. Des conditions continentales sont rétablies au Paléocène dans tout le fossé, excepté sur sa bordure atlantique. La structuration syn-dépôt des premiers bassins crétacé inférieur s'observe par le développement, la nature et la répartition des faciès syn-rift dans des structures en hémi-grabens : dépôts de cônes confinés dans les bassins étroits, enchaînements faciès de cônes proximaux. De plaine fluviatile et plaine deltai͏̈que, de deltas distaux avec décantation distale (mais parfois mise en place turbide) de dépôts lacustres ou palustres. Dans des bassins disjoints Les relations entre tectonique et sédimentation se précisent avec la répartition des méagabrèches et conglomérats de pied de faille, discordances progressives et déformation syn-sédiment : blocs basculés, microfailles hydroplastiques. La découverte dans le NE du fossé d'un volcanisme précoce bi-modal fini-jurassique et syn-sédiment crétacé, tous deux d'affinité transitionnelle, précise le cadre tectonique distensif. Dans la période post-rift majeur, une migration progressive des dépôt-centres confine les bassins crétacé supérieur sur la bordure ouest du fossé alors que se produit uneinversion structurale des premiers bassins situés sur le bord oriental, au cours du Santonien. Un volcanisme alcalin affecte à cette époque la partie SW du fossé. La structure en grand du fossé de la Bénoué s'articule autour des grandes familles de failles héritées de l'orogenèse panafricaine : failles de transfert transafricaines ENE-WSW sur lesquelles jouent par rifting oblique les fractures NNE-SSW et WNW-ESE. Cette fracturation syn-rift s'irradie à l'intérieur du continent africain lors de la fragmentation crétacée du Gondwana et constitue l'immense système des rifts médio-africains (M. A. R. S. ). Ces fractures continentales qui régissent la cinématique intraplaque conditionnent ensemble les premiers mouvements interplaques, l'implantation et la direction des premiers segments transformants océaniques dans le golfe de Guinée. Le fossé de la Bénoué devient ainsi un marqueur complémentaire dans les reconstitutions sédimentaire. Tectonique et cinématique de l'évolution crétacée de l'océan Atlantique sud

Ce travail de thèse est essentiellement consacré à l'investigation des populations de zircons et de l'âge des protolithes des formations varisques grâce à une datation U-Pb in situ systématique par le couplage Laser/MC-ICPMS, dans des formations orthodérivées, métasédimentaires et magmatiques tardi-orogéniques du Massif Central français et du domaine Sud armoricain. Cette étude a montré que les populations de zircons sont largement héritées, s'étalant de l'Archéen au Paléozoïque inférieur. L'ensemble des pics d'âges obtenus, ainsi que l'absence d'âge mésoprotérozoïque (1.7-1.1 Ga) accréditent l'affinité gondwanienne de ces formations. Des âges maximum de dépôt sont proposés pour les métasédiments des principales unités définies dans le Massif Central. Ces âges maximum de dépôt sont décroissants suivant l'empilement lithotectonique reconnu. En marge de ce travail, nous avons pu déterminer que les âges de mise en place des granitoïdes du Sillon Houiller (Massif Central) et du golfe du Morbihan se situent aux alentours de 300 Ma. Des âges plus anciens, principalement obtenus sur monazite lors d'études antérieures, ont été réinterprétés comme des âges hérités. L'abondance de zircons hérités d'âge ordovicien et néoprotérozoïque met en évidence la large contribution des métagranites et métasédiments fertiles de ces périodes dans la source des granites tardi-orogéniques varisques. Le granite de Sarzeau expose de plus les traces d'un protolithe silurien. Le recyclage de la croûte continentale est caractérisé par une évolution polycyclique au cours de l'orogénèse varisque, avec tout d'abord l'érosion de formations du craton africain, majoritairement d'âge néoprotérozoïque, puis la superposition des événements magmatiques cambro-ordoviciens et syn-orogéniques varisques.

Les grands systèmes magmatiques siliciques peuvent produire des éruptions dévastatrices avec des volumes émis supérieurs à 100 km3. La forme et l'organisation des zones de stockage du magma sont toujours source de débats. Or, la forme utilisée modifie les résultats des modélisations faites. Nous avons choisi de prendre le problème sous un angle nouveau, en réfléchissant aux raisons physiques qui expliquent la formation d'une zone de stockage pré-éruptive, ceci afin d'avoir une meilleure compréhension des contraintes exercées par la chambre magmatique sur son environnement. Notre hypothèse est que le magma silicique, sous l'effet de sa flottabilité positive, remonte dans la partie ductile de la croûte continentale sous forme de diapir. Il est ensuite stoppé par la transition fragile-ductile de la croûte continentale, provoquant la formation d'une zone de stockage. Ce travail de thèse suit trois axes. Dans le premier nous adaptons un modèle de déformation de plaque issu de la littérature, afin de pouvoir décrire les contraintes et les déplacements dus à un liquide de flottabilité positive stocké sous une plaque élastique. Le second axe est expérimental. Un montage expérimental a été créé afin de reproduire le stockage supposé en laboratoire. Nos résultats permettent de tester notre modèle. Troisièmement, afin de valider l'hypothèse de départ, les prédictions théoriques sont comparées à des données issues de cas naturels. Nous montrons d'abord que notre modèle permet de reproduire la répartition des grandes déformations observées au volcan Uturuncu en Bolivie. Plus généralement, nous montrons que les contraintes produites par la chambre magmatique dans la plaque, peuvent provoquer des failles circulaires au-dessus de la chambre magmatique. Ce résultat a des implications directes pour la compréhension de la formation des caldeiras lors des éruptions majeures des grands systèmes siliciques
Huge silicic magmatic systems can produce devastating eruptions with emitted volumes above 100 km³. The shape and the organisation of magmatic storage are still debated, despite their crucial influence on the results of theoretical predictions. We choose to look at the problem in a new light, by thinking about the physical reasons that explain the formation of a storage zone. This is in order to better understand stresses produced by the magma chamber in the host rocks. Our hypothesis is that silicic magma, due to its positive buoyancy, rises through the ductile continental crust as a diapir, and is then stopped by the brittle-ductile transition. This PhD follows three axes. The first presents the adaptation of a published theoretical plate-model which enables us to describe the stresses and displacements due to the storage of a buoyant liquid under an elastic plate. The second axis is an experimental one. An apparatus was built to reproduce experimentally the storage configuration assumed. Our results, allow us to test our model. Thirdly, in order to validate our starting hypothesis, numerical results are compared with data from natural systems. We show that our model can reproduce the large observed deformation at Uturuncu volcano in Bolivia. Then, more broadly, we show that stresses produced in the plate by the magma chamber can produce circular faults above the storage zone. This result has direct implications for the understanding of caldera formation during large silicic eruptions

Les clinopyroxenes du massif central francais montrent une diversite morphologique se resumant en deux classes: les clinopyroxenes magnesiens en equilibre avec le basalte hote et les clinopyroxenes riches en fer en desequilibre; ceux-ci traduisent dans les basaltes alcalins le caractere generalise du processus d'hybridation magmatique dans la basaltogenese alcaline. Differents niveaux de differenciation sont decrits et caracterisent les districts volcaniques du massif central