Dissertations / Theses on the topic 'Magma emplacement'

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

This thesis is a study of the mechanisms by which granitic magmas rise through the crust to be empiaced at a level above their source, with particular reference to diapirisni. and how these mechanisms may be analysed by combined structural and petrological studies. The Northern Arran granite is used as an example of how this problem may be approached. The Northern Arran granite is a two component granite of Tertiary age intruded into structurally heterogeneous upper crustal rocks under regional tension. A synform concentric to the granite, synchronous with the development of a narrow thermal aureole, records the vertical ascent of a single body of magma with a hemispherical upper surface. Post ascent, radial expansion of this body, indicated by flattening strains parallel to its surface and superimposed on the concentric synform records a change in shape of the pluton. This was permitted by the reactivation of an existing fault which the pluton intersected during its ascent. Petrological studies of the outer coarse unit of the northern granite indicate that it is a single body of magma derived by differentiation of a crustally contaminated basaltic source. Theoretical modelling of the crystallisation of the coarse granite shows that textural and chemical variations, are consistent with solidification by sidewall crystallisation (liquid fractionation) but not fractional crystallisation. The inner (younger) fine granite is also a single body of magma derived from the same or a similar source as the coarse granite. The sharp undeformed contacts between the coarse and fine granites and the presence of internal sheets in the fine granite parallel to its contacts with the coarse granite are consistent with emplacement of the fine granite as a series of pulses which filled a propagating ring dyke fracture within the coarse granite. Theoretical modelling of the ascent of the coarse granite using the Hot Stokes equation indicates that bouyancy driven ascent aided by a reduction in wall rock viscosity controlled by the rate of heat loss of from the granite is a viable ascent mechanism. The patterns of strain in the aureole of the Northern Arran granite result from the ascent and emplacement of a single diapiric body. They provide examples of the types of structure which may be used to recognise and distinguish between diapiric ascent and radial expansion. This has important implications for the study of ballooning diapirs. The reactivation of an existing fault system during emplacement suggests that existing crustal structure can influence the final geometry of an intrusive body. It is shown that the complete evolution of the Northern Arran granite can be determined using a combination of structural and petrological data. Structural data provides constraints on the later stages of ascent and the emplacement of granitic plutons. Petrological data can be used to constrain the origin, early stages of ascent and the crystallisation of a magma body.

The emplacement processes of the Whin Sill complex and its associated dykes have been studied using a combined approach of detailed field and magnetic investigations. Regional palaeodirectionai results show two different but consistent palaeomagnetic signatures and allow the sill complex to be subdivided into three geographically separate intrusions: the 'Holy Island Sill', the 'Alnwick Sill' and the ' Hadrian's Wall-Pennine Sill', The Little Whin Sill has been identified previously as a fourth, separate sill on petrological evidence. On the basis of the palaeodirectional results it is also possible to relate the exposed contemporaneous dykes to the individual intrusions. The magma flow geometries within the individual Whin Sill intrusions were detennined by AMS analyses, and both traditional and newly identified magma flow indicators. For the Holy Island Sill the results suggest that the magma flow was homogeneously southwards directed and that the Holy Island Dyke acted as the feeder to this intrusion. In the Alnwick Sill the magma flow was homogeneously westwards directed and it is proposeU that an offshore, en echelon segment of the High Green Dyke fed the Alnwick Sill. The magma flow geometry for the Hadrian's Wall-Pennine Sill is complex. The results suggest that this intrusion was fed by the Hett Dyke and that the magma flow within the sill was generally north and north westwards directed. It is proposed that the intrusion was emplaced during a phase of late Variscan, E-W compression and that pre-existing faults at a high angle to this compression direction acted as a magma flow barrier along which the magma became deflected. The geometry of the sills is approximately that of a quarter- to half-saucer-shape, with the dykes being situated at the saucer truncation. From the feeder dykes magma was injected into the individual sill bodies and the magma flow was generally up dip or parallel to the strike of the host rock bedding, thus out of the basin centres and into levels of lower lithospheric pressure. It is proposed that a compressional stress field and locally overpressured horizons had a significant impact on the initiation and emplacement of the Whin Sill complex.

Exposed rhyolitic dykes at eroded volcanoes arguably provide in situ records of conduit processes during rhyolitic eruptions, thus bridging the gap between surface and sub-surface processes. This study involved micro- to macro-scale analysis of the textures and water content within shallow (emplacement depths <500 m) rhyolitic dykes at two Icelandic central volcanoes. It is demonstrated that dyke propagation commenced with the intrusion of gascharged currents that were laden with particles, and that the distribution of intruded particles and degree of magmatic overpressure required for dyke propagation were governed by the country rock permeability and strength, with pre-existing fractures playing a pivotal governing role. During this stage of dyke evolution significant amounts of exsolved gas may have escaped. Furthermore, during later magma emplacement within the dyke interiors, particles that were intruded and deposited during the initial phase were sometimes preserved at the dyke margins, forming dykemarginal external tuffisite veins, which would have been capable of facilitating persistent outgassing during dyke growth. It is further demonstrated that following initial dyke-opening, geochemically homogenous dykes grew via the incremental emplacement of magma, with fluctuations in the shallow-dyke permeability occurring via bubble collapse, and this is deemed to have been critical in dictating pressure within the deeper magma source region and fragmentation. Of further significance, it is also shown that shear deformation was localised during magma emplacement, with localised vesiculation occurring along emplacement boundary layers via viscous heating, which temporarily promoted magma ascent, but with later bubble collapse culminating in brittle failure of bubble-free magma, after shear zone migration. However, in some instances high strain rates during viscous bubble deformation resulted in ductile-brittle transitions, with resultant slip triggering micro-tensile failure of bubbly magma, as the slipped magmatic plug experienced decompression. This tensile failure probably occurred distal to shear zones, where bubbles where relatively isolated. Interlinking of the micro-cracks formed extensive internal tuffisite vein networks, which acted as efficient outgassing pathways, given their access to significant quantities of preexsolved volatiles. The models presented in this thesis are relevant to the conduit processes that take place during rhyolitic eruptions; insight is provided into how rhyolitic magma ascends through the shallow (<500 m deep) crust and also into how the magma deforms during its ascent and into the processes that govern magmatic outgassing.

The volcanic island of Fogo belongs to the Cape Verde archipelago, a two-tiered chain of islands situated 500 km west of the African coast. Fogo is regarded as one of the most active volcanoes in the world with 10 eruptions during the last 250 years. The former shield volcano Monte Amarelo reached 3500 m.a.s.l. before it collapsed into the Atlantic Ocean. The massive landslide event occurred between 124 and 86 ka, forming the Bordeira cliffs and the high plateau Cha das Caldeiras on Fogo. We have collected rock samples from the Bordeira dikes, which intruded into the Bordeira wall prior to collapse. The purpose of the project is to produce a magmatic storage model for Fogo using mineral chemistry and thermobarometric methods. Additionally, I aim to determine the processes prevailing in the magmatic system, the link between the volcanic and plutonic system. Previous studies on the magma storage beneath Fogo have focused on the volcanics, which show crystallization pressures between 0.45 to 0.68 GPa using clinopyroxene-melt thermobarometry on rims. The Bordeira dikes are basanitic to nephelinitic in composition. The mineral assemblage of the 20 dike samples consist of phenocrystic clinopyroxene ± olivine ± plagioclase ± xenocrystic amphibole. Accessory minerals are titanomagnetite, apatite, nepheline, plagioclase and alkali feldspar in a microcrystalline groundmass. Clinopyroxene displays a large compositional variation, ranging from Mg#38 to Mg#85, with a mean of Mg#71±10 2s.d. (n=614). Xenocrystic amphibole varies from Mg#37 to Mg#72, with a mean of Mg#62±15 2s.d. (n=78). Interstitial feldspar forms two groups, one of An#24 to An#79, with a mean of An#66±19 2s.d., (n=125) and a second with Or#19 to 100 with a mean of Or#69±42 2s.d.(n=71). Bulk geochemistry of the 20 samples range from 1.82 to 11.5 MgO wt%. Our clinopyroxene-melt thermobarometry show crystallization pressures ranging from 0.02 to 0.85 GPa, with a mean of 0.47±0.29 2s.d. (n=502) (Putirka et al. 2003). Structural data from the intrusive dikes in the Bordeira contain three preferred orientations, N-S, NW-SE and E-W (n=371). The main process occurring in the magmatic system is fractional crystallization, however there is some evidence for phenocryst accumulation and magma recharge. Our magma storage model show that clinopyroxene crystallization initiates in the lithospheric mantle, between 15 to 28 km depth. Significant clinopyroxene rim and microcryst crystallization occur above Moho, between 9 to 12 km, implying that magma storage levels do exist in the oceanic crust. The intrusive and extrusive rocks present on Fogo show common storage levels, suggesting that they are formed in the same system but the difference being their residence time in the crustal level storage. Our structural data and 3D model suggest that the Monte Amarelo rift zone was composed of three components, being oriented NW-SE, N-NE and E-W. The flank collapse was caused by dike intrusions of N-S orientation which enabled a E-W extension of the shield volcano.
Vulkanön Fogo är en del av ögruppen Kap Verde i Atlanten. Ögruppen bildar en två delad arkipelag positionerad 500 km väster om det afrikanska fastlandet. Ön, tillika vulkanen Fogo har på senare tid varit en av de mest aktiva vulkanerna i världen med 10 utbrott under de senaste 250 åren. Ön byggdes upp av sköldvulkanen Monte Amarelo nådde 3500 m ö h innan delar av den kollapsade ned i Atlanten. Det massiva skredet som skedde mellan 86 och 124 tusen år sedan skapade högplatån Cha das Caldeiras samt den omringande klippsektionen Bordeira. Vi har samlat stenprover från de plutoniska bergarter som har trängt in sig i klippsektionen Bordeira. Målet med vår studie är att skapa en modell för hur magma lagringen fungerar under Fogo. Vi ämnar kartlägga magmalagringsdjupet med hjälp av kemiska variation i mineral som kan användas för att kartlägga kristalliseringstryck och temperatur som i t.ex. klinopyroxen. Vi är samtidigt intresserade av att veta vilka processer som sker i det magmatiska systemet och sambandet mellan vulkanska bergarter t.ex. lava och plutoniska bergarter. Tidigare studier av Fogos magmalagring har använt vulkaniska bergarter, som kristalliserar sig mellan 0.45 till 0.68 GPa när man undersökt kemin på kristallkanter av klinopyroxen. 20 prover har analyserats från Bordeiraklipporna och de innehåller låga kiselhalter, mellan 37 till 47% samt höga mängder alkaliska oxider så som kalium och natrium. Provernas mineralinnehåll består främst av större kristaller av silikatmineralen klinopyroxen ± olivin± fältspat ± främmande amfibolkristaller. De större kristallerna är omringande av en mikrokristallin grundmassa bestående av järn-titanoxider, apatit och fältspatoider. Klinopyroxen har en relativt stor kemisk variation, med Mg#37 till Mg#85, med ett medelvärde på Mg#71. Vi har även två olika sorter av fältspat, en grupp med ett kalciumrik rikt innehåll klassificeras som anortit, och en annan med ett kaliumrikt innehåll, som ortoklas. Vår analys av klinopyroxen-smälta har gett oss kristalliseringstryck som sträcker sig mellan 0.02 till 0.85 GPa med ett medelvärde på 0.47 GPa. Detta innebär att den dominerande processen i magmalagringssystemet är fraktionerad kristallisering då vi kan se ett linjärt avtagande för många ämnen när de jämförs mot magnesiumhalten. Vår magmalagringsmodell för vulkanen Fogo visar att klinopyroxenkrystallisering påbörjas i den litosfäriska manteln, mellan 15 och 28 km djup. Kristallisering av kanter på klinopyroxenkristaller samt mindre kristaller i grundmassan sker ytligare och visar på att det finns en eller flera magmalagringsnivåer i den oceaniska jordskorpan, mellan 9 till 12 km djup. Vulkaniska och plutoniska bergarter vittnar om ett delat magmasystem, vilket indikerar att skillnaden mellan de två bergarterna främst är tiden de befinner sig på respektive lagringsnivå. Vår strukturgeologiska data samt 3D modell visar att den intrusiva aktiviteten var primärt orienterad NV-SO, N-NO och O-Vriktning. Monte Amarelo-vulkanens skred och kollaps orsakades av intruderande gångar med en generell N-S orientering vilket ledde till ett skred på östsidan.

The Dolomites form the central-eastern portion of the Southern Alps, in Northern Italy. The stratigraphic framework of the Dolomitic area includes mainly Permian to Cretaceous terrains, while it is largely dominated by the magnificent Triassic carbonate platforms and basinal systems. The area of the Dolomites recorded several tectonic and magmatic events, from Permian up to Cretaceous. During the Middle Triassic transtensional tectonics, associated with differential subsidence and uplifting, the south-western part of the Dolomites has witnessed a massive and short-lived Ladinian (Middle Triassic) tectono-magmatic event, forming a series of significant magmatic features. The Monzoni, Predazzo and Cima Pape Intrusive Complexes are situated in the southwestern Dolomites and represent the main intrusive expressions of the Ladinian magmatism. This PhD project offers new insights regarding the emplacement mechanisms of the Monzoni Intrusive Complex, by combining fieldwork data and analogue models on magma emplacement. The Monzoni pluton is located parallel to San Pellegrino Valley and appears elongated, with an NE-SW orientation, covering an area of approximately 4.0 km2. The main characteristics of Monzoni pluton, that is the elongated shape and the shoshonitic orogenic affinity, suggest a potential correlation and emplacement control by the Triassic developing and/or reactivated inherited strike-slip structure. The generation, ascent and emplacement of Monzoni pluton and its relation to strike-slip faulting, is still a matter of debate. The lack of direct field observations attributed to the volcano-tectonic activity, keeps the mechanisms of magma–strike-slip fault interactions poorly understood. Updated geological maps, based on field campaign data, bring new insights regarding intrusion, fault-controlled boundaries and deformational pattern of the pluton and host-rock formations. In addition, investigations on anisotropy of magnetic susceptibility (AMS) on Monzoni pluton, reveal zonation within the pluton and indicate the presence of magmatic feeder in the north-eastern part of the intrusion. Finally, the 3D modelling of the Monzoni Intrusive Complex, projecting all geological data, constrains the pluton’s volume to 4.35km3 and offers a simplified profile-view projection of the pluton/host-rock system. The Monzoni Intrusive Complex, due to its excellent three-dimensional exposure, is particularly suited for the study of volcano-tectonic systems allowing the application and comparison to analogue models. During this project we conducted sandbox-type analogue modelling experiments on magma emplacement along crustal scale strike-slip fault zones. We investigate two tectonic regimes, strike–slip and transtension, and three temporal relationships between magmatism and tectonics; pre-tectonic, syn-tectonic and post-tectonic intrusion. Experimental results show that there is a strong interaction between tectonic structures, evolving or inherited, and magmatism and that the geometrical characteristics of the experimental plutons represent a good indicator for the classification of plutons, defining the timing and tectonic setting of emplacement. The combination of all applied methodologies suggests magmatic emplacement in transtensional tectonic regime with two possible kinematic scenarios; a left lateral strike –slip direction along the N70° fault set or a right-lateral strike slip direction, along the N30° faults.
Le Dolomiti formano la parte centro-orientale delle Alpi meridionali, nel Nord Italia. L’organizzazione stratigrafica dell'area dolomitica comprende principalmente terreni dal Permiano al Cretaceo, mentre è in gran parte dominata dalle magnifiche piattaforme carbonatiche triassiche e dai relativi bacini. L'area delle Dolomiti ha registrato numerosi eventi tettonici e magmatici, dal Permiano fino al Cretaceo. Durante la tettonica transtensionale del Triassico medio, associata a subsidenza differenziale, la parte sud-occidentale delle Dolomiti è stata soggetta a un evento tettonico-magmatico Ladinico (Medio Triassico) di breve durata, sviluppando una serie di rilevanti strutture magmatiche. I complessi intrusivi di Monzoni, Predazzo e Cima Pape sono situati nelle Dolomiti sud-occidentali e rappresentano le principali espressioni intrusive del magmatismo Ladinico. Questo progetto di dottorato offre nuove conoscenze sui meccanismi di messa in posto del Complesso Intrusivo dei Monzoni, combinando dati sul campo e modelli analogici su postazioni di magma. Il plutone dei Monzoni, che si trova parallelo alla Valle di San Pellegrino, appare allungato, con orientamento NE-SO, coprendo un'area di circa 4,0 km2. Le principali caratteristiche del plutone dei Monzoni, la forma allungata e l'affinità shoshonitica orogenica, suggeriscono una potenziale correlazione e controllo della messa in posto da parte di strutture trascorrenti ereditate o medio-triassiche. La generazione, l'ascesa e la messa in posto del plutone dei Monzoni e il suo rapporto con le strutture trascorrenti, sono ancora oggetto di dibattito. La mancanza di osservazioni dirette sul campo attribuite all'attività vulcano-tettonica, rende difficile la comprensione dei meccanismi di interazione tra faglie e magmatismo. Le mappe geologiche aggiornate basate sui nuovi dati di terreno, forniscono nuove informazioni sui limiti di intrusione controllati da faglie e sul modello deformativo delle formazioni incassanti e del plutone. Inoltre, indagini su anisotropia di suscettività magnetica (AMS) sull’intrusione dei Monzoni, rivelano la zonazione all'interno del plutone e indicano la presenza di un condotto di alimentazione principale nella parte nord-orientale dell'intrusione. Infine, la modellazione geologica 3D del complesso intrusivo dei Monzoni, coerente con i dati geologici, limita il volume del plutone a 4.35 km3 e offre una visione - semplificata dei rapporti tra plutone e rocce incassanti. Il Complesso Intrusive di Monzoni, grazie alla sua eccellente esposizione tridimensionale, è particolarmente adatto allo studio di sistemi vulcano-tettonici, permettendo anche l'applicazione e il confronto con modelli analogici. Durante questo progetto, sono stati condotti esperimenti di modellazione analogica di tipo sandbox, su lungo zone di taglio trascorrenti a scala crostale. Sono stati distinti due regimi tettonici, trascorrenza pura e transtensione, e tre relazioni temporali tra magmatismo e tettonica; intrusione pre-tettonica, sin-tettonica e post-tettonica. I risultati sperimentali mostrano che esiste una forte interazione tra le strutture tettoniche, in evoluzione o ereditate, ed il magmatismo e che le caratteristiche geometriche dei plutoni sperimentali rappresentano un buon indicatore per la classificazione dei plutoni, definendo i tempi e l'ambiente tettonico della messa in posto. La combinazione di tutte le metodologie applicate, suggerisce la messa in posto del plutone durante un regime tettonico transtensionale con due possibili scenari cinematici; una transtensione sinistra - direzione N70° o una transtensione destra lungo faglie N30°.

The vast distribution and long duration of the Late Mesozoic magmatism in the eastern part of South China presents a unique case in the world. This offers a natural laboratory to study the process of magma genesis, the magma emplacement mode, the relationship between magmatism and tectonics, the geodynamic role on the magma emplacement and lithospheric evolution. Since 50's, particularly 90's of the last century, geoscientists have made important efforts in geological cartography and carried out numerous studies with remarkable scientific achievements, building a solid background to understand the tectonic evolution of the South China Block (SCB). However, certain fundamental questions mentioned above remain unsolved and/or are in hot debate. In order to make progress in these scientific issues, we have carried out in a multi-disciplinary study in the Late Mesozoic Qingyang-Jiuhua massif, Hengshan massif and Fujian coastal zone according to their distance with respect to the paleo subduction zone of the Paleo-Pacific plate, the ages of granitic massifs and related tectonics, including field observation on the structure geology, micro-observation on thin section, U-Pb dating on monazite, AMS, paleomagnetism, gravity modeling and P condition concern the granite emplacement. In the view of deformation in these granitic massifs and their country rocks, mode and influence of regional tectonics on the emplacement, though each studied zone reveals its distinguished characteristics, they show some intrinsic and common relationships between them. With our new results and integrating previous data, in this thesis, we discuss the tectonic context of emplacement of these Late Mesozoic magmatic massifs and the geodynamic evolution of the SCB., We propose a 3-step geodynamic model: (1) during 145-130 Ma period, the Paleo-Pacific plate subducted northwestwardly, the West Philippines micro-continent, approaching to SCB, important subduction-related arc volcanism was produced in the coastal areas of Southeast China coast (Zhejiang-Fujian-Guangdong), forming a back-arc extension tectonic system in SCB; (2) during 130-110 Ma period, due to the collision between the West Philippines microcontinent and SCB, the compressional tectonic structures were developed in the Changle-Na'ao coastal zone, producing ductile deformation zones. However, the inland of the eastern part of SCB was under a NW-SE extensional tectonic regime; (3) during 105-90 Ma period, a new subduction zone was developed in the SE flank of the West Philippines micro-continent, the subducting slab reached the Changle-Nan'ao tectonic belt, with the possible break-off of slab, the asthenospheric ascent was responsible for the important emplacement of plutonic massifs and dykes. The tectonics of the eastern part of SCB was characterized by a general extensional system in this period. This tectonic pattern has been significantly disturbed by the Oligocene-Eocene opening of the South China sea,and the Miocene shortening of the SCB margin in Taiwan. Of course, this model should be improved by more geological, geophysical and geochemical investigations.

The Station Creek Igneous Complex (SCIC) is one of the largest Middle-Late Triassic plutonic bodies in the northern New England Orogen of Eastern Australia. The igneous complex comprises of five plutons - the Woonga Granodiorite (237 Ma), Woolooga Granodiorite (234 Ma), Rush Creek Granodiorites (231 Ma) and Gibraltar Quartz Monzodiorite and Mount Mucki Diorite (227 Ma respectively), emplaced as high-level or epizonal bodies within the Devonian-Carboniferous subduction complex that resulted from a westward subduction along the east Australian margin. Composition of the SCIC ranges from monzogabbro to monzogranite, and includes diorite, monzodiorite, quartz monzodiorite and granodiorite. The SCIC has the typical I-type granitoid mineralogy, geochemistry and isotopic compositions. Its geochemistry is characteristics of continental arc magma, and has a depleted-upper mantle signature with up to 14 wt% supracrustal components (87Sr/86Srinitial = 0.70312 to 0.70391; Nd = +1.35 to +4.9; high CaO, Sr, MgO; and low Ni, Cr, Ba, Rb, Zr, Nb, Ga and Y). The SCIC (SiO2 47%-76%) has similar Nd and Sr isotopic values to island-arc and continentalised island-arc basalts, which suggests major involvement of upper mantle sourced melts in its petrogenesis. SCIC comprises of two geochemical groups - the Woolooga-Rush Greek Granodiorite group (W-RC) and the Mount Mucki Diorite-Gibraltar Quartz Monzodiorite group (MMD-GQM). The W-RC Group is high-potassium, calc-alkalic and metaluminous, whereas the MMD-GQM Group is medium to high potassium, transitional calc-alkalic to tholeiitic and metaluminous. The two geochemical groups of the SCIC magmas are generated from at least two distinct sources - an isotopically evolved Neoproterozoic mantle-derived source with greater supracrustal component (10-14 wt%), and an isotopically primitive mafic source with upper mantle affinity. Petrogenetic modeling using both major and trace elements established that the variations within respective geochemical group resulted from fractional crystallisation of clinopyroxene, amphibole and plagioclase from mafic magma, and late fractionation of alkalic and albitic plagioclase in the more evolved magma. Volcanic rocks associated with SCIC are the North Arm Volcanics (232 Ma), and the Neara Volcanics (241-242 Ma) of the Toogoolawah Group. The major and trace element geochemistry of the North Arm Volcanics is similar to the SCIC, suggesting possible co-magmatic relationship between the SCIC and the volcanic rock. The age of the North Arm Volcanics matches the age of the fractionated Rush Creek Granodiorite, and xenoliths of the pluton are found within epiclastic flows of the volcanic unit. The Neara Volcanics (87Sr/86Sr= 0.70152-0.70330, 143Nd/144Nd = 0.51253-0.51259) differs isotopically from the SCIC, indicating a source region within the HIMU mantle reservoir (commonly associated with contaminated upper mantle by altered oceanic crust). The Neara Volcanics is not co-magmatic to the SCIC and is derived from partial melting upper-mantle with additional components from the subducting oceanic plate. The high levels emplacement of an isotopically primitive mantle-derived magma of the SCIC suggest periods of extension during the waning stage of convergence associated with the Hunter Bowen Orogeny in the northern New England Orogen. The geochemical change between 237 to 227 Ma from a depleted-mantle source with diminishing crustal components, to depleted-mantle fractionate, reflects a fundamental change in the source region that can be related to the tectonic styles. The decreasing amount of supracrustal component suggests either thinning of the subduction complex due to crustal attenuation, leading to the late Triassic extension that enables mantle melts to reach subcrustal levels.

The Station Creek Igneous Complex (SCIC) is one of the largest Middle-Late Triassic plutonic bodies in the northern New England Orogen of Eastern Australia. The igneous complex comprises of five plutons - the Woonga Granodiorite (237 Ma), Woolooga Granodiorite (234 Ma), Rush Creek Granodiorites (231 Ma) and Gibraltar Quartz Monzodiorite and Mount Mucki Diorite (227 Ma respectively), emplaced as high-level or epizonal bodies within the Devonian-Carboniferous subduction complex that resulted from a westward subduction along the east Australian margin. Composition of the SCIC ranges from monzogabbro to monzogranite, and includes diorite, monzodiorite, quartz monzodiorite and granodiorite. The SCIC has the typical I-type granitoid mineralogy, geochemistry and isotopic compositions. Its geochemistry is characteristics of continental arc magma, and has a depleted-upper mantle signature with up to 14 wt% supracrustal components (87Sr/86Srinitial = 0.70312 to 0.70391; Nd = +1.35 to +4.9; high CaO, Sr, MgO; and low Ni, Cr, Ba, Rb, Zr, Nb, Ga and Y). The SCIC (SiO2 47%-76%) has similar Nd and Sr isotopic values to island-arc and continentalised island-arc basalts, which suggests major involvement of upper mantle sourced melts in its petrogenesis. SCIC comprises of two geochemical groups - the Woolooga-Rush Greek Granodiorite group (W-RC) and the Mount Mucki Diorite-Gibraltar Quartz Monzodiorite group (MMD-GQM). The W-RC Group is high-potassium, calc-alkalic and metaluminous, whereas the MMD-GQM Group is medium to high potassium, transitional calc-alkalic to tholeiitic and metaluminous. The two geochemical groups of the SCIC magmas are generated from at least two distinct sources - an isotopically evolved Neoproterozoic mantle-derived source with greater supracrustal component (10-14 wt%), and an isotopically primitive mafic source with upper mantle affinity. Petrogenetic modeling using both major and trace elements established that the variations within respective geochemical group resulted from fractional crystallisation of clinopyroxene, amphibole and plagioclase from mafic magma, and late fractionation of alkalic and albitic plagioclase in the more evolved magma. Volcanic rocks associated with SCIC are the North Arm Volcanics (232 Ma), and the Neara Volcanics (241-242 Ma) of the Toogoolawah Group. The major and trace element geochemistry of the North Arm Volcanics is similar to the SCIC, suggesting possible co-magmatic relationship between the SCIC and the volcanic rock. The age of the North Arm Volcanics matches the age of the fractionated Rush Creek Granodiorite, and xenoliths of the pluton are found within epiclastic flows of the volcanic unit. The Neara Volcanics (87Sr/86Sr= 0.70152-0.70330, 143Nd/144Nd = 0.51253-0.51259) differs isotopically from the SCIC, indicating a source region within the HIMU mantle reservoir (commonly associated with contaminated upper mantle by altered oceanic crust). The Neara Volcanics is not co-magmatic to the SCIC and is derived from partial melting upper-mantle with additional components from the subducting oceanic plate. The high levels emplacement of an isotopically primitive mantle-derived magma of the SCIC suggest periods of extension during the waning stage of convergence associated with the Hunter Bowen Orogeny in the northern New England Orogen. The geochemical change between 237 to 227 Ma from a depleted-mantle source with diminishing crustal components, to depleted-mantle fractionate, reflects a fundamental change in the source region that can be related to the tectonic styles. The decreasing amount of supracrustal component suggests either thinning of the subduction complex due to crustal attenuation, leading to the late Triassic extension that enables mantle melts to reach subcrustal levels.

Evolution de l'ile de la reunion, exemple de volcanisme intraplaque, plus particulierement du piton de la fournaise ou au cours des 500 000 dernieres annees, la tectonique d'effondrement a ete sub-continue et s'est manifestee par des caldeiras. Leur migration marque un deplacement des reservoirs dans l'edifice. Le glissement du grand brule semble sub-contemporain de la phase d'effondrement de l'enclos. L'activite recente est concentree sur le cone central a sa base et le long des rifts zones. Les transferts de magma entre les zones profondes et le reservoir superficiel ne sont pas continus. Les donnees de surveillance, depuis 1980 permettent de caracteriser les phenomenes precurseurs des eruptions et les mecanismes des intrusions et de la fracturation. Les consequences volcanotectoniques des deformations remanentes associees aux intrusions frequentes dans la zone centrale sont etudiees en terme de prevision et de l'evolution du volcan a moyen terme

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Various igneous bodies have intruded the Palmer area throughout the Delamerian Orogeny. The earliest, the Rathjen Gneiss, intruded either before or during D1 which gave it the prominent foliation. D1 was also responsible for crenulations in migmatite veins throughout the area. These crenelated migmatite veins are in areas folded by D2 mesoscale folds. Some pegmatite veins are also folded by D2 folds. The Palmer Granite intruded during D2 as is seen by shearing in a semi-crystalline state and a tectonic foliation that has been folded. The ballooning of the granite during emplacement deforms the surrounding sediments and the pre-granite folds hence their axes lie parallel to the contact of the granite. The effect of the granite intruding during the deformation has lead to the axis of the D2 folds forming after the granite to have a degree of randomness about their axis. Migmatite grade was reached again after the intrusion of the granite causing melt veins to develop to disrupt the foliation. D3 formed a regional syncline of the area combined with some small scale folding within the granite, however a foliation did not form. The emplacement of the granite and some other igneous bodies throughout the area has been controlled by using the bedding plane of the Kanmantoo. The geochemical trends throughout the Palmer Granite is formed by two different groups fractionally crystallising zircon, amphibole and biotite. This results in a decrease of normally incompatible elements. The two groups form by one group from a homogeneous source and the other a heterogeneous source. The xenoliths crystallised from a mafic magma. The amphibolites form two groups according to their differentiation and genetic relationship. They both form by fractional crystallisation however U and Pb are decreasing cannot be explained by this. Another possible mechanism is liquid un-mixing. To tie all of the groups together a model of a mafic pluton that crystallises the xenoliths as a chilled margin. The mafic magma evolves some of the Palmer Granite whilst turbulently convecting hence homogenising the magma. A magma recharge forms the more evolved mafic and this forms more Palmer Granite which convects in a laminar fashion forming heterogeneities. Part of the mafics evolve enough to be caught up in the Palmer Granite and as it does not crystallise zircons all the fractional crystallisation of the Palmer Granite must have occurred in the mafic plution.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Earth and Environmental Sciences, 1993