Littérature scientifique sur le sujet « Structural analysis, calc-alkaline magmatism »

Research subject. An analysis of regional and global geological material characterizing the spatio-temporal relationships between alkaline magmatic and saline complexes allowed the author to propose and justify a new geological-genetic model of alkaline magmatism. This model considers saline complexes, located along the paths of the upward movement of deep magmas, as additional sources of alkaline and volatile components.Materials and methods. Three articles are devoted to the discussion and justification of this model. Two articles were devoted to geological aspects of the problem. The prerequisites and signs of the participation of ancient saline complexes in alkaline magmatism were characterized. It was shown that the presence of saline rocks in the deep zones of the earth's crust along the paths of the upward movement of deep magma flows is a geologically natural and common phenomenon. Natural alkaline-salt associations (spatio-temporal combinations of alkaline and salt objects) were indicated; their tectonic types were distinguished. A global overview of their different-age analogues (neo- and paleogeodynamic) was given.Results and discussion. The collected data made it possible to evaluate older (than magmas) salt-bearing complexes (deeply buried in the substrate) as a possible important and active participant in the ontogenesis of alkaline complexes, to give a positive assessment of the geological aspects of the “magma halocontamination” model and salt-magmatic interactions; to formulate the main geological-genetic provisions of this model.Conclusion. This article focuses on the discussion of the genetic aspects of the proposed model with an assessment of the probable role and significance of various halophilic components in the formation of alkaline magmas and their features. For this purpose, the similarity features in the spatial and quantitative distribution of halophilic and foydaphilic components in salt and alkaline rocks are considered; the probable role of various halophilic components in the formation of alkaline specialization of magmas, in the emergence of a rich set of unusual features of alkaline rocks (material, structural, morphological, etc.) is discussed. The probability of participation of the complex of paragenic (non-salt) members of the halophilic community (dolomites, anhydrites, black shales and associated ore components) in the interaction with hot magma is estimated. A comparative analysis of some basic provisions of the model under consideration with other geological-genetic models of alkaline petrogenesis is performed. The advantages of this model and its predictive capabilities are evaluated. A number of ideas have been proposed by the author for the first time, thus requiring further elucidation.

The paper reports the isotope-geochronological and petrological-geochemical studies of the Pliocene moderately-acid volcanism of the Akhalkalaki Plateau in the central part of the Lesser Caucasus (Javakheti highland, Georgia). K-Ar dating showed that young dacitic lavas and pyroclastic rocks were formed in the Mid-Pliocene (3.28 ± 0.10 Ma) in relation with the explosive–effusive eruptions of small composite volcanic cones and formation of minor extrusive domes confined mainly to the eastern margin of the region. Isotope-geochronological data in the combination with results of structural drilling indicate that the considered short-term pulse of the volcanic activity occurred during a short gap between two phases of the Pliocene–Early Pleistocene mafic magmatism widespread within the Akhalkalaki plateau. The studied Pliocene dacites were erupted at the post-collisional stage of the evolution of the Lesser Caucasus, but bear petrological-geochemical affinity of adakitic series. They are characterized by the steady presence of amphibole phenocrysts, the elevated contents of Sr, Ba, LILE and the lowered contents of Y, Nb, Ta, and HREE, and have depleted Sr isotopic composition (87Sr/86Sr < 0.7045). Analysis of petrogenetic models earlier proposed to explain the generation of adakite-like magmas in the modern collision zones showed that the origin of the Pliocene dacitic lavas of the Akhlkalaki plateau is best described by the crystallization differentiation of watersaturated calc-alkaline basaltic melts with removal of common mafic rock-forming minerals (first of all, amphibole and pyroxene) and accessory phases (apatite, titanite, Ti-magnetite) as cumulus minerals. Crustal assimilation of evolved magmas only insignificantly contributed to the petrogenesis of the dacites.

Geological mapping in the Chéticamp granitoid belt in combination with petrographic and geochemical studies and U–Pb (zircon) dating by laser ablation inductively coupled plasma mass spectrometry have resulted in major reinterpretation of the geology in the western part of the Ganderian Aspy terrane of Cape Breton Island. Nine new U–Pb (zircon) ages show that the former “Chéticamp pluton” consists of 10 separate plutons of five different ages: late Neoproterozoic (ca. 567 Ma), Cambrian–Ordovician (490–482 Ma), Ordovician–Silurian (442–440 Ma), mid-Silurian (ca. 428 Ma), and late Devonian (366 Ma). The three late Neoproterozoic granodioritic to monzogranitic plutons are older than the adjacent metavolcanic and metasedimentary rocks of the Jumping Brook Metamorphic Suite, whereas the tonalitic to quartz dioritic Cambrian–Ordovician plutons intruded those metamorphic rocks. Petrographic characteristics and approximately 100 whole-rock chemical analyses show that with the exception of the mid-Silurian Grand Falaise alkali-feldspar granite, which has A-type within-plate characteristics, the plutonic units have calc-alkaline affinity and were emplaced in a volcanic-arc tectonic setting. These results are evidence that fragments of a long history of episodic subduction-related magmatism and terrane collision are preserved in this small part of Ganderia. Eight new Sm–Nd isotopic analyses are consistent with the Ganderian affinity of the Chéticamp plutonic belt. The ca. 490–482 Ma plutons are the first direct evidence in Cape Breton Island for the Penobscottian event recognized in the Exploits Subzone of central Newfoundland and in New Brunswick. However, the structural relationship of the Chéticamp plutonic belt to the rest of the Aspy and Bras d’Or terranes remains enigmatic, as is the apparent absence of effects of Devonian deformation and metamorphism in the older plutonic units.

Geological and geophysical studies recently conducted in the White Sea and the adjacent territory have provided new data on the deep structure of this region. Our study aims to conduct complex analysis of the anomalous magnetic field and the geological and geophysical data on the Onega-Kandalaksha paleorift located in the White Sea basin and the adjacent southeastern land area, and to develop a model showing its deep structure. The basis for analysing the magnetic field is the anomalous magnetic field (AMF) map constructed by the authors using the magnetic survey data consolidated by the Marine Arctic Geological Expedition (MAGE) in 2003–2008 and supplemented by the survey data of the Institute of Oceanology RAS in 2001–2004. The parameters of the magnetically active layer are estimated by the independent complementary methods of quantitative interpretation developed by the Laboratory of Geophysical Fields, P.P. Shirshov Institute of Oceanology RAS. This article describes a model showing the structure and formation of the magnetically active layer of the White Sea paleorift. Our study shows that the magnetically active layer of the paleorift system has a complex structure reflecting all the main stages in the evolution of tectonic activity in the White Sea region, from the Middle and Late Riphean to the last glaciation of the Quaternary period. The model includes three structural layers, each corresponding to a certain stage. The bottom structural layer is the base of the magnetically active layer, which reflects the continental rifting stage in the evolution of the White Sea mobile belt in the Middle and Late Riphean. The middle structural layer reflects the Middle Paleozoic (Late Devonian) stage of rifting reactivation, which is characterized by alkaline-ultrabasic magmatism and represented by swarms of alkaline dykes and diatremes, including kimberlite pipes. The top structural layer reflecting a high-frequency component of the AMF is related to the highly magnetic sources of anomalies located in the upper part of this structural layer. The characteristics of the top structural layer suggest that it formed in the Late Pleistocene – Holocene and developed during the final stage the tectonic activation of this region. The deep crustal structure of the White Sea basin is specified in our model showing the magnetically active layer for the low-frequency component of the AMF. In the southeastern part of the basin, magmatism products of the basic (Riphean – Vendian) and alkaline-ultrabasic (Middle Paleozoic) composition are abundant in the crust and provide for a strong magnetic source of anomalies, the lower edges of which are traced at the depths to 30 km. This probably reflects the most active plume-lithospheric interaction. Wedging and uplifting of the magnetically active layer northwestward along the Onega-Kandalaksha rift is related to the White Sea (Belomorsky) deep fault. This fault is a long-lived conduit that channels magma from the central portion of the plume, as evidenced by the igneous bodies of the basic composition in the basement and central parts of the sedimentary wedge in the Kandalaksha graben. The complex analysis of the AMF in the White Sea region suggests the presence of morphologically different igneous bodies in the upper crust in the study region.

The article presents the results of diamond prospecting studies in the Subotska structure of the Inhulskyi megablock of Ukrainian Shield. For the results, it is indicated that the Subotska structure is mimicked by crater rocks and in some cases by manifestations of the vent facies with signs of kimberlitic-lamproitic magmatism. The typical local features of manifestations of explosive structures from maar volcanism in Subotska area are determined. The article presents the results of petrographic and mineralogical study of the core material from exploratory wells on the Subotska structure, the results of study of material composition of the clay fraction, X-ray diffraction analysis of the pellet fraction. Data of the X-ray structural analysis of the pellet fraction of samples taken from the core material from exploratory wells on the Subotska structure indicates the obvious mechanical sum, the head folder of such is calciferous montmorilonite, and also saponite, nontronite, hydromica and kaolinit. The availability of the nontronite and saponite is confirmed by the results of electronic-microscopic reports. Also the article presents the results of studying the secondary lithochemical halos of Cr, Ni, Mg, Co, Ti, V, Fe, covering the geochemical spectrum inherent in alkaline-ultrabasic rocks and their weathering crust. These halos are combined with negative gravitational anomalies associated with the explosive structures in the Subotska area. The structural control of the great part of the detected geochemical anomalies, geochemical halos are determined. According to the degree of manifestation of the complex of criteria five potential diamond-prospective structures are discovered on the Subotska area. There were developed recommendations for further research on the Subotska area.

AbstractThe Assynt Culmination of the Moine Thrust Belt, in the northwest Scottish Highlands, contains a variety of Caledonian alkaline and calc-alkaline intrusions that are mostly of Silurian age. These include a significant but little-studied suite of dykes and sills, the Northwest Highlands Minor Intrusion Suite. We describe the structural relationships of these minor intrusions and suggest a classification into seven swarms. The majority of the minor intrusions can be shown to pre-date movement in the Moine Thrust Belt, but some appear to have been intruded during the period of thrusting. A complex history of magmatism is thus recorded within this part of the Moine Thrust Belt. New geochemical data provide evidence of a subduction-related component in the mantle source of the minor intrusions.

A new plate-tectonic model accounts for lithological relations and regional structural patterns in late Archean supracrustal domains of the Slave Province. Multiple dykes and pillowed mafic flows, most common in the western part of the province, suggest sea-floor spreading. The mafic volcanics, lying in narrow homoclinal belts stratigraphically below more extensive turbidites, are viewed as megaxenolithic remnants of oceanic crust preserved on the periphery of granitoid plutons and blocks of sialic crust. Closure of an oceanic basin was marked by emplacement of the granitoid plutons and coeval felsic volcanics, the latter predominating over mafic volcanics in northeastern domains. The felsic calc-alkaline magmas may have risen from a shallow-dipping subduction zone. Westerly verging folds, westerly convex fold arcs, and inclinations of later foliations, particularly in lower level rocks of higher metamorphic grade, are in accord with underthrusting to the east. The zone of underthrusting shifted progressively westward, and calc-alkaline magmatism swept across the western part of the province. Plutons followed crustal fracture systems, some of which were inherited from initial rifts, producing a rectilinear zigzag pattern of contacts between plutons, and mafic volcanics. The fracture systems and rising plutons redirected stresses, resulting in distinctive sets of regional and local foliations that reflect crustal compression only indirectly related to the sense of subduction.

Geologi Pulau Bangka disusun oleh variasi granit sebagai Granitoid Klabat yang tersebar di berbagai lokasi. Unsur jejak dapat diaplikasikan dalam diskriminasi magmatisme dalam pembentukan granitoid tersebut. Tujuan penelitian ini adalah mengetahui karakteristik granitoid yang tersebar di Pulau Bangka berdasarkan geokimia unsur jejak untuk diaplikasikan dalam mempelajari magmatisme, sumber dan situasi tektoniknya.Metode analisis geokimia yang diaplikasikan dengan menggunakan Analisis Aktivasi Neutron (AAN) dan portableX-Ray Fluorescence (pXRF) untuk analisis kualitatif dan kuantitatif pada 27 sampel dari Granitoid Klabat di Pulau Bangka.Hasil penelitian ini menyimpulkan Granitoid Bangka Utara (Belinyu) dan Bangka Tengah sebagai percampuran kerak-mantel dengan afinitas Calc-Alkaline, karakteristik Tipe I sedangkan Granitoid Bangka Selatan dan Barat asal kerak dengan afinitas High-KCalc-Alkaline sebagai Tipe S. Diharapkan diskrimasi magmatisme granitoid bermanfaat dalam memberikan panduan eksplorasi bahan galian nuklir di Pulau Bangka. Geology of Bangka Island consists by variation of granite as Klabat Granitoid scattered in various locations. Trace elements can be applied in magmatism discrimination of granitoid.The purpose of this study was to determine the characteristics Bangka Island granitoid based on trace element geochemistry to be applied in the study of magmatism, source and tectonic situation. Geochemical analyses method used are the Neutron Activation Analysis (NAA) and portableX-Ray Fluorescence (pXRF) for qualitative and quantitative analyses on 27 samples of Klabat granitoid on Bangka Island. This study concluded granitoid East Bangka (Belinyu) and Central Bangka as crust-mantle mixing with affinityCalc-Alkaline, characteristic of I Type while South and West Bangka granitoid crust origin with affinity high K Calc-Alkaline as S Type. Expectedmagmatismdiscrimination ofgranitoidhelpfulin providingradioactive mineral explorationguidein BangkaIsland.

The North Caribou terrane of the western Superior Province attained continental thickness (~35 km) by 2997 Ma. It records a subsequent 300 million years history of continental fragmentation, arc magmatism, and terrane accretion. At Lake Winnipeg the ~2978 Ma Lewis–Storey quartzite–komatiite–iron formation assemblage marks Mesoarchean breakup. Unlike the relatively continuous 2980–2735 Ma stratigraphic record of the Red Lake and Birch–Uchi greenstone belts to the east, little of this interval is recorded at Lake Winnipeg. Rather, two belts of younger, juvenile rocks are tectonically juxtaposed: the Black Island assemblage of isotopically depleted, 2723 Ma basalt, and calc-alkaline andesite; and Rice Lake greenstone belt of basalt, calc-alkaline andesite, and dacite (2731–2729 Ma). Collectively these terranes represent a short-lived island-arc–back-arc system that docked with the southwestern North Caribou margin along a northwest-trending, dextral, transpressive, D1 suture. This zone is marked by the highly deformed coarse clastic Guano Island sequence (<2728 Ma) that contains detritus of North Caribou affinity and is interpreted as a strike-slip basin deposit. Younger clastic sequences, including the Hole River (<2708 Ma), San Antonio (<2705 Ma), and English River (<2704 Ma) assemblages, occur in east–west belts that may have been deposited during the terminal collision (D2, D3) between the North Caribou terrane and continental crust of the Winnipeg River terrane to the south. Several terrane docking events within a framework of north-dipping subduction and continental arc magmatism appear necessary to explain structural and stratigraphic relationships in the 2735–2700 Ma interval.

AbstractThe South Altyn Orogenic Belt (SAOB) is one of the most important orogenic belts in NW China, consisting of the South Altyn Continental Block and the Apa–Mangya Ophiolitic Mélange Belt. However, its Palaeozoic tectonic evolution is still controversial. Here, we present petrological, geochemical, zircon U–Pb and Lu–Hf isotopic data for the Mangya plutons with the aim of establishing the Palaeozoic tectonic evolution. We divide the Early Palaeozoic magmatism in the Apa–Mangya Ophiolitic Mélange Belt into four episodes and propose a plate tectonic model for the formation of these rocks. During 511–494 Ma, the South Altyn Ocean (SAO) was in a spreading stage, and some shoshonite series, I-type granitic rocks were generated. From 484 to 458 Ma, the oceanic crust of the SAO subducted northward, accompanied by large-scale magmatic events resulting in the generation of vast high-K calc-alkaline series, I-type granitic rocks. During 450–433 Ma, the SAO closed, and break-off of the subducted oceanic slab occurred, with the generation of some high-K calc-alkaline series, I–S transitional type granites. The SAOB was in post-orogenic extensional environment from 419 to 404 Ma, and many A-type granites were generated.

The timing of the Alpine deformation in the Central Southern Alps (CSA or Orobic Alps) has always been a debated topic, since the scarcity of reliable absolute age constraints has prevented an accurate chronological reconstruction of the evolution of this sector of the European Alps. In this work, detailed structural analyses performed in different areas of the CSA allowed us to distinguish different compressive features within both the crystalline basement and the sedimentary cover. The integration of these field data with new isotopic data provides time constraints for the reconstruction of the evolution of the CSA during the Alpine orogeny. In the northern sector of the belt a Variscan polymetamorphic basement is stacked southward on the Permian to Mesozoic cover along two main regional faults (Orobic and Porcile thrusts). Fault zones, slightly postdating a first folding event of Alpine age (D3), experienced a complex evolution through the ductile and brittle deformation regime, showing greenschists facies mylonites overprinted by a penetrative cataclastic deformation (D4). Generation of fault-related pseudotachylytes marks the onset of brittle conditions, lasting up to the youngest episodes of fault activity. Thrusting along this structures also produced thrusting within the Permian-Triassic cover with the formation of different south-verging thrust stacks. This first thrusting event was followed by the activation of new deeper thrust surfaces leading to the emplacement of three regional anticlines (Orobic Anticlines) which tilted to the south the previously stacked units. During this long compressive stage (Orobic-Porcile thrusts and Orobic Anticlines) the sedimentary cover of the CSA was also involved in thrusting and different stacks of Mesozoic units were emplaced to the south. 40Ar/39Ar dating of the pseudotachylyte matrix of 9 samples from both the Orobic and Porcile thrusts give two separated age clusters: Late Cretaceous (80-68 Ma) and Early to Middle Eocene (55-43 Ma). These new data provide evidence that the pre-Adamello evolution of the CSA was characterized by the superposition of different tectonic events accompanying the exhumation of the deepest part of the belt through the brittle-ductile transition. The oldest pseudotachylyte ages demonstrate that south-verging regional thrusting in the CSA was already active during the Late Cretaceous, concurrently with both the HP metamorphism that affected the Austroalpine units of the eastern Alps, and the development of a syn-orogenic foredeep basin where the Upper Cretaceous Lombardian Flysch was deposited. In the Early to Middle Eocene a minor reactivation of the Orobic and Porcile thrusts occurred, as testified by the youngest pseudotachylyte ages obtained by 40Ar/39Ar dating. This event was probably related with the closure of the Ligurian-Piedmont and the ongoing of the Europe-Adria collision. South of the Orobic Anticlines system the Triassic sedimentary succession is stacked into several units bounded by south-verging low-angle thrust faults, which are related to different steps of crustal shortening. Different thrust stacks occur within the Triassic cover between the Como Lake to the west and the Adamello batholith to the east. They usually have an antiformal arrangement and are separated by each other by different N-S trending transverse zones, such as the poorly known Grem-Vedra Transverse Zone (GVTZ), formed during complex deformational phenomena in a transpressional regime coeval with thrust emplacement. The GVTZ formed during the southward imbrication of the older thrust sheets of the Menna-Arera group, strongly interacting with syn-thrust ductile structures, and was reactivated during the growth of the Orobic Anticlines belt. The GVTZ and other transverse zones of the CSA probably reflect the occurrence of pre-existing fault systems that characterize the Norian to Jurassic rifting history of the Lombardian basin, and were reactivated as strike-slip features during Alpine tectonics. In the Gandino and Presolana areas thrust surfaces are cut by high-angle extensional and strike-slip faults, which controlled the emplacement of hypabissal magmatic intrusions that post-date thrusts motions. Intrusion ages based on SHRIMP U-Th-Pb zircon dating span between 42±1 and 39±1 Ma, suggesting close time relationships with the earliest Adamello intrusion stages and, more in general, with the widespread calc-alkaline magmatism described in the Southern Alps. Fission track ages of magmatic apatites are indistinguishable from U-Pb crystallization ages of zircons, suggesting that the intrusion occurred in country rocks already exhumed above the partial annealing zone of apatite (depth < 2-4 km). These data indicate that the northern and central sectors of the CSA were already structured and largely exhumed in the Middle Eocene and no major internal deformations has occurred in these areas after the Bartonian. Neogene deformations were instead concentrated further south, along the frontal part of the belt (Milano Belt). These new data provide a direct evidence that thrusting and nappe stacking were active during Late Cretaceous times not only in the Eastern Alps, but also in the CSA, significantly extending southward the sector of the Alpine belt affected by the Cretaceous orogenic event. In this view, the Late Cretaceous Southern Alps can be interpreted as the south-verging retrobelt of a pre-collisional orogenic wedge, which formed during the subduction of the Alpine Tethys beneath the attenuated northern Adria margin.