Articles de revues : « Structures and metamorphism »

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Roy, A. B., et A. R. Das. « A Study on the Time Relations Between Movements, Metamorphism and Granite Emplacement in the Middle Proterozoic Delhi Supergroup Rocks of Rajasthan ». Journal Geological Society of India 26, n^o 10 (1 octobre 1985) : 726–33. http://dx.doi.org/10.17491/jgsi/1985/261004.

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Abstract The Delhi Supergroup rocks, an important tectonic-stratigraphic unit of the Aravalli Mountains, are affected by multiple folding and polyphase metamorphism. Three generations of deformational structures have been recognized in these rocks (DFI, DF2 and DF3). DFI structures which include a set of isoclinal folds with penetrative axial planar schistosity, were deformed successively by DF2 and DF3 folding movements. Microtcxtural studies of rocks yielded evidence for two metamorphic events-a progressive metamorphism (M1) rising upto epidote amphibolite facies, followed by a retrogressive phase (M3) under greenschist facies condition. Field and microscopic studies indicate that both DF1 and DF2 structures were formed during Ml phase, and DF3 structures were formed during the retrogressive phase (M2). The metamorphic climax of M1 phase was during the early phase of DF2, and the granites emplaced coevally with this event indicated ca. 1450 Ma Rb/Sr isochron age. No definite relationship could be established for the 850-700 Ma secondary (mineral) ages with DF3 deformation and M2 metamorphism.

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Webster, Ewan Russell, et David R. M. Pattison. « Spatially overlapping episodes of deformation, metamorphism, and magmatism in the southern Omineca Belt, southeastern British Columbia ». Canadian Journal of Earth Sciences 55, n^o 1 (janvier 2018) : 84–110. http://dx.doi.org/10.1139/cjes-2017-0036.

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The southeastern Omineca Belt of the Canadian Cordillera preserves a record of overlapping Barrovian and Buchan metamorphism spanning 180–50 Ma. This paper documents the timing, character, and spatial relationships that define separate domains of Middle Jurassic, Early Cretaceous, and Late Cretaceous deformation and metamorphism, and the nature of the geological interfaces that exist between them. A domain of Early Jurassic deformation (D1) and regional greenschist-facies metamorphism (M1) is cross-cut by Middle Jurassic (174–161 Ma) intrusions. Associated contact aureoles are divided into lower pressure (cordierite-dominated; ∼2.5–3.3 kbar; 1 kbar = 100 MPa) and higher pressure (staurolite-bearing; 3.5–4.2 kbar) subtypes; contact metamorphic kyanite occurs rarely in some staurolite-bearing aureoles. Jurassic structures are progressively overprinted northwards by Early Cretaceous deformation and metamorphism (D2M2), manifested in a tightening of Jurassic structures, development of more pervasive ductile fabrics, and Barrovian metamorphism. The D2M2 domain is the southerly continuation of the 600 km long Selkirk–Monashee–Cariboo metamorphic belt. Mid-Cretaceous intrusions (118–90 Ma) were emplaced throughout the D2M2 domain, the earliest of which contain D2 fabrics, but cut M2 isograds. The D2M2 domain makes a continuous, southeasterly transition into a domain of Late Cretaceous regional Barrovian metamorphism and deformation (D3M3; 94–76 Ma). The interface between these two domains is obscured by the coaxial nature of the deformation and the apparent continuity of the metamorphic zones, resulting in a complex and cryptic interface. Similarities between the D3M3 domain and the Selkirk Crest of Idaho and Washington suggest that this domain is the northerly continuation of the northward-plunging Priest River Complex.

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Monger, J. W. H. « Correlation of Settler Schist with Darrington Phyllite and Shuksan Greenschist and its tectonic implications, Coast and Cascade mountains, British Columbia and Washington ». Canadian Journal of Earth Sciences 28, n^o 3 (1 mars 1991) : 447–58. http://dx.doi.org/10.1139/e91-039.

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Amphibolite-facies Settler Schist in the southeastern Coast Mountains of British Columbia has long been correlated with Chiwaukum Schist of the Cascade metamorphic core, North Cascade Mountains, northwestern Washington. The additional correlation proposed here of Settler Schist with Darrington Phyllite and Shuksan Greenschist (and blueschist) of the Northwest Cascade System in Washington is based on along-strike near-continuity of outcrop areas, a similar protolith composition range, the same structural position relative to the Shuksan fault zone, and distinctive irregular structures in variably metamorphosed sandstone and pelite of both Darrington Phyllite and Settler Schist. If this correlation is valid, then the record of Early Cretaceous; subduction-related blueschist metamorphism of Shuksan–Darrington rocks was destroyed in Settler Schist by overprinting by early Late Cretaceous Barrovian metamorphism; only some distinctive, premetamorphic structures remain. The implication is that within the southeastern Coast Mountains, a cryptic record of subduction is overprinted by Barrovian metamorphism.

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Primmer, T. J. « A transition from diagenesis to greenschist facies within a major Variscan fold/thrust complex in SW England ». Mineralogical Magazine 49, n^o 352 (juin 1985) : 365–74. http://dx.doi.org/10.1180/minmag.1985.049.352.07.

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AbstractThe north coast of Cornwall, from Bude to Newquay, provides a continuous section through a major Variscan fold/thrust complex. Illite crystallinity studies have revealed a transition from diagenesis in the north to greenschist facies metamorphism in the south in the Upper Palaeozoic succession. More detailed studies of mineral assemblages in both metabasites and pelitic rocks support the regional pattern of metamorphism indicated by illite crystallinity, and show that locally in the Tintagel district, the grade of metamorphism may have reached middle to upper greenschist facies. An attempt to correlate the above data with temperatures (108–985°C) derived from O-isotope geothermometers is made. Interpretation of the metamorphic data presented helps to emphasize the tectonic importance of the major structures seen in the fold/thrust complex.

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Tarhan, Niyazi. « THE WORLDS RICHEST METABLASTIC ORE DEPOSITS ARE ASSOCIATED WITH CARBONATOBLASTIC ROCKS OF METAMORPHIC ORIGIN (KNOWN AS CARBONATITES) ». International Journal of Advanced Research 12, n^o 12 (31 décembre 2024) : 1061–85. https://doi.org/10.21474/ijar01/20114.

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In the Geology/Earth Sciences literature, carbonatites are accepted as rocks of magmatic origin that are rarely seen in nature as intrusions, carbonatite dykes, veins, pegmatitics, stocks, sills and lenses. The view that carbonatites are of magmatic origin and rarely seen in nature is definitely not true. On the contrary Carbonatites are rootless, metamorphic origin, a new type of modern metamorphic rocks, pure-impure leucocratoblastic (light colored, rock composed of different carbonate origin/based crystalloblast neominerals) carbonatoblastic rocks / carbonatoblastites / carbonatoblastic rock series and their derivatives. Pure-impure leucocratoblastic carbonatoblastic rocks / carbonatoblastites, whose primary source rocks are different (pure-impure carbonate/limestone) They form a type of leucocratoblastic metablastic rocks, which are of metamorphic origin, rootless, a type of new modern metamorphic rocks, with a granite mineralogical composition and composed of silicate origin/based crystalloblast neominerals. Carbonatoblastic rocks (known as carbonatites, rarely seen in nature, which are of magmatic origin until today) They developed in the last/second closing stage of the regional dynamothermal Tarhan metamorphism cycle, and they developed in the changing physical conditions (P/T) of the facies and sub-facies of Abukuma type reversed regional regressive dynamothermal metamorphism, where temperatures are effective in proportion to/compared to pressures (T>P, P, Pressure T, Temperature Temperatures put their stamp on the metamorphism). Previously existing primary source rock units pure-impure carbonate/limestones They developed in the first initial phase of the regional dynamothermal Tarhan metamorphism cycle, within pure and impure classical marbles, which are the metamorphic equivalents of Barrow type regional progressive dynamothermal metamorphism, where pressures are effective compared to/in proportion to temperatures (P>T, pressures put their stamp/mark on the metamorphism). Carbonatoblastic rocks/carbonatoblastites were derived in solid phase and in-situ (autochthonous) from the pure-impure classical marbles in the Abukuma type inverted regional regressive dynamothermal metamorphism type/phase that developed in the second/last closing phase of the metamorphism cycle. Metamorphic origin leucocratoblastic pure-impure carbonatoblastic rocks derived from pure-impure carbonate/limestones of different primary rocks They constitute a new type of modern metamorphic rocks of metamorphic origin, rootless, leucocratoblastic metablastites / metablastic rocks / metablastic rock series and their derivatives, defined for the first time under the general name, named and different types. Pure-impure carbonatoblastic rocks / carbonatoblastites / carbonatoblastic rock series and derivatives under the name of many carbonatoblastite type metablastic rocks (alkali metablastites, syenitoblastite, monzonitoblastite, calcitoblastite, calcito-dolomitoblastite, dolomitoblastite, sideritoblastite, stroncianitoblastite etc.) and carbonate origin / based rock forming main-secondary-trace crystalloblast neominerals (calcitoblast, dolomitoblast, witheritoblast, stroncianitoblast etc.) etymologically redefined for the first time, named, classified, physical-chemical properties determined. Carbonatoblastic rocks / Carbonatoblastites / Carbonatoblastic rock series and their derivatives It has been determined for the first time that they have very rich and widespread potential in terms of metablastic ore/mine deposits of metamorphic origin, defined and named for the first time. Different carbonatoblastic rock types develop depending on the composition and contents of pure-impure carbonate/limestones, which are pre-existing primary origin rock units. In the changing physical conditions (P/T) of the facies and sub-facies of the Abukuma type reversed regional regressive dynamothermal metamorphism, which developed in the last/second/closing phase of the regional dynamothermal Tarhan metamorphism cycle, where temperatures are effective compared to pressures (T>P, temperatures put their stamp on the metamorphism), protominerals (minerals of primary source rocks) - metaprotominerals (classical metamorphic minerals of metamorphic equivalent rocks of primary source rocks) lose their stability and are partially and completely gradually dissolved in the solid phase and in-situ. With dissolution in the solid phase solid neo-solutions with different chemical compositions, anhydrous, unstable and disordered structures, consisting of free and unstable ions (cation, anion) of different elements with increased electrically charged and atomic diffusion rates are widely developed. In the current physicochemical conditions of the facies and subfacies of the abukuma type inverted regional regressive dynamothermal metamorphism, recrystallization by metablastization commonly develops from unstable solid neo-solutions with different chemical compositions due to the temperatures effective in the environment. With the recrystallization developing by metablastation from solid neo-solutions rock-forming main-secondary-trace element cations, metallic-non-metallic ore cations, radioactive and rare earth elements/REE, which are incompatible elements with large ionic radii, are electrically charged, have increased atomic diffusion rates, and are free and unstable cations They combine with root carbonate anion (CO3)2-, root silicate anion (smooth surface silicon tetrahedral) (SiO4)4- / or [(Si, Al)O4]4- and oxygen (O2-) anions (due to polarization, solid-solid chemical reactions) and develop free blast/embryo/nucleus/bud and blast/embryo aggregates of their own unique/belonging minerals. In this way, the ions become electrically neutral and become stable. Blast/embryo, blast/embryo aggregates with the same and similar geochemical properties formed in the current/setting physicochemical conditions of Abukuma type reversed regional regressive dynamo-thermal metamorphism where temperatures are effective compared to pressures (T>P, temperatures put their stamp on the metamorphism) group among themselves and add to each other, and gradually grow as crystalloblast, porphyroblast and megacrystalloblast type rock-forming main-secondary-trace, metallic-non-metallic ore-forming, strategic, radioactive and light-heavy rare earth elements/REE free/independent crystalloblast neominerals. In this way, metallic-non-metallic ore crystalloblast neominerals, strategic crystalloblast neominerals, radioactive-rare earth elements/REE from incompatible elements with large ionic radii that cannot easily enter the crystal structures of rock-forming main-secondary-trace crystalloblast neominerals are naturally produced by their own crystalloblast neominerals. They become enriched and visible in the environment/setting. They are naturally enriched and become visible in the form of crystalloblast neominerals in leucocratoblastic carbonatoblastic rocks, which are rootless and a type of new modern metamorphic rocks of metamorphic origin. It has been determined and suggested for the first time that they develop the richest metamorphic-origin metablastic ore/mine deposits in the world by growing freely in the form of metallic-non-metallic crystalloblast neominerals that are unique/belonging to them. Carbonatoblastic rocks / Carbonatoblastites / Carbonatoblastic rock series and their derivatives are also very rich in terms of their contents of precious and semi-precious stones, ornamental stones (Gemology) and natural colored-patterned building stones (natural ceramic stones). Carbonatoblastic rocks They form the 2nd generation metamorphic rocks, the 3rd generation rocks, allotropic superionic metablastic matter / mine / rock / minerals and the 5th classical state of matter. All these geological phenomena appear before us as the products of the metamorphic view that constitutes the regional dynamo-thermal Tarhan metamorphism cycle that has been determined and suggested for the first time. They are not the products of the magmatic view. They are not the products of logical imagination models either. Therefore, if material, moral and temporal losses are not desired for scientific studies on similar topics, but reversible economic gains are desired, the magmatic view should be rejected. In contrast, the metamorphic view should be accepted.

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KHUDOLEY, ANDREI K., et SERGEI D. SOKOLOV. « Structural evolution of the northeast Asian continental margin : an example from the western Koryak fold and thrust belt (northeast Russia) ». Geological Magazine 135, n^o 3 (mai 1998) : 311–30. http://dx.doi.org/10.1017/s0016756898008747.

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The western Koryak fold and thrust belt consists of a set of tectonostratigraphic terranes that contain units ranging from Lower Palaeozoic to Cenozoic. Three deformational events have been identified in the study area. The first event structures are folds, domes and shear zones with related high-pressure/low-temperature metamorphism. These structures are early Carboniferous and are only recognized in the metamorphic terranes. The second event structures are imbricate fans of thrusts and folds with southeast vergence, broken formation and serpentinite mélange. These are latest Jurassic to early Cretaceous (early Albian) and occur throughout the study area. During this event, thrusting was accompanied by dextral strike-slip faulting. The second event structures are overlapped by the Upper Albian sedimentary rocks with an angular unconformity at the base. During metamorphism associated with the first and second deformational events, some of the rocks were metamorphosed to blueschist grade and were affected by strain with axial ratios of up to 15[ratio ]1. The third deformational event is characterized by significant sinistral strike-slip displacement at higher crustal levels. This resulted in a new set of structures and rotation of pre-existing structures. The age of the sinistral strike-slip faults is interpreted to be late Cretaceous to Cenozoic. The kinematics of the second and third deformational events correspond to assumed proto-Pacific plate motions based on palaeomagnetic data.

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Umar, U. S. « METAMORPHISM AND DEFORMATION OF GOLD-BEARING NEOPROTEROZOIC WONAKA SCHIST BELT, NORTHWEST-NIGERIA. » Open Journal of Physical Science (ISSN : 2734-2123) 5, n^o 1 (10 juillet 2024) : 1–17. http://dx.doi.org/10.52417/ojps.v5i1.626.

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The role of metamorphism and deformation is indispensable in the occurrences of gold mineralization worldwide. In this work, deformation and metamorphic conditions for gold-bearing Neoproterozoic Wonaka Schist Belt; located around Kutcheri town of Tsafe Local Government of Zamfara State, was investigated. This is achieved using metamorphic litho-minerals obtained from ternary plots via X-Ray fluorescence (XRF) geochemical data, and directly using minerals phases from X-Ray Diffraction (XRD) technique. Index minerals identified from petrographic analysis previously suggest low to medium-grade metamorphism (M1). XRD analysis indicates quartz, albite, oligoclase, microcline, chlorite, and biotite, suggesting greenschist to lower amphibolite facies (M2). Sillimanite, andalusite, kyanite, staurolite, chlorite, biotite, and garnet were identified from the ternary plots using XRF major oxides, indicating upper amphibolite to granulite facies metamorphism (M3). This is typical of prograde metamorphism, granulite facie metamorphic grade is indicated. Na2O/Al2O3 versus K2O/Al2O3 for petrogenetic character suggests shale provenance, while the trace elements spider diagram indicates Wonaka litho-units as co-genetic compositionally, as high concentrations of V and Cr linked the petrogenetic affinity to mafic sources. Three circles of deformations are indicated; ductile deformation (D1) of the paleosome Schist producing foliations and lineation, brittle type (D2) in mid Pan-African and was accompanied by several fractures and felsic intrusions. Late Pan-African (D3) involves the folding of banded orthogneisses, the development of boudinage as well as intense shearing (ductile fault). Geospatial analysis of the fractures suggests that they represent regional Pan-African sutures cross-cutting Nigeria into the Atlantic and up to South American plate. The research therefore concludes that Au-fluid emanating through this regional event, utilizes D2 as channel ways and loci. D3 with M3 engulfed the entire structures repositioning the geometry to its present disposition.

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Webster, Ewan Russell, David Pattison et S. Andrew DuFrane. « Geochronological constraints on magmatism and polyphase deformation and metamorphism in the southern Omineca Belt, British Columbia ». Canadian Journal of Earth Sciences 54, n^o 5 (mai 2017) : 529–49. http://dx.doi.org/10.1139/cjes-2016-0126.

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The Omineca Belt between Nelson and Creston in southeastern British Columbia was affected by overlapping pulses of Mesozoic magmatism, metamorphism, and deformation. U–Pb geochronological data from zircon and monazite were collected by laser ablation – inductively coupled plasma – mass spectrometry (LA–ICP–MS) to constrain the timing of these events. The Porcupine Creek stock (162.3 ± 1.3 Ma) intruded across folds and fabrics associated with the earliest phase of regional deformation and metamorphism (D1M1), restricting it to the Early–Middle Jurassic. The Jurassic structures are overprinted northwards by Early Cretaceous deformation and metamorphism (D2M2). The Baldy pluton (117.8 ± 1.2 Ma) crosscuts the regional 144–134 Ma M2 isograds, yet was pervasively affected by the D2 deformation, indicating that D2 deformation outlasted M2 metamorphism but had ceased by 111 Ma, the age of an undeformed pluton. Monazite dates from a kyanite-bearing rock in the contact aureole of the Middle Jurassic Wall stock overlap with the age of the intrusion (167 Ma), indicating a contact rather than regional origin for the kyanite. In the southeast part of the study area, three samples from the regional sillimanite zone contain monazite intergrown with sillimanite that yield dates between 80 and 69 Ma, indicating an episode of Late Cretaceous (M3) Barrovian metamorphism and deformation (D3). To the north of this domain, in an area characterized by the older D2M2 deformation, a sillimanite zone schist contains two main monazite age populations, suggestive of overlapping effects of Early Cretaceous and Late Cretaceous metamorphic episodes.

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Ziultsle, O. V., et V. V. Ziultsle. « Breed Associations of the Gaisin Block of the Ukrainian Shield ». Geochemistry and ore formation, n^o 42 (2021) : 61–70. http://dx.doi.org/10.15407/gof.2021.42.061.

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The Gaysins block is characterized by a wide range of both metamorphic and ultrametamorphic formations. Ultrametamorphic formations are represented by an association of rocks with a transition from charnockitoids to two-feldspar granites. Remnants of metamorphic rocks are composed of diafluorinated varieties to varying degrees. Geological surveys of the last decades have discovered on the territory of the Gaysin block structures of variegated composition, which are represented by both metamorphic and ultrametamorphic rocks. The most studied are structures in the area of the settlements of Chagiv, Tyagun, Sitkovtsi, Naraevka, Tsibuliv and Popudnya. The wide variety of the mineral composition of the rocks of the Gaysinsky block is due to the metamorpho-metasomatic transformations of the primary parageneses formed under the conditions of the granulite facies. These transformations are taking place against the background of a decrease in the PT parameters of regional metamorphism.

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Gal, L. P., et E. D. Ghent. « Metamorphism in the Solitude Range, southwestern Rocky Mountains, British Columbia : comparison with adjacent Omineca Belt rocks and tectonometamorphic implications for the Purcell Thrust ». Canadian Journal of Earth Sciences 27, n^o 11 (1 novembre 1990) : 1511–20. http://dx.doi.org/10.1139/e90-161.

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Rocks of the Solitude Range, British Columbia, have been metamorphosed from chloritoid–chorite-zone to kyanite-zone conditions. The grade of metamorphism increases southwestward toward the Rocky Mountain Trench (RMT) and the Omineca Belt. Isograds crosscut lithologies and trend more northerly than deformation 2 (D2) structures and the RMT. They are thought to have been quenched syn- to post-D2. Pelitic (Mahto Formation) and calc-pelitic (Tsar Creek unit) rocks contain assemblages that reflect the increase in metamorphic grade. Physical conditions of metamorphism are estimated to be approximately 450–540 °C from the garnet to the kyanite zone; pressures averaged 6–7 kbar (1 kbar = 100 MPa). The pressures, temperatures, and metamorphic assemblages are very similar to those of the Adamant Range, which lies across the Purcell Thrust, to the southwest. This is in contrast with the Big Bend area, to the northwest, where differences in pressure across the Purcell Thrust (PT) have been documented. Two possible models to explain these contrasting relationships are presented. One model suggests that there was post-movement heating on the PT, which reduced the metamorphic contrast across the PT. The second model suggests that a combination of thrust and normal faulting, including warping of isobaric surfaces, has produced an apparently unbroken metamorphic sequence across the PT.

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Frater, Kenneth Maxwell. « Mineralization at the Golden Grove Cu – Zn deposit, Western Australia. I : Premetamorphic textures of the opaque minerals ». Canadian Journal of Earth Sciences 22, n^o 1 (1 janvier 1985) : 1–14. http://dx.doi.org/10.1139/e85-001.

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The competent opaque minerals in the Archaean Golden Grove deposit, pyrite and magnetite, retain pre-regional metamorphic textures despite the lower greenschist-facies grade of metamorphism. The pre-regional metamorphic textures and structures recognized include the development of pyrite and magnetite overgrowths, the replacement of pyrrhotite by pyrite, the conversion of a primary hematite–goethite mineralogy to magnetite and, as a result of thermal metamorphism, further local replacement of pyrrhotite (and sphalerite) by magnetite. Comparisons between pyrite from the Cu-rich mineralization at the base of the deposit and that from the Zn-rich mineralization in the hanging wall indicate that postdepositional modification and recrystallization were more extreme at the base of the deposit. The pre-regional metamorphic textures and structures indicate that pyrite and magnetite overgrowths developed almost immediately after primary precipitation ceased and that overgrowths continued to develop into the late hydrothermal–diagenetic stage of mineralization. A large proportion of the sulphide–Fe-oxide mineralization was formed at shallow depth within the volaniclastic host rocks, but at two horizons (the base and hanging wall) the mineralization formed at or very near the sea floor. These two periods of near sea-floor sulphide precipitation are separated by an oxide-dominated opaque-mineral assemblage, originally hematite–goethite and secondary marcasite but now converted to magnetite and secondary pyrite. The microtextural evidence supports a three-stage evolution of the ore deposit, two sulphide exhalative phases of mineralization separated by a stage of more oxidized hydrothermal activity, or, alternatively, sea-floor weathering during which hematite–goethite formed and marcasite partly replaced earlier formed sulphides.

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Godin, Laurent, Mark Ahenda, Djordje Grujic, Ross Stevenson et John Cottle. « Protolith affiliation and tectonometamorphic evolution of the Gurla Mandhata core complex, NW Nepal Himalaya ». Geosphere 17, n^o 2 (8 mars 2021) : 626–46. http://dx.doi.org/10.1130/ges02326.1.

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Abstract Assigning correct protolith to high metamorphic-grade core zone rocks of large hot orogens is a particularly important challenge to overcome when attempting to constrain the early stages of orogenic evolution and paleogeography of lithotectonic units from these orogens. The Gurla Mandhata core complex in NW Nepal exposes the Himalayan metamorphic core (HMC), a sequence of high metamorphic-grade gneiss, migmatite, and granite, in the hinterland of the Himalayan orogen. Sm-Nd isotopic analyses indicate that the HMC comprises Greater Himalayan sequence (GHS) and Lesser Himalayan sequence (LHS) rocks. Conventional interpretation of such provenance data would require the Main Central thrust (MCT) to be also outcropping within the core complex. However, new in situ U-Th/Pb monazite petrochronology coupled with petrographic, structural, and microstructural observations reveal that the core complex is composed solely of rocks in the hanging wall of the MCT. Rocks from the core complex record Eocene and late Oligocene to early Miocene monazite (re-)crystallization periods (monazite age peaks of 40 Ma, 25–19 Ma, and 19–16 Ma) overprinting pre-Himalayan Ordovician Bhimphedian metamorphism and magmatism (ca. 470 Ma). The combination of Sm-Nd isotopic analysis and U-Th/Pb monazite petrochronology demonstrates that both GHS and LHS protolith rocks were captured in the hanging wall of the MCT and experienced Cenozoic Himalayan metamorphism during south-directed extrusion. Monazite ages do not record metamorphism coeval with late Miocene extensional core complex exhumation, suggesting that peak metamorphism and generation of anatectic melt in the core complex had ceased prior to the onset of orogen-parallel hinterland extension at ca. 15–13 Ma. The geometry of the Gurla Mandhata core complex requires significant hinterland crustal thickening prior to 16 Ma, which is attributed to ductile HMC thickening and footwall accretion of LHS protolith associated with a Main Himalayan thrust ramp below the core complex. We demonstrate that isotopic signatures such as Sm-Nd should be used to characterize rock units and structures across the Himalaya only in conjunction with supporting petrochronological and structural data.

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Manby, G. M. « Mid-Palaeozoic metamorphism and polyphase deformation of the Forland Complex, Svalbard ». Geological Magazine 123, n^o 6 (novembre 1986) : 651–63. http://dx.doi.org/10.1017/s001675680002416x.

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AbstractThe Forland Complex of Prins Karls Forland has been subjected to mid-Palaeozoic greenschist facies metamorphism and polyphase deformation. Metamorphism was initiated prior to D1 deformation and gave rise to a parallelism of stratigraphic and metamorphic reaction surfaces. D1 gave rise to imbricately thrust, southwest-directed fold nappes which have not noticeably disturbed the isograd surfaces. D2, interpreted as belonging to the mid-Cenozoic West Spitsbergen Orogeny (WSO) which was coaxial but not coplanar with D1, produced crenulation folds and pressure solution cleavages and some thrusting. D3 structures are related to the formation of the Prins Karls Forland–Forlandsundet–Oscar II horst and graben system which is a late expression of the WSO. The rejuvenation of the Prins Karls Forland horst along NNW–SSE faults, the large scale E–W flexures and ENE–WSW faults in the Forland Complex and Tertiary graben deposits are assigned to D3.

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Castonguay, Sébastien, et Alain Tremblay. « Tectonic evolution and significance of Silurian Early Devonian hinterland-directed deformation in the internal Humber zone of the southern Quebec Appalachians ». Canadian Journal of Earth Sciences 40, n^o 2 (1 février 2003) : 255–68. http://dx.doi.org/10.1139/e02-045.

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In the southern Quebec Appalachians, the early tectonic history of the Laurentian margin (Humber zone) comprises foreland-propagating, northwest-directed thrust faulting, nappe emplacement, and regional prograde metamorphism in response to the obduction of large ophiolitic nappes during the Taconian orogeny. In the internal Humber zone, this event is dated at 462 ± 3 Ma (late Middle Ordovician), which is interpreted to represent the timing of near-peak Taconian metamorphism. Superimposed hinterland-directed structures are accompanied by retrograde metamorphism and consist of back thrusts and normal faults, which respectively delimit the northwestern and southeastern limbs of the Sutton and Notre-Dame mountains anticlinoria, both salient structures of the internal Humber zone of southern Quebec. Geochronologic data on the timing of hinterland-directed deformation vary from 431 to 411 Ma. Two tectonic models are presented and discussed, which may account for the Silurian Early Devonian evolution of the Laurentian margin: (1) back thrusting and syn- to post-compressional crustal extension in response to the tectonic wedging of basement-cored duplexes inducing delamination of supracrustal rocks; (2) tectonic exhumation of the internal Humber zone by extensional collapse. Evidence for Silurian Early Devonian extensional tectonism in the Humber zone provides the basement infrastructures necessary for the creation and the onset of sedimentation in the Gaspé Belt basins (e.g., Connecticut Valley Gaspé synclinorium). Several structural, metamorphic features in the internal Humber zone of the northwestern New England Appalachians yield analogous characteristics with those of southern Quebec and may have shared a similar Silurian Early Devonian tectonic evolution.

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Ings, S. J., et J. V. Owen. « ‘Decompressional’ reaction textures formed by isobaric heating : an example from the thermal aureole of the Taylor Brook Gabbro Complex, western Newfoundland ». Mineralogical Magazine 66, n^o 6 (décembre 2002) : 941–51. http://dx.doi.org/10.1180/0026461026660069.

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Abstract Reaction textures including corona structures in granulites from the Proterozoic Long Range Inlier of western Newfoundland are spatially associated with a Silurian (0.34 Ga) mafic intrusion, the Taylor Brook Gabbro Complex. They comprise, in metabasites and tonalitic gneiss, coronal orthopyroxene and plagioclase on garnet and, in metapelites, cordierite and spinel formed at the expense of sillimanite, garnet and quartz. Although generally interpreted to indicate near-isothermal decompression (ITD) following regional metamorphism, which in the inlier occurred at ˜1.10–1.03 Ga, these features appear to be absent elsewhere. Therefore they are interpreted to be products of contact metamorphism (near-isobaric heating – IBH) within the thermal aureole of the gabbro. Thus, there is a ˜0.7 Ga difference (i.e. mid-Proterozoic vs. mid-Silurian) between the age of the regional metamorphic mineral assemblages and the contact aureole assemblages. The observation that classic ITD features occur in this aureole environment underscores the fact that P-sensitive reactions can progress during IBH as well as by pressure release.

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Pandeya, Lokendra, et Kabi Raj Paudyal. « Precise Location and Mapping of the Main Central Thrust Zone in Reference to Micro-Structures and Deformation along Khudi-Tal Area of Marsyangdi Valley ». Bulletin of the Department of Geology 22 (15 décembre 2020) : 33–40. http://dx.doi.org/10.3126/bdg.v22i0.33414.

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Geological mapping was carried out along Marsyangdi valley in the Khudi - Dahare -Tal area on a scale of 1: 50,000 covering about 142 square kilometers. Recent study aims to locate the Main Central Thrust (MCT) precisely based on lithostratigraphy, micro-structures, deformation, and metamorphism. Several thin sections were observed to study the metamorphism, deformation, and micro-structures developed in the rocks. The rocks sequences in both the Higher Himalaya and the Lesser Himalaya have undergone polyphase metamorphism and deformation. The Lesser Himalaya experienced first burial metamorphism (M1) followed by garnet grade inverted metamorphism related to the MCT activity (M2) followed by retrograde metamorphism (M3) whereas the Higher Himalaya has undergone regional high-pressure/ high-temperature kyanite/ sillimanite- grade prograde regional metamorphism (M1) followed by the (M2) related to ductile sharing which in turn is overprinted by the later post-tectonic retrograde garnet to chlorite grade metamorphism during exhumation. The polyphase deformation is indicated by the cross-cutting foliation and many other features. The deformation phase D1 is associated with the development of the bedding parallel foliation due to burial in both the Higher Himalaya and the Lesser Himalaya. Isoclinal folds and crenulation cleavage were developed before the collision is categorized as D2. Development of nearly N- S trending mineral and stretching lineation, south vergent drag folds, folded S2 cleavage and microscopic shear sense indicators, rotated syn- tectonic garnet grains, etc. were developed during the deformation D3 related to the ductile shearing through the MCT. Various brittle faults and shear zones cross-cutting all earlier features were developed during D4 during the upheaval. The rocks in the MCT zone are affected by intense sharing and mylonitization as indicated by the presence of many mylonitic structures in the thin sections throughout the Lesser Himalaya in the area. Features like polygonization and ribbon quartz with evidence of sub-grain rotation, mica fish, syn-tectonic rotated garnet grains indicate the ductile shearing in the MCT area suggesting the dynamic recrystallization in the MCT zone whereas rocks of the Higher Himalaya show the evidence of recrystallization under static condition. The MCT zone was mapped precisely based on the microstructures and deformation.

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Pandeya, Lokendra, et Kabi Raj Paudyal. « Precise Location and Mapping of the Main Central Thrust Zone in Reference to Micro-Structures and Deformation along Khudi-Tal Area of Marsyangdi Valley ». Bulletin of the Department of Geology 22 (15 décembre 2020) : 33–40. http://dx.doi.org/10.3126/bdg.v22i0.33414.

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Geological mapping was carried out along Marsyangdi valley in the Khudi - Dahare -Tal area on a scale of 1: 50,000 covering about 142 square kilometers. Recent study aims to locate the Main Central Thrust (MCT) precisely based on lithostratigraphy, micro-structures, deformation, and metamorphism. Several thin sections were observed to study the metamorphism, deformation, and micro-structures developed in the rocks. The rocks sequences in both the Higher Himalaya and the Lesser Himalaya have undergone polyphase metamorphism and deformation. The Lesser Himalaya experienced first burial metamorphism (M1) followed by garnet grade inverted metamorphism related to the MCT activity (M2) followed by retrograde metamorphism (M3) whereas the Higher Himalaya has undergone regional high-pressure/ high-temperature kyanite/ sillimanite- grade prograde regional metamorphism (M1) followed by the (M2) related to ductile sharing which in turn is overprinted by the later post-tectonic retrograde garnet to chlorite grade metamorphism during exhumation. The polyphase deformation is indicated by the cross-cutting foliation and many other features. The deformation phase D1 is associated with the development of the bedding parallel foliation due to burial in both the Higher Himalaya and the Lesser Himalaya. Isoclinal folds and crenulation cleavage were developed before the collision is categorized as D2. Development of nearly N- S trending mineral and stretching lineation, south vergent drag folds, folded S2 cleavage and microscopic shear sense indicators, rotated syn- tectonic garnet grains, etc. were developed during the deformation D3 related to the ductile shearing through the MCT. Various brittle faults and shear zones cross-cutting all earlier features were developed during D4 during the upheaval. The rocks in the MCT zone are affected by intense sharing and mylonitization as indicated by the presence of many mylonitic structures in the thin sections throughout the Lesser Himalaya in the area. Features like polygonization and ribbon quartz with evidence of sub-grain rotation, mica fish, syn-tectonic rotated garnet grains indicate the ductile shearing in the MCT area suggesting the dynamic recrystallization in the MCT zone whereas rocks of the Higher Himalaya show the evidence of recrystallization under static condition. The MCT zone was mapped precisely based on the microstructures and deformation.

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Puliaev, N. А. « Tectonic pozition and metamorphism of rocks in the Chara-Tokko iron ore region ». Vestnik of North-Eastern Federal University Series "Earth Sciences", n^o 1 (25 mars 2024) : 53–61. http://dx.doi.org/10.25587/2587-8751-2024-1-53-61.

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The article provides the material on the geological structure, structures, tectonic position and metamorphism of rocks developed within the Chara-Tokko iron ore region, identified in the western part of the Aldan-Stanovoy shield. Most researchers agree that the Chara-Tokko iron ore region is confined to a trough that formed on the site of one of the long-lived deep fault zones of meridional strike. This zone received various names (Derbegelakhskaya, Charo-Imalykskaya, Tarynakhskaya, etc.). Currently, the fault zone and the trough trough are called Chara-Tokko after the name of the iron ore region. It is assumed that to the south, the iron ore deposits of the Chara group (Sulumatskoye, Nizhne-Sakukanskoye) are confined to the continuation of this trough trough. The nature and conditions of metamorphism can be judged by the mineral associations of some rocks of the sedimentary productive complex, such as metapelites or ferruginous quartzites. It is noted that metapelites, now represented by gneisses and mica schists with high-alumina minerals, are especially important for this. Based on their characteristic mineral associations, three temperature facies were identified in the Chara-Tokko iron ore region: staurolite, biotite-muscovite gneiss and orthoclase-biotite-sillimanite. The internal structure of the Chara-Tokko iron ore field were not sufficiently studied and is extremely controversial. The boundaries of metamorphic zones are mainly parallel to the general strike of the structures and only in rare cases intersect the stripes of productive sedimentary units. After the temperature maximum of progressive metamorphism and past granitization, a significant period of cooling began, which was heated and in some places impregnated with layer-by-layer injections of migmatites and bodies of granites of the Borsala sequence. Gradual cooling was accompanied by regressive metamorphism with the formation of low-temperature minerals at the expense of higher-temperature ones. It is believed that ferruginous quartzites of the western part of the Aldan-Stanovoi shield are part of sedimentary-volcanogenic rock complexes that fill suture troughs or troughs confined to long-lived deep fault zones. The processes of regressive metamorphism manifested themselves most intensively in tectonic zones. The nature of the processes depends on the composition of the host rocks. The regressive phenomena of continuous chloritization and epidotization are associated with the local movement of components, mainly calcium.

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Tamang, Shashi, Sandeep Thapa, Kabi Raj Paudyal, Frédéric Girault et Frédéric Perrier. « Geology and mineral resources of Khudi-Bahundanda area of west-central Nepal along Marshyangdi Valley ». Journal of Nepal Geological Society 58 (25 juin 2019) : 97–103. http://dx.doi.org/10.3126/jngs.v58i0.24592.

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Geological study was carried out along the Khudi-Bahundanda area of the Marshyangdi Valley in the west central Nepal. The area lies partly in the Main Central Thrust (MCT) zone and partly in the Higher Himalayan Crystalline Zone. The aim of the study was to prepare a detail geological map and cross section in the scale of 1:25,000 to work out on stratigraphy, metamorphism and mineral resource potential of the area. The rocks of the Higher Himalaya have been mapped under a single unit as Formation I. This unit consists of kyanite-garnet para-gneiss. The lithological units of the MCT zone are mapped into three units as the Benighat Slate, the Malekhu Formation and the Robang Formation from the bottom to the top, respectively. The Benighat Slate consists of dark grey to black schist with some carbonate beds as members. The Malekhu Formation consists of creamy white siliceous dolomite marble with parting of schist. The Robang Formation comprises of light grey psammitic schist with garnet and white micaceous quartzite in various proportion. Many secondary structures are observed in the study area, but primary structures are missing due to extreme metamorphism. The large-scale structures are the MCT, which separates the Lesser Himalayan rocks to the south from the Higher Himalaya to the north, and the Bahundanda Thrust (BT). Numerous outcrop-scale structures like meso-scale folds, quartz veins, boudinage and ptygmatic folds are abundant. Folds in the MCT zone are mostly E-W trending, and rocks have experienced multiple metamorphism and dynamic crystallization of minerals. The Lesser Himalayan rocks resemble the garnet zone while the Higher Himalayan rocks resemble to the kyanite grade of metamorphism. As in the other sections of the Himalaya, the present section also clearly shows the inverted metamorphism in the MCT zone. The MCT zone is considered as the potential site for precious and semi-precious stones, of which the most potential ones are the garnet and kyanite.

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Kalsbeek, F., P. N. Taylor et R. T. Pidgeon. « Unreworked Archaean basement and Proterozoic supracrustal rocks from northeastern Disko Bugt, West Greenland : implications for the nature of Proterozoic mobile belts in Greenland ». Canadian Journal of Earth Sciences 25, n^o 5 (1 mai 1988) : 773–82. http://dx.doi.org/10.1139/e88-072.

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Whole-rock Rb–Sr and Pb–Pb and zircon U–Pb isotope data yield an age of approximately 2800 Ma for the Atâ granite from northeastern Disko Bugt, West Greenland. Field observations and isotope data suggest that the surrounding gneisses were formed by deformation and recrystallization of granitoid rocks similar to the Atâ granite some 100 Ma after the emplacement of the granite. Rb–Sr whole-rock data on siltstones at low metamorphic grade give an age of 1760 ± 185 Ma, which is interpreted as the time of closure of the isotope systems after metamorphism. The initial 87Sr/86Sr ratio demonstrates that the sediments were probably deposited during the early Proterozoic.Field observations and isotope data show that Proterozoic (Hudsonian s.l.) deformation and metamorphism were weak in the investigated area. The Archaean basement is well preserved, and the metasediments have well-preserved sedimentary structures. The area lies between the Proterozoic Nagssugtoqidian and Rinkian mobile belts of West Greenland, which are thus separated by an area of Archaean rocks little affected by mid-Proterozoic tectono-thermal events. Thus the two belts form separate tectonic units and do not constitute a single contiguous vast area of Archaean rocks reworked by Proterozoic deforma-tion and metamorphism. It is suggested that the Proterozoic mobile belts of West Greenland are orogenic zones of restricted width (a few hundreds of kilometres) that may be interpreted in terms of modern plate tectonic processes.

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Boyanov, Ivan, Milka Ruseva, Violeta Toprakčieva et Ekaterina Dimitrova. « Lithostratigraphy of the Mesozoic rocks from the Eastern Rhodopes ». Geologica Balcanica 20, n^o 5 (30 octobre 1990) : 3–28. http://dx.doi.org/10.52321/geolbalc.20.5.3.

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Mesozoic slightly metamorphosed sedimentary and volcano- sedimentary rocks have been proven in the south-eastern part of the Eastern Rhodopes around the villages of Bregovec, Orešino, Mandrica and Dolno Lukovo. They are subdivided into two groups: Mandrica Group (new group) - with supposed Triassic or Triassic - Early Jurassic age, and Maglenica Group (new group) - with Early Jurassic and Late Cretaceous age. Mandrica Group is built up of sedimentary and sedimentary-volcanogenic rocks accompanied by basic and ultrabasic magmatic rocks, all subjected to greenschist facies to epidote-amphibolite facies regional metamorphism. Two formations are subdivided within this group: Orešinovo Formation (new formation) mainly metasedimentary, with intrusive basic igneous rocks, and Gorno-Lukovo Formation (new formation), predominantly volcanogenic. Maglenica Group is represented by two formations: Dolno-Lukovo Formation (new formation), and Meden-buk Formation (new formation). Dolno-Lukovo Formation is of sedimentaryvolcanogenic composition and Early Jurassic age. It contains olistostroms with Upper Permian and MiddleUpper Triassic silicites and organogenic limestones as well as boudinaged bodies of tholeiitic basalts - clastolavas. The degree of alteration is anchimetamorphic. Meden-buk Formation belongs to the Upper Senonian (Campanian). It is sedimenrary volcanogenic, non-metamorphic, with basaltoid andesites. It is intruded by diorite-granodioritic hypabyssal intrusions. The rocks of Mandrica Group and Dolno-Lukovo Formation from the Eastern Rhodopes on Bulgarian territory are correlated after their composition, metamorphism and age with the lithostratigraphic units Makri and Drimos-Melia, described on Greek territory in Western Thrace. The Mesozoic rocks in the Eastern Rhodopes build together with the high-grade Precambrian metamorphics an allochthonous sheet (East-Rhodope complex nappe) which has been thrust over paraautochthonous in tensely diaphthorized Precambrian ultrametamorphics. The presence of Precambrian, Paleozoic, Triassic, Jurassic and Upper Cretaceous rocks in complex allochthonous structures supports the idea about the existence of a complex collisional orogen.

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Karim, Kamal Haji. « Geology of Zagros metamorphosed volcaniclastic sandstones : a key for changing the Mawat Ophiolite Complex to a metamorphic core complex, Kurdistan Region, NE-Iraq ». IOP Conference Series : Earth and Environmental Science 906, n^o 1 (1 novembre 2021) : 012024. http://dx.doi.org/10.1088/1755-1315/906/1/012024.

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Abstract Mawat Ophiolite Complex is located about 36 km to the northeast of Sulaimani city and directly to the east-northeast of Mawat town near the border of Iran in the northeastern Iraq. The complex has about 600-km2 surface area and consists of high mountain terrains that subjected to intense geological investigations from the fiftieth of previous century till now. According to previous studies, the complex contains tens of igneous rocks such as basalt, metabasalt, tuff, diabase, metadiabase, diorite dykes, periodotite, serpentinite, serpentinite-matrix mélange, gabbro, metagabbro, harzbergite, pyroxenite, plagiogranite, pegmatite, granitiod rocks and dunite. They added occurrences of the volcanic and subvolcanic rocks in the form of dykes or basaltic flows. The present study tries to change the petrology and tectonics of whole complex from Ophiolite Complex to Metamorphic Core Complex. The revision includes refusal of all the above igneous rocks, instead they considered as medium grade regional metamorphism of different types of volcaniclastic sandstones (volcanic wackes), arenites and greywackes (impure sandstones) which sourced predominantly from remote volcanic source area inside Iran. The revision depended on several conjugate field and laboratory evidences inside the complex. These evidences such as absence of pillow basalt, volcanic flows, glass shards, volcanic cones, dykes, sills, contact metamorphism, dilatational structures and flow structures. Other evidences are presence of cross beddings, erosional surfaces, lensoidal channel fills, metamorphosed conglomerate, exposures of thousands of laminated planar beds and transition from fresh volcaniclastic sandstones to its medium grade metamorphosed counterparts, which previously considered as igneous rocks of ophiolite types. Another, evidence, in contrast to ophiolite section, the basalt location is at the base of the claimed ophiolite section while plutonic (dunite and peridotite) rocks located at its top. These locations of the two rocks contradict the definition of ophiolites. Accordingly, the present study changed the geological map of the whole Mawat area from igneous outcrops to metamorphosed volcaniclastic sandstones, arenites and greywackes that belong to Walash-Naoperdan Series. The parent rocks of the series transformed to different types of regionally metamorphosed rocks by deep burial during Eocene. During the burial, diageneses and metamorphisms enhanced by complex mixture of materials from different source areas and seawaters environments. Later, they uplifted, unroofed and exhumed during Pliocene as a core complex.

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Searle, Michael P., et Thomas N. Lamont. « Compressional metamorphic core complexes, low-angle normal faults and extensional fabrics in compressional tectonic settings ». Geological Magazine 157, n^o 1 (2 avril 2019) : 101–18. http://dx.doi.org/10.1017/s0016756819000207.

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AbstractMetamorphic core complexes (MCCs) are interpreted as domal structures exposing ductile deformed high-grade metamorphic rocks in the core underlying a ductile-to-brittle high-strain detachment that experienced tens of kilometres of normal sense displacement in response to lithospheric extension. Extension is supposedly the driving force that has governed exhumation. However, numerous core complexes, notably Himalayan, Karakoram and Pamir domes, occur in wholly compressional environments and are not related to lithospheric extension. We suggest that many MCCs previously thought to form during extension are instead related to compressional tectonics. Pressures of kyanite-and sillimanite-grade rocks in the cores of many of these domes are c. 10–14 kbar, approximating to exhumation from depths of c. 35–45 km, too great to be accounted for solely by isostatic uplift. The evolution of high-grade metamorphic rocks is driven by crustal thickening, shortening, regional Barrovian metamorphism, isoclinal folding and ductile shear in a compressional tectonic setting prior to regional extension. Extensional fabrics commonly associated with all these core complexes result from reverse flow along an orogenic channel (channel flow) following peak metamorphism beneath a passive roof stretching fault. In Naxos, low-angle normal faults associated with regional Aegean extension cut earlier formed compressional folds and metamorphic fabrics related to crustal shortening and thickening. The fact that low-angle normal faults exist in both extensional and compressional tectonic settings, and can actively slip at low angles (< 30°), suggests that a re-evaluation of the Andersonian mechanical theory that requires normal faults to form and slip only at high angles (c. 60°) is needed.

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Brachaniec, Tomasz. « Shock structures in the Morasko meteorite - preliminary SEM data ». Contemporary Trends in Geoscience 3, n^o 1 (1 septembre 2014) : 1–4. http://dx.doi.org/10.2478/ctg-2014-0016.

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Abstract This paper is a preliminary review of main shock deformations in the Morasko meteorite. Three main types of metamorphism structures occur in the investigated material: (i) brittle, (ii) plastic and (iii) thermal. Their interpretation may indicate, that Morasko meteorite reveals several stages of shock, eg.: extraterrestrial collisions and fall on the Earth

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Dee, S. J., et S. Roberts. « Late-kinematic gold mineralisation during regional uplift and the role of nitrogen : an example from the La Codosera area, W. Spain ». Mineralogical Magazine 57, n^o 388 (septembre 1993) : 437–50. http://dx.doi.org/10.1180/minmag.1993.057.388.07.

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AbstractVein formation occurred throughout a deformation sequence which involved early transpressive ductile deformation through to late-kinematic transpressive brittle structures which host a series of gold prospects. Fluid inclusion data from (S1) fabric parallel veins associated with early deformation suggest that a low-salinity aqueous fluid, with a mean salinity of 6.4 wt.%, was present during peak metamorphism, Pelite mineralogy and isochores constrain peak metamorphism to the lowermost part of the upper greenschist facies at 325 to 425°C and 1.4 to 3.4 kbar.Fluid inclusion data from auriferous and barren late-kinematic quartz veins, both containing unmixing assemblages of aqueo-carbonic inclusions with low salinities of ≈2.7 wt.% NaCl equiv., indicate unmixing occurred at 300°C and 1.5 kbar.Volatiles (CO2, N2, CH4) are observed in all the late-kinematic veins. The N2contents of veins with elevated gold grades are typically higher than those with low gold grades. N2reaches 8.7 mole% in a vein with 0.49−4.6 p.p.m. Au compared to <1 mole% in a vein with <0.05 p.p.m. Au. The CH4content of late kinematic veins is generally less than 1 mole% and shows no relative enrichment in mineralised veins. The generation of N2in the mineralising fluid most likely results from interaction of fluid with the ammonium ion, NH4+, in micas and feldspars. This interaction could take place either at source, due to metamorphic devolatisation reactions, or along those structures which acted as fluid conduits due to fluid-rock interaction.

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Gorter, J. D. « THE PETROLEUM POTENTIAL OF AUSTRALIAN PHANEROZOIC IMPACT STRUCTURES ». APPEA Journal 38, n^o 1 (1998) : 159. http://dx.doi.org/10.1071/aj97009.

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This paper examines proven, probable, possible and speculative impact structures in Australian Phanerozoic strata and their petroleum potential. There are two classes of crater: simple and complex. The former usually assumes a bowl shaped depression with a raised and overturned rim with a diameter rarely more than three kilometres, with complex structures generally occurring above diameters of two kilometres in sedimentary rocks and four kilometres in crystalline rocks. Complex craters are characterised by a central uplifted area and a classic 'sombrero' structure and can be very large and have diameters of over 800 km. Criteria for the identification of terrestrial impact structures include: (a) circular plan; (b) faulted rim structure; (c) flat floor with central uplift (may not always be present) or interior ring(s); (d) negative gravity anomaly; (e) magnetic low with subdued magnetic relief; (f) brecciated crater fill; (g) low seismic velocities in the crater fill; (h) shock metamorphism (coesite, multiple sets of planar shock lamellae in quartz grains, shatter cones); (i) meteoric material; (j) presence of melt rock; (k) distal ejecta; and occurrence of an annular trough between the central uplift and the outer faulted rim.Proven impact features, like Gosses Bluff and Mt Toondina, are well exposed, contain indisputable evidence of shock metamorphism, and have had extensive geophysical surveys conducted over them: these structures provide models to interpret completely buried structures. Subsurface impact structures have been detected in areas where there has been intensive seismic surveying in the search for hydrocarbons. The Tookoonooka, Talundilly and Mulkarra structures all occur in the Cooper-Eromanga Basin, an area of high intensity exploration. The best known wholly subsurface impact feature is the 66 km diameter Tookoonooka Structure in southwestern Queensland, which exhibits several of the accepted criteria for an impact origin, including shock metamorphism, and is classified as a probable complex type impact structure. The Talundilly Structure, a possible impact feature, lies 300 km to the northeast of the Tookoonooka Structure and is of the same general age. The two structures could reflect the impact of fragments of the same bolide. The Yallallie Structure lies in a moderately explored hydrocarbon province in the central Perth Basin. It has a classic 'sombrero' shape in section view with a central uplift and evidence of shock metamorphism. Yallallie is a probable complex impact structure. The Mulkarra Structure, located in northeastern South Australia, has been classified as a simple type of impact crater lacking a central uplift, but recent geophysical work indicates a probable complex impact origin. Other possible and speculative impact related features described here owe their recognition to good quality seismic surveying.'there are yet some geologists who, adhering to Lyellian dogma, devoutly refuse to accept that large objects fall out of the sky.' (Shoemaker, 1997).

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Scarpelli, Wilson, et Élio Hiromi Horikava. « Gold, iron and manganese in central Amapá, Brazil ». Brazilian Journal of Geology 47, n^o 4 (décembre 2017) : 703–21. http://dx.doi.org/10.1590/2317-4889201720170114.

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ABSTRACT: Greenstone belts with deposits of gold, iron and manganese are common in the Paleoproterozoic Maroni-Itacaiunas Tectonic Province of the Guiana Shield. In Brazil, in the State of Amapá and northwest of Pará, they are represented by the Vila Nova Group, constituted by a basal unit of metabasalts, covered by metasediments of clastic and chemical origin. The basal metasediments, the Serra do Navio Formation, are made of a cyclothem with lenses of manganese marbles at the top of each cycle. Under the intense weathering of the Amazon, these lenses were oxidized to large deposits of high-grade manganese oxides. The exploitation of these oxides left behind the manganese carbonates and low-grade oxides. The overlaying Serra da Canga Formation presents a calcium and magnesium domain grading to an iron domain with banded silicate and oxide iron formations, mined for iron ores. Overlapping structures and superposed metamorphic crystallizations indicate two phases of dynamothermal metamorphism, the first one with axis to north-northeast and the second one to northwest, with an intermediate phase of thermal metamorphism related to syntectonic granitic intrusions. Shears oriented north-south, possibly formed during the first dynamothermal metamorphism and reactivated in the second, are ideal sites for hydrothermalism and gold mineralization, which is greater when occurs in iron formation and carbonate-bearing rocks, as it happened at the Tucano mine. Layered mafic-ultramafic intrusions in the greenstones represent a potential for chromite and platinum group elements. Pegmatites are source of cassiterite and tantalite exploited from alluvial deposits.

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Oziegbe, E. J., S. A. Babarinde, O. Oziegbe et O. T. Kayode. « Retrograde Assemblages in the Muscovite-Biotite Gneiss of Oluyole Southwestern Nigeria, an Indication of Shear-Zone Environment ». IOP Conference Series : Earth and Environmental Science 1342, n^o 1 (1 mai 2024) : 012037. http://dx.doi.org/10.1088/1755-1315/1342/1/012037.

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Abstract Petrographic and whole-rock geochemical study of biotite-muscovite gneiss was determined in order to interpret the metamorphic evolution of the Basement Complex of Southwestern, Nigeria. The gneiss shows a millimetric banding, and in some cases the quartzo-feldspathic bands running up to 10 cm. The gneiss has mineral assemblage biotite + plagioclase + quartz + garnet + K-feldspar + muscovite + chlorite + ilmenite ±titanite. Chlorite occurs along cleavage planes of biotite, and in some cases forms reaction rims around porphyroblasts of garnet. K-feldspar crystals are surrounded by muscovite. Titanite crystals are sub-idioblastic to xenoblastic in form, and have inclusions of ilmenite. Titanite, where present, occurs in close association with biotite and opaque minerals (ilmenite). Also, titanite forms a reaction rim around apatite. Mylonitic texture, fine-grained matrix of mica and quartz ribbons were observed. In addition, there is stretching of the quartz crystals. The SiO2 content is greater than 60 wt %, while CaO ranges from 3.05-6.91 wt %. The M1 foliation comprise of mineral biotite some of which are included in the opaque mineral, M2 represents the metamorphism which gave rise to porphyroblasts of ilmenite, while the M3 gave rise to foliations that forms a wraparound structure on the porphyroblasts of ilmenite. The last metamorphism gave rise to retrograde minerals; chlorite, titanite, and muscovite. The study suggests that this area of the Basement Complex has been subjected to multiple deformations, as well as multiple episodes of metamorphism. The structures observed are similar to those associated with shear zone environment.

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Lucas, S. B., et M. R. St-Onge. « Terrane accretion in the internal zone of the Ungava orogen, northern Quebec. Part 2 : Structural and metamorphic history ». Canadian Journal of Earth Sciences 29, n^o 4 (1 avril 1992) : 765–82. http://dx.doi.org/10.1139/e92-065.

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The tectonic history of the early Proterozoic Ungava orogen is marked by structural–metamorphic episodes that both predate and postdate a collision between a magmatic arc terrane and the northern continental margin of the Superior Province. Distinct precollisional tectonic histories are documented for the rocks forming the lower plate of the Ungava orogen (the Archean Superior Province basement and an Early Proterozoic rift-to-drift margin sequence) and the orogenic upper plate (Early Proterozoic ophiolitic and magmatic arc units). The lower-plate units preserved in the external part of the orogen (Cape Smith Thrust Belt) record the development of a foreland thrust belt characterized by south-verging faults ramping up from a basal décollement located at the basement–cover contact. The plutonic core of the magmatic arc contains structures and metamorphic assemblages indicative of an episode of dextral transcurrent deformation contemporaneous with granulite-facies metamorphism and arc plutonism. The "tectonically suspect" ophiolitic and arc units were accreted to the thrust belt along south-verging faults, which reimbricated the foreland thrust belt and which resulted in at least 100 km of displacement of upper-plate units with respect to the autochthonous basement. Collisional thickening and consequent exhumation resulted in relatively high-pressure, greenschist- to amphibolite-facies metamorphism of lower-plate cover units, and in the retrogression of high-grade assemblages in the arc rocks. Postaccretion shortening resulted in folding of both the allochthonous rocks and the footwall basement.

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Piccolo, Andrea, Manuele Faccenda, Rodolfo Carosi, Chiara Montomoli et Dario Visonà. « Crustal strength control on structures and metamorphism in collisional orogens ». Tectonophysics 746 (octobre 2018) : 470–92. http://dx.doi.org/10.1016/j.tecto.2017.09.018.

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Reinhardt, J. « Low-pressure, high-temperature metamorphism in a compressional tectonic setting : Mary Kathleen Fold Belt, northeastern Australia ». Geological Magazine 129, n^o 1 (janvier 1992) : 41–57. http://dx.doi.org/10.1017/s0016756800008116.

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AbstractThe Mary Kathleen Fold Belt in northeastern Australia consists of highly deformed, Mid-Proterozoic sedimentary and volcanic sequences as well as intrusives, which were metamorphosed under low-pressure, high-temperature conditions. In the light of current controversy on tectono-thermal settings of low-pressure metamorphic terrains, the interrelations of progressive deformation and metamorphism have been closely examined. Remarkably, there is no direct evidence for syn-metamorphic extensional deformation nor is any significant intrusive activity recorded.Syn-metamorphic structures indicate lateral, bulk coaxial shortening of at least 50–60%. Tight upright folds, pervasive axial planar fabrics, undulating fold axes, and a vertical mineral lineation characterize this deformation. The metamorphic textures, particularly those in andalusite- and/or cordierite-bearing schists, reveal the sequential growth of metamorphic minerals that was synchronous with progressively increasing bulk rock strain. The corresponding metamorphic reactions constrain a prograde P–T path segment that crossed the andalusite and sillimanite stability fields while temperature and pressure increased. After reaching the metamorphic peak, the region cooled down near-isobarically, before major decompression occurred. The prograde–retrograde P–T path forms a complete anticlockwise loop.Due to the lack of evidence for crustal thinning and large-scale magmatism in the upper crust, alternative models are discussed in order to explain the transient high geothermal gradient. These are in particular convective thinning of the lithospheric mantle and fast decompression of crustal sections, possibly linked to tectonic processes preceeding the low-pressure/high-temperature orogenic event.

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Chen, Zhen-Yu, Li-Fei Zhang, Zeng Lü et Jin-Xue Du. « Episodic Fluid Action in Chinese Southwestern Tianshan HP/UHP Metamorphic Belt : Evidence from U–Pb Dating of Zircon in Vein and Host Eclogite ». Minerals 9, n^o 12 (25 novembre 2019) : 727. http://dx.doi.org/10.3390/min9120727.

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Fluid plays a key role in metamorphism and magmatism in subduction zones. Veins in high-pressure (HP) to ultrahigh-pressure (UHP) rocks are the products of fluid–rock interactions and can thus provide important constraints on fluid processes in subduction zones. In this study, we present an integrated study of zircon in situ U–Pb dating, trace element and mineral inclusion analysis for a complex vein and its host eclogite in the southwestern Tianshan UHP terrane, aiming to decipher the episodic fluid action during slab subduction and exhumation. Both zircon in eclogite and vein have euhedral, prismatic morphology similar to those crystallized from metamorphic fluid. Zircon in eclogite shows core–rim structures with distinct bounds and mineral inclusions. Zircon in the vein shows sector zoning or weak zoning, with bright rims around most zircon grains, which suggests recrystallization of the zircon crystals after their formation and multiple evolution of the vein. Eclogite zircon rims yield a weighted mean of 311 ± 3 Ma and cores yield a range from 413 ± 4 to 2326 ± 18 Ma, respectively. Vein zircon yields four groups of age (~355 Ma, ~337 Ma, ~315 Ma, and ~283 Ma), which date four episodes of fluid flow involving zircon growth. The first two groups of age may represent prograde epidote–amphibolite facies and amphibolite/blueschist facies metamorphism stage, respectively. The third group is similar to that of eclogite zircon rims, which is thought to date the eclogitic facie metamorphism (320–305 Ma), and the fourth group dates a later retrograde metamorphism after greenschist facies. The vein-forming fluid system was supposed to be an open system indicated by trace element of vein zircon and mineral assemblage of the vein. The coexistence of rutile, zircon, and garnet in prograde vein and the heavy rare earth elements (HREE) enrichment characteristic of vein zircon suggest that the vein-forming fluid are enriched in high field strength elements (HFSE) and HREE, and such fluid could be formed under low P–T conditions.

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Xue, Guang Wu, Hong Fu Liu et Jing Lin Guo. « A Study on the Occurrence Law of Coal Seam Gas in Shanxi ». Advanced Materials Research 594-597 (novembre 2012) : 2244–50. http://dx.doi.org/10.4028/www.scientific.net/amr.594-597.2244.

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The thickness of overlaying bedrock is the main control factor of preservation, in addition, magmatic activities, different structural types and hydrogeological conditions are also control factors. Regional magmatic thermometamorphism appeared as a kind of superimposed effect on the background of deep metamorphism. Two Eastwest （EW）metamorphic belts in Shanxi tallied with the distribution of magmatic rock masses, resulting in coal seam gas exploitation and utilization bases of Yangquan and Jincheng mining areas. From gas-accumulation structural setting, closed type structures make high gas content in coal reservoir such as Qinnan, Gujiao, Xingjiashe, Dongshe areas in the Qinshui Basin, Shilou, Daning-Jixian and Sanjiao-Liulin areas in the Hedong Basin, gas content is 10-15 m³/t or even higher. There are convenient channels for coal seam gas effusion in the open structures; with lower gas content, such as Lishi mining area in Hedong Basin, Huoxi structural area in Qinshui Basin, Hunyuan, and Wutai coal districts.

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Carson, C. J., M. Hand et P. H. G. M. Dirks. « Stable coexistence of grandidierite and kornerupine during medium pressure granulite facies metamorphism ». Mineralogical Magazine 59, n^o 395 (juin 1995) : 327–39. http://dx.doi.org/10.1180/minmag.1995.059.395.16.

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AbstractPetrological and mineral chemical data are presented for two new occurrences of co-existing borosilicate minerals in the Larsemann Hills, East Antarctica. The assemblages contain kornerupine and the rare borosilicate, grandidierite (Mg,Fe)A13BSiO9. Two distinct associations occur: (1) At McCarthy Point, 1–10 mm thick tourmaline-kornerupine-grandidierite layers are hosted within quartzofeldspathic gneiss; and (2) Seal Cove, where coexisting kornerupine and grandidierite occur within coarse-grained, metamorphic segregations with Mg-rich cores of cordierite-garnet-spinel-biotite-ilmenite and variably developed plagioclase halos. The segregations are hosted within biotite-bearing, plagio-feldspathic gneiss. Textural relationships from these localities indicate the stability of co-existing kornerupine and grandidierite.The grandidierite- and kornerupine-bearing segregations from Seal Cove largely postdate structures developed during a crustal thickening event (D2) which was coeval with peak metamorphism. At McCarthy Point, grandidierite, kornerupine and late-tourmaline growth predates, or is synchronous, with F3 fold structures developed during a extensive granulite grade, normal shearing event (D3) which occurred prior to, and synchronous with, near-isothermal decompression. Average pressure calculations on assemblages that coexist with the borosilicates at Seal Cove, indicate the prevailing conditions were 5.2–5.5 kbar at ∼ 750°C for formation of the grandidierite-kornerupine assemblage.

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Bonnet, G., P. Agard, S. Angiboust, P. Monié, M. Fournier, B. Caron et J. Omrani. « Structure and metamorphism of a subducted seamount (Zagros suture, Southern Iran) ». Geosphere 16, n^o 1 (21 novembre 2019) : 62–81. http://dx.doi.org/10.1130/ges02134.1.

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Abstract Millions of seamounts on modern and past seafloor end up being subducted, and only small pieces are recovered in suture zones. How they are metamorphosed and deformed is, however, critical to understand how seamount subduction can impact subduction zone geometry, fluid circulation or seismogenic conditions, and more generally to trace physical conditions along the subduction boundary. Since geophysical studies mostly reach the shallowest subducted seamounts and miss internal structures due to low resolution, there is a high need for fossil seamount exposures. We herein report on a fully exposed, 3D example of seamount that we discovered in the Siah Kuh massif, Southern Iran. Through a series of sections across the whole massif and the combination of magmatic-metamorphic-sedimentary petrological data, we document several distinct stages associated with seamount build-up on the seafloor and with subduction. In particular, we constrain different stages of metamorphism and associated mineralogy, with precise conditions for subduction-related metamorphism around 250 °C and 0.7 GPa, in the middle of the seismogenic zone. Extensive examination of the seismogenic potential of the Siah Kuh seamount reveals that it was not a large earthquake asperity (despite the report of a rare example of cm-scale, high-pressure pseudotachylyte in this study), and that it possibly behaved as a barrier to earthquake propagation. Finally, we discuss the nature of high-pressure fluid circulation preserved in this seamount.

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Ebner, P. P., M. Schneebeli et A. Steinfeld. « Tomography-based monitoring of isothermal snow metamorphism under advective conditions ». Cryosphere Discussions 9, n^o 1 (18 février 2015) : 1021–45. http://dx.doi.org/10.5194/tcd-9-1021-2015.

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Abstract. Time-lapse X-ray micro-tomography was used to investigate the structural dynamics of isothermal snow metamorphism exposed to an advective airflow. Diffusion and advection across the snow pores were analysed in controlled laboratory experiments. The 3-D digital geometry obtained by tomographic scans was used in direct pore-level numerical simulations to determine the effective transport properties. The results showed that isothermal advection with saturated air have no influence on the coarsening rate that is typical for isothermal snow metamorphism. Diffusion originating in the Kelvin effect between snow structures dominates and is the main transport process in isothermal snow packs.

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Cook, Nigel J., Christopher Halls et Alan P. Boyle. « Deformation and metamorphism of massive sulphides at Sulitjelma, Norway ». Mineralogical Magazine 57, n^o 386 (mars 1993) : 67–81. http://dx.doi.org/10.1180/minmag.1993.057.386.07.

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AbstractThe copper-bearing stratabound pyritic massive sulphide bodies contained in metamorphosed basic eruptives of Ordovician age at Sulitjelma in Nordland County, Norway, form one of the important fields of sulphide mineralisation within the Köli Nappe Complex. The sulphide bodies and their enclosing rocks were subject to successive stages of penetrative deformation and recrystallisation during the cycle of metamorphism and tectonic transport caused by the Scandian Orogeny. Textures within the ores and the immediate envelope of schists show that strain was focused along the mineralised horizons. The marked contrast in competence between the massive pyritic sulphides and their envelopes of alteration composed dominantly of phyllosilicates, and the metasediments of the overlying Furulund Group, led to the formation of macroscale fold and shear structures. On the mesoto microscale, a variety of textures have been formed within the pyrite-pyrrhotite-chalcopyrite-sphalerite sulphide rocks as a result of strain and recrystallisation. Variations in pyrite:pyrrhotite ratios and in the texture and proportions of associated gangue minerals evidently governed the strength and ductility of the sulphide rocks so that the same sulphide mineral can behave differently, displaying different textures in different matrices. In massive pyritic samples there is evidence of evolution towards textural equilibrium by recrystallisation, grain growth and annealment during the prograde part of the metamorphic cycle. Later, brittle deformation was superimposed on these early fabrics and the textural evidence is clearly preserved. By comparing published data on the brittle-ductile transformation boundaries of sulphide minerals with the conditions governing metamorphism at Sulitjelma, it is concluded that most of the brittle deformation in the sulphides took place during or after D3under retrograde greenschist conditions. Grain growth of pyrite in matrices of more ductile sulphides during the prograde and early retrograde stages of metamorphism produced the coarse metablastic textures for which Sulitjelma is well-known. In some zones of high resolved shear stress, pyrite shows ductile behaviour which could be explained by a dislocation flow mechanism operating at conditions close to the metamorphic peak. In those horizons in which pyrrhotite is the dominant iron sulphide, the contrast in ductility between silicates, pyrite and pyrrhotite has led to the development of spectacular tectonoclastic textures in which fragments of wall rock have been broken, deformed, rolled and rotated within the ductile pyrrhotite matrix.

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Relf, C. « Two distinct shortening events during late Archean orogeny in the west-central Slave Province, Northwest Territories, Canada ». Canadian Journal of Earth Sciences 29, n^o 10 (1 octobre 1992) : 2104–17. http://dx.doi.org/10.1139/e92-167.

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Late Archean structures in the west-central part of the Slave Province formed during two separate orogenic events. Evidence for early folding and thrusting in an accretionary prism is confined to a narrow belt along the east margin of an older microcontinent (the Anton terrane) in the west part of the province. Structures related to this event are overprinted by regional low-pressure metamorphism. Subsequent shortening occurred in a continental-arc setting in which folding and faulting was accompanied by calc-alkaline magmatism and regional low-pressure metamorphism. Although the entire region was affected, the bulk of shortening during the second orogenic event occurred east of the early fold and thrust belt. The first orogenic event produced a suture zone between old continental crust to the west and juvenile rocks to the east, and during the second orogenic event rocks on either sides of the suture were tectonically underplated and intruded.

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Guo, Fenghui, Wei Xu, Minghui Tang, Ziqi Zhang, Zean Chen et Hao Lu. « Investigation of the mineralogical composition and origin analysis of black jadeite ». Journal of Physics : Conference Series 2790, n^o 1 (1 juillet 2024) : 012003. http://dx.doi.org/10.1088/1742-6596/2790/1/012003.

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Abstract Analyzing black jadeite variety is helpful not only to distinguish black jadeite from the common omphacite jadeite, but also to learn the origin of jadeite by studying these special specimens. The basic gemological properties, mineral composition, structural characteristics, spectroscopic features, and color-causing mechanisms of black jadeite were studied through testing methods such as Polarizing microscope, X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy. Recent studies have confirmed the distinct crystalloblastic textures of black jadeite—namely columnar, granular, and fibrous—alongside its characteristic fracture structures. This research further identifies the presence of secondary minerals, primarily opaque black impurities consisting of a graphite and disordered graphite mixture, as revealed by Raman spectroscopy and X-ray diffraction analyses. These analyses have elucidated that the variable distribution of graphite mixtures contributes to differing degrees of light absorption, thereby forming various shades of black, a critical factor in the gemstone’s coloration. This study suggests that these carbonaceous materials are a byproduct of the carbonization of organic matter within metamorphic fluids, subsequently forming dispersed graphite structures within the mineral matrix and its fractures, indicative of multi-stage metamorphic processes. Additionally, the identification of graphite supports the theory that jadeite’s protolith originated from the subduction processes of the Neotethys Ocean, accompanied by metasomatism and high-pressure metamorphism of olivinite.

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Kozakov, I. K., A. M. Kozlovsky, V. V. Yarmolyuk, T. I. Kirnozova, M. M. Fugzan, Ts Oyunchimeg et Ch Erdenezhargal. « Geodynamic environments of the origin of poly- and monometamorphic complexes in the Southern Altai metamorphic belt, Central Asian orogenic belt ». Петрология 27, n^o 3 (19 mai 2019) : 233–57. http://dx.doi.org/10.31857/s0869-5903273233-257.

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Tectonic sheets of various size along the southern slope of the Mongolian and Chinese Altai ranges and in eastern Kazakhstan include high-grade metamorphic rocks, which are collectively referred to as the Southern Altai Metamorphic Belt. Rocks of the sheets show traces of amphibolite-facies elevated-pressure metamorphism of the kyanite–sillimanite type M2. Some of the tectonic sheets display evidence of polymetamorphism: the rocks preserve textures and mineral assemblages of an earlier metamorphic episode (of elevated temperature and relatively low pressure) of the andalusite–sillimanite facies series M1. The earlier metamorphic episode occurred at 390–385 Ma, and the later one, at ~370–356 Ma. The protoliths of the high-grade metamorphic rocks were mostly Early Paleozoic terrigenous rocks and subordinate amounts of volcanic rocks analogous to the weakly metamorphosed or unmetamorphosed rocks in their northern surroundings. Typical rocks of the tectonic sheets are mafic dikes and massifs of the Gashun Nuur Complex, which were emplaced between metamorphic episodes M1 and M2. According to their geochemistry and Nd isotopic parameters, most of the metabasites are similar to enriched basalts of mid-oceanic ridges and oceans plateaus. The quantitatively subordinate group of the layered mafic bodies displays geochemical characteristics of subduction-related rocks. Correlations between the metamorphic events and magmatism in the continental (Mongolian and Chinese Altai) and paleoceanic (Trans-Altai Gobi and eastern Junggar) regions led us to suggest a geodynamic model for the development of the Southern Altai Metamorphic Belt. The volcano-terrigenous rocks, which were later metamorphosed, were accumulated mostly in the Early Paleozoic as an accretion wedge on an active continental margin. The earlier episode of high-temperature metamorphic M1 and coeval large-scale calc–alkaline magmatism occurred at the same active continental margin after the magmatic front shifted southward (in modern coordinates). The emplacement of the swarms of mafic bodies of the Gashun Nuur Complex and simultaneous rifting in the southern Chinese Altai were triggered by the subduction of an spreading ridge of an oceanic or backarc basin beneath the active margin. The second metamorphic episode (elevated-pressure metamorphism) M2 and overthrusting in the structures of the Altai are correlated with deformations at low angles and the transition from oceanic to continental volcanism in the Trans-Altai Gobi and Junggar. These tectonic processes were induced by the accretion of a system of mid-Paleozoic ensimatic island arcs of the Trans-Altai Gobi and Junggar to the Altai margin of the Siberian paleocontinent.

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Timmermann, Hilke, Rebecca A. Jamieson, Randall R. Parrish et Nicholas G. Culshaw. « Coeval migmatites and granulites, Muskoka domain, southwestern Grenville Province, Ontario ». Canadian Journal of Earth Sciences 39, n^o 2 (1 février 2002) : 239–58. http://dx.doi.org/10.1139/e01-076.

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We present new field observations and petrologic and geochronological data from the Muskoka domain in the southwestern Grenville Province of Ontario in an attempt to constrain the relationship between amphibolite-facies and granulite-facies gneisses in areas of transitional metamorphic grade, and to examine their implication for tectonometamorphic models for the Grenville Province of Ontario. The predominant medium-grained amphibolite-facies migmatitic orthogneisses of the Muskoka domain contain several generations of leucosome, some of which are related to southeast-directed extensional structures. The amphibolite-facies granitoid gneisses contain numerous mafic enclaves with granulite-facies assemblages recrystallized from anhydrous precursors during Grenvillian metamorphism. Other associated granulites are characterized by their patchy occurrence and gradational contacts, similar to the charnockites in southern India. Patchy granulites, leucocratic vein networks in mafic enclaves, and crosscutting leucocratic granulite veins are interpreted to have formed as a result of local differences in reaction sequences and (or) fluid compositions. The UPb zircon lower intercept age of the patchy granulites overlaps with the previously determined range of 10801060 Ma for high-grade metamorphism in the Muskoka domain, while zircon and titanite from a crosscutting granulite vein crystallized at about 10651045 Ma, supporting a Grenvillian age for granulite formation. Peak metamorphic conditions of 750850°C and 1011.5 kbar (1 kbar = 100 MPa) were determined from the mafic enclaves, whereas the more felsic migmatites reequilibrated at somewhat lower temperatures. The high temperatures caused extensive migmatization and facilitated rheological weakening of the Muskoka domain 1025 million years after the start of the Ottawan orogeny in the Central Gneiss Belt.

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Moser, D. E. « The geology and structure of the mid-crustal Wawa gneiss domain : a key to understanding tectonic variation with depth and time in the late Archean Abitibi–Wawa orogen ». Canadian Journal of Earth Sciences 31, n^o 7 (1 juillet 1994) : 1064–80. http://dx.doi.org/10.1139/e94-096.

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The amphibolite-facies central Wawa gneiss domain (CWGD) preserves structures that developed at the mid-crustal level of the ca. 2.7 Ga Abitibi–Wawa orogen in the southern Superior Province. The relative ages of these domainal structures are documented and brackets on their absolute ages established using existing U–Pb age data. Correlation of tectonic events within the CWGD, and comparison of these events with the evolution of other structural levels of the orogen, has led to subdivision of orogenesis into five stages. During stage 1 (2700–2680 Ma), 2.9 and 2.7 Ga rocks were tightly folded and (or) thrusted at all crustal levels in at least one thick-skinned compression event. During stage 2 (2680–2670 Ma), folding and thrusting of Timiskaming-age sediments at high levels of the orogen was thin-skinned and had no effect on CWGD gneisses. During stage 3 (2670–2660 Ma), while the upper crust was relatively stable, a 1 km thick package of volcanics and sediments, the Borden Lake belt, was underthrust northwards to depths of 30 km and in-folded with orthogneiss of the CWGD. During stage 4 (2660–2637 Ma), coeval east–west extension and granulite metamorphism of the middle crust produced gently dipping shear zones that overprinted earlier fold structures in the CWGD and lower structural levels of the orogen. This took place with minimal effect on the upper crust. Stage 5 (2630–2580 Ma) marks a period of east–west shortening and (or) fault reactivation in the Kapuskasing uplift and upper-crustal greenstone belts that allowed penetration of deep-crustal metamorphic fluids into the latter. In general, analysis of the structural evolution of the CWGD indicates that deformation and metamorphism in the middle crust of the Abitibi–Wawa orogen outlasted that at upper-crustal levels, resulting in the generally shallower dips of planar fabrics in the deeper structural levels of the Kapuskasing uplift crustal cross section.

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Adhikari, Drona, Champak Babu Silwal et Lalu Prasad Paudel. « Review of the Geology of the Arun-Tamor Region, Eastern Nepal : Present Understndings, Controversies and Research Gaps ». Journal of Institute of Science and Technology 26, n^o 2 (29 décembre 2021) : 79–97. http://dx.doi.org/10.3126/jist.v26i2.41439.

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Systematic study of the eastern Nepal Himalaya was started after 1950 when Nepal opened up for foreigners. Thereafter, several geological studies have been carried out in the Arun-Tamor region of eastern Nepal Himalaya. The Tibetan-Tethys sedimentary sequence, the Higher Himalayan amphibolite to granulite facies metamorphic crystalline sequence, the Lesser Himalayan sedimentary and greenschist facies metasedimentary sequences, and the Siwalik foreland molassic sedimentary sequence are the four major tectonic units of this area. The individual nomenclature schemes of stratigraphic units, the correlational dispute, the positions and interpretations of regional geological structures are some examples that have created controversies regarding the lithostratigraphy and structural arrangements. The difference in age and genesis of the Main Central Thrust and its effects in the metamorphism of the eastern Nepal Himalaya are the exemplification of the contradiction in the interpretation of the tectonometamorphic history. There is a gap in research in the tectonics and episodic metamorphic evolution of the area owing to the bare approach in the microstructural and geochronological investigation. Future investigations should be focused on solving the above mentioned controversies and narrowing down the research gaps in tectonic and metamorphic evolution.

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Thapa, Sandeep, Shashi Tamang, Kabi Raj Paudyal, Frédéric Girault et Frédéric Perrier. « Geology and micro-structure analysis of the MCT zone along Khudi- Bahundanda area of Lamjung District, west-central Nepal ». Journal of Nepal Geological Society 58 (25 juin 2019) : 105–10. http://dx.doi.org/10.3126/jngs.v58i0.24593.

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Geological mapping was carried out along the Marsyangdi Valley in the Khudi-Bahundanda area of west-central Nepal covering the Main Central Thrust (MCT) zone. The main objectives of the study were to draw a clear picture of lithology, geological structures and micro-tectonics in the rocks. A detail survey on stratigraphy and correlation with central Nepal reveals geological rock units such as the Benighat Slate, the Malekhu Formation and the Robang Formation of the Lesser Himalaya and the Formation I of the Higher Himalaya. Both regional and small-scale geological structures have been studied. The MCT zone has been mapped as a major regional structure in the region. The Bahundanda Thrust (BT), which has brought the older Malekhu Formation over the younger Robang Formation, is an another significant structure mapped. The BT is marked on the basis of fault breccia, slickensides as well as large deposits of debris mass at the fault zone. The study area has undergone poly-metamorphism and dynamic crystallization of minerals. The Lesser Himalayan rocks resemble the garnet zone while the Higher Himalaya rocks resemble to the kyanite grade of metamorphism. The present section clearly shows the inverted metamorphism in the MCT zone as in the other sections of the Himalaya. Microscopic features like ribbon-quartz, polygonization of quartz crystals, grain boundary reduction, mica-fish and rotated garnet grains indicates the ductile shearing in the MCT zone suggesting the dynamic recrystallization during thrust propagation. Numerous outcrop-scale structures like meso-scalefolds, quartz veins, boudinage and ptygmatic folds are abundant folds in the MCT zone and these are mostly E-W trending.

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Couëslan, Chris G., et David R. M. Pattison. « Low-pressure regional amphibolite-facies to granulite-facies metamorphism of the Paleoproterozoic Thompson Nickel Belt, Manitoba ». Canadian Journal of Earth Sciences 49, n^o 10 (octobre 2012) : 1117–53. http://dx.doi.org/10.1139/e2012-029.

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The Thompson Nickel Belt is a ca. 35 km × 400 km northeast-trending segment of the northwest margin of the Archean Superior craton in Manitoba, bounded to the west by the Paleoproterozoic Reindeer Zone. The belt was metamorphosed and deformed during the Trans-Hudson orogeny (ca. 1.9–1.7 Ga). Mineral assemblages in metamorphosed pelite, aluminous greywacke, mafic igneous rock, iron formation, and ferruginous wacke define regional metamorphic domains, separated by mineral isograds, that are subparallel to the strike of the belt and to regional-scale D3 structures. An elongate, ca. 5 km × 73 km, central zone of middle amphibolite-facies rocks is characterized by the following: muscovite-bearing mineral assemblages in pelites containing combinations of staurolite, andalusite, and sillimanite; muscovite-free, staurolite + cordierite + garnet-bearing mineral assemblages in greywackes; hornblende-bearing mineral assemblages in mafic metaigneous rocks; and grunerite-bearing mineral assemblages in iron formation. Pressure–temperature (P–T) conditions of the middle amphibolite-facies zone are ca. 550–620 °C and 3.0–5.0 kbar (1 kbar = 100 MPa), with pressure increasing to the northeast. The middle amphibolite-facies zone is bordered to the east and west by an upper amphibolite-facies zone, ca. 5 km wide on the east and ca. 3–5 km on the west. The upper amphibolite-facies zone is characterized by variably migmatitic K-feldspar + sillimanite-bearing mineral assemblages in pelites; migmatitic, garnet + cordierite + sillimanite-bearing mineral assemblages in greywackes; orthopyroxene-free, hornblende-bearing mineral assemblages in mafic rocks; and orthopyroxene-bearing mineral assemblages in iron formations. Pressure–temperature conditions of the upper amphibolite-facies zone are ca. 640–710 °C and 3.0–5.5 kbar in the southeast, and 675–755 °C and 4.5–6.0 kbar in the northwest. The outermost metamorphic zone is of the granulite facies, characterized by migmatitic garnet + cordierite + K-feldspar-bearing assemblages in pelites and greywackes, orthopyroxene + clinopyroxene ± garnet-bearing mineral assemblages in mafic rocks, and orthopyroxene + K-feldspar-bearing mineral assemblages in iron formation in which biotite is unstable. Pressure–temperature conditions of the granulite-facies zone are ca. 775–830 °C and 5.0–7.0 kbar. The P–T paths in the Thompson Nickel Belt appear to be broadly clockwise, except for some domains where they are close to isobaric. The peak P–T conditions, combined with local but widespread development of andalusite, imply relatively steep geothermal gradients of ca. 33–51 °C/km during metamorphism. Regional bathozones (domains of uniform peak-metamorphic pressure) correspond in general but not in detail with the metamorphic-facies zones. They reveal an increase in pressure towards the northeast, suggesting greater degrees of postmetamorphic exhumation in that region. Microstructural analysis suggests that peak metamorphism coincided with, and possibly outlasted, the D2 deformation event. Metamorphic isograds were deformed by D3–D4 structures. These features are consistent with a tectonic model in which the Superior craton moved in a northwest or west-northwest direction relative to the Reindeer Zone, with greatest convergence and tectonic burial occurring at the Thompson promontory.

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Cheeney, R. F. « The plutonic igneous and high-grade metamorphic rocks of southern Liverpool Land, central East Greenland, part of a supposed Caledonian and Precambrian complex ». Rapport Grønlands Geologiske Undersøgelse 123 (31 décembre 1985) : 1–39. http://dx.doi.org/10.34194/rapggu.v123.7878.

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Layered metamorphic formations in southern Liverpool Land are disposed in dome-shaped structures with inclinations everywhere gentie. Meta-igneous and meta-sedimentary rock types are represented as well as formations of less clear affinities. Rocks of eclogitic character comprise part of one of the formations which, together with others of the layered succession, passes laterally into a less-deformed complex containing discordant contact relations of plutonic igneous type. The Hurry Inlet granite is a posttectonic, post-metamorphic formation cropping out in the extreme north-west of the area. The earliest detectable events indicate emplacement of granodiorite into sedimentary or metasedimentary formations. Subsequently, these rocks of igneous and sedimentary origin were raised to upper amphibolite or granulite facies with some deformation leading to the prominent layering and including, perhaps, their juxtaposition with eclogitic rocks originating at great depth. Retrogression to 'normal' amphibolite facies followed, with widespread development of hydrous minerals, pegmatites, etc. The position of limited cataclasis with respect to this later stage of metamorphism is not abundantly clear. Emplacement of the Hurry Inlet granite entirely post-dates all of these events.

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Baltybaev, Sh K., V. M. Savatenkov et M. E. Petrakova. « T-t Evolution of the Early Proterozoic Rocks in the Northern Ladoga Region from the Data on U-Pb, Rb-Sr and Sm-Nd Systems in Minerals ». Geodynamics & ; Tectonophysics 15, n^o 3 (18 juin 2024) : 0759. http://dx.doi.org/10.5800/gt-2024-15-3-0759.

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This paper presents the results of a study of isotopic systems in minerals and rocks in southern margin of the epi-Archean Karelian craton in the zone of its junction with the Svecofennian mobile belt. U-Pb, Sm-Nd and Rb-Sr mineral ages of metamorphic rocks allowed reconstructing a T-t trend during ~1.88–1.61 Ga, which reflects a wide-ranging cooling history of metamorphic rocks from the peak values of about 650–700 °C at 1.88–1.79 Ga (U-Pb age of monazites and apparent oldest Sm-Nd age of amphiboles) to 300–400 °C at 1.61 Ga (model Rb-Sr age of biotites) in zones of low- and medium-temperature metamorphism. The specificity of removal of deep-seated rocks to the present-day erosion surface and the reconstructed T-t trend comply with the development of thrust-nappe structures during the exhumation of the Svecofennids. It is also assumed that differential vertical block movements played a significant role during the post-orogenic extensional collapse and neorifting.

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Raven, J. G. M., et Ben A. Van Der Pluijm. « Metamorphic fluids and transtension in the Cantabrian Mountains of northern Spain : an application of the conodont colour alteration index ». Geological Magazine 123, n^o 6 (novembre 1986) : 673–81. http://dx.doi.org/10.1017/s0016756800024183.

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AbstractConodont colour alteration index (CAI) values from Upper Paleozoic rocks in the Cantabrian zone of northern Spain show that temperatures during Hercynian metamorphism locally exceeded 300 °C. Various temperature domains have been defined, which are generally separated by fundamental structures. These domains do not correspond with the tripartite subdivision based on stratigraphic analysis.The observed CAI values of conodonts are in general agreement with the mineral paragenesis. Areas with high CAI values display extensive alteration and mineralization, and where CAI values exceed 4–4.5 (>200 °C) slaty cleavage has developed.The Cantabrian zone is an area of very low grade metamorphism, where peak conditions were reached in Upper Carboniferous to Lower Permian times. The characteristics of the metamorphism and its spatial relationship with major faults suggest that fluids were the main source for regional heating and that fluid transport was focussed along crustal-scale structural features.The overall deformation regime in this part of the Variscan orogen of western Europe is interpreted to be large-scale transtension. This is in agreement with earlier proposed models for the formation of Upper Palaeozoic basins in this area.

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Corti, Luca, Davide Zanoni, G. Diego Gatta et Michele Zucali. « Strain partitioning in host rock controls light rare earth element release from allanite-(Ce) in subduction zones ». Mineralogical Magazine 84, n^o 1 (22 janvier 2020) : 93–108. http://dx.doi.org/10.1180/mgm.2020.4.

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AbstractCombined microstructural, mineral chemical, X-ray maps and X-ray single-crystal diffraction analyses are used to reveal the behaviour of individual grains of magmatic allanite relicts hosted in variably deformed metagranitoids at Lago della Vecchia (inner part of the Sesia-Lanzo Zone, Western Alps, Europe), which experienced high-pressure and low-temperature metamorphism during the Alpine subduction. X-ray single-crystal diffraction shows that none of the allanite crystals, irrespective of the strain state of the host rock, record any evidence of plastic deformation (i.e. intracrystalline deformation), as indicated by the shape of the Bragg diffraction spots, the atomic site positions, and their displacement around the centre of gravity. On the contrary, strong plastic deformation affected matrix minerals, such as quartz, white mica and feldspar of the hosting rocks, during the development of the Alpine eclogitic- and blueschist-facies metamorphism. Despite the strain-free atomic structures of allanite, different patterns of chemical zoning, as a function of strain accumulated in the rock matrix, are observed. As allanite occurs in magmatic and metamorphic rocks and it is stable at high-pressure and low-temperature conditions, we infer that allanite could behave as one of the main carriers of light rare earth elements into the mantle wedge during subduction of continental crust. In particular, the release of light rare earth elements from allanite, under high-pressure conditions in subduction zones, is facilitated by high strain accumulated in the host rock.

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Li, Xiangchun, Zhongbei Li, Fan Zhang, Qi Zhang, Baisheng Nie et Yangyang Meng. « Nanopore Structure of Different Rank Coals and Its Quantitative Characterization ». Journal of Nanoscience and Nanotechnology 21, n^o 1 (1 janvier 2021) : 22–42. http://dx.doi.org/10.1166/jnn.2021.18728.

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Based on gas adsorption theory, high-pressure mercury intrusion (HPMI), low-temperature liquid nitrogen gas adsorption (LT-N2GA), CO2 adsorption, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and small-angle X-ray scattering (SAXS) techniques were used to analyze the pore structures of six coal samples with different metamorphisms in terms of pore volume, specific surface area (SSA), pore size distribution (PSD) and pore shape. Combined with the gas adsorption constant a, the influence and mechanism of the pore structure of different coal ranks on gas adsorption capacity were analyzed. The results show that there are obvious differences in the pore structure of coals with different ranks, which leads to different adsorption capacities. To a large extent, the pore shapes observed by SEM are consistent with the LT-N2GA isotherm analysis. The pore morphology of coal samples with different ranks is very different, indicating the heterogeneity among the coal surfaces. Adsorption analysis revealed that mesopore size distributions are multimodal and that the pore volume is mainly composed of mesopores of 2–15 nm. The adsorption capacity of the coal body micropores depends on the 0.6–0.9 nm and 1.5–2.0 nm aperture sections. The influence of coal rank on gas desorption and diffusion is mainly related to the difference in pore structure. The medium metamorphic coal sample spectra show that the number of peaks in the high-wavenumber segment is small and that it is greater in the high metamorphic coal. The absorption intensity of the C–H stretching vibration peak of naphthenic or aliphatic hydrocarbons varies significantly among the coal samples. Over a small range of angles, as the scattering angle increases, the scattering intensity of each coal sample gradually decreases, and as the degree of metamorphism increases, the scattering intensity gradually increases. That is, the degree of metamorphism of coal samples is directly proportional to the scattering intensity. The influence of coal rank on gas adsorption capacity is mainly related to the difference in pore structure. The gas adsorption capacity shows an asymmetric U-shaped relationship with coal rank. For higher rank coals (Vdaf < 15%), the gas adsorption consistently decreases significantly with increasing Vdaf. In the middle and low rank coal stages (Vdaf > 15%), it increases slowly with the increase of Vdaf. We believe that the results of this study will provide a theoretical basis and practical reference value for effectively evaluating coal-rock gas storage capacity, revealing the law of CBM enrichment and the development and utilization of CBM resources.

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