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1
Brown, Michael. "Metamorphic geology: Tectonics of metamorphism." Nature 318, no. 6044 (November 1985): 314–15. http://dx.doi.org/10.1038/318314a0.
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Tracy, Robert J. "Metamorphic geology." Reviews of Geophysics 25, no. 5 (1987): 1115. http://dx.doi.org/10.1029/rg025i005p01115.
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PEACOCK, SIMON M. "Metamorphic Geology." Reviews of Geophysics 29, S2 (January 1991): 486–99. http://dx.doi.org/10.1002/rog.1991.29.s2.486.
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Li, Zhen, Hao Wang, Qian Zhang, Meng-Yan Shi, Jun-Sheng Lu, Jia-Hui Liu, and Chun-Ming Wu. "Ultra-High Pressure Metamorphism and Geochronology of Garnet Clinopyroxenite in the Paleozoic Dunhuang Orogenic Belt, Northwestern China." Minerals 11, no. 2 (January 24, 2021): 117. http://dx.doi.org/10.3390/min11020117.
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Ultra-high pressure (UHP) metamorphism is recorded by garnet clinopyroxenite enclaves enclosed in an undeformed, unmetamorphosed granitic pluton, northeastern Paleozoic Dunhuang orogenic belt, northwestern China. The protoliths of the garnet clinopyroxenite might be basic or ultrabasic volcanic rocks. Three to four stages of metamorphic mineral assemblages have been found in the garnet clinopyroxenite, and clockwise metamorphic pressure–temperature (P-T) paths were retrieved, indicative of metamorphism in a subduction environment. Peak metamorphic P-T conditions (790–920 °C/28–41 kbar) of garnet clinopyroxenite suggest they experienced UHP metamorphism in the coesite- or diamond-stability field. The UHP metamorphic event is also confirmed by the occurrence of high-Al titanite enclosed in the garnet, along with at least three groups of aligned rutile lamellae exsolved from the garnet. Secondary ion mass spectrometry (SIMS) U-Pb dating of metamorphic titanite indicates that the post-peak, subsequent tectonic exhumation of the UHP rocks occurred in the Devonian period (~389–370 Ma). These data suggest that part of the Paleozoic Dunhuang orogenic belt experienced UHP metamorphism, and diverse metamorphic facies series prevailed in this Paleozoic orogen. It can be further inferred that most of the UHP rocks in this orogen remain buried.
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Lardeaux, Jean-Marc. "Deciphering orogeny: a metamorphic perspective. Examples from European Alpine and Variscan belts." Bulletin de la Société Géologique de France 185, no. 2 (February 1, 2014): 93–114. http://dx.doi.org/10.2113/gssgfbull.185.2.93.
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AbstractIn this paper we review and discuss, in a synthetic historical way, the main results obtained on Alpine metamorphism in the western Alps. First, we describe the finite metamorphic architecture of the western Alps and discuss its relationships with subduction and collision processes. Second, we portray the progressive metamorphic evolution through time and space with the presentation of 5 metamorphic maps corresponding to critical orogenic periods, namely 85-65 Ma, 60-50 Ma, 48-40 Ma, 38-33 Ma and 30-20 Ma. We underline the lack of temporal data on high-pressure/low-temperature metamorphic rocks as well as the severe uncertainties on the sizes of rock units that have recorded the same metamorphic history (i.e. coherent P-T-t/deformation trajectories). We discuss the role of subduction-driven metamorphism in ocean-derived protoliths and the conflicting models that account for the diachrony of continental subductions in the western Alps.
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APARICIO, A., M. A. BUSTILLO, R. GARCIA, and V. ARAÑA. "Metasedimentary xenoliths in the lavas of the Timanfaya eruption (1730–1736, Lanzarote, Canary Islands): metamorphism and contamination processes." Geological Magazine 143, no. 2 (March 2006): 181–93. http://dx.doi.org/10.1017/s0016756806001713.
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We report on the investigation of contact metamorphism provoked by the emplacement of a shallow magma chamber during the Timanfaya eruption of Lanzarote from 1730 to 1736 AD. The study was carried out on metamorphic xenoliths from basaltic Timanfaya lavas, and shows how the primary basanitic magma was contaminated by sedimentary and metamorphic rocks. Mineralogical and chemical studies allowed the definition of several xenolith types. Silica xenoliths (quartz, tridymite, cristobalite or a mixture of these, constituting more than 50 % of the xenolith) and calc-silicate xenoliths (wollastonite, sometimes the 2M type, diopside, forsterite or mixture of these, constituting more than 50 % of the xenolith) are the most frequent. Other minerals recognized were calcite, dolomite, augite, enstatite, hypersthene, spinel and scapolite. The mineralogy and some textures of the metamorphic forsteritic xenoliths are identical to those found in ultrabasic xenoliths (dunites) and point to a possible metamorphic origin for some of them. Major and trace elements showed a diversity of composition, controlled by the mineralogy. The REE composition of the metamorphic xenoliths is high, compared with the sedimentary xenoliths not affected by metamorphism. The mineral assemblages define metamorphic facies of low, medium and high grade, depending on the distance of the sedimentary rocks from the magma chamber border. The IGPETWIN-MIXING program was used to verify the contamination process, taking the xenoliths as representative of the sedimentary/metamorphic rocks that were melted. The results indicated that sedimentary/metamorphic rock contamination of a basanitic magma can produce tholeiitic compositions.
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MAJKA, JAROSLAW, STANISLAW MAZUR, MACIEJ MANECKI, JERZY CZERNY, and DANIEL K. HOLM. "Late Neoproterozoic amphibolite-facies metamorphism of a pre-Caledonian basement block in southwest Wedel Jarlsberg Land, Spitsbergen: new evidence from U–Th–Pb dating of monazite." Geological Magazine 145, no. 6 (September 10, 2008): 822–30. http://dx.doi.org/10.1017/s001675680800530x.
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AbstractSouthwest Spitsbergen, Wedel Jarlsberg Land, consists of two Proterozoic crustal blocks with differing metamorphic histories. Both blocks experienced Caledonian greenschist-facies metamorphism, but only the southern block records an earlier pervasive M1 amphibolite-facies metamorphism and strong deformational fabric. In situ EMPA total-Pb monazite geochronology from both matrix and porphyroblast inclusion results indicate that the older M1 metamorphism occurred at 643 ± 9 Ma, consistent with published cooling ages of c. 620 Ma (hornblende) and 580 Ma (mica) obtained from these same rocks. This region thus contains a lithostratigraphic profile and metamorphic history which are unique within the Svalbard Archipelago. Documentation of a pervasive late Neoproterozoic Barrovian metamorphism is difficult to reconcile with a quiescent non-tectonic regime typically inferred for this region, based on the occurrence of rift-drift sequences on the Baltic and Laurentian passive margins. Instead, our new metamorphic age implies an exotic origin of the pre-Devonian basement exposed in SW Spitsbergen and supports models of terrane assembly postulated for the Svalbard Archipelago.
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RAMÍREZ-SÁNCHEZ, ELISA, KATJA DECKART, and FRANCISCO HERVÉ. "Significance of 40Ar–39Ar encapsulation ages of metapelites from late Palaeozoic metamorphic complexes of Aysén, Chile." Geological Magazine 145, no. 3 (December 17, 2007): 389–96. http://dx.doi.org/10.1017/s0016756807004220.
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AbstractThe ages obtained by the 40Ar–39Ar encapsulation technique (retention and total gas ages) on <2 μm fractions of five metapelites from the Eastern Andean Metamorphic Complex and two from the Chonos Metamorphic Complex allow discussion of the latest recorded metamorphic event in each zone. The Kübler Index (KI) of illite/muscovite (principal component of the metapelites) varies between 0.15° and 0.45° Δ°2θ, indicating regional variation from diagenetic to epizonal metamorphic grade. The 40Ar–39Ar encapsulation analyses reveal 39Ar loss varying between 21 and 25%, which shows a limited positive correlation with KI values. The obtained retention and total gas metapelite ages reflect distinct metamorphic conditions. Retention ages most probably indicate burial or regional metamorphic events without plutonic influence in the southern Eastern Andean Metamorphic Complex. Total gas ages reflect contact ages for metapelites close to intrusions in the northern and southern Eastern Andean Metamorphic Complex and in the Chonos Metamorphic Complex. The thermal overprinting of metapelites occurred in Early Cretaceous times at 130 Ma and 145 Ma and is related to the contact metamorphism of an emplacement pulse of the North Patagonian Batholith. Total gas metapelite ages obtained from the western belt of the Chonos Metamorphic Complex suggest a thermal event related to a distinct pulse of the North Patagonian Batholith.
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Yan, Jun, Ying Cui, and Xiaoyu Liu. "Evolution of Contact Metamorphic Rocks in the Zhoukoudian Area: Evidence from Phase Equilibrium Modelling." Minerals 13, no. 8 (August 10, 2023): 1056. http://dx.doi.org/10.3390/min13081056.
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The Yanshan intraplate tectonic belt is a tectonic-active area in the central part of the North China Craton that has undergone long-term orogenic evolution. Detailed studies on magmatic activity and metamorphism of this belt are significant for restoring its orogenic thermal evolution process. The Fangshan pluton in the Zhoukoudian area within this tectonic belt is a product of the late Mesozoic Yanshan event. However, there is a lack of detailed research on the metamorphic evolution history of the ancient terrane surrounding the Fangshan pluton subjected to contact thermal metamorphism. To further constrain the metamorphic P–T evolution of contact metamorphism associated with the Fangshan pluton, we collected rock samples in the andalusite–biotite contact metamorphic zone of the Fangshan pluton, and conducted petrographic investigations, geochemical and mineral composition analysis, and phase equilibrium modeling. The phase equilibrium modeling in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O system indicates that the peak mineral assemblages of andalusite–biotite schists are pl + q + mu + bi + and ± kfs + ilm + mt, formed at 550 to 610 °C, 1 to 3.5 kbar, and the peak mineral assemblage of garnet–andalusite–cordierite–biotite schists is gt + pl + q + bi + and + cord + ilm + mt, formed at 580 to 620 °C, 1.5 to 2.1 kbar. Therefore, we believe that the rocks in the andalusite biotite contact metamorphic zone of the Fangshan pluton underwent low pressure and medium temperature metamorphism, with the peak metamorphic conditions of about 550–610 °C, <3.5 kbar. The results show that the rocks in contact with the thermal metamorphic zone were rapidly heated by the heat released by the Fangshan pluton, and after reaching the peak metamorphic temperature, they were cooled down simultaneously with the cooling of the rock mass, defined in a nearly isobaric P–T trajectory.
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XIAO, LING-LING, GUO-DONG WANG, HAO WANG, ZONG-SHENG JIANG, CHUN-RONG DIWU, and CHUN-MING WU. "Zircon U–Pb geochronology of the Zanhuang metamorphic complex: reappraisal of the Palaeoproterozoic amalgamation of the Trans-North China Orogen." Geological Magazine 150, no. 4 (April 30, 2013): 756–64. http://dx.doi.org/10.1017/s001675681300006x.
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AbstractAmphibolites and metapelites exposed in the Zanhuang metamorphic complex situated in the south-middle section of the Trans-North China Orogen (TNCO) underwent upper-amphibolite-facies metamorphism and record clockwise P–T paths including retrograde isothermal decompression. High-resolution zircon U–Pb geochronological analyses indicate that the metamorphic peak occurred during ~ 1840–1860 Ma, which is in accordance with the ubiquitous metamorphic ages of ~ 1850 Ma retrieved by miscellaneous geochronologic methods throughout the metamorphic terranes of the northern TNCO, confirming that the south-middle section of the TNCO was involved in the amalgamation of the Eastern and Western Blocks of the North China Craton during the Palaeoproterozoic.
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Hara, Hidetoshi, Hiroshi Mori, Kohei Tominaga, and Yuki Nobe. "Progressive Low-Grade Metamorphism Reconstructed from the Raman Spectroscopy of Carbonaceous Material and an EBSD Analysis of Quartz in the Sanbagawa Metamorphic Event, Central Japan." Minerals 11, no. 8 (August 8, 2021): 854. http://dx.doi.org/10.3390/min11080854.
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Low-grade metamorphic temperature conditions associated with the Sanbagawa metamorphic event were estimated by the Raman spectroscopy of carbonaceous material (RSCM) in pelitic rocks and an electron backscatter diffraction (EBSD) analysis of the quartz in siliceous rocks. Analytical samples were collected from the Sanbagawa metamorphic complex, the Mikabu greenstones, and the Chichibu accretionary complex in the eastern Kanto Mountains, central Japan. Previously, low-grade Sanbagawa metamorphism was only broadly recognized as pumpellyite–actinolite facies assigned to the chlorite zone. The RSCM results indicate metamorphic temperatures of 358 °C and 368 °C for the chlorite zone and 387 °C for the garnet zone of the Sanbagawa metamorphic complex, 315 °C for the Mikabu greenstones, and 234–266 °C for the Chichibu accretionary complex. From the EBSD analyses, the diameter of the quartz grains calculated by the root mean square (RMS) approximation ranges from 55.9 to 69.0 μm for the Sanbagawa metamorphic complex, 9.5 to 23.5 μm for the Mikabu greenstones, and 2.9 to 7.3 μm for the Chichibu accretionary complex. The opening angles of the c-axis fabric approximate 40–50°, presenting temperatures of 324–393 °C for the Sanbagawa metamorphic complex and the Mikabu greenstones. The temperature conditions show a continuous increase with no apparent gaps from these low-grade metamorphosed rocks. In addition, there exists an empirical exponential relationship between the estimated metamorphic temperatures and the RMS values of the quartz grains. In this study, integrated analyses of multiple rock types provided valuable information on progressive low-grade metamorphism and a similar approach may be applied to study other metamorphic complexes.
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Prakash, D., DK Patel, MK Yadav, B. Vishal, S. Tewari, R. Yadav, SK Rai, and CK Singh. "Prograde polyphase regional metamorphism of pelitic rocks, NW of Jamshedpur, eastern India: constraints from textural relationship, pseudosection modelling and geothermobarometry." Geological Magazine 157, no. 7 (November 11, 2019): 1045–67. http://dx.doi.org/10.1017/s0016756819001171.
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AbstractThe study area belongs to the Singhbhum metamorphic belt of Jharkhand, situated in the eastern part of India. The spatial distribution of the index minerals in the pelitic schists of the area shows Barrovian type of metamorphism. Three isograds, viz. garnet, staurolite and sillimanite, have been delineated and the textural study of the schists has revealed a time relation between crystallization and deformation. Series of folds with shifting values of plunges in the supracrustal rocks having axial-planar schistosity to the folds have been widely cited. Development of these folds could be attributed to the second phase of deformation. In total, two phases of deformation, D1 and D2, in association with two phases of metamorphism, M1 and M2, have been lined up in the study area. Chemographic plots of reactant and product assemblages corresponding to various metamorphic reactions suggest that the pattern of metamorphic zones mapped in space is in coherence with the temporal-sequential change during prograde metamorphism. The prograde P–T evolution of the study area has been obtained using conventional geothermobarometry, internally consistent winTWQ program and Perple_X software in the MnNCKFMASHTO model system. Our observations suggest that the progressive metamorphism in the area is not related to granitic intrusion or migmatization but that it was possibly the ascending plume that resulted in the M1 phase of metamorphism followed by D1 deformation. The second and prime metamorphic phase, M2, with its possible heat source generated by crustal overloading, was preceded by D1 and it lasted until late- to post-D2 deformation.
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Lardeaux, Jean-Marc. "Deciphering orogeny: a metamorphic perspective Examples from European Alpine and Variscan belts." Bulletin de la Société Géologique de France 185, no. 5 (May 1, 2014): 281–310. http://dx.doi.org/10.2113/gssgfbull.185.5.281.
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AbstractIn this paper we review and discuss, in a synthetic historical way, the main results obtained on Variscan metamorphism in the French Massif Central. First, we describe the pre-orogenic architecture of the French Massif Central on the base of available lithostratigraphic and geochemical constraints. Second, we portray the progressive metamorphic evolution through time and space with the presentation of 6 metamorphic maps corresponding to critical orogenic periods, namely 430–400 Ma, 400–370 Ma, 370–360 Ma, 360–345 Ma, 340–325 Ma and 320–290 Ma. We discuss the role of multiple subductions in orogeny, the metamorphic effects of continental collision (i.e. regional development of intermediate-pressure metamorphic series) as well as the links between post-thickening tectonics and the regional development of low-pressure metamorphic series coeval with crustal partial melting. As it was the case for the western Alps, we emphasize the lack of temporal data on high-pressure/low-temperature metamorphic rocks as well as the uncertainties on the sizes of rock units that have recorded the same metamorphic history (i.e. coherent P-T-t/deformation trajectories). Finally, we underline the main differences and similarities between the metamorphic evolutions of the western Alps and the French Massif Central.
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Li, Yunshuai, Jianxin Zhang, Shengyao Yu, Yanguang Li, Hu Guo, Jian Zhang, Changlei Fu, Hui Cao, Mengqi Jin, and Zhihui Cai. "Petrological, geochronological, and geochemical potential accounting for continental subduction and exhumation: A case study of felsic granulites from South Altyn Tagh, northwestern China." GSA Bulletin 132, no. 11-12 (April 22, 2020): 2611–30. http://dx.doi.org/10.1130/b35459.1.
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Abstract Deciphering the formation and geodynamic evolution of high-pressure (HP) granulites in a collisional orogeny can provide crucial constraints on the geodynamic evolution of subduction-exhumation. To fully exploit the geodynamic potential of metamorphic rocks, it is necessary to constrain the metamorphic ages, although it is difficult to link zircon and monazite ages to metamorphic evolution. A good case study for understanding these geodynamic processes is felsic granulites in the Bashiwake area, South Altyn Tagh. Petrographic observations suggest that the studied felsic granulites have suffered multi-stage metamorphism, and the distinct metamorphic events were documented by compositional zoning and high Y + heavy rare earth element (HREE) concentrations in the large garnet porphyroblast. Zircon U-Pb dating yielded two major age clusters: one age cluster at ca. 900 Ma represents the age of the protolith for the felsic granulite, and another age cluster at ca. 500 Ma represents the post-UHT (ultrahigh temperature) stage based on the rare earth element distribution coefficients between zircon and garnet. Meanwhile, in situ monazites U-Pb dating yielded a weighted mean 206Pb/238U age of 482 ± 3.5 Ma, and the monazite U-Pb age was interpreted to be in agreement with the metamorphic zircon rims data, which together with zircon recorded the cooling time after the UHT stage. Whole-rock major and trace elements as well as Sr-Nd isotopes suggest that the protolith of the felsic granulite derived from partial melting of ancient crustal materials with the addition of mantle materials. Integrating these results along with previous studies, we propose that the felsic granulites metamorphosed from the Neoproterozoic granitic rocks, and the granitic rocks with associated mafic-ultramafic rocks suffered a common high-pressure–ultrahigh temperature (HP-UHT) metamorphism and subsequent granulite-facies metamorphism. A tentative model of subduction-relamination was proposed for the geodynamic evolution of the Bashiwake unit, South Altyn Tagh.
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Dallmeyer, R. D. "Partial thermal resetting of 40Ar/39Ar mineral ages in western Spitsbergen, Svalbard: possible evidence for Tertiary metamorphism." Geological Magazine 126, no. 5 (September 1989): 587–93. http://dx.doi.org/10.1017/s001675680002286x.
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AbstractHornblende and muscovite within high-grade Caledonian metamorphic rocks exposed in Albert I Land (northwest Spitsbergen) display internally concordant 40Ar/39Ar age spectra providing no record of any post-Silurian thermal history. Both minerals record 420−425 Ma plateau ages indicating that relatively rapid cooling followed Silurian metamorphism. Muscovite within some metamorphic sequences exposed within the Tertiary tectonic zone of Oscar II Land (west-central Spitsbergen) displays a low-temperature 40Ar/39Ar age spectra discordance suggestive of a slight, post-Caledonian thermal disturbance. This is consistent with a post-Palaeozoic thermal overprint documented elsewhere in Oscar II Land and indicates that, at least locally, metamorphism may have accompanied the Tertiary orogeny.
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ÇELİK, ÖMER FARUK. "Metamorphic sole rocks and their mafic dykes in the eastern Tauride belt ophiolites (southern Turkey): implications for OIB-type magma generation following slab break-off." Geological Magazine 144, no. 5 (July 6, 2007): 849–66. http://dx.doi.org/10.1017/s0016756807003573.
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The metamorphic sole rocks observed between the Pozantı–Karsantı ophiolite and the melange unit are located on the eastern part of the Tauride carbonate platform. They consist of ortho-amphibolites at the top and metasedimentary lithologies at the base. Amphibolites from the metamorphic sole rocks are represented by OIB, MORB and IAT based on their major, trace and REE compositions. The isolated dolerite dykes intrude both the metamorphic sole rocks and the ophiolitic unit at different structural levels. The dolerite dykes cutting the metamorphic sole rocks are enriched in LILE and depleted in HFSE. Enrichment in LILE such as Th, relative to HFSE, is indicative of the presence of a subduction component. Flat-lying REE patterns of the dolerite dykes also confirm an IAT source. Pyroxenite and albitite dykes also cut the metamorphic sole rocks. REE patterns of pyroxenite dykes show prominent LREE enrichment, similar to that observed in within-plate alkaline basalts. The alkaline isolated pyroxenite dykes were probably the result of late-stage magmatism fed by melts that originated from an OIB source, shortly before the emplacement of the Pozantı–Karsantı ophiolite onto the Tauride carbonate platform. A hydrothermal alteration stage is characterized by albitite formation in the joints of the metamorphic sole rocks and by secondary mineralization along the contact zones of dolerite dykes. Mineral parageneses in the metamorphic sole rocks exhibit amphibolite and greenschist-facies assemblages. Geothermobarometric studies based on a newly recognized mineral assemblage (e.g. kyanite) and chemical compositions of minerals in the metamorphic sole rocks indicate that the metamorphic temperature during the metamorphism was 570–580°C and the pressure was around 5–6 kbar.
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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, no. 1 (January 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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Lanari, Pierre, Silvio Ferrero, Philippe Goncalves, and Eugene G. Grosch. "Metamorphic geology: progress and perspectives." Geological Society, London, Special Publications 478, no. 1 (December 10, 2018): 1–12. http://dx.doi.org/10.1144/sp478-2018-186.
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D´Eramo, F. J., J. J. Esteban, M. Demartis, E. Aragón, J. E. Coniglio, and L. P. Pinotti. "Time lag between metamorphism and crystallization of anatectic granites (Córdoba, Argentina)." Geologica Acta 18 (November 2, 2020): 1–14. http://dx.doi.org/10.1344/geologicaacta2020.18.17.
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SHRIMP and LA-ICP-MS analyses carried out on zircons from the Río de los Sauces granite revealed their metamorphic and igneous nature. The metamorphic zircons yielded an age of 537±4.8 (2σ)Ma that probably predates the onset of the anatexis during the Pampean orogeny. By contrast, the igneous zircons yielded a younger age of 529±6 (2σ)Ma and reflected its crystallization age. These data point to a short time lag of ca. 8Myr between the High Temperature (HT) metamorphic peak and the subsequent crystallization age of the granite. Concordia age of 534±3.8 (2σ)Ma, for both types of zircon populations, can be considered as the mean age of the Pampean HT metamorphism in the Sierras de Córdoba.
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Pognante, U., D. Castelli, P. Benna, G. Genovese, F. Oberli, M. Meier, and S. Tonarini. "The crystalline units of the High Himalayas in the Lahul–Zanskar region (northwest India): metamorphic–tectonic history and geochronology of the collided and imbricated Indian plate." Geological Magazine 127, no. 2 (March 1990): 101–16. http://dx.doi.org/10.1017/s0016756800013807.
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AbstractIn the High Himalayan belt of northwest India, crustal thickening linked to Palaeogene collision between India and Eurasia has led to the formation of two main crystalline tectonic units separated by the syn-metamorphic Miyar Thrust: the High Himalayan Crystallines sensu stricto (HHC) at the bottom, and the Kade Unit at the top. These units are structurally interposed between the underlying Lesser Himalaya and the very low-grade sediments of the Tibetan nappes. They consist of paragneisses, orthogneisses, minor metabasics and, chiefly in the HHC, leucogranites. The HHC registers: a polyphase metamorphism with two main stages designated as M1 and M2; a metamorphic zonation with high-temperature recrystallization and migmatization at middle structural levels and medium-temperature assemblages at upper and lower levels. In contrast, the Kade Unit underwent a low-temperature metamorphism. Rb–Sr and U–Th–Pb isotope data point to derivation of the orthogneisses from early Palaeozoic granitoids, while the leucogranites formed by anatexis of the HHC rocks and were probably emplaced during Miocene time.Most of the complicated metamorphic setting is related to polyphase tectonic stacking of the HHC with the ‘cooler’ Kade Unit and Lesser Himalaya during the Himalayan history. However, a few inconsistencies exist for a purely Himalayan age of some Ml assemblages of the HHC. As regards the crustal-derived leucogranites, the formation of a first generation mixed with quartzo-feldspathic leucosomes was possibly linked to melt-lubricated shear zones which favoured rapid crustal displacements; at upper levels they intruded during stage M2 and the latest movements along the syn-metamorphic Miyar Thrust, but before juxtaposition of the Tibetan nappes along the late- metamorphic Zanskar Fault.
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Schmidt, William L., and John P. Platt. "Metamorphic Temperatures and Pressures across the Eastern Franciscan: Implications for Underplating and Exhumation." Lithosphere 2020, no. 1 (November 9, 2020): 1–19. http://dx.doi.org/10.2113/2020/8853351.
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Abstract The Eastern Belt of the Franciscan Complex in the northern California Coast Ranges consists of coherent thrust sheets predominately made up of ocean floor sediments subducted in the Early Cretaceous and then accreted to the overriding plate at depths of 25-40 km. Progressive packet accretion resulted in the juxtaposition of a series of thrust sheets of differing metamorphic grades. This study utilizes laser Raman analysis of carbonaceous material to determine peak metamorphic temperatures across the Eastern Belt and phengite barometry to determine peak metamorphic pressures. Locating faults that separate packets in the field is difficult, but they can be accurately located based on differences in peak metamorphic temperature revealed by Raman analysis. The Taliaferro Metamorphic Complex in the west reached 323-336°C at a minimum pressure of ~11 kbar; the surrounding Yolla Bolly Unit 215–290°C; the Valentine Springs Unit 282-288°C at 7.8±0.7 kbar; the South Fork Mountain Schist 314–349°C at 8.6–9.5 kbar, a thin slice in the eastern portion of the SFMS, identified here for the first time, was metamorphosed at ~365°C and 9.7±0.7 kbar; and a slice attributed to the Galice Formation of the Western Klamath Mountains at 281±13°C. Temperatures in the Yolla Bolly Unit and Galice slice were too low for the application of phengite barometry. Microfossil fragments in the South Fork Mountain Schist are smaller and less abundant than in the underlying Valentine Springs Unit, providing an additional method of identifying the boundary between the two units. Faults that record a temperature difference across them were active after peak metamorphism while faults that do not were active prior to peak metamorphism, allowing for the location of packet bounding faults at the time of accretion. The South Fork Mountain Schist consists of two accreted packets with thicknesses of 300 m and 3.5 km. The existence of imbricate thrust faults both with and without differences in peak metamorphic temperature across them provides evidence for synconvergent exhumation.
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Yılmaz, Yücel. "Southeast Anatolian Orogenic Belt revisited (geology and evolution)." Canadian Journal of Earth Sciences 56, no. 11 (November 2019): 1163–80. http://dx.doi.org/10.1139/cjes-2018-0170.
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The Southeast Anatolian Orogenic Belt consists of the Arabian Platform, a zone of imbrication, and a nappe zone. The Arabian Platform is represented by a thick marine succession. The zone of imbrication is a narrow belt sandwiched between the Arabian Platform and the nappes. The nappes are the highest tectonic unit. They consist of two continental slivers separated by ophiolitic associations representing oceanic environments. They were involved in the orogenic development and formed two metamorphic belts. The oceanic environment survived by the end of Middle Eocene. A northward subduction began in this ocean and generated the Elbistan–Yüksekova arc built above the Göksun ophiolite. Development of the Southeastern Anatolian Orogenic Belt began in the north, where the Binboğa–Malatya metamorphic massif, collided with the Elbistan volcanic arc to the end of Early Eocene period. Later new tectonic entities were accreted to this progressively growing and southerly transporting nappe stack. In the lower plate, the southern continental sliver that was attached to the oceanic slab subducted together and underwent high-pressure metamorphism. The subducting oceanic slab retreated. Asthenospheric inflow caused high-temperature metamorphism, which superimposed on the previous high-pressure metamorphism. The oceanic and continental fragments formed the Bitlis Massif and the Berit metaophiolite when exhumed. A younger volcanic arc was built on the ocean floor to the south. Accretion of the volcanic arc to the nappe pile occurred during the Late Eocene period. The orogenic belt was formed when the nappes collided with the Arabian plate during the Late Miocene.
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Vallance, T. "Achievement in Isolation: A.W. Howitt, Pioneering Investigator of Metamorphism in Australia." Earth Sciences History 5, no. 1 (January 1, 1986): 39–49. http://dx.doi.org/10.17704/eshi.5.1.3h10521520544830.
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The earliest coherent observations of metamorphic phenomena in Australia were made by a policemagistrate, stationed in a remote part of Victoria and largely self-taught in geology. In a series of reports and papers issued between 1875 and 1892 that magistrate, Alfred William Howitt, recorded details of metamorphic progressions found in the mountains of eastern Victoria - from folded Palaeozoic strata to crystalline schists and gneisses, and of different sorts of granitic bodies in the regional metamorphic association.Howitt worked at a time when the metamorphic status of crystalline schists was far from generally accepted in Europe and America; some still regarded them as portions of unchanged Primitive crust. Like George Barrow in Scotland - whose work in some ways he anticipated, Howitt, however, through the influence of Lyell's writings, began as a believer in metamorphism. But whereas Barrow is respected for innovative contributions to metamorphic thought and method, Howitt's isolation in Australia kept his work little known. In fact, as recent studies show, Howitt was investigating a regional metamorphism different in style from that of Barrow. Howitt not only pioneered metamorphic petrology in Australia, he really began the study of what is now termed low-pressure regional metamorphism.This paper seeks to set Howitt's metamorphic investigations in the contexts of his career and the then condition of his chosen subject. The principal influences on his approaches to petrography and metamorphism are seen to be German in origin. Howitt may have had no formal training in science but as a boy he lived in Germany for some years and learned the language. It was to be a most useful acquisition.
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KEMP, S. J., and R. J. MERRIMAN. "Polyphase low-grade metamorphism of the Ingleton Group, northern England, UK: a case study of metamorphic inversion in a mudrock succession." Geological Magazine 146, no. 2 (November 14, 2008): 237–51. http://dx.doi.org/10.1017/s0016756808005542.
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AbstractA series of boreholes in Horton Quarry, northwest Yorkshire (Horton-in-Ribblesdale Inlier) penetrated mudstones and slates belonging to the Austwick Formation (Windermere Supergroup) overlying laminated mudstones of the Ingleton Group. Illite (IC) and chlorite (ChC) crystallinity measurements indicate a metamorphic inversion between the two groups of mudrocks. The Windermere Supergroup mudrocks are mostly in the high anchizone or epizone, whereas the Ingletonian samples are lower grade in terms of IC, and are mostly deep diagenetic zone or low anchizone. Hence younger strata at higher grades rest on older strata at lower grades, creating a metamorphic inversion. Ingletonian slates exposed at Pecca Falls on the River Twiss show epizonal and anchizonal IC values, and greywacke samples from Ingleton Quarry contain pumpellyite. This suggests that grade in the Ingletonian may increase to the NW from the Horton to Ingleton inliers. K-white mica b cell dimensions show further differences between the Ingleton Group and the Windermere Supergroup. The Ingletonian samples are characterized by low b cell values (8.989–9.035, mean 9.007 Å), whereas the Windermere Supergroup has higher values in the range 9.022–9.034, mean 9.027 Å. The Windermere Supergroup values are similar to those recorded from the Windermere Supergroup of the southern Lake District, and Lower Palaeozoic rocks from the Scottish Southern Uplands, and are consistent with metamorphism in a low heat flow, convergent geotectonic setting. The Ingletonian b cell values suggest metamorphism in a higher heat flow setting, most likely an extensional basin. The metamorphic inversion at Horton and differences in K-white mica b cell dimensions suggest that the Ingleton Group and Windermere Supergroup strata evolved in different geotectonic settings and record two separate metamorphic events. The discovery of the metamorphic inversion at Horton provides further evidence in favour of an Ordovician rather than Neoproterozoic depositional age for the controversial Ingleton Group.
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Lihter, Iva, Kyle P. Larson, Sudip Shrestha, John M. Cottle, and Alex D. Brubacher. "Contact metamorphism of the Tethyan Sedimentary Sequence, Upper Mustang region, west-central Nepal." Geological Magazine 157, no. 11 (April 24, 2020): 1917–32. http://dx.doi.org/10.1017/s0016756820000229.
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AbstractThe Upper Mustang region of west-central Nepal contains exposures of metamorphosed Tethyan Sedimentary Sequence rocks that have been interpreted to reflect either contact metamorphism related to the nearby Mugu pluton or regional metamorphism associated with the North Himalayan domes. New monazite geochronology results show that the Mugu leucogranite crystallized at c. 21.3 Ma, while the dominant monazite age peaks from the surrounding garnet ± staurolite ± sillimanite schists range between c. 21.7 and 19.4 Ma, generally decreasing in age away from the pluton. Metamorphic temperature estimates based on Ti-in-biotite and garnet–biotite thermometry are highest in the specimens closest to the pluton (648 ± 24°C and 615 ± 25°C, respectively) and lowest in those furthest away (578 ± 24°C and 563 ± 25°C, respectively), while pressure estimates are all within uncertainty of one another, averaging 5.0 ± 0.5 kbar. These results are interpreted to be consistent with contact metamorphism of the rocks in proximity to the Mugu pluton, which was emplaced at c. 18 ± 2 km depth after local movement across the South Tibetan detachment system had ceased. While this new dataset helps to characterize the metamorphic rocks of the Tethyan Sedimentary Sequence and provides new constraints on the thickness of the upper crust, it also emphasizes the importance of careful integration of metamorphic conditions and inferred processes that may affect interpretation of currently proposed Himalayan models.
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Merriman, R. J., T. C. Pharaoh, N. H. Woodcock, and P. Daly. "The metamorphic history of the concealed Caledonides of eastern England and their foreland." Geological Magazine 130, no. 5 (September 1993): 613–20. http://dx.doi.org/10.1017/s0016756800020914.
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AbstractWhite mica (illite) crystallinity data, derived mostly from borehole samples, have been used to generate a contoured metamorphic map of the concealed Caledonide fold belt of eastern England and the foreland formed by the Midlands Microcraton. The northern subcrop of the fold belt is characterized by epizonal phyllites and quartzites of possible Cambrian age, whereas anchizonal grades characterize Silurian to Lower Devonian strata of the Anglian Basin in the southern subcrop of the fold belt. Regional metamorphism in the Anglian Basin resulted from deep burial and Acadian deformation beneath a possible overburden of 7 km, assuming a metamorphic field gradient of 36 °C km-1. Late Proterozoic volcaniclastic rocks forming the basement of the microcraton show anchizonal to epizonal grades that probably developed during late Avalonian metamorphism. Cambrian to Tremadoc strata, showing late diagenetic alteration, rest on the basement with varying degrees of metamorphic discordance. During early Palaeozoic times, much of the microcraton was a region of slow subsidence with overburden thicknesses of 3.3–5.5 km. However, concealed Tremadoc strata in the northeast of the microcraton reach anchizonal grades and may have been buried to depths of 7 km beneath an overburden of uncertain age.
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Zhang, Min, Richen Zhong, Chang Yu, and Hao Cui. "The Immobility of Uranium (U) in Metamorphic Fluids Explained by the Predominance of Aqueous U(IV)." Minerals 13, no. 3 (March 17, 2023): 427. http://dx.doi.org/10.3390/min13030427.
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The solubility of uranium (U) in hydrothermal fluid is thought to be controlled by oxidation. In general, uranium is mainly transported as U(VI) in oxidized fluid, but precipitated as U(IV) in reduced fluid. However, many geological observations indicate that metamorphic fluids, which are buffered by metamorphic rocks with oxidized protoliths such as oxidized pelite or altered marine basalt, are not enriched in U. To explore the reason of the low solubility of U in metamorphic fluids, we simulated the hydrous speciation and solubility of U in fluids that are in equilibrium with rocks. The simulations were conducted at pressure–temperature (P-T) conditions of greenschist and amphibolite facies metamorphism. The results show that U is mainly dissolved as U(IV), instead of U(VI), in metamorphic fluids. The solubility of U remains at a low level of ~10−12 molal, and is not significantly influenced by metamorphic temperature, pressure, and fluid salinity. This result is consistent with geological observations and, thus, can explain the low-U nature of natural metamorphic fluids. The simulation also shows high solubility of U(VI) (1.3 × 10−7 molal) in oxidized pelite-buffered fluids at low temperature (<250 °C), consistent with the geological fact that U can be mobilized by low-temperature geofluids.
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Banno, Shohei, and Chihiro Sakai. "Geology and metamorphic evolution of the Sanbagawa metamorphic belt, Japan." Geological Society, London, Special Publications 43, no. 1 (1989): 519–32. http://dx.doi.org/10.1144/gsl.sp.1989.043.01.50.
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Selyatitskii, A. Yu, O. P. Polyansky, and R. A. Shelepaev. "A High-Pressure Thermal Aureole of the Bayan-Kol Gabbro–Monzodiorite Intrusion (Western Sangilen, Southeastern Tuva): Evidence for Lower-Crust Mafic Magma Chambers." Russian Geology and Geophysics 62, no. 9 (September 1, 2021): 987–1005. http://dx.doi.org/10.2113/rgg20194157.
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Abstract —Thermal metamorphism produced an aureole near the early Paleozoic Bayan-Kol gabbro–monzodiorite intrusion in the Erzin shear zone of western Sangilen (Tuva–Mongolia microcontinent, Central Asian Orogenic Belt). Field observation of intrusive contact, structure–textural and mineral transformations of metamorphic rocks, regular changes in the chemical composition of minerals with approaching the intrusive contact, and high temperature gradient from intrusive to wallrocks verified the occurrence of a contact aureole near the Bayan-Kol intrusion. The high-gradient thermal metamorphism (M2) affected garnet–staurolite–kyanite schists that formed during earlier regional metamorphism (M1) at 6.2–7.9 kbar and 600–670 ºC. The 0.5 km wide M2 metamorphic aureole mapped along the northwestern intrusion margin consists of a muscovite–sillimanite zone adjacent to the sedimentary country rocks and a cordierite–K-feldspar zone on the side of the intrusion. The M2 metamorphic reactions occurred within the granulite facies temperature range 880–910 ºC along the contact with monzodiorites and at ~950 ºC along the boundary with gabbronorites; the temperature on the aureole periphery was about 640 ºC. Pressure estimates indicate deep-seated high-grade metamorphism at 6.9–7.8 kbar, while the intrusion itself crystallized at 7.7–7.8 kbar. The suggested numerical model implying the formation of a thermal aureole at a depth of 26 km (7 kbar) in the lower crust is consistent with the temperature pattern determined by geothermobarometry for several key points of the metamorphic zoning and confirms its deep-level origin. Thus, the aureole near the Bayan-Kol intrusion represents a rare case of contact metamorphism in the lower continental crust. The obtained results, along with published petrological and geochronological evidence, reveal two depth levels of the early Paleozoic M2 metamorphism in the Sangilen area: upper (7–15 km, 2–4 kbar) and lower (26–30 km, 7–8 kbar) crust. The Bayan-Kol gabbro–monzodiorite intrusion is likely a small apophysis or a fragment of a deep-crust intermediate magma chamber, while the moderate-pressure (7–8 kbar) M2 granulites in the Erzin shear zone are products of high-gradient metamorphism related to the Cambrian–Ordovician collisional mafic magmatism in the Sangilen area.
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Bozkaya, Ömer, and Hüseyin Yalçın. "Mineral Chemistry of Low-Temperature Phyllosilicates in Early Paleozoic Metaclastic Rocks, Eastern Tauride Belt, Türkiye." Minerals 12, no. 9 (August 28, 2022): 1088. http://dx.doi.org/10.3390/min12091088.
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The mineral chemistry of illite/mica and chlorites, together with the evaluation of textural data of low-temperature metaclastic rocks, plays an important role in determining their origin and metamorphic grade. This study aimed to investigate the chemical properties of phyllosilicates in early Paleozoic metaclastic rocks in the Eastern Tauride Belt, Türkiye. The textural (electron microscopy) and chemical (mineral chemistry analysis) analyses were performed on the samples representing different grades of metamorphism. The illites/micas and chlorites are observed as detrital (chlorite–mica stacks) and neoformation origin. Trioctahedral chlorites (chamosite) exhibit different chemistry for detrital and neoformed origin as well as the metamorphic grade. Tetrahedral Al and octahedral Fe + Mg increase, whereas octahedral Al decreases together with the increasing grade of metamorphism. The detrital chlorites have higher tetrahedral Al and Fe contents than their neoformed counterparts. Chlorite geothermometry data (detrital: 241–≥350 °C; neoformed: 201–268 °C) are compatible with the texture and illite Kübler index data. Illite/white-mica compositions display muscovite and Na-K mica. Tetrahedral Al and interlayer K + Na contents of illites/micas increase with metamorphic grade. Na-K mica and paragonite are observed as replacement-type developments within the detrital CMS. The obtained data indicate that phyllosilicate chemistry can be used effectively for determining the geological evolution of low-grade metamorphic sequences.
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LU, JUN-SHENG, GUO-DONG WANG, HAO WANG, HONG-XU CHEN, TAO PENG, and CHUN-MING WU. "Zircon SIMS U–Pb geochronology of the Lushan terrane: dating metamorphism of the southwestern terminal of the Palaeoproterozoic Trans-North China Orogen." Geological Magazine 152, no. 2 (October 22, 2014): 367–77. http://dx.doi.org/10.1017/s0016756814000430.
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AbstractHigh-resolution SIMS U–Pb dating of metamorphic zircons of the TTG gneisses, gneissic granitoid and amphibolites of the Lushan terrane, Taihua metamorphic complex, suggests that the metamorphism had taken place at least as early as ~1.96–1.86 Ga. These new dates, along with reference data, demonstrate that the southern and middle terranes of the Trans-North China Orogen had been involved in the continent–continent collision between the Western Block and the Eastern Block of the North China Craton. This orogenic process started as early as 1.96 Ga and lasted as late as 1.80 Ga.
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Bonev, Nikolay, Richard Spikings, and Robert Moritz. "40Ar/39Ar age constraints for an early Alpine metamorphism of the Sakar unit, Sakar–Strandzha zone, Bulgaria." Geological Magazine 157, no. 12 (September 14, 2020): 2106–12. http://dx.doi.org/10.1017/s0016756820000953.
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AbstractWe investigated the Sakar unit metamorphic rocks of the Sakar–Strandzha zone in Bulgaria, using 40Ar/39Ar dating of amphibole from the polymetamorphic basement and white mica in the overlying upper Permian metasedimentary rocks of the Paleokastro Formation. The amphibole and white mica revealed plateau ages of 140.50 ± 1.75 Ma and 126.19 ± 1.29 Ma, respectively, indicating an Early Cretaceous cooling history of the regional amphibolite-facies metamorphism to greenschist-facies conditions. Similar metamorphic grades and cooling histories of the Sakar unit share evidence with the nearby Rhodope Massif for the northern Aegean region-wide early Alpine tectonometamorphic event.
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Wu, Shangjing, Changqing Yin, Donald W. Davis, Jian Zhang, Jiahui Qian, Hengzhong Qiao, Yanfei Xia, and Jingna Liu. "Metamorphic evolution of high-pressure felsic and pelitic granulites from the Qianlishan Complex and tectonic implications for the Khondalite Belt, North China Craton." GSA Bulletin 132, no. 11-12 (March 16, 2020): 2253–66. http://dx.doi.org/10.1130/b35502.1.
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Abstract High-pressure felsic granulites in association with pelitic granulites are widely distributed in the Qianlishan Complex of the Khondalite Belt, North China Craton. A link between “inter-layered” felsic and pelitic granulites was established based on comprehensive metamorphic analysis, revealing that they record similar metamorphic stages: peak pressure (M1), post-peak decompression (M2), and late retrograde cooling (M3) stages. Felsic granulites experienced high-pressure metamorphism up to ∼12 kbar, while pelitic granulites estimated peak pressure is 11–15 kbar. The decompression stage (M2) is indicated by cordierite + sillimanite symplectite and/or cordierite coronae with conditions of 5.7–6.5 kbar/800–835 °C in pelitic granulites, and by garnet-sillimanite assemblages with conditions of >6.5 kbar/810–865 °C in felsic granulites. The later cooling stage (M3) is marked by sub-solidus biotite-quartz-plagioclase symplectite and later melt crystallization. These mineral assemblages and pressure-temperature (P-T) conditions define clockwise P-T paths involving near-isothermal decompression and near-isobaric cooling, suggesting a continent-continent collisional event. Secondary ion mass spectrometry zircon U-Pb dating yields a consistent metamorphic age of ca. 1.95 Ga, interpreted as peak metamorphism. The results, combined with available data, suggest that the Khondalite Belt formed by collision between the Yinshan and Ordos blocks at ca. 1.95 Ga.
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Biševac, Vanja, Kadosa Balogh, Dražen Balen, and Darko Tibljaš. "Eoalpine (Cretaceous) very low- to low-grade metamorphism recorded on the illite-muscovite-rich fraction of metasediments from South Tisia (eastern Mt Papuk, Croatia)." Geologica Carpathica 61, no. 6 (December 1, 2010): 469–81. http://dx.doi.org/10.2478/v10096-010-0029-9.
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Eoalpine (Cretaceous) very low- to low-grade metamorphism recorded on the illite-muscovite-rich fraction of metasediments from South Tisia (eastern Mt Papuk, Croatia)Eoalpine very low- to low-grade metamorphism related to Cretaceous orogenesis has been investigated in the Slavonian Mts, Croatia. Samples belonging to the Psunj metamorphic complex (PMC), the Radlovac metamorphic complex (RMC) and Permian-Triassic and Triassic sedimentary sequences (PTSS) were studied. The Kübler and Árkai indices of all the analysed samples indicate high-anchizonal to epizonal metamorphism. The degree of Eoalpine metamorphism tends to be constant in all samples implying that the different complexes passed through and recorded the same event. Measurements of illite-white K-micab0-parameter of the RMC samples imply transitional low- to medium-pressure character of the metamorphism. These data together with K-Ar ages (~100-80 Ma) measured on illite-white K-mica rich < 2 μm grain-size fractions point to Late Cretaceous very low- to low-grade regional metamorphism presumably related to the main nappe-forming compressional events in the Pannonian Basin and the Carpathians. The P-T-t (pressure-temperature-time) evolution of the studied area is in good agreement with similar scenarios in the surrounding areas of Tisia, but also in Eastern Alps, Carpathians and Pannonian Basin (ALCAPA).
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Tholt, Andrew, Sean R. Mulcahy, William C. McClelland, Sarah M. Roeske, Vinícius T. Meira, Patricia Webber, Emily Houlihan, Matthew A. Coble, and Jeffrey D. Vervoort. "Metamorphism of the Sierra de Maz and implications for the tectonic evolution of the MARA terrane." Geosphere 17, no. 6 (October 12, 2021): 1786–806. http://dx.doi.org/10.1130/ges02268.1.
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Abstract The Mesoproterozoic MARA terrane of western South America is a composite igneous-metamorphic complex that is important for Paleozoic paleogeographic reconstructions and the relative positions of Laurentia and Gondwana. The magmatic and detrital records of the MARA terrane are consistent with a Laurentian origin; however, the metamorphic and deformation records lack sufficient detail to constrain the correlation of units within the MARA terrane and the timing and mechanisms of accretion to the Gondwana margin. Combined regional mapping, metamorphic petrology, and garnet and monazite geochronology from the Sierra de Maz of northwest Argentina suggest that the region preserves four distinct litho-tectonic units of varying age and metamorphic conditions that are separated by middle- to lower-crustal ductile shear zones. The Zaino and Maz Complexes preserve Barrovian metamorphism and ages that are distinct from other units within the region. The Zaino and Maz Complexes both record metamorphism ca. 430–410 Ma and show no evidence of the regional Famatinian orogeny (ca. 490–455 Ma). In addition, the Maz Complex records an earlier granulite facies event at ca. 1.2 Ga. The Taco and Ramaditas Complexes, in contrast, experienced medium- and low-pressure upper amphibolite to granulite facies metamorphism, respectively, between ca. 470–460 Ma and were later deformed at ca. 440–420 Ma. The Maz shear zone that bounds the Zaino and Maz Complexes records sinistral oblique to sinistral deformation between ca. 430–410 Ma. The data suggest that at least some units in the MARA terrane were accreted by translation, and the Gondwana margin of northwest Argentina transitioned from a dominantly convergent margin to a highly oblique margin in the Silurian.
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GEUSEBROEK, P. A., and N. A. DUKE. "An Update on the Geology of the Lupin Gold Mine, Nunavut, Canada." Exploration and Mining Geology 13, no. 1-4 (January 1, 2004): 1–13. http://dx.doi.org/10.2113/gsemg.13.1-4.1.
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Abstract The Lupin mine, located in the central Slave province just east of the western boundary of Nunavut Territory, is a world-class example of a Neoarchean-aged banded iron formation (BIF)-hosted lode-gold deposit. At the minesite the gold-mineralized Lupin BIF, separating stratigraphically underlying psammitic wacke and overlying argillaceous turbidite sequences, delineates the Lupin dome, a hammerhead-shaped F2/F3 interference fold structure occurring at the greenschist to amphibolite facies metamorphic transition within the thermal aureole of the Contwoyto batholith. Detailed paragenetic relationships indicate that peak thermal metamorphism coincided with the switch from regional D2 compression to rapid D3 unroofing of the Neoarchean orogenic infrastructure. Gold initially precipitated with pyrrhotite, replacing amphibolitic BIF at the apex of the Lupin deformation zone, separating the east and west lobes of the Contwoyto batholith. Over the course of associated prograde/retrograde metasomatic overprints, gold was further remobilized during garnet and loellingite/arsenopyrite growth in chlorite-altered selvages of late-forming ladder quartz veins. A metamorphic model of ore genesis, with gold being scavenged and transported by metamorphic fluid that was shed and structurally trapped at the amphibolite recrystallization front, is favored over the previously proposed syngenetic and exogenic models of gold concentration that have tended to polarize genetic interpretations to date.
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McFarlane, C. RM, and D. RM Pattison. "Geology of the Matthew Creek metamorphic zone, southeast British Columbia: a window into Middle Proterozoic metamorphism in the Purcell Basin." Canadian Journal of Earth Sciences 37, no. 7 (July 1, 2000): 1073–92. http://dx.doi.org/10.1139/e00-018.
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Southwest of Kimberley, southeastern British Columbia, the Matthew Creek metamorphic zone occupies the core of a structural dome in Mesoproterozoic rocks of the Lower Aldridge formation (lower Purcell Supergroup). It comprises (1) a core zone of ductilely deformed sillimanite-grade metapelites, thin foliated mafic sills, and sheared quartz-plagioclase-tourmaline pegmatites; and (2) a thin transition zone of ductilely deformed metasediments which marks a textural and metamorphic transition between the core zone and overlying regionally extensive, brittlely deformed, biotite-grade semipelitic Lower Aldridge formation metasediments and thick Moyie sills. The core zone and transition zone in combination cover an area of 30 km2. The deepest exposed rocks in the core zone have a strong foliation and lineation (D1 deformation) formed during late M1 metamorphism at conditions of 580650°C and 3.5 ± 0.5 kbar. The timing of this metamorphic-structural episode is constrained to the interval 13521341 Ma based on near-concordant UPb ages from monazite in pelitic schist near the mouth of Matthew Creek. Later, weaker metamorphic and deformation episodes variably overprinted the rocks of the Matthew Creek metamorphic zone. The juxtaposition of low-grade, weakly deformed rocks above high-grade, strongly deformed rocks across a zone of ductile deformation is interpreted to be due to a subhorizontal shear zone.
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Hervé, Francisco, Jorge Lobato, Ignacio Ugalde, and Robert J. Pankhurst. "The geology of Cape Dubouzet, northern Antarctic Peninsula: continental basement to the Trinity Peninsula Group?" Antarctic Science 8, no. 4 (December 1996): 407–14. http://dx.doi.org/10.1017/s0954102096000582.
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Cape Dubouzet is mainly composed of a volcanic-subvolcanic complex of extrusive rhyolitic breccias, a banded rhyolite and a semi-annular body of dacite porphyry rich in xenoliths of metamorphic rocks. Major and REE geochemistry indicate that the volcanic rocks are calc-alkaline and that they are genetically related by fractional crystallization of a plagioclase-bearing assemblage from a common magma. Rb-Sr data suggest that the rhyolitic complex is of Middle-to-Late Jurassic age, and that it is intruded by Late Cretaceous stocks of banded diorite and gabbro. All these rocks are partially covered by moraines whose clasts are of local provenance. Xenoliths in the dacite porphyry suggest that the northern tip of the Antarctic Peninsula is underlain by a metamorphic complex composed of amphibolites, meta-tonalites and pelitic gneiss containing garnet, sillimanite, cordierite, hercynite, and andalucite. Such rocks are not known in the Scotia metamorphic complex, nor in the Trinity Peninsula Group and its low grade metamorphic derivatives, which also occur as rare xenoliths in the dacite. Previous dating of xenoliths collected from the moraines suggested a late Carboniferous age for this amphibolite-grade metamorphism. Both the Jurassic-Cenozoic magmatic arc of the Antarctic Peninsula and the accretionary complex rocks of the Trinity Peninsula Group were thus developed, at least in part, over pre-existing continental crust.
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Kondor, Henrietta, and Tivadar M Tóth. "Contrasting metamorphic and post-metamorphic evolutions within the Algyő basement high (Tisza Mega-unit, SE Hungary). Consequences for structural history." Central European Geology 64, no. 2 (May 29, 2021): 91–112. http://dx.doi.org/10.1556/24.2021.00004.
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AbstractThe Algyő High (AH) is an elevated crystalline block in southeastern Hungary covered by thick Neogene sediments. Although productive hydrocarbon reservoirs are found in these Neogene sequences, numerous fractured reservoirs also occur in the pre-Neogene basement of the Pannonian Basin. Based on these analogies, the rock body of the AH might also play a key role in fluid storage and migration; however, its structure and therefore the reservoir potential is little known. Based on a comprehensive petrologic study in conjunction with analysis of the spatial position of the major lithologies, the AH is considered to have been assembled from blocks with different petrographic features and metamorphic history. The most common lithologies of garnet-kyanite gneiss and mica schist associated with garnetiferous amphibolite are dominant in the northwestern and southeastern parts of the AH. The first regional amphibolite facies metamorphism of the gneiss and mica schist was overprinted by a contact metamorphic (metasomatic) event during decompression in the stability field of kyanite. Garnet-bearing amphibolite experienced amphibolite facies peak conditions comparable with the host gneiss. Regarding the similarities in petrologic features, the northwestern and southeastern parts of the area represent disaggregated blocks of the same rock body. The central part of the AH area is characterized by an epidote gneiss-dominated block metamorphosed along with a greenschist-facies retrograde pathway as well as a chlorite schist-dominated block formed by greenschist-facies progressive metamorphism. The independent evolution of these two blocks is further confirmed by the presence of a propylitic overprint in the chlorite schists. The different metamorphic blocks of the northwestern, southeastern and central parts of the AH probably became juxtaposed along post-metamorphic normal faults developed due to extensional processes. The supposed brittle structural boundaries between the blocks could have provided hydrocarbon migration pathways from the adjacent over-pressured sub-basins, or could even represent suitable reservoirs.
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Dutta, Upama, Ayan Kumar Sarkar, Sadhana M. Chatterjee, Anirban Manna, Alip Roy, and Subhrajyoti Das. "Petrological implications of element redistribution during metamorphism: insights from meta-granite of the South Delhi Fold Belt, Rajasthan, India." Geological Magazine 159, no. 5 (February 23, 2022): 735–60. http://dx.doi.org/10.1017/s0016756821001345.
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AbstractMeta-granites of the South Delhi Fold Belt, northwestern India, contain spectacular reaction textures formed during the metamorphic replacement of primary minerals. Textural relationships imply that amphibole was replaced sequentially in two stages. Epidote + titanite + quartz symplectite formed syn-tectonically on amphibole grain boundaries/fractures, followed by post-deformational growth of euhedral garnet overprinting amphibole grains. Besides occurring as symplectite grown during deformation, titanite in this rock also developed as a post-tectonic corona around magnetite. Parent magnetite contains exsolutions of ilmenite and/or ultrafine lamellae of Ti-rich oxide (Ti-oxd). Textures involving coronal titanite suggest their formation through a magnetite + ilmenite(/Ti-oxd) + plagioclase → titanite reaction. Compositional attributes and the calculation of the gain versus loss of components during the reaction suggest that the Mn2+ for garnet (XSpss = 0.23–0.29) that grew replacing amphibole was supplied by ilmenite (Mn2+ is 0.118–0.128 apfu) as it disintegrated to form coronal titanite. The redistribution of components between the metamorphic reaction sites connects the texturally unrelated domains and suggests that these zones were in chemical equilibrium during metamorphism. We estimated the P–T conditions of metamorphism for these post-tectonic assemblages as ∼650–700 °C from pseudosection modelling and conventional thermometry. Zircon data from this study suggest that the granitic rock crystallized at 988.8 ± 8.8 Ma. We propose that the metamorphic phases replaced the primary minerals during the mid Neoproterozoic tectonic activity reported from this terrane. The syn-tectonic symplectitic assemblage formed as the temperature increased during prograde metamorphism, and the post-tectonic minerals developed at peak conditions following the cessation of deformation.
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Talarico, F. M., E. Stump, B. F. Gootee, K. A. Foland, R. Palmeri, W. R. Van Schmus, P. K. Brand, and C. A. Ricci. "First evidence of a “Barrovian”-type metamorphic regime in the Ross orogen of the Byrd Glacier area, central Transantarctic Mountains." Antarctic Science 19, no. 4 (August 2, 2007): 451–70. http://dx.doi.org/10.1017/s0954102007000594.
Full textAbstract:
AbstractThe Selborne Group comprises two metamorphic rock units, the muscovite±dolomite bearing Madison Marble and the biotite-muscovite±quartz-calcite Contortion Schist, which contains thick lenses of variably deformed metabasalts and metaconglomerates. Petrological and structural data indicate a polyphase metamorphic evolution including: i) an early stage of upper greenschist regional metamorphism (P = ~0.15–0.3 GPa; T = ~380–450°C), ii) prograde metamorphism during D1 up to amphibolite facies peak conditions (P = 0.58–0.8 GPa, T = ~560–645°C), iii) syn-D2 unloading-cooling retrograde metamorphism, iv) a post-D2 contact metamorphic overprint at variable T between 450 and 550°C and ~0.2 GPa connected to the emplacement of granitic plutons and felsic dyke swarms. Geochronological data constrain the polyphase syn-D1/D2 evolution between ~ 510 and 492 Ma. A similar metamorphic path, including a medium P stage but at lower T conditions, is documented in greenschist facies metabasalts within the Byrd Group in the Mount Dick area. The metamorphic pattern and close lithostratigraphic matching between Selborne Group and Byrd Group sharply contrast with the high-grade Horney Formation that is exposed north of the Byrd Glacier and corroborate the hypothesis that the Byrd Glacier discontinuity marks a first-order crustal tectonic boundary crossing the Ross orogen.
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BRÖCKER, MICHAEL, REINER KLEMD, ELLEN KOOIJMAN, JASPER BERNDT, and ALEXANDER LARIONOV. "Zircon geochronology and trace element characteristics of eclogites and granulites from the Orlica-Śnieżnik complex, Bohemian Massif." Geological Magazine 147, no. 3 (November 6, 2009): 339–62. http://dx.doi.org/10.1017/s0016756809990665.
Full textAbstract:
AbstractU–Pb zircon geochronology and trace element analysis was applied to eclogites and (ultra)high-pressure granulites that occur as volumetrically subordinate rock bodies within orthogneisses of the Orlica-Śnieżnik complex, Bohemian Massif. Under favourable circumstances such data may help to unravel protolith ages and yet-undetermined aspects of the metamorphic evolution, for example, the time span over which eclogite-facies conditions were attained. By means of ion-probe and laser ablation techniques, a comprehensive database was compiled for samples collected from prominent eclogite and granulite occurrences. The 206Pb/238U dates for zircons of all samples show a large variability, and no single age can be calculated. The protolith ages remain unresolved due to the lack of coherent age groups at the upper end of the zircon age spectra. The spread in apparent ages is interpreted to be mainly caused by variable and possibly multi-stage Pb-loss. Further complexities are added by metamorphic zircon growth and re-equilibration processes, the unknown relevance of inherited components and possible mixing of different aged domains during analysis. A reliable interpretation of igneous crystallization ages is not yet possible. Previous studies and the new data document the importance of a Carboniferous metamorphic event at c. 340 Ma. The geological significance of this age group is controversial. Such ages have previously either been related to peak (U)HP conditions, the waning stages of eclogite-facies metamorphism or the amphibolite-facies overprint. This study provides new arguments for this discussion because, in both rock types, metamorphic zircon is characterized by very low total REE abundances, flat HREE patterns and the absence of an Eu anomaly. These features strongly suggest contemporaneous crystallization of zircon and garnet and strengthen interpretations proposing that the Carboniferous ages document late-stage eclogite-facies metamorphism, and not amphibolite-facies overprinting.
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Nitkina, E. A., O. A. Belyaev, D. V. Dolivo-Dobrovol’skii, N. E. Kozlov, T. V. Kaulina, and N. E. Kozlova. "Metamorphism of the Korvatundra Structure of the Lapland–Kola Orogen (Arctic Zone of the Fennoscandian Shield)." Russian Geology and Geophysics 63, no. 4 (April 1, 2022): 503–18. http://dx.doi.org/10.2113/rgg20214404.
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Abstract We study the P–T conditions and age of metamorphic evolution of the rocks that make up the Korvatundra structure in the northeast of the Fennoscandian Shield. The rocks underwent progressive metamorphism of the amphibolite facies at 625–660 ºC and 8.7–8.8 kbar 1945 ± 34 Ma (Sm–Nd data). The pegmatite cutting the metamorphic paragenesis that formed at this stage has an age of 1917 ± 6 Ma (zircon U–Pb data). Metamorphic transformations after 1917 Ma are manifested locally as discrete zones of blastomylonites in the rocks of the northern part and some inner sites of the Korvatundra structure. Both local increases and decreases in temperature and pressure are possible in these zones. The formation of light titanite with an age of 1863 ± 44 Ma marks the next stage of shear strain. Low-temperature alterations (chloritization and silicification) took place in the zones of final deformations 1722 ± 5 Ma (Rb–Sr data). Beginning from 1.94 Ga, the general deformational and metamorphic history of the Korvatundra structure, Lapland Granulite Belt, and Tana Belt confirms the assumption of the formation of a single inverted metamorphic zoning within the Korvatundra structure and the overlying Lapland–Kolvitsa Collision Belt in the Paleoproterozoic. The obtained data supplement the idea of the Paleoproterozoic geodynamic evolution of the Lapland–Kola orogen.
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Lamont, Thomas N., Michael P. Searle, David J. Waters, Nick M. W. Roberts, Richard M. Palin, Andrew Smye, Brendan Dyck, Phillip Gopon, Owen M. Weller, and Marc R. St-Onge. "Compressional origin of the Naxos metamorphic core complex, Greece: Structure, petrography, and thermobarometry." GSA Bulletin 132, no. 1-2 (June 4, 2019): 149–97. http://dx.doi.org/10.1130/b31978.1.
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Abstract The island of Naxos, Greece, has been previously considered to represent a Cordilleran-style metamorphic core complex that formed during Cenozoic extension of the Aegean Sea. Although lithospheric extension has undoubtedly occurred in the region since 10 Ma, the geodynamic history of older, regional-scale, kyanite- and sillimanite-grade metamorphic rocks exposed within the core of the Naxos dome is controversial. Specifically, little is known about the pre-extensional prograde evolution and the relative timing of peak metamorphism in relation to the onset of extension. In this work, new structural mapping is presented and integrated with petrographic analyses and phase equilibrium modeling of blueschists, kyanite gneisses, and anatectic sillimanite migmatites. The kyanite-sillimanite–grade rocks within the core complex record a complex history of burial and compression and did not form under crustal extension. Deformation and metamorphism were diachronous and advanced down the structural section, resulting in the juxtaposition of several distinct tectono-stratigraphic nappes that experienced contrasting metamorphic histories. The Cycladic Blueschists attained ∼14.5 kbar and 470 °C during attempted northeast-directed subduction of the continental margin. These were subsequently thrusted onto the more proximal continental margin, resulting in crustal thickening and regional metamorphism associated with kyanite-grade conditions of ∼10 kbar and 600–670 °C. With continued shortening, the deepest structural levels underwent kyanite-grade hydrous melting at ∼8–10 kbar and 680–750 °C, followed by isothermal decompression through the muscovite dehydration melting reaction to sillimanite-grade conditions of ∼5–6 kbar and 730 °C. This decompression process was associated with top-to-the-NNE shearing along passive-roof faults that formed because of SW-directed extrusion. These shear zones predated crustal extension, because they are folded around the migmatite dome and are crosscut by leucogranites and low-angle normal faults. The migmatite dome formed at lower-pressure conditions under horizontal constriction that caused vertical boudinage and upright isoclinal folds. The switch from compression to extension occurred immediately following doming and was associated with NNE-SSW horizontal boudinage and top-to-the-NNE brittle-ductile normal faults that truncate the internal shear zones and earlier collisional features. The Naxos metamorphic core complex is interpreted to have formed via crustal thickening, regional metamorphism, and partial melting in a compressional setting, here termed the Aegean orogeny, and it was exhumed from the midcrust due to the switch from compression to extension at ca. 15 Ma.
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Breitfeld, H. Tim, and Robert Hall. "Metamorphic rocks of the Kuching Zone Sarawak: Comment on Najiatun Najla Mohamad et al. (2020) The geology and stratigraphic framework of the Kuching Zone Sarawak: Current understanding and unresolved issues." Warta Geologi 47, no. 2 (August 30, 2021): 126–27. http://dx.doi.org/10.7186/wg472202104.
Full textAbstract:
Metamorphic rocks of Sarawak have been dated and are not Upper Carboniferous or older rocks nor are they correlatives of the Pinoh Metamorphics of Kalimantan. Two newly-dated rocks are Triassic and are named the West Sarawak Metamorphics and a third sample is Cretaceous.
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Itano, Keita, Kenta Ueki, Tsuyoshi Iizuka, and Tatsu Kuwatani. "Geochemical Discrimination of Monazite Source Rock Based on Machine Learning Techniques and Multinomial Logistic Regression Analysis." Geosciences 10, no. 2 (February 6, 2020): 63. http://dx.doi.org/10.3390/geosciences10020063.
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Detrital monazite geochronology has been used in provenance studies. However, there are complexities in the interpretation of age spectra due to their wide occurrence in both igneous and metamorphic rocks. We use the multinomial logistic regression (MLR) and cross-validation (CV) techniques to establish a geochemical discrimination of monazite source rocks. The elemental abundance-based geochemical discrimination was tested by selecting 16 elements from granitic and metamorphic rocks. The MLR technique revealed that light rare earth elements (REEs), Eu, and some heavy REEs are important discriminators that reflect elemental fractionation during magmatism and/or metamorphism. The best model yielded a discrimination rate of ~97%, and the CV method validated this approach. We applied the discrimination model to detrital monazites from African rivers. The detrital monazites were mostly classified as granitic and of garnet-bearing metamorphic origins; however, their proportion of metamorphic origin was smaller than the proportion that was obtained by using the elemental-ratio-based discrimination proposed by Itano et al. in Chemical Geology (2018). Considering the occurrence of metamorphic rocks in the hinterlands and the different age spectra between monazite and zircon in the same rivers, a ratio-based discrimination would be more reliable. Nevertheless, our study demonstrates the advantages of machine-learning-based approaches for the quantitative discrimination of monazite.
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Taylor, Richard J. M., Tim E. Johnson, Chris Clark, and Richard J. Harrison. "Persistence of melt-bearing Archean lower crust for >200 m.y.—An example from the Lewisian Complex, northwest Scotland." Geology 48, no. 3 (December 17, 2019): 221–25. http://dx.doi.org/10.1130/g46834.1.
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Abstract Geochronological data from zircon in Archean tonalite–trondhjemite–granodiorite (TTG) gneisses are commonly difficult to interpret. A notable example is the TTG gneisses from the Lewisian Gneiss Complex, northwest Scotland, which have metamorphic zircon ages that define a more-or-less continuous spread through the Neoarchean, with no clear relationship to zircon textures. These data are generally interpreted to record discrete high-grade events at ca. 2.7 Ga and ca. 2.5 Ga, with intermediate ages reflecting variable Pb loss. Although ancient diffusion of Pb is commonly invoked to explain such protracted age spreads, trace-element data in zircon may permit identification of otherwise cryptic magmatic and metamorphic episodes. Although zircons from the TTG gneiss analyzed here show a characteristic spread of Neoarchean ages, they exhibit subtle but key step changes in trace-element compositions that are difficult to ascribe to diffusive resetting, but that are consistent with emplacement of regionally extensive bodies of mafic magma. These data suggest suprasolidus metamorphic temperatures persisted for 200 m.y. or more during the Neoarchean. Such long-lived high-grade metamorphism is supported by data from zircon grains from a nearby monzogranite sheet. These preserve distinctive trace-element compositions consistent with derivation from a mafic source, and they define a well-constrained U-Pb zircon age of ca. 2.6 Ga that is intermediate between the two previously proposed discrete metamorphic episodes. The persistence of melt-bearing lower crust for hundreds of millions of years was probably the norm during the Archean.
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“Beraki, W. H., F. F. Bonavia, T. Getachew, R. Schmerold, and T. Tarekegn. "The Adola Fold and Thrust Belt, southern Ethiopia: a re-examination with implications for Pan-African evolution." Geological Magazine 126, no. 6 (November 1989): 647–57. http://dx.doi.org/10.1017/s0016756800006944.
Full textAbstract:
AbstractThe Adola Fold and Thrust Belt of Ethiopia is a late- Precambrian, north–south trending belt of volcanic-sedimentary and ophiolite–like units overlying ‘basement rocks’ (gneisses and granitic gneisses). Detailed structural analysis and data from microfabrics have documented two thrusting events (D1, D5) and three folding phases (F2, F3, F4). All deformations have affected both the rocks of the Adola Fold and Thrust Belt and the ‘older basement’. The structural history is recorded as follows: (1) formation of ductile shear zones (Dl); (2) progressive development of recumbent folds (F2); (3) a superimposed north-south trending, tight to isoclinal, upright penetrative folding phase (F3), which imparted the dominant structure of the region; (4) F3 was later gently affected, at right angles to F3, by the last folding event (F4). Metamorphic conditions during this deformational cycle reached highest greenschist to medium amphibolite facies conditions. Close to major thrust contacts imbrication produced metamorphic discontinuities. Marked retrograde metamorphism in narrow horizons indicates a continuation of minor movements along the major thrusts, postdating the metamorphic peak. Evidence of a second thrusting/faulting episode (D5) is only recorded at the mesoscale. This latter event was accompanied by very low to low grade metamorphic conditions. In accepting the previously reported age of 1030±40 Ma for the volcanic–sedimentary unit of the Adola Fold and Thrust Belt, and the interpretation of these units as an immature island arc, a possibly early Pan-African oceanic accretion is postulated.
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Zi, Jian-Wei, Birger Rasmussen, Janet R. Muhling, Wolfgang D. Maier, and Ian R. Fletcher. "U-Pb monazite ages of the Kabanga mafic-ultramafic intrusions and contact aureoles, central Africa: Geochronological and tectonic implications." GSA Bulletin 131, no. 11-12 (April 15, 2019): 1857–70. http://dx.doi.org/10.1130/b35142.1.
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AbstractMafic-ultramafic rocks of the Kabanga-Musongati alignment in the East African nickel belt occur as Bushveld-type layered intrusions emplaced in metasedimentary sequences. The age of the mafic-ultramafic intrusions remains poorly constrained, though they are regarded to be part of ca. 1375 Ma bimodal magmatism dominated by voluminous S-type granites. In this study, we investigated igneous monazite and zircon from a differentiated layered intrusion and metamorphic monazite from the contact aureole. The monazite shows contrasting crystal morphology, chemical composition, and U-Pb ages. Monazite that formed by contact metamorphism in response to emplacement of mafic-ultramafic melts is characterized by extremely high Th and U and yielded a weighted mean 207Pb/206Pb age of 1402 ± 9 Ma, which is in agreement with dates from the igneous monazite and zircon. The ages indicate that the intrusion of ultramafic melts was substantially earlier (by ∼25 m.y., 95% confidence) than the prevailing S-type granites, calling for a reappraisal of the previously suggested model of coeval, bimodal magmatism. Monazite in the metapelitic rocks also records two younger growth events at ca. 1375 Ma and ca. 990 Ma, coeval with metamorphism during emplacement of S-type granites and tin-bearing granites, respectively. In conjunction with available geologic evidence, we propose that the Kabanga-Musongati mafic-ultramafic intrusions likely heralded a structurally controlled thermal anomaly related to Nuna breakup, which culminated during the ca. 1375 Ma Kibaran event, manifested as extensive intracrustal melting in the adjoining Karagwe-Ankole belt, producing voluminous S-type granites. The Grenvillian-aged (ca. 990 Ma) tin-bearing granite and related Sn mineralization appear to be the far-field record of tectonothermal events associated with collision along the Irumide belt during Rodinia assembly. Since monazite is a ubiquitous trace phase in pelitic sedimentary rocks, in contact aureoles of mafic-ultramafic intrusions, and in regional metamorphic belts, our study highlights the potential of using metamorphic monazite to determine ages of mafic-ultramafic intrusions, and to reconstruct postemplacement metamorphic history of the host terranes.
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Searle, Michael P., and Thomas N. Lamont. "Compressional metamorphic core complexes, low-angle normal faults and extensional fabrics in compressional tectonic settings." Geological Magazine 157, no. 1 (April 2, 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.