Samuele Papeschi, Federico Rossetti, Jesse B. Walters
Chloritoid and kyanite coexist in metapelites from the high-pressure/low-temperature Massa Unit in the Alpi Apuane metamorphic complex (Northern Apennines, Italy). The composition of chloritoid is extremely variable throughout the Massa Unit. Fe-chloritoid occurs in association with hematite-free, graphite-bearing schists, whereas strongly zoned Fe-Mg chloritoid is found with hematite and kyanite. We investigated the effect of different bulk Fe2O3 contents in controlling chloritoid composition through phase equilibria modelling of four selected samples, representative of the different chloritoid-bearing parageneses found in the Massa Unit. The ferric iron content, measured through wet chemical titration, ranges from 0 (graphite-chloritoid schist) to 73% of the total iron (hematite-chloritoid schist). We show that Mg-rich chloritoid compositions and stability of kyanite at greenschist to blueschist facies conditions can be reproduced in the MnO–Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (MnNKFMASHTO) chemical system only considering the presence of significant amounts of ferric iron as part of the bulk composition. The stabilization of kyanite at lower grade is directly linked to the presence of Fe2O3, which renders the reactive bulk rock composition effectively enriched in Al2O3 with respect to Fe and Mg. We also document that high Fe2O3 contents exacerbate the effect of chloritoid fractionation, producing strongly zoned Fe-Mg-chloritoid grains. Finally, the P–T modelling of the Massa Units performed in this study allows, for the first time, the recognition of a two-stage evolution at peak conditions, with an earlier pressure peak (1.2–1.3 GPa at 350–400°C), and a later thermal peak (0.7–1.1 GPa at 440–480°C), compatible with subduction, underthrusting and exhumation of the Adria continental margin during growth of the Northern Apennine orogenic wedge.
{"title":"Growth of kyanite and Fe-Mg chloritoid in Fe2O3-rich high-pressure–low-temperature metapelites and metapsammites: A case study from the Massa Unit (Alpi Apuane, Italy)","authors":"Samuele Papeschi, Federico Rossetti, Jesse B. Walters","doi":"10.1111/jmg.12736","DOIUrl":"10.1111/jmg.12736","url":null,"abstract":"<p>Chloritoid and kyanite coexist in metapelites from the high-pressure/low-temperature Massa Unit in the Alpi Apuane metamorphic complex (Northern Apennines, Italy). The composition of chloritoid is extremely variable throughout the Massa Unit. Fe-chloritoid occurs in association with hematite-free, graphite-bearing schists, whereas strongly zoned Fe-Mg chloritoid is found with hematite and kyanite. We investigated the effect of different bulk Fe<sub>2</sub>O<sub>3</sub> contents in controlling chloritoid composition through phase equilibria modelling of four selected samples, representative of the different chloritoid-bearing parageneses found in the Massa Unit. The ferric iron content, measured through wet chemical titration, ranges from 0 (graphite-chloritoid schist) to 73% of the total iron (hematite-chloritoid schist). We show that Mg-rich chloritoid compositions and stability of kyanite at greenschist to blueschist facies conditions can be reproduced in the MnO–Na<sub>2</sub>O–K<sub>2</sub>O–FeO–MgO–Al<sub>2</sub>O<sub>3</sub>–SiO<sub>2</sub>–H<sub>2</sub>O–TiO<sub>2</sub>–O (MnNKFMASHTO) chemical system only considering the presence of significant amounts of ferric iron as part of the bulk composition. The stabilization of kyanite at lower grade is directly linked to the presence of Fe<sub>2</sub>O<sub>3</sub>, which renders the reactive bulk rock composition effectively enriched in Al<sub>2</sub>O<sub>3</sub> with respect to Fe and Mg. We also document that high Fe<sub>2</sub>O<sub>3</sub> contents exacerbate the effect of chloritoid fractionation, producing strongly zoned Fe-Mg-chloritoid grains. Finally, the P–T modelling of the Massa Units performed in this study allows, for the first time, the recognition of a two-stage evolution at peak conditions, with an earlier pressure peak (1.2–1.3 GPa at 350–400°C), and a later thermal peak (0.7–1.1 GPa at 440–480°C), compatible with subduction, underthrusting and exhumation of the Adria continental margin during growth of the Northern Apennine orogenic wedge.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 8","pages":"1049-1079"},"PeriodicalIF":3.4,"publicationDate":"2023-05-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12736","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41583070","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The integration of garnet-based petrologic constraints with multimineral geochronologic data in eclogites and blueschists allows the timing and rate of subduction zone metamorphism to be constrained. We present a combined garnet Lu–Hf/Sm–Nd and zircon/rutile U–Pb geochronology study on three eclogites, a garnet-bearing blueschist, and a micaschist from the Changning–Menglian orogenic belt, a newly discovered ultrahigh-pressure metamorphic belt in southeast Tibet, in order to characterize tectono-metamorphic events and determine the duration of Paleo-Tethys oceanic subduction. Integration of phase equilibrium modelling and conventional thermobarometry for the eclogites defines a clockwise P–T path evolving from blueschist facies conditions at ~1.4 GPa and ~505–530°C to peak eclogite facies conditions at ~2.8 GPa and ~630–640°C, followed by isothermal decompression to amphibolite facies at ~1.0 GPa and ~630–650°C. The Lu–Hf ages of c. 239–236 Ma obtained for the eclogites and the blueschist are indistinguishable from the rutile U–Pb age of c. 239 Ma obtained for the eclogites and, combined with the observation of well-preserved Rayleigh-fractionation-style Mn and Lu zoning profiles in garnet, reflect the timing of early prograde garnet growth. The Sm–Nd ages of c. 242–236 Ma reflect a later period of garnet growth, evidenced by flat and/or M-shaped Sm zoning profiles. Each of the Sm–Nd ages overlaps, within uncertainty, with its corresponding Lu–Hf age (i.e., from the same garnet fraction). The consistency of the Lu–Hf and Sm–Nd ages indicates a short overall duration of garnet growth from blueschist to eclogite facies metamorphism, reflecting rapid subduction of the oceanic slab. The magmatic zircon U–Pb dates of c. 247 Ma constrain the protolith age of these metabasaltic rocks. The close protolith and the high-pressure metamorphic ages, together with the consistent garnet Lu–Hf and Sm–Nd ages and the overlapping youngest and oldest metamorphic ages of the oceanic-type and continental-type eclogites, respectively, suggest a fast tectonic transition from divergence to convergence highlighted by rapid oceanic subduction, continuous transition from oceanic to continental subduction, and a rapid cooling of the subduction interface.
{"title":"Consistent garnet Lu–Hf and Sm–Nd ages indicate short-lived high-pressure metamorphism and rapid subduction in oceanic subduction belt: An example from the Changning–Menglian orogenic belt, southeastern Tibetan Plateau","authors":"Ying Zhou, Hao Cheng, Zhi-min Peng, Besim Dragovic, Yu-zhen Fu, Kai-Yang Du","doi":"10.1111/jmg.12735","DOIUrl":"10.1111/jmg.12735","url":null,"abstract":"<p>The integration of garnet-based petrologic constraints with multimineral geochronologic data in eclogites and blueschists allows the timing and rate of subduction zone metamorphism to be constrained. We present a combined garnet Lu–Hf/Sm–Nd and zircon/rutile U–Pb geochronology study on three eclogites, a garnet-bearing blueschist, and a micaschist from the Changning–Menglian orogenic belt, a newly discovered ultrahigh-pressure metamorphic belt in southeast Tibet, in order to characterize tectono-metamorphic events and determine the duration of Paleo-Tethys oceanic subduction. Integration of phase equilibrium modelling and conventional thermobarometry for the eclogites defines a clockwise <i>P–T</i> path evolving from blueschist facies conditions at ~1.4 GPa and ~505–530°C to peak eclogite facies conditions at ~2.8 GPa and ~630–640°C, followed by isothermal decompression to amphibolite facies at ~1.0 GPa and ~630–650°C. The Lu–Hf ages of c. 239–236 Ma obtained for the eclogites and the blueschist are indistinguishable from the rutile U–Pb age of c. 239 Ma obtained for the eclogites and, combined with the observation of well-preserved Rayleigh-fractionation-style Mn and Lu zoning profiles in garnet, reflect the timing of early prograde garnet growth. The Sm–Nd ages of c. 242–236 Ma reflect a later period of garnet growth, evidenced by flat and/or M-shaped Sm zoning profiles. Each of the Sm–Nd ages overlaps, within uncertainty, with its corresponding Lu–Hf age (i.e., from the same garnet fraction). The consistency of the Lu–Hf and Sm–Nd ages indicates a short overall duration of garnet growth from blueschist to eclogite facies metamorphism, reflecting rapid subduction of the oceanic slab. The magmatic zircon U–Pb dates of c. 247 Ma constrain the protolith age of these metabasaltic rocks. The close protolith and the high-pressure metamorphic ages, together with the consistent garnet Lu–Hf and Sm–Nd ages and the overlapping youngest and oldest metamorphic ages of the oceanic-type and continental-type eclogites, respectively, suggest a fast tectonic transition from divergence to convergence highlighted by rapid oceanic subduction, continuous transition from oceanic to continental subduction, and a rapid cooling of the subduction interface.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 8","pages":"1031-1047"},"PeriodicalIF":3.4,"publicationDate":"2023-05-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47703195","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Luc de Hoÿm de Marien, Pavel Pitra, Marc Poujol, Nathan Cogné, Florence Cagnard, Benjamin Le Bayon
Two eclogite samples from the Haut-Allier record a prograde evolution from ~20 kbar, 650°C to 750°C, 22–23 kbar followed by heating up to 850–875°C and partial melting. Incipient decompression in high-pressure granulite facies conditions (19.5 kbar, 875°C) was followed by exhumation to high-temperature amphibolite facies conditions (<9 kbar, 750–850°C). Following a detailed geochemical, petrological, and geochronological investigation using trace-element data and laser ablation inductively coupled plasma mass spectrometry U–Pb dating of zircon, apatite, and rutile, the eclogites reveal an Ordovician (c. 490 Ma) rifting event followed by Devonian (c. 370–360 Ma) subduction and Carboniferous (c. 350 Ma) exhumation in this part of the French Massif Central. The previously proposed Silurian age for the subduction, which strongly influenced many tectonic models, is definitively rejected. In the light of other geological data from the French Massif Central, including the lithological and geochemical zoning of calc-alkaline Devonian volcanism, we propose a southward polarity of the subduction and question the very existence of the so-called Massif Central Ocean. Furthermore, we infer that following subduction, the eclogites were relaminated to the upper plate and exhumed at the rear of the magmatic arc pointing to similarities with the geodynamics of the Bohemian Massif.
The petrochronological record of zircon is particularly complex. Metamorphic zircon with clear eclogitic rare-earth elements patterns (no Eu anomaly and flat heavy rare-earth elements) and inclusions (garnet, rutile, and omphacite) shows concordant apparent ages that spread from c. 380 down to c. 310 Ma. This apparent age pattern strongly contrasts with the well-defined age of apatite and rutile of c. 350 Ma. Apparent zircon ages younger than 350 Ma unequivocally testify that zircon can recrystallize outside the conditions of the eclogite facies, which resets the U–Pb while preserving an apparent eclogitic signature. Local fractures filled by analcite, thomsonite, plagioclase, and biotite testify to late interaction of the eclogites with alkaline fluids at relatively low temperatures. This interaction, possibly at c. 310 Ma or later, could lead to the recrystallization of zircon while leaving apatite unaffected.
{"title":"Complex geochronological record of an emblematic Variscan eclogite (Haut-Allier, French Massif Central)","authors":"Luc de Hoÿm de Marien, Pavel Pitra, Marc Poujol, Nathan Cogné, Florence Cagnard, Benjamin Le Bayon","doi":"10.1111/jmg.12733","DOIUrl":"10.1111/jmg.12733","url":null,"abstract":"<p>Two eclogite samples from the Haut-Allier record a prograde evolution from ~20 kbar, 650°C to 750°C, 22–23 kbar followed by heating up to 850–875°C and partial melting. Incipient decompression in high-pressure granulite facies conditions (19.5 kbar, 875°C) was followed by exhumation to high-temperature amphibolite facies conditions (<9 kbar, 750–850°C). Following a detailed geochemical, petrological, and geochronological investigation using trace-element data and laser ablation inductively coupled plasma mass spectrometry U–Pb dating of zircon, apatite, and rutile, the eclogites reveal an Ordovician (c. 490 Ma) rifting event followed by Devonian (c. 370–360 Ma) subduction and Carboniferous (c. 350 Ma) exhumation in this part of the French Massif Central. The previously proposed Silurian age for the subduction, which strongly influenced many tectonic models, is definitively rejected. In the light of other geological data from the French Massif Central, including the lithological and geochemical zoning of calc-alkaline Devonian volcanism, we propose a southward polarity of the subduction and question the very existence of the so-called Massif Central Ocean. Furthermore, we infer that following subduction, the eclogites were relaminated to the upper plate and exhumed at the rear of the magmatic arc pointing to similarities with the geodynamics of the Bohemian Massif.</p><p>The petrochronological record of zircon is particularly complex. Metamorphic zircon with clear eclogitic rare-earth elements patterns (no Eu anomaly and flat heavy rare-earth elements) and inclusions (garnet, rutile, and omphacite) shows concordant apparent ages that spread from c. 380 down to c. 310 Ma. This apparent age pattern strongly contrasts with the well-defined age of apatite and rutile of c. 350 Ma. Apparent zircon ages younger than 350 Ma unequivocally testify that zircon can recrystallize outside the conditions of the eclogite facies, which resets the U–Pb while preserving an apparent eclogitic signature. Local fractures filled by analcite, thomsonite, plagioclase, and biotite testify to late interaction of the eclogites with alkaline fluids at relatively low temperatures. This interaction, possibly at c. 310 Ma or later, could lead to the recrystallization of zircon while leaving apatite unaffected.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 7","pages":"967-995"},"PeriodicalIF":3.4,"publicationDate":"2023-05-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12733","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48211930","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Jack James Percival, Jiří Konopásek, Pedro Oyhantçabal, Jiří Sláma, Robert Anczkiewicz
The Dom Feliciano Belt of southern Brazil and Uruguay represents part of a larger Neoproterozoic orogenic system formed during the amalgamation of Western Gondwana. The hinterland and foreland domains in parts of the belt preserve deformation structures and metamorphic assemblages that developed during early crustal thickening from c. 650 Ma. However, the metamorphic history of the southern foreland, in Uruguay, and its relationship with the hinterland, is not so well understood. We show that metamorphism in the southern hinterland is characterized by near-isothermal decompression from ~10 kbar (~770°C) down to ~6 kbar, reflecting exhumation from depths of ~40 km during convergent thrusting and crustal thickening. This metamorphic event and associated magmatism is constrained by garnet Lu–Hf and zircon U–Pb dating to c. 655–640 Ma, supporting age and P–T constraints from previous studies. In contrast, prograde metamorphism in the foreland supracrustal rocks reached maximum lower-amphibolite facies conditions (~6–7 kbar and ~550–570°C) and is constrained by garnet Lu–Hf dating to 582 ± 23 Ma. An exposed sheet of imbricated foreland basement rocks reached partial melting at upper-amphibolite facies conditions, and metamorphism is similarly constrained to c. 585–570 Ma by monazite U–Pb dating. The data indicate that metamorphism in the foreland occurred during a sinistral transpressional event c. 55–85 Ma after the start of crustal thickening recorded in the hinterland, whereby strain partitioning during sinistral transpression led to imbrication in the foreland and oblique thrusting of the basement over more distal supracrustal rocks. This event is coeval with transpressional deformation in the Kaoko and Gariep belts, indicating a distinct two-stage tectonic history driven by the three-way convergence between the Congo, Kalahari, and South American cratons.
{"title":"Garnet growth and mineral geochronology constrains the diachronous Neoproterozoic convergent evolution of the southern Dom Feliciano Belt, Uruguay","authors":"Jack James Percival, Jiří Konopásek, Pedro Oyhantçabal, Jiří Sláma, Robert Anczkiewicz","doi":"10.1111/jmg.12734","DOIUrl":"10.1111/jmg.12734","url":null,"abstract":"<p>The Dom Feliciano Belt of southern Brazil and Uruguay represents part of a larger Neoproterozoic orogenic system formed during the amalgamation of Western Gondwana. The hinterland and foreland domains in parts of the belt preserve deformation structures and metamorphic assemblages that developed during early crustal thickening from c. 650 Ma. However, the metamorphic history of the southern foreland, in Uruguay, and its relationship with the hinterland, is not so well understood. We show that metamorphism in the southern hinterland is characterized by near-isothermal decompression from ~10 kbar (~770°C) down to ~6 kbar, reflecting exhumation from depths of ~40 km during convergent thrusting and crustal thickening. This metamorphic event and associated magmatism is constrained by garnet Lu–Hf and zircon U–Pb dating to c. 655–640 Ma, supporting age and P–T constraints from previous studies. In contrast, prograde metamorphism in the foreland supracrustal rocks reached maximum lower-amphibolite facies conditions (~6–7 kbar and ~550–570°C) and is constrained by garnet Lu–Hf dating to 582 ± 23 Ma. An exposed sheet of imbricated foreland basement rocks reached partial melting at upper-amphibolite facies conditions, and metamorphism is similarly constrained to c. 585–570 Ma by monazite U–Pb dating. The data indicate that metamorphism in the foreland occurred during a sinistral transpressional event c. 55–85 Ma after the start of crustal thickening recorded in the hinterland, whereby strain partitioning during sinistral transpression led to imbrication in the foreland and oblique thrusting of the basement over more distal supracrustal rocks. This event is coeval with transpressional deformation in the Kaoko and Gariep belts, indicating a distinct two-stage tectonic history driven by the three-way convergence between the Congo, Kalahari, and South American cratons.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 7","pages":"997-1030"},"PeriodicalIF":3.4,"publicationDate":"2023-05-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12734","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44265544","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Yi-Peng Li, Lin Ding, Alexander C. Robinson, De-Liang Liu, Jing Xie, Li-Yun Zhang, Chen-Yuan Zhao, Ya-Hui Yue, Yiduo Liu, Ilhomjon Oimuhammadzoda, Mustafo Gadoev, Negmat Rajabov
One of the most striking geological features of the Pamir is the south-dipping lithospheric slab beneath the orogen characterized by an intracontinental Wadati-Benioff zone. A widely accepted hypothesis over the past 40 years interprets the slab to represent southward subducted cratonic Asian continental lithosphere, which predicts significant cratonic Asia-sourced crustal materials (e.g., Tarim Basin) beneath the Pamir. Alternatively, recent studies have interpreted the slab to be lithosphere delaminated from the base of the Pamir. To test these hypotheses, depth–tectonic affinity relations of crustal xenoliths carried by Miocene volcanic rocks in the eastern Pamir, interpreted to be sourced from the Pamir deep lithosphere, are used to determine whether they represented Asian affinity cratonic crust. Thermodynamic calculations, zircon U–Pb geochronology combined with rare earth element analysis, and whole-rock major-trace element and Sr–Nd isotopic analyses document that (1) eclogite and pyroxenite xenoliths (~31–43 kbar/~960–1170°C) are the deepest sourced portions of the lithosphere from ~100 to 140 km depth, the protoliths of which represent the mid-lower crustal rocks of the Cretaceous Pamir magmatic arc, rather than material from cratonic Asia, and (2) granulite xenoliths (~20 kbar/~900°C) represent the Cenozoic lower crustal rocks of Pamir terranes from ~70 km depth. These results indicate the south-dipping slab represents delaminated Pamir lower crust and mantle lithosphere, rather than intracontinental subduction of Asian lithosphere, and further support the hypothesis of minimal Cenozoic northward translation of the Pamir.
{"title":"Was cratonic Asia deeply subducted beneath the Pamir? Evidence from P–T conditions and tectonic affinities of Cenozoic Pamir crustal xenoliths","authors":"Yi-Peng Li, Lin Ding, Alexander C. Robinson, De-Liang Liu, Jing Xie, Li-Yun Zhang, Chen-Yuan Zhao, Ya-Hui Yue, Yiduo Liu, Ilhomjon Oimuhammadzoda, Mustafo Gadoev, Negmat Rajabov","doi":"10.1111/jmg.12723","DOIUrl":"10.1111/jmg.12723","url":null,"abstract":"<p>One of the most striking geological features of the Pamir is the south-dipping lithospheric slab beneath the orogen characterized by an intracontinental Wadati-Benioff zone. A widely accepted hypothesis over the past 40 years interprets the slab to represent southward subducted cratonic Asian continental lithosphere, which predicts significant cratonic Asia-sourced crustal materials (e.g., Tarim Basin) beneath the Pamir. Alternatively, recent studies have interpreted the slab to be lithosphere delaminated from the base of the Pamir. To test these hypotheses, depth–tectonic affinity relations of crustal xenoliths carried by Miocene volcanic rocks in the eastern Pamir, interpreted to be sourced from the Pamir deep lithosphere, are used to determine whether they represented Asian affinity cratonic crust. Thermodynamic calculations, zircon U–Pb geochronology combined with rare earth element analysis, and whole-rock major-trace element and Sr–Nd isotopic analyses document that (1) eclogite and pyroxenite xenoliths (~31–43 kbar/~960–1170°C) are the deepest sourced portions of the lithosphere from ~100 to 140 km depth, the protoliths of which represent the mid-lower crustal rocks of the Cretaceous Pamir magmatic arc, rather than material from cratonic Asia, and (2) granulite xenoliths (~20 kbar/~900°C) represent the Cenozoic lower crustal rocks of Pamir terranes from ~70 km depth. These results indicate the south-dipping slab represents delaminated Pamir lower crust and mantle lithosphere, rather than intracontinental subduction of Asian lithosphere, and further support the hypothesis of minimal Cenozoic northward translation of the Pamir.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 7","pages":"925-965"},"PeriodicalIF":3.4,"publicationDate":"2023-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12723","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47860595","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Changqing Yin, Xiaoman Wang, Shoufa Lin, Chris G. Couëslan, Christian O. Böhm, Jiahui Qian, Jian Zhang, Yanfei Xia, Wenxuan Li
The Pikwitonei Granulite Domain (PGD), located in the northwestern Superior Province, is one of the largest Neoarchean high-grade metamorphic domains in the world, and is a key to understanding the Neoarchean crustal evolution of the Superior Province. Here we report results of a study on ultrahigh temperature (UHT) granulites with a sapphirine + quartz-bearing peak assemblage from the Sipiwesk Lake area in the PGD. Three stages of metamorphic assemblage development are recognized based on petrographic observations: Pre-peak stage is marked by garnet, sillimanite, K-feldspar, and biotite inclusions within sapphirine and orthopyroxene. Peak stage is charactered by the typical UHT associations of sapphirine + quartz and sapphirine + orthopyroxene. The retrograde stage is represented by the retrograde formation of cordierite, biotite and sillimanite in the matrix. Phase equilibrium modelling based on the bulk compositions of sapphirine-bearing granulites suggest that the rocks have experienced extensional UHT metamorphism in excess of 1035°C at pressures of 7.9–9.0 kbar, followed by isobaric cooling process. SHRIMP U–Pb dating of metamorphic zircons in sapphirine-bearing granulite record the timing of the UHT event in the PGD with a weighted mean 207Pb/206Pb age of 2681 ± 13 Ma. This study in combination with other metamorphic P–T paths and age information reveals that this UHT metamorphism at 2.68 Ga in PGD was generated by upwelling asthenosphere due to slab break-off in collision during the amalgamation of the Superior Province.
{"title":"Archean ultrahigh temperature event in the northwestern Superior Province, Canada: Metamorphic evolution and tectonic implications of the Pikwitonei Granulite Domain","authors":"Changqing Yin, Xiaoman Wang, Shoufa Lin, Chris G. Couëslan, Christian O. Böhm, Jiahui Qian, Jian Zhang, Yanfei Xia, Wenxuan Li","doi":"10.1111/jmg.12722","DOIUrl":"10.1111/jmg.12722","url":null,"abstract":"<p>The Pikwitonei Granulite Domain (PGD), located in the northwestern Superior Province, is one of the largest Neoarchean high-grade metamorphic domains in the world, and is a key to understanding the Neoarchean crustal evolution of the Superior Province. Here we report results of a study on ultrahigh temperature (UHT) granulites with a sapphirine + quartz-bearing peak assemblage from the Sipiwesk Lake area in the PGD. Three stages of metamorphic assemblage development are recognized based on petrographic observations: Pre-peak stage is marked by garnet, sillimanite, K-feldspar, and biotite inclusions within sapphirine and orthopyroxene. Peak stage is charactered by the typical UHT associations of sapphirine + quartz and sapphirine + orthopyroxene. The retrograde stage is represented by the retrograde formation of cordierite, biotite and sillimanite in the matrix. Phase equilibrium modelling based on the bulk compositions of sapphirine-bearing granulites suggest that the rocks have experienced extensional UHT metamorphism in excess of 1035°C at pressures of 7.9–9.0 kbar, followed by isobaric cooling process. SHRIMP U–Pb dating of metamorphic zircons in sapphirine-bearing granulite record the timing of the UHT event in the PGD with a weighted mean <sup>207</sup>Pb/<sup>206</sup>Pb age of 2681 ± 13 Ma. This study in combination with other metamorphic P–T paths and age information reveals that this UHT metamorphism at 2.68 Ga in PGD was generated by upwelling asthenosphere due to slab break-off in collision during the amalgamation of the Superior Province.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 7","pages":"899-923"},"PeriodicalIF":3.4,"publicationDate":"2023-04-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43299671","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Melt inclusions (MIs) in high-temperature metamorphic rocks provide a unique window into crustal anatexis in collisional orogenic belts and have been widely used to characterize compositions of anatectic melts as well as melting mechanisms. In this study, MIs hosted by peritectic garnet were for the first time identified in an Al2SiO5-free graywacke-type paragneiss from the Namche Barwa Complex, the Eastern Himalaya, Southeast Tibet. These MIs occur as nanogranites in the rims of porphyroblastic garnet, exhibit negative crystal shapes with an average diameter of ~12 μm and consist of a mineral assemblage of biotite + quartz + plagioclase + K-feldspar ± muscovite. Re-homogenization experiments of these nanogranites were conducted at a pressure of 1.5 GPa and temperatures of 800°C, 850°C and 900°C and produced homogeneous glasses at 850°C. The homogenized glasses are strongly peraluminous and calc-alkalic in composition, with 66.43–71.31 wt.% SiO2, 12.64–15.06 wt.% Al2O3, high alkaline (5.41–7.22 wt.%) and low ferromagnesian (2.72–4.46 wt.%) contents. They are lower in silica and CaO but higher in K2O compared with MI produced by fluid-present melting of metasedimentary rocks, thus indicating fluid-absent melting. These glasses are also characterized by enrichment of large ion lithophile elements (particularly Cs and Rb), depletion of Ba and Sr, low contents of light rare earth elements (3.6 to 33.7 ppm), high Rb/Sr ratios (6.19–37.3) and low Nb/Ta ratios (2.55–18.7). In combination with phase equilibrium modelling, these compositional features suggest that a sequential dehydration melting of muscovite and biotite was responsible for the production of MI during prograde metamorphism of the studied paragneiss. By compiling MI data published in the literature, we show that dehydration melting of metasedimentary rocks from the Himalayan orogen can produce initial melts with various peraluminous and granitic compositions.
{"title":"The origin and compositions of melt inclusions in an Al2SiO5-free paragneiss from the Namche Barwa Complex in the Eastern Himalayan Syntaxis","authors":"Qiang Liu, Penglei Liu, Xin Li, Junfeng Zhang","doi":"10.1111/jmg.12721","DOIUrl":"10.1111/jmg.12721","url":null,"abstract":"<p>Melt inclusions (MIs) in high-temperature metamorphic rocks provide a unique window into crustal anatexis in collisional orogenic belts and have been widely used to characterize compositions of anatectic melts as well as melting mechanisms. In this study, MIs hosted by peritectic garnet were for the first time identified in an Al<sub>2</sub>SiO<sub>5</sub>-free graywacke-type paragneiss from the Namche Barwa Complex, the Eastern Himalaya, Southeast Tibet. These MIs occur as nanogranites in the rims of porphyroblastic garnet, exhibit negative crystal shapes with an average diameter of ~12 μm and consist of a mineral assemblage of biotite + quartz + plagioclase + K-feldspar ± muscovite. Re-homogenization experiments of these nanogranites were conducted at a pressure of 1.5 GPa and temperatures of 800°C, 850°C and 900°C and produced homogeneous glasses at 850°C. The homogenized glasses are strongly peraluminous and calc-alkalic in composition, with 66.43–71.31 wt.% SiO<sub>2</sub>, 12.64–15.06 wt.% Al<sub>2</sub>O<sub>3</sub>, high alkaline (5.41–7.22 wt.%) and low ferromagnesian (2.72–4.46 wt.%) contents. They are lower in silica and CaO but higher in K<sub>2</sub>O compared with MI produced by fluid-present melting of metasedimentary rocks, thus indicating fluid-absent melting. These glasses are also characterized by enrichment of large ion lithophile elements (particularly Cs and Rb), depletion of Ba and Sr, low contents of light rare earth elements (3.6 to 33.7 ppm), high Rb/Sr ratios (6.19–37.3) and low Nb/Ta ratios (2.55–18.7). In combination with phase equilibrium modelling, these compositional features suggest that a sequential dehydration melting of muscovite and biotite was responsible for the production of MI during prograde metamorphism of the studied paragneiss. By compiling MI data published in the literature, we show that dehydration melting of metasedimentary rocks from the Himalayan orogen can produce initial melts with various peraluminous and granitic compositions.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 7","pages":"879-898"},"PeriodicalIF":3.4,"publicationDate":"2023-04-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47036071","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Central Qilian terrane (CQT) of the northern Tibetan Plateau played a key role in the tectonic evolution of the Proto-Tethyan Ocean in the Tethysides, but its formation and tectonic attribution have been hotly debated. Here, we report the discovery of eclogites and HP mafic granulites in the Beidahe Complex of the western CQT. These occur as blocks of various sizes within a sequence of metavolcanic–sedimentary rocks, exhibiting typical a ‘block-in-matrix’ and thrust imbrication structure. The eclogite facies metamorphic rocks preserve distinct mineral assemblages and textures corresponding to prograde, peak, and retrograde metamorphism. By combining phase equilibrium modelling with SHRIMP and LA-ICPMS U–Pb dating of metamorphic zircon, sphene, and rutile, the Late Neoproterozoic–Cambrian (c. 553–516 Ma) eclogite facies peak with low thermal gradients of 10–14°C/km, Cambrian (c. 515–506 Ma) post-peak decompression and Ordovician (c. 495–455 Ma) cooling histories for these metabasic rocks have been restored. These constitute hairpin-type clockwise pressure–temperature–time (P–T–t) paths depicting in detail the sequence of deep subduction and subsequent exhumation in Central Qilian during the Late Neoproterozoic to Early Palaeozoic. Our new findings suggest that the CQT represents a Japan-type arc-accretionary system that formed as a result of the North Qilian oceanic plate, one of the major branches of the Proto-Tethys Ocean, being subducted southward. Eclogites in the Beidahe Complex in western CQT offer the earliest (c. 553 Ma) metamorphic record of subduction in the Qilian orogen, indicating that the North Qilian Ocean commenced subducting southward prior to the Late Neoproterozoic.
{"title":"Late Neoproterozoic–Cambrian eclogites and high-pressure granulites in the Central Qilian terrane (China) record the earliest subduction of Proto-Tethyan Ocean in the eastern Tethysides","authors":"Hao Wang, Wenjiao Xiao, Rui Li, Hongxu Chen, Zhou Tan, Qigui Mao","doi":"10.1111/jmg.12720","DOIUrl":"10.1111/jmg.12720","url":null,"abstract":"<p>The Central Qilian terrane (CQT) of the northern Tibetan Plateau played a key role in the tectonic evolution of the Proto-Tethyan Ocean in the Tethysides, but its formation and tectonic attribution have been hotly debated. Here, we report the discovery of eclogites and HP mafic granulites in the Beidahe Complex of the western CQT. These occur as blocks of various sizes within a sequence of metavolcanic–sedimentary rocks, exhibiting typical a ‘block-in-matrix’ and thrust imbrication structure. The eclogite facies metamorphic rocks preserve distinct mineral assemblages and textures corresponding to prograde, peak, and retrograde metamorphism. By combining phase equilibrium modelling with SHRIMP and LA-ICPMS U–Pb dating of metamorphic zircon, sphene, and rutile, the Late Neoproterozoic–Cambrian (c. 553–516 Ma) eclogite facies peak with low thermal gradients of 10–14°C/km, Cambrian (c. 515–506 Ma) post-peak decompression and Ordovician (c. 495–455 Ma) cooling histories for these metabasic rocks have been restored. These constitute hairpin-type clockwise pressure–temperature–time (<i>P–T–t</i>) paths depicting in detail the sequence of deep subduction and subsequent exhumation in Central Qilian during the Late Neoproterozoic to Early Palaeozoic. Our new findings suggest that the CQT represents a Japan-type arc-accretionary system that formed as a result of the North Qilian oceanic plate, one of the major branches of the Proto-Tethys Ocean, being subducted southward. Eclogites in the Beidahe Complex in western CQT offer the earliest (c. 553 Ma) metamorphic record of subduction in the Qilian orogen, indicating that the North Qilian Ocean commenced subducting southward prior to the Late Neoproterozoic.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 6","pages":"849-878"},"PeriodicalIF":3.4,"publicationDate":"2023-03-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45631065","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Matthew Rioux, Joshua M. Garber, Michael Searle, James L. Crowley, Sally Stevens, Mark Schmitz, Andrew Kylander-Clark, Kayla Leal, Tyler Ambrose, Andrew J. Smye
<p>High-precision dating of the metamorphic sole of ophiolites can provide insight into the tectonic evolution of ophiolites and subduction zone processes. To understand subduction initiation beneath a young, well-preserved and well-characterized ophiolite, we performed coupled zircon laser-ablation inductively coupled mass spectrometry trace element analyses and high-precision isotope dilution-thermal ionization mass spectrometry U–Pb dating on 25 samples from the metamorphic sole of the Samail ophiolite (Oman-United Arab Emirates). Zircon grains from amphibolite- to granulite-facies (0.8–1.3 GPa, ~700–900°C), garnet- and clinopyroxene-bearing amphibolite samples (<i>n</i> = 18) show systematic trends of decreasing heavy rare earth element slope (HREE; Yb/Dy) with decreasing Yb concentration, reflecting progressive depletion of the HREE during prograde garnet growth. For half of the garnet-clinopyroxene amphibolite samples, Ti-in-zircon temperatures increase, and U–Pb dates young with decreasing HREE slope, consistent with coupled zircon and garnet growth during prograde metamorphism. In the remaining samples, there is no apparent variation in Ti-in-zircon temperature with decreasing HREE slope, and the combined U–Pb and geochemical data suggest zircon crystallization along either the prograde to peak or prograde to initial retrograde portions of the metamorphic <i>P–T–t</i> path. The new data bracket the timing of prograde garnet and zircon growth in the highest grade rocks of the metamorphic sole between 96.698 ± 0.094 and 95.161 ± 0.064 Ma, in contrast with previously published geochronology suggesting prograde metamorphism at ~104 Ma. Garnet-free amphibolites and leucocratic pods from lower grade (but still upper amphibolite facies) portions of the sole are uniformly HREE enriched (Yb/Dy > 5) and are ~0.5–1.3 Myr younger than the higher grade rocks from the same localities, constraining the temporal offset between the metamorphism and juxtaposition of the higher and lower grade units. Positive zircon ε<sub>Hf</sub> (+6.5 to +14.6) for all but one of the dated amphibolites are consistent with an oceanic basalt protolith for the sole. Our new data indicate that prograde sole metamorphism (96.7–95.2 Ma) immediately predated and overlapped growth of the overlying ophiolite crust (96.1–95.2 Ma). The ~600 ky offset between the onset of sole metamorphism in the northern portion of the ophiolite versus the start of ophiolite magmatism is an order of magnitude shorter than previously proposed (~8 Ma) and is consistent with either spontaneous subduction initiation or an abbreviated period of initial thrusting during induced subduction initiation. Taken together, the sole and ophiolite crust preserve a record of the first ~1.5 Myr of subduction. A gradient in the initiation of high-grade metamorphism from the northwest (96.7 Ma) to southeast (96.0–95.7 Ma) may record propagation of the nascent subduction zone and/or variations in subduction rate alo
{"title":"The temporal evolution of subduction initiation in the Samail ophiolite: High-precision U–Pb zircon petrochronology of the metamorphic sole","authors":"Matthew Rioux, Joshua M. Garber, Michael Searle, James L. Crowley, Sally Stevens, Mark Schmitz, Andrew Kylander-Clark, Kayla Leal, Tyler Ambrose, Andrew J. Smye","doi":"10.1111/jmg.12719","DOIUrl":"10.1111/jmg.12719","url":null,"abstract":"<p>High-precision dating of the metamorphic sole of ophiolites can provide insight into the tectonic evolution of ophiolites and subduction zone processes. To understand subduction initiation beneath a young, well-preserved and well-characterized ophiolite, we performed coupled zircon laser-ablation inductively coupled mass spectrometry trace element analyses and high-precision isotope dilution-thermal ionization mass spectrometry U–Pb dating on 25 samples from the metamorphic sole of the Samail ophiolite (Oman-United Arab Emirates). Zircon grains from amphibolite- to granulite-facies (0.8–1.3 GPa, ~700–900°C), garnet- and clinopyroxene-bearing amphibolite samples (<i>n</i> = 18) show systematic trends of decreasing heavy rare earth element slope (HREE; Yb/Dy) with decreasing Yb concentration, reflecting progressive depletion of the HREE during prograde garnet growth. For half of the garnet-clinopyroxene amphibolite samples, Ti-in-zircon temperatures increase, and U–Pb dates young with decreasing HREE slope, consistent with coupled zircon and garnet growth during prograde metamorphism. In the remaining samples, there is no apparent variation in Ti-in-zircon temperature with decreasing HREE slope, and the combined U–Pb and geochemical data suggest zircon crystallization along either the prograde to peak or prograde to initial retrograde portions of the metamorphic <i>P–T–t</i> path. The new data bracket the timing of prograde garnet and zircon growth in the highest grade rocks of the metamorphic sole between 96.698 ± 0.094 and 95.161 ± 0.064 Ma, in contrast with previously published geochronology suggesting prograde metamorphism at ~104 Ma. Garnet-free amphibolites and leucocratic pods from lower grade (but still upper amphibolite facies) portions of the sole are uniformly HREE enriched (Yb/Dy > 5) and are ~0.5–1.3 Myr younger than the higher grade rocks from the same localities, constraining the temporal offset between the metamorphism and juxtaposition of the higher and lower grade units. Positive zircon ε<sub>Hf</sub> (+6.5 to +14.6) for all but one of the dated amphibolites are consistent with an oceanic basalt protolith for the sole. Our new data indicate that prograde sole metamorphism (96.7–95.2 Ma) immediately predated and overlapped growth of the overlying ophiolite crust (96.1–95.2 Ma). The ~600 ky offset between the onset of sole metamorphism in the northern portion of the ophiolite versus the start of ophiolite magmatism is an order of magnitude shorter than previously proposed (~8 Ma) and is consistent with either spontaneous subduction initiation or an abbreviated period of initial thrusting during induced subduction initiation. Taken together, the sole and ophiolite crust preserve a record of the first ~1.5 Myr of subduction. A gradient in the initiation of high-grade metamorphism from the northwest (96.7 Ma) to southeast (96.0–95.7 Ma) may record propagation of the nascent subduction zone and/or variations in subduction rate alo","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 6","pages":"817-847"},"PeriodicalIF":3.4,"publicationDate":"2023-03-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12719","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46013658","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Toru Takeshita, Takeshi Imayama, Mizuho Ando, Yuto Kimura, Marie Python
We investigated the tectonic evolution of amphibolite and blueschist tectonic blocks in the serpentinite- or pelite-matrix mélange, which are distributed at the highest structural level of the high-P/T type Kamuikotan metamorphic rocks in northern Japan. The tectonic blocks in this study area are divided into six rock types: garnet-epidote amphibolite, epidote amphibolite, amphibolite, plagioclase-poor amphibolite, epidote blueschist and glaucophane-bearing quartz schist. Based on phase equilibrium modelling, garnet-epidote amphibolite and epidote amphibolite experienced peak metamorphism at pressure and temperature conditions of 1.1–1.25 GPa and 550–590°C, and 0.8–1.3 GPa and 475–550°C, respectively, (at an apparent thermal gradient ranging between 13 to 17°C/km). By contrast, although the peak-metamorphic conditions for each one of amphibolite, plagioclase-poor amphibolite, and glaucophane-bearing quartz schist are not well constrained, they may have been originally metamorphosed at amphibolite to epidote-amphibolite facies at thermal gradients exceeding 20°C/km, inferred from the core composition of amphibole (edenite/magnesiohornblende/barroisite). The epidote blueschist experienced peak metamorphism at pressure and temperature conditions of 0.8–1.6 GPa and 360–520°C (most probably 0.8–0.85 GPa and 360–480°C). Although different types of tectonic blocks experienced a variety of peak metamorphism under different P/T conditions, all of them underwent epidote blueschist facies metamorphism at the peak or retrograde stage (as shown by the glaucophane rims of amphibole with different core compositions). The overall P–T paths appear counter-clockwise, which could be interpreted to reflect the cooling history of the subduction channel from the beginning to the steady state of subduction. The geothermal gradient could have changed from 15–17° to ~10°C/km over ~20–25 Myr, as estimated by previously reported radiometric ages. The protoliths to the tectonic blocks could have begun to subduct into the subduction channel at different times (where the thermal structure evolved with time), acquiring different prograde P–T paths. Subsequently, these tectonic blocks were juxtaposed at a certain depth and incorporated into the overlying serpentinite during the subduction stage. Finally, the serpentinite- or pelite-matrix mélange, including these tectonic blocks, were exhumed together with the coherent accretionary units as the former was emplaced over the latter.
{"title":"Pressure–temperature paths of tectonic blocks in mélange: Recording thermal evolution of a subduction channel at an initial stage of subduction","authors":"Toru Takeshita, Takeshi Imayama, Mizuho Ando, Yuto Kimura, Marie Python","doi":"10.1111/jmg.12718","DOIUrl":"10.1111/jmg.12718","url":null,"abstract":"<p>We investigated the tectonic evolution of amphibolite and blueschist tectonic blocks in the serpentinite- or pelite-matrix mélange, which are distributed at the highest structural level of the high-<i>P/T</i> type Kamuikotan metamorphic rocks in northern Japan. The tectonic blocks in this study area are divided into six rock types: garnet-epidote amphibolite, epidote amphibolite, amphibolite, plagioclase-poor amphibolite, epidote blueschist and glaucophane-bearing quartz schist. Based on phase equilibrium modelling, garnet-epidote amphibolite and epidote amphibolite experienced peak metamorphism at pressure and temperature conditions of 1.1–1.25 GPa and 550–590°C, and 0.8–1.3 GPa and 475–550°C, respectively, (at an apparent thermal gradient ranging between 13 to 17°C/km). By contrast, although the peak-metamorphic conditions for each one of amphibolite, plagioclase-poor amphibolite, and glaucophane-bearing quartz schist are not well constrained, they may have been originally metamorphosed at amphibolite to epidote-amphibolite facies at thermal gradients exceeding 20°C/km, inferred from the core composition of amphibole (edenite/magnesiohornblende/barroisite). The epidote blueschist experienced peak metamorphism at pressure and temperature conditions of 0.8–1.6 GPa and 360–520°C (most probably 0.8–0.85 GPa and 360–480°C). Although different types of tectonic blocks experienced a variety of peak metamorphism under different <i>P/T</i> conditions, all of them underwent epidote blueschist facies metamorphism at the peak or retrograde stage (as shown by the glaucophane rims of amphibole with different core compositions). The overall <i>P–T</i> paths appear counter-clockwise, which could be interpreted to reflect the cooling history of the subduction channel from the beginning to the steady state of subduction. The geothermal gradient could have changed from 15–17° to ~10°C/km over ~20–25 Myr, as estimated by previously reported radiometric ages. The protoliths to the tectonic blocks could have begun to subduct into the subduction channel at different times (where the thermal structure evolved with time), acquiring different prograde <i>P–T</i> paths. Subsequently, these tectonic blocks were juxtaposed at a certain depth and incorporated into the overlying serpentinite during the subduction stage. Finally, the serpentinite- or pelite-matrix mélange, including these tectonic blocks, were exhumed together with the coherent accretionary units as the former was emplaced over the latter.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 6","pages":"787-816"},"PeriodicalIF":3.4,"publicationDate":"2023-03-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45659968","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}