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Yongfei Zheng - One of the best experts on this subject based on the ideXlab platform.
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Protolith control on fluid availability for zircon growth during continental subduction zone metamorphism in the dabie orogen
Journal of Asian Earth Sciences, 2013Co-Authors: Yongfei Zheng, Yixiang ChenAbstract:Abstract Different episodes of zircon growth are recognized by a combined study of CL images, mineral inclusions, U–Pb ages, trace elements and Lu–Hf isotopes for ultrahigh-pressure (UHP) eclogite-facies metamorphic rocks in the Dabie orogen. The results provide insights into the effect of Protolith property on fluid liberation during continental collision. Fluid availability from premetamorphic Protoliths of different origins is recognized as a key to the zircon growth. Zircon U–Pb dating for UHP metabasalt and metasediment (eclogite and its host paragneiss) yields two groups of ages at 244 ± 3 and 225 ± 2 Ma, respectively. Mineral inclusion, trace element and Lu–Hf isotope analyses also suggest that these two groups of zircon grew from hydrous melt during the final subduction and supercritical fluid during the initial exhumation, respectively. In contrast, zircon U–Pb dating for UHP metaintrusive rocks (granitic orthogneiss and its hosted eclogite) gave only one group of age at 222 ± 2 Ma. Mineral inclusion, trace element and Lu–Hf isotope analyses suggest that the metamorphic zircon grew from aqueous fluid during the initial exhumation. The difference between the two groups of zircon U–Pb dates is attributed to the difference in their Protolith origin. Volcanic and sedimentary rocks contain large amounts of water primarily in the form of molecular water (pore fluid), so that considerable amounts of aqueous fluid can be released from them during subduction. This episode of fluid action was recorded by the growth of anatectic zircon at ∼244 Ma. In contrast, intrusive rock only contains small amounts of water primarily in the form of structural hydroxyl in crystalline minerals, so that little fluid can be released from them during subduction. Nevertheless, large amounts of retrograde fluid were released from UHP metamorphic rocks regardless of their Protolith origin during decompression exhumation. This episode of fluid action was recorded by the growth of metamorphic and anatectic zircons at 220–225 Ma. Therefore, the Protolith property is a key to the liberation of aqueous fluid from metamorphosing rocks during subduction, which has great bearing on partial melting, element transport and mineral growth during continental subduction-zone metamorphism.
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zircon u pb age and hf isotope evidence for contrasting origin of bimodal Protoliths for ultrahigh pressure metamorphic rocks from the chinese continental scientific drilling project
Journal of Metamorphic Geology, 2007Co-Authors: Yongfei Zheng, Renxu Chen, Zifu Zhao, Jun Tang, Xiaoming LiuAbstract:A combined study of zircon morphology, U–Pb ages and Hf isotopes as well as whole-rock major and trace elements was carried out for ultrahigh-pressure (UHP) eclogite and felsic gneiss from the main hole (MH) of the Chinese Continental Scientific Drilling (CCSD) project in the Sulu orogen. The results show contrasting Hf isotope compositions for bimodal UHP metaigneous rocks, pointing to contrasting origins for their Protoliths (thus dual-bimodal compositions). The samples of interest were from two continuous core segments from CCSD MH at depths of 734.21–737.16 m (I) and 929.67–932.86 m (II) respectively. Zircon U–Pb dating for four samples from the two core segments yields two groups of ages at 784 ± 17 and 222 ± 3 Ma, respectively, corresponding to Protolith formation during supercontinental rifting and metamorphic growth during continental collision. Although the Triassic UHP metamorphism significantly reset the zircon U–Pb system of UHP rocks, the Hf isotope compositions of igneous zircon can be used to trace their Protolith origin. Contrasting types of initial Hf isotope ratios are, respectively, correlated with segments I and II, regardless of their lithochemistry. The first type shows positive ɛHf(t) values of 7.8 ± 3.1 to 6.0 ± 3.0, with young Hf model age of 1.03 and 1.11 Ga. The second type exhibits negative ɛHf(t) values of −6.9 ± 1.6 to −9.1 ± 1.1, with old Hf model ages of 2.11 and 2.25 Ga. It appears that the UHP rocks from the two segments have Protoliths of contrasting origin. Consistent results are also obtained from their trace element compositions suggesting that mid-Neoproterozoic Protoliths of bimodal UHP metaigneous rocks formed during supercontinental rifting at the northern margin of the South China Block. Thus, the first type of bimodal magmatism formed by rapid reworking of juvenile crust, whereas the second type of bimodal magmatism was principally generated by rift anatexis of Paleoproterozoic crust. Melting of orogenic lithosphere has potential to bring about bimodal magmatism with contrasting origins. Because arc–continent collision zones are the best place to accumulate both juvenile and ancient crusts, the contrasting types of bimodal magmatism are proposed to occur in an arc–continent collision orogen during the supercontinental rifting, in response to the attempted breakup of the supercontinent Rodinia at c. 780 Ma.
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geochronology and geochemistry of metamorphic rocks in the jiaobei terrane constraints on its tectonic affinity in the sulu orogen
Precambrian Research, 2007Co-Authors: Yongfei Zheng, Jun Tang, Bing Gong, Xiaoming LiuAbstract:Abstract Tectonic affinity of tectono-lithological units close to ultrahigh-pressure metamorphic belt is a key issue for understanding the geodynamics of continental collision. This is particularly so for the Jiaobei terrane northeast of the Dabie-Sulu orogenic belt in China. New data from LA-ICPMS zircon U–Pb dating, whole-rock elements and Nd–Sr isotopes, and mineral O isotopes are presented for metamorphic rocks from this terrane. The results place geochronological and geochemical constraints on their Protolith nature and metamorphic timing and thus on its tectonic affinity to one of the two Triassic collided continents, the North and the South China Blocks. Protolith ages for TTG gneiss, amphibolite and mafic granulite are ∼2.7, ∼2.5 and ∼2.4 Ga, respectively; regional metamorphism took place extensively at ∼1.76 Ga. Protolith of the TTG gneiss was generated by partial melting of mantle-derived rocks at the root of a thickened crust. Protolith of the amphibolite was probably a product of arc-like magmatism; Protolith of the mafic granulite was derived from a depleted mantle source. Both of them were locally contaminated by supracrustal materials. Protoliths of paragneiss and schist in the Fenzishan Group were mostly derived from supracrustal sources, but Protolith of amphibolite in the Fenzishan Group is of mantle-derived signature. Unlike the UHP metaigneous rocks in the Dabie-Sulu orogenic belt that show unusual 18O-depletion, the Jiaobei metamorphic rocks have basically preserved their original mantle-like O isotope compositions. In general, the nature and timing of geological events recorded in the metamorphic rocks from the Jiaobei terrane are comparable with those from the North China Block rather than the South China Block. Thus, the Jiaobei terrane is concluded to have tectonic affinity to the former, but behave like a micro-continent during the Triassic continental collision. The ∼1.76 Ga regional metamorphism in the Jiaobei terrane is likely related to reworking of the arc-continent collisional orogen in the periphery of the North China Block rather than the ∼1.85 Ga collision event between the eastern and western North China Blocks. The present study lends support to the common assumption that the suture boundary between the North and South China Blocks in the Sulu orogen is located along the Wulian-Yantai fault. Tectonic mingling along the Wulian-Yantai fault is probably related to subduction erosion during the continental collision.
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zircon u pb age hf and o isotope constraints on Protolith origin of ultrahigh pressure eclogite and gneiss in the dabie orogen
Chemical Geology, 2006Co-Authors: Yongfei Zheng, Zifu Zhao, Yuanbao Wu, Shaobing Zhang, Fuyuan WuAbstract:Abstract Zircon U–Pb dating, Hf and O isotope analyses were carried out for ultrahigh-pressure eclogite and felsic gneiss from the Dabie orogen in China. The results provide constraints on their Protolith origin, with significance for continental growth by rift magmatism during splitting of the Yangtze Block from the supercontinent Rodinia. Mafic and felsic Protoliths formed as a bimodal volcanic suite at about 750 Ma, with incorporation of ca. 2.15 Ga crust into the felsic Protolith. δ 18 O values of − 4.1‰ to 4.4‰ suggest differential involvement of meteoric water in Protolith magmas. Initial Hf isotope ratios are subdivided into two groups, with positive e Hf ( t ) values of 12.9 ± 0.7 to 5.9 ± 0.9 and neutral e Hf ( t ) values of 2.3 ± 0.3 to − 2.7 ± 0.6, respectively. The positive e Hf ( t ) values correspond to depleted mantle Hf model ages of 0.82 to 1.24 Ga, whereas the negative e Hf ( t ) values correspond to crust Hf model ages of 1.82 ± 0.07 Ga. A few zircons from the gneiss have strongly negative e Hf ( t ) values of − 22.6 ± 0.6, which are associated not only with consistent depleted mantle and crust Hf model ages of 2.2 Ga but also with the old U–Pb ages falling on a 2147 ± 22 Ma discordia line. Thus the felsic Protolith contains Paleoproterozoic crustal relicts, some of which were originally derived from coeval depleted mantle. On the other hand, strongly positive e Hf ( t ) values are associated with young U–Pb ages falling on the ∼750 Ma discordia lines. This suggests that both eclogite and gneiss Protoliths principally formed by immediate reworking of mid-Neoproterozoic juvenile crust, with contrasting features only in petrochemistry. The highest e Hf ( t ) values of 12.7 to 14.4 for the eclogite correspond to the youngest Hf model ages of 0.75 to 0.82 Ga relative to the depleted mantle reservoir. These Hf model ages are not only close to the timing of zircon growth from the mafic magma, but also similar to ages for bimodal magmatism in the periphery of the Yangtze Block. This demonstrates new addition of the depleted mantle material to the continental crust by rift magmatism in the northern margin of the Yangtze Block, with coeval crust–mantle interaction in the mid-Neoproterozoic rifting tectonic zone. Therefore, the bimodal magmatism at about 750 Ma transports both heat and material from the mantle to the crust, resulting in reworking of both the Paleoproterozoic old crust and the meteoric-hydrothermally altered juvenile crust along the active rifting zone.
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low grade metamorphic rocks in the dabie sulu orogenic belt a passive margin accretionary wedge deformed during continent subduction
International Geology Review, 2005Co-Authors: Yongfei Zheng, Jianbo Zhou, Zhi XieAbstract:Greenschist-facies metasedimentary and meta-igneous rocks occur continuously along the northern margin, and sporadically in the interior, of the Dabie-Sulu orogenic belt in east-central China. An integrated study of geochronological, petrological, and paleontological observations demonstrates that precursors of flysch-facies metasedimentary rocks were deposited along the northern, passive continental margin of the Yangtze plate prior to the Triassic, and that Protoliths of the meta-igneous rocks are a product of Middle Neoproterozoic bimodal magmatism along the northern margin of this plate. Except for the striking contrast in metamorphic grade, these low-grade rocks generally can be correlated in Protolith origin and age with ultrahigh-pressure metamorphic rocks within the orogenic belt. Relationships in time and space between these rocks of contrasting grades can be reasonably interpreted through an accretionary wedge model that links their evolution with continent subduction. The low-grade metamorphic ...
Pettke Thomas - One of the best experts on this subject based on the ideXlab platform.
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Petrology and Geochemistry of Serpentinites Associated with the Ultra-High Pressure Lago di Cignana Unit (ItalianWestern Alps)
'Oxford University Press (OUP)', 2022Co-Authors: Gilio M., Scambelluri M., Agostini S., Godard M., Peters Daniel, Pettke ThomasAbstract:In the Western Alps, the ophiolitic Zermatt–Saas Zone (ZSZ) and the Lago di Cignana Unit (LCU) record oceanic lithosphere subduction to high (540°C, 2·3GPa) and ultra-high pressure (600°C, 3·2GPa), respectively. The top of the Zermatt–Saas Zone in contact with the Lago di Cignana Unit consists of olivine þ Ti-clinohumite-bearing serpentinites (the Cignana serpentinite) hosting olivine þ Ti-clinohumite veins and dykelets of olivine þ Ti-chondrodite þ Ti-clinohumite. The composition of this serpentinite reveals a refertilized oceanic mantle peridotite Protolith that became subsequently enriched in fluid-mobile elements (FME) during oceanic serpentinization. The olivine þ Ti-clinohumite veins in the Cignana serpentinite display Rare Earth Element (REE) and FME compositions quite similar to the host-rock, which suggests closed-system dehydration of this serpentinite during subduction. The Ti-chondrodite-bearing dykelets are richer in REE and FME than the host-rock and the dehydration olivine þ Ti-clinohumite veins: their Nd composition points to a mafic Protolith, successively overprinted by oceanic metasomatism and by subduction zone recrystallization. These dykelets are comparable in composition to eclogites within the ultra-high pressure LCU that derive from subducted oceanic mafic crust. Different from the LCU, serpentinites from the core domains of the ZSZ display REE compositions indicating a depleted mantle Protolith. The oceanic serpentinization of these rocks led to an increase in FME and to seawater-like Sr isotope compositions. The serpentinites sampled at increasing distance from the ultra-high pressure LCU reveal different mantle Protoliths, still preserve an oceanic geochemical imprint and contain mafic dykelets affected by oceanic metasomatism. The subduction zone history of these rocks thus occurred under relatively closed system conditions, the only possible change during subduction being an enrichment in As and Sb recorded by the serpentinites closer to the crustal LCU. The ZSZ and Cignana serpentinites thus likely evolved in a slab setting and were weakly exposed to interaction with slab-derived fluids characteristic of plate interface settings. Our data suggest two possible scenarios for the evolution of the studied ZSZ and Cignana serpentinites. They are either part of a coherent ophiolite unit whose initial lithospheric mantle was variably affected by depletion and re-fertilization processes, or they belong to separate tectonic slices derived from two different oceanic mantle sections. In the Cignana serpentinite atop the ZSZ, the presence of Ti-chondrodite dykelets similar in composition to the LCU eclogites suggests these two domains were closely associated in the oceanic lithosphere and shared the same evolution to ultra-high pressure conditions during Alpine subduction
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Petrology and Geochemistry of Serpentinites Associated with the Ultra-High Pressure Lago di Cignana Unit (Italian Western Alps)
Oxford University Press (OUP), 2019Co-Authors: Gilio Mattia, Peters Daniel, Scambelluri Marco, Agostini Samuele, Godard Marguerite, Pettke ThomasAbstract:International audienceIn the Western Alps, the ophiolitic Zermatt–Saas Zone (ZSZ) and the Lago di Cignana Unit (LCU) record oceanic lithosphere subduction to high (540°C, 2·3GPa) and ultra-high pressure (600°C, 3·2GPa), respectively. The top of the Zermatt–Saas Zone in contact with the Lago di Cignana Unit consists of olivine + Ti-clinohumite-bearing serpentinites (the Cignana serpentinite) hosting olivine + Ti-clinohumite veins and dykelets of olivine + Ti-chondrodite + Ti-clinohumite. The composition of this serpentinite reveals a refertilized oceanic mantle peridotite Protolith that became subsequently enriched in fluid-mobile elements (FME) during oceanic serpentinization. The olivine + Ti-clinohumite veins in the Cignana serpentinite display Rare Earth Element (REE) and FME compositions quite similar to the host-rock, which suggests closed-system dehydration of this serpentinite during subduction. The Ti-chondrodite-bearing dykelets are richer in REE and FME than the host-rock and the dehydration olivine + Ti-clinohumite veins: their Nd composition points to a mafic Protolith, successively overprinted by oceanic metasomatism and by subduction zone recrystallization. These dykelets are comparable in composition to eclogites within the ultra-high pressure LCU that derive from subducted oceanic mafic crust. Different from the LCU, serpentinites from the core domains of the ZSZ display REE compositions indicating a depleted mantle Protolith. The oceanic serpentinization of these rocks led to an increase in FME and to seawater-like Sr isotope compositions. The serpentinites sampled at increasing distance from the ultra-high pressure LCU reveal different mantle Protoliths, still preserve an oceanic geochemical imprint and contain mafic dykelets affected by oceanic metasomatism. The subduction zone history of these rocks thus occurred under relatively closed system conditions, the only possible change during subduction being an enrichment in As and Sb recorded by the serpentinites closer to the crustal LCU. The ZSZ and Cignana serpentinites thus likely evolved in a slab setting and were weakly exposed to interaction with slab-derived fluids characteristic of plate interface settings. Our data suggest two possible scenarios for the evolution of the studied ZSZ and Cignana serpentinites. They are either part of a coherent ophiolite unit whose initial lithospheric mantle was variably affected by depletion and re-fertilization processes, or they belong to separate tectonic slices derived from two different oceanic mantle sections. In the Cignana serpentinite atop the ZSZ, the presence of Ti-chondrodite dykelets similar in composition to the LCU eclogites suggests these two domains were closely associated in the oceanic lithosphere and shared the same evolution to ultra-high pressure conditions during Alpine subduction
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Stability of antigorite serpentinite and geochemical exchange with oceanic crustal rocks during ultrahigh- pressure subduction-zone metamorphism (Lago di Cignana Unit, Italian Western Alps)
'Oxford University Press (OUP)', 2019Co-Authors: Gilio Mattia, Peters Daniel, Scambelluri Marco, Agostini Samuele, Godard Marguerite, Pettke ThomasAbstract:International audienceIn the Western Alps, the ophiolitic Zermatt–Saas Zone (ZSZ) and the Lago di Cignana Unit (LCU) record oceanic lithosphere subduction to high (540°C, 2·3GPa) and ultra-high pressure (600°C, 3·2GPa), respectively. The top of the Zermatt–Saas Zone in contact with the Lago di Cignana Unit consists of olivine + Ti-clinohumite-bearing serpentinites (the Cignana serpentinite) hosting olivine + Ti-clinohumite veins and dykelets of olivine + Ti-chondrodite + Ti-clinohumite. The composition of this serpentinite reveals a refertilized oceanic mantle peridotite Protolith that became subsequently enriched in fluid-mobile elements (FME) during oceanic serpentinization. The olivine + Ti-clinohumite veins in the Cignana serpentinite display Rare Earth Element (REE) and FME compositions quite similar to the host-rock, which suggests closed-system dehydration of this serpentinite during subduction. The Ti-chondrodite-bearing dykelets are richer in REE and FME than the host-rock and the dehydration olivine + Ti-clinohumite veins: their Nd composition points to a mafic Protolith, successively overprinted by oceanic metasomatism and by subduction zone recrystallization. These dykelets are comparable in composition to eclogites within the ultra-high pressure LCU that derive from subducted oceanic mafic crust. Different from the LCU, serpentinites from the core domains of the ZSZ display REE compositions indicating a depleted mantle Protolith. The oceanic serpentinization of these rocks led to an increase in FME and to seawater-like Sr isotope compositions. The serpentinites sampled at increasing distance from the ultra-high pressure LCU reveal different mantle Protoliths, still preserve an oceanic geochemical imprint and contain mafic dykelets affected by oceanic metasomatism. The subduction zone history of these rocks thus occurred under relatively closed system conditions, the only possible change during subduction being an enrichment in As and Sb recorded by the serpentinites closer to the crustal LCU. The ZSZ and Cignana serpentinites thus likely evolved in a slab setting and were weakly exposed to interaction with slab-derived fluids characteristic of plate interface settings. Our data suggest two possible scenarios for the evolution of the studied ZSZ and Cignana serpentinites. They are either part of a coherent ophiolite unit whose initial lithospheric mantle was variably affected by depletion and re-fertilization processes, or they belong to separate tectonic slices derived from two different oceanic mantle sections. In the Cignana serpentinite atop the ZSZ, the presence of Ti-chondrodite dykelets similar in composition to the LCU eclogites suggests these two domains were closely associated in the oceanic lithosphere and shared the same evolution to ultra-high pressure conditions during Alpine subduction
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Aluminous websterite and granulite xenoliths from the Chyulu Hills volcanic field, Kenya: gabbro-troctolitic cumulates subjected to lithospheric foundering
2018Co-Authors: Ulianov Alexey, Kalt Angelika, Pettke ThomasAbstract:Whole rock major and trace element abundances in aluminous garnet-spinel websterite, sapphirine-bearing Mg-Al granulite and hibonite-bearing Ca-Al granulite xenoliths from the Chyulu Hills volcanic field, Kenya, suggest that the samples represent a meta-igneous suite linked by fractionation. The incompatible major element contents increase from the websterites to the Mg-Al granulites and further to the Ca-Al granulites. High bulk rock Mg#s and very low concentrations of most incompatible trace elements indicate that the rocks are cumulates rather than crystallized melts. Elevated Ni abundances, impoverishment in Cr and HFSE and high contents of normative plagioclase and olivine in the granulites indicate that their Protoliths were similar to troctolite. The textures and metamorphic reaction paths recorded in the granulites suggest igneous emplacement in the crust and cooling from igneous to ambient crustal temperatures accompanied or followed by compression. For the websterite xenoliths, there is an apparent contradiction between the results of P-T calculations that suggest high P and T of crystallization of early generation pyroxenes and elevated P-T conditions during final equilibration (1.4-2.2GPa/740-980°C) on the one hand and the positive Eu anomaly that suggests shallow-level plagioclase accumulation on the other hand. This contradiction can be reconciled by a model of compression of a plagioclase-bearing (gabbroic) Protolith to mantle depths where it recrystallized to an ultramafic assemblage, which requires foundering of dense lower crustal material into the mantl
Sato, Ana María - One of the best experts on this subject based on the ideXlab platform.
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Evolución del basamento paleozoico de los Andes Norpatagónicos en el área de San Martín de los Andes (Neuquén, Argentina): petrología, edad y correlaciones
2020Co-Authors: Serra Varela Samanta, González, Pablo Diego, Giacosa, Raúl E., Heredia Nemesio, Pedreira David, Martín González Fidel, Sato, Ana MaríaAbstract:In San Martín de los Andes area (Argentinian Patagonia) the Palaeozoic basement consists of metamorphic and plutonic rocks. The metamorphic rocks include strongly deformed schists, gneisses and migmatites. Their geochemical and petrographic characteristics suggest that the protholith could have been a sequence of pelites and greywackes. Detrital zircon analysis (U-Pb Q-ICP-LA-MS) yielded a maximum depositional age of 501±14 Ma (Series 3 Cambrian) for this sedimentary Protolith. Metasedimentary rocks are affected by a regional foliation defined by the minerals of the metamorphic peak. This is a S2 foliation, since relics of a former foliation are present in some samples. This regional foliation is locally affected by open folds that develop an incipient crenulation cleavage (S3). The high-grade metamorphism includes partial melting processes, where the incipient segregates intrude parallel to the regional foliation and also cut it in presence of abundant melt. Zircons from anatectic granites formed during this partial melting process yielded a U-Pb Concordia age of 434.1±4.5 Ma (Llandovery-Wenlock, Silurian). The age of maximum sedimentation and the anatectic age constrain the metamorphic evolution of the basement into the lower Palaeozoic (between upper Cambrian and lower Silurian). The igneous rocks of the basement are granodiorites, tonalities, and some gabbros that cut the metamorphic basement and contain xenoliths and roof pendants from the country rocks. These plutonic rocks are affected by low-grade metamorphism, with the development of discrete, centimetric to hectometric, brittle-ductile shear zones. The age of these igneous rocks has been constrained through U-Pb zircons analysis, carried out by several authors between ca. 370 and 400 Ma (Devonian). The maximum sedimentation age for the Protolith and its metamorphic evolution seems to be related to an early Palaeozoic orogenic event, probably the Patagonian Famatinian orogeny. In contrast, the Devonian igneous rocks of San Martín de los Andes could represent a Devonian magmatic arc, related to subduction processes developed at the beginning of the Gondwanan orogenic cycle, which culminates with the Gondwanan orogeny.En el entorno de San Martín de los Andes (Patagonia argentina), el basamento Paleozoico está constituido por rocas metamórficas e ígneas. Las rocas metamórficas de esta área incluyen esquistos, gneises y migmatitas intensamente deformadas. Según sus características geoquímicas y petrográficas, el protolito de las mismas es considerado una alternancia de pelitas y grauvacas. Análisis de circones detríticos (U-Pb Q-ICP-LA-MS) permiten definir una edad máxima de sedimentación de 501±14 Ma (Series 3, Cámbrico) para este protolito sedimentario. Las rocas metasedimentarias se encuentran afectadas por una foliación regional definida por la asociación principal del pico metamórfico. Esta es una foliación de tipo S2, ya que se encuentran relictos de una foliación anterior en algunas muestras. Esta foliación regional, está localmente afectada por pliegues abiertos donde se desarrolla un clivaje de crenulación incipiente (S3). El metamorfismo de alto grado incluye un evento de fusión parcial, donde los primeros segregados se introducen a favor de la foliación S2 y la cortan en presencia de abundante fundido. Circones obtenidos de un granito anatéctico producido durante este evento de fusión parcial arrojaron una edad Concordia U-Pb de 434.1±4.5 Ma (Llandovery-Wenlock, Silúrico). La edad máxima de sedimentación y la edad de anatexis permiten acotar la evolución de las rocas metamórficas del basamento al Paleozoico inferior (entre el Cámbrico tardío y el Silúrico temprano). Las rocas ígneas del basamento son granodioritas y tonalitas, con menor proporción de gabros, las cuales cortan las rocas metamórficas y contienen xenolitos y roof pendants de las mismas. Por otra parte, estas rocas plutónicas fueron afectadas por un metamorfismo de bajo grado que llevan asociadas zonas de cizalla frágil-dúctil centimétricas a hectométricas. Edades U-Pb SHRIMP en circones restringe la edad de estas rocas entre 370 y 400 Ma (Devónico). La edad máxima de sedimentación de los protolitos de las rocas metamórficas y su evolución metamórfica podría estar relacionada a un evento orogénico del Paleozoico temprano, probablemente a la orogenia Famatiniana Patagónica. En contraste, las rocas plutónicas de San Martín de los Andes representarían un arco magmático, relacionado a una subducción que se habría producido al comienzo del ciclo orogénico Gondwánico, el cual culmina con la orogenia Gondwánica.Centro de Investigaciones Geológica
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Evolución del basamento paleozoico de los Andes Norpatagónicos en el área de San Martín de los Andes (Neuquén, Argentina): petrología, edad y correlaciones
'Pontificia Universidad Catolica de Valparaiso', 2019Co-Authors: Serra Varela Samanta, González, Pablo Diego, Heredia Nemesio, Pedreira David, Martín González Fidel, Giacosa, Raul Eduardo, Sato, Ana MaríaAbstract:En el entorno de San Martín de los Andes (Patagonia argentina), el basamento Paleozoico está constituido por rocas metamórficas e ígneas. Las rocas metamórficas de esta área incluyen esquistos, gneises y migmatitas intensamente deformadas. Según sus características geoquímicas y petrográficas, el protolito de las mismas es considerado una alternancia de pelitas y grauvacas. Análisis de circones detríticos (U-Pb Q-ICP-LA-MS) permiten definir una edad máxima de sedimentación de 501±14 Ma (Series 3, Cámbrico) para este protolito sedimentario. Las rocas metasedimentarias se encuentran afectadas por una foliación regional definida por la asociación principal del pico metamórfico. Esta es una foliación de tipo S2, ya que se encuentran relictos de una foliación anterior en algunas muestras. Esta foliación regional, está localmente afectada por pliegues abiertos donde se desarrolla un clivaje de crenulación incipiente (S3). El metamorfismo de alto grado incluye un evento de fusión parcial, donde los primeros segregados se introducen a favor de la foliación S2 y la cortan en presencia de abundante fundido. Circones obtenidos de un granito anatéctico producido durante este evento de fusión parcial arrojaron una edad Concordia U-Pb de 434.1±4.5 Ma (Llandovery-Wenlock, Silúrico). La edad máxima de sedimentación y la edad de anatexis permiten acotar la evolución de las rocas metamórficas del basamento al Paleozoico inferior (entre el Cámbrico tardío y el Silúrico temprano). Las rocas ígneas del basamento son granodioritas y tonalitas, con menor proporción de gabros, las cuales cortan las rocas metamórficas y contienen xenolitos y roof pendants de las mismas. Por otra parte, estas rocas plutónicas fueron afectadas por un metamorfismo de bajo grado que llevan asociadas zonas de cizalla frágil-dúctil centimétricas a hectométricas. Edades U-Pb SHRIMP en circones restringe la edad de estas rocas entre 370 y 400 Ma (Devónico). La edad máxima de sedimentación de los protolitos de las rocas metamórficas y su evolución metamórfica podría estar relacionada a un evento orogénico del Paleozoico temprano, probablemente a la orogenia Famatiniana Patagónica. En contraste, las rocas plutónicas de San Martín de los Andes representarían un arco magmático, relacionado a una subducción que se habría producido al comienzo del ciclo orogénico Gondwánico, el cual culmina con la orogenia Gondwánica.In San Martín de los Andes area (Argentinian Patagonia) the Palaeozoic basement consists of metamorphicand plutonic rocks. The metamorphic rocks include strongly deformed schists, gneisses and migmatites. Their geochemicaland petrographic characteristics suggest that the protholith could have been a sequence of pelites and greywackes.Detrital zircon analysis (U-Pb Q-ICP-LA-MS) yielded a maximum depositional age of 501±14 Ma (Series 3 Cambrian)for this sedimentary Protolith. Metasedimentary rocks are affected by a regional foliation defined by the minerals of themetamorphic peak. This is a S2 foliation, since relics of a former foliation are present in some samples. This regionalfoliation is locally affected by open folds that develop an incipient crenulation cleavage (S3). The high-grade metamorphismincludes partial melting processes, where the incipient segregates intrude parallel to the regional foliation and also cut itin presence of abundant melt. Zircons from anatectic granites formed during this partial melting process yielded a U-PbConcordia age of 434.1±4.5 Ma (Llandovery-Wenlock, Silurian). The age of maximum sedimentation and the anatecticage constrain the metamorphic evolution of the basement into the lower Palaeozoic (between upper Cambrian and lowerSilurian). The igneous rocks of the basement are granodiorites, tonalities, and some gabbros that cut the metamorphicbasement and contain xenoliths and roof pendants from the country rocks. These plutonic rocks are affected by low-grademetamorphism, with the development of discrete, centimetric to hectometric, brittle-ductile shear zones. The age ofthese igneous rocks has been constrained through U-Pb zircons analysis, carried out by several authors between ca. 370and 400 Ma (Devonian). The maximum sedimentation age for the Protolith and its metamorphic evolution seems to berelated to an early Palaeozoic orogenic event, probably the Patagonian Famatinian orogeny. In contrast, the Devonianigneous rocks of San Martín de los Andes could represent a Devonian magmatic arc, related to subduction processesdeveloped at the beginning of the Gondwanan orogenic cycle, which culminates with the Gondwanan orogeny.Fil: Serra Varela, Samanta. Universidad Nacional de Río Negro. Sede Alto Valle. Instituto de Investigaciones en Paleobiología y Geología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Gonzalez, Pablo Diego. Universidad Nacional de Río Negro. Sede Alto Valle. Instituto de Investigaciones en Paleobiología y Geología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Giacosa, Raul Eduardo. Secretaria de Industria y Mineria. Servicio Geológico Minero Argentino. Instituto de Geología y Recursos Minerales; ArgentinaFil: Heredia, Nemesio. Instituto Geológico y Minero de España; EspañaFil: Pedreira, David. Universidad de Oviedo; EspañaFil: Martín González, Fidel. Universidad Rey Juan Carlos; EspañaFil: Sato, Ana Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; Argentin
Renxu Chen - One of the best experts on this subject based on the ideXlab platform.
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tectonic evolution from oceanic subduction to continental collision during the closure of paleotethyan ocean geochronological and geochemical constraints from metamorphic rocks in the hong an orogen
Gondwana Research, 2015Co-Authors: Ligang Zhou, Renxu Chen, Qiongxia Xia, Yueheng YangAbstract:Abstract A combined study of whole-rock major-trace elements and Sr–Nd isotopes as well as zircon U–Pb ages, trace elements and Lu–Hf isotopes was carried out for high-pressure (HP) and ultrahigh-pressure (UHP) eclogite-facies metamorphic rocks in the Hong'an orogen, China. The results provide insights into the tectonic evolution from oceanic subduction to continental collision during the closure of Paleotethyan ocean between the South and North China Blocks. Based on the whole-rock geochemistry, eclogites in the Hong'an orogen are categorized into continental- and oceanic-type, respectively. The continental-type eclogites widely occur in the whole orogen, exhibiting general enrichments of LILE and LREE but depletion of HFSE and HREE. Zircon U–Pb dating yields Protolith ages of about 750 to 1200 Ma, demonstrating a tectonic affinity to the South China Block. They have both positive and negative e Nd (t) and e Hf (t) values for whole-rock and zircons, suggesting that their Protoliths were originated from both juvenile and ancient crustal rocks. The oceanic-type eclogites only occur in the northwestern edge of the orogen, exhibiting mid-ocean ridge basalt (MORB)-like flat REE patterns and arc-like trace element patterns. Zircon U–Pb dating on the relict zircon cores of magmatic origin yields Protolith ages of ~ 420 Ma. They have variable e Nd (t) and positive e Hf (t) values and slightly high ( 87 Sr/ 86 Sr) i ratios. These geochemical features suggest that their Protoliths are equivalent to backarc basin basalts (BABB) in a continental margin. The eclogite-facies metamorphic ages and grade are also different for these two types of eclogites. The oceanic-type eclogites were only metamorphosed under HP conditions in the Carboniferous whereas the continental-type eclogites were mostly metamorphosed under HP to UHP conditions in the Triassic. Thus, the two types of eclogites record the tectonic transition from oceanic subduction to continental collision during the closure of the Paleotethyan ocean between the South and North China Blocks. It is possible that the backarc basins were developed in the northern margin of the SCB during the Early Paleozoic, and then became a new subduction zone in which both backarc basin basalts and overlying terrigenous sediments were carried to a depth in the Carboniferous for the HP eclogite-facies metamorphism. Afterwards the continental crust would start to subduct northwards, eventually leading to the HP to UHP eclogite-facies metamorphism in the Triassic. Therefore, the Hong'an orogen is a composite one that is tectonically different not only from the Qinling–Tongbai orogens to the west but also from the Dabie–Sulu orogens to the east. The occurrence of both oceanic-type and continental-type eclogites in the Hong'an orogen suggests that the subduction of continental crust would be gravitationally pulled by the subduction of oceanic crust at the same subduction zone and the HP eclogite-facies metamorphic melange would be exhumed together along the same subduction channel.
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fluid rock interaction and geochemical transport during Protolith emplacement and continental collision a tale from qinglongshan ultrahigh pressure metamorphic rocks in the sulu orogen
American Journal of Science, 2014Co-Authors: Yixiang Chen, Long Li, Renxu ChenAbstract:Ultrahigh-pressure (UHP) metamorphic rocks from the Qinglongshan region of the Sulu orogen are comprehensively studied for their whole-rock geochem- istry, mineral O isotopes and zirconology. The metamorphic minerals, which experi- enced eclogite- to amphibolite-facies metamorphism, exhibit low to negative 18 O values, suggesting that the 18 O-depletion of UHP rocks was acquired from their igneous Protolith due to high-T meteoric-hydrothermal alteration during the Neopro- terozoic. The O isotope heterogeneity in the Protolith was not homogenized during the Triassic UHP metamorphism, indicating very limited fluid flow during orogenesis. However, the fluid flow is locally significant during exhumation of the UHP rocks, resulting in the formation of quartz veins, symplectites and coronas. Geochemical transport due to fluid action is evident in whole-rock geochemistry and mineralogical composition. The UHP rocks exhibit unreasonably low 87 Sr/ 86 Sr ratios at t1 750 Ma but much radiogenic Sr isotopes at t2 230 Ma, suggesting the mobility of water- soluble elements due to hydrothermal alteration during Protolith emplacement and metamorphic dehydration during continental collision. Fluid-rock interaction during exhumation would also have mobilized Al, Si, Ca and LREE, resulting in the formation of high-pressure veins in the UHP eclogites. The Protolith zircon of magmatic origin underwent different types of metamorphic recrystallization in response to fluid- mineral interaction, leading to differential redistribution of trace elements and O-Hf isotopes. Newly grown zircons of metamorphic origin exhibit negative 18 O values, indicating precipitation from negative 18 O fluids that were likely generated by metamor- phic dehydration of the hydrothermally altered negative 18 O rock-forming minerals during the Triassic. The metamorphic zircons exhibit relatively homogeneous Hf isotope compositions, suggesting that fluid Hf isotopes originated from the same Hf isotope composition of the Protolith. Relict zircon domains of magmatic origin exhibit both positive Hf (t) and negative Hf (t) values, indicating that the Protolith of UHP rocks formed by reworking of both juvenile and ancient crustal rocks.
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zircon u pb age and hf isotope evidence for contrasting origin of bimodal Protoliths for ultrahigh pressure metamorphic rocks from the chinese continental scientific drilling project
Journal of Metamorphic Geology, 2007Co-Authors: Yongfei Zheng, Renxu Chen, Zifu Zhao, Jun Tang, Xiaoming LiuAbstract:A combined study of zircon morphology, U–Pb ages and Hf isotopes as well as whole-rock major and trace elements was carried out for ultrahigh-pressure (UHP) eclogite and felsic gneiss from the main hole (MH) of the Chinese Continental Scientific Drilling (CCSD) project in the Sulu orogen. The results show contrasting Hf isotope compositions for bimodal UHP metaigneous rocks, pointing to contrasting origins for their Protoliths (thus dual-bimodal compositions). The samples of interest were from two continuous core segments from CCSD MH at depths of 734.21–737.16 m (I) and 929.67–932.86 m (II) respectively. Zircon U–Pb dating for four samples from the two core segments yields two groups of ages at 784 ± 17 and 222 ± 3 Ma, respectively, corresponding to Protolith formation during supercontinental rifting and metamorphic growth during continental collision. Although the Triassic UHP metamorphism significantly reset the zircon U–Pb system of UHP rocks, the Hf isotope compositions of igneous zircon can be used to trace their Protolith origin. Contrasting types of initial Hf isotope ratios are, respectively, correlated with segments I and II, regardless of their lithochemistry. The first type shows positive ɛHf(t) values of 7.8 ± 3.1 to 6.0 ± 3.0, with young Hf model age of 1.03 and 1.11 Ga. The second type exhibits negative ɛHf(t) values of −6.9 ± 1.6 to −9.1 ± 1.1, with old Hf model ages of 2.11 and 2.25 Ga. It appears that the UHP rocks from the two segments have Protoliths of contrasting origin. Consistent results are also obtained from their trace element compositions suggesting that mid-Neoproterozoic Protoliths of bimodal UHP metaigneous rocks formed during supercontinental rifting at the northern margin of the South China Block. Thus, the first type of bimodal magmatism formed by rapid reworking of juvenile crust, whereas the second type of bimodal magmatism was principally generated by rift anatexis of Paleoproterozoic crust. Melting of orogenic lithosphere has potential to bring about bimodal magmatism with contrasting origins. Because arc–continent collision zones are the best place to accumulate both juvenile and ancient crusts, the contrasting types of bimodal magmatism are proposed to occur in an arc–continent collision orogen during the supercontinental rifting, in response to the attempted breakup of the supercontinent Rodinia at c. 780 Ma.
Keiko Hattori - One of the best experts on this subject based on the ideXlab platform.
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geochemistry of subduction zone serpentinites a review
Lithos, 2013Co-Authors: Fabien Deschamps, Stéphane Guillot, Marguerite Godard, Keiko HattoriAbstract:Abstract Over the last decades, numerous studies have emphasized the role of serpentinites in the subduction zone geodynamics. Their presence and role in subduction environments are recognized through geophysical, geochemical and field observations of modern and ancient subduction zones and large amounts of geochemical database of serpentinites have been created. Here, we present a review of the geochemistry of serpentinites, based on the compilation of ~ 900 geochemical data of abyssal, mantle wedge and exhumed serpentinites after subduction. The aim was to better understand the geochemical evolution of these rocks during their subduction as well as their impact in the global geochemical cycle. When studying serpentinites, it is essential to determine their Protoliths and their geological history before serpentinization. The geochemical data of serpentinites shows little mobility of compatible and rare earth elements (REE) at the scale of hand-specimen during their serpentinization. Thus, REE abundance can be used to identify the Protolith for serpentinites, as well as magmatic processes such as melt/rock interactions before serpentinization. In the case of subducted serpentinites, the interpretation of trace element data is difficult due to the enrichments of light REE, independent of the nature of the Protolith. We propose that enrichments are probably not related to serpentinization itself, but mostly due to (sedimentary-derived) fluid/rock interactions within the subduction channel after the serpentinization. It is also possible that the enrichment reflects the geochemical signature of the mantle Protolith itself which could derive from the less refractory continental lithosphere exhumed at the ocean–continent transition. Additionally, during the last ten years, numerous analyses have been carried out, notably using in situ approaches, to better constrain the behavior of fluid-mobile elements (FME; e.g. B, Li, Cl, As, Sb, U, Th, Sr) incorporated in serpentine phases. The abundance of these elements provides information related to the fluid/rock interactions during serpentinization and the behavior of FME, from their incorporation to their gradual release during subduction. Serpentinites are considered as a reservoir of the FME in subduction zones and their role, notably on arc magma composition, is underestimated presently in the global geochemical cycle.
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geochemistry of subduction zone serpentinites a review
Lithos, 2013Co-Authors: Fabien Deschamps, Stéphane Guillot, Marguerite Godard, Keiko HattoriAbstract:Abstract Over the last decades, numerous studies have emphasized the role of serpentinites in the subduction zone geodynamics. Their presence and role in subduction environments are recognized through geophysical, geochemical and field observations of modern and ancient subduction zones and large amounts of geochemical database of serpentinites have been created. Here, we present a review of the geochemistry of serpentinites, based on the compilation of ~ 900 geochemical data of abyssal, mantle wedge and exhumed serpentinites after subduction. The aim was to better understand the geochemical evolution of these rocks during their subduction as well as their impact in the global geochemical cycle. When studying serpentinites, it is essential to determine their Protoliths and their geological history before serpentinization. The geochemical data of serpentinites shows little mobility of compatible and rare earth elements (REE) at the scale of hand-specimen during their serpentinization. Thus, REE abundance can be used to identify the Protolith for serpentinites, as well as magmatic processes such as melt/rock interactions before serpentinization. In the case of subducted serpentinites, the interpretation of trace element data is difficult due to the enrichments of light REE, independent of the nature of the Protolith. We propose that enrichments are probably not related to serpentinization itself, but mostly due to (sedimentary-derived) fluid/rock interactions within the subduction channel after the serpentinization. It is also possible that the enrichment reflects the geochemical signature of the mantle Protolith itself which could derive from the less refractory continental lithosphere exhumed at the ocean–continent transition. Additionally, during the last ten years, numerous analyses have been carried out, notably using in situ approaches, to better constrain the behavior of fluid-mobile elements (FME; e.g. B, Li, Cl, As, Sb, U, Th, Sr) incorporated in serpentine phases. The abundance of these elements provides information related to the fluid/rock interactions during serpentinization and the behavior of FME, from their incorporation to their gradual release during subduction. Serpentinites are considered as a reservoir of the FME in subduction zones and their role, notably on arc magma composition, is underestimated presently in the global geochemical cycle.
