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Jan Wijbrans - One of the best experts on this subject based on the ideXlab platform.
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40ar 39ar geochronological study and the genesis of extraneous 40ar in yuka hp uhp Phengite north qaidam nw china
Earth Science Frontiers, 2014Co-Authors: Hua-ning Qiu, Jan Wijbrans, Fraukje M. Brouwer, Min WangAbstract:Six Phengites from a restrict area in the Yuka terrane of the north Qaidam high/ultrahigh pressure (HP/UHP) metamorphic belt were dated using the 40Ar/39Ar method by laser stepwise heating, in order to investigate their geochronological significances and decipher the genesis of extraneous 40Ar in Phengite. New 40Ar/39Ar age results show that the extraneous 40Ar exists in UHP metamorphic rocks (eclogite and amphibolite) Phengite but not in their country rocks (gneiss and schist) Phengite. High-Si and Mg-rich Phengites from eclogite and Phengite-bearing amphibolite yield disturbed age spectra, anomalously old apparent ages and a large variation in total gas ages (708-534 Ma) and isochron ages (681-513 Ma), indicating the existence of heterogeneous extraneous 40Ar. In contrast, relative low-Si and Fe-rich Phengites from granitic gneiss and schist yield flat age spectra and geologically significant plateau ages (454.4 Ma and 418.0 Ma). The data points constituting the age plateaux yield isochrons with concordantly intercept ages of 456.2±2.3 and 418.1±2.2 Ma, corresponding to initial 40Ar/36Ar ratios of 177.3±151.2 and 287.1±16.9, respectively, implying that there is no extraneous 40Ar inside the minerals. The plateau age of 454.4 Ma recorded by Phengite from granitic gneiss is interpreted as the age of cooling through its retention temperature for argon (ca. 400°C), and responding to the time of the Yuka metamorphic rocks exhumed from upper-mantle depth to middle-lower crustal levels (ca. 10 km in depth) after deep subduction. In contrast, the plateau age of 418 Ma from the schist should correspond to a Later Silurian ductile shearing deformation event. Considering the protoliths characteristics of the eclogite and amphibolite, the extremely high p-T metamorphic condition and aqueous fluid shortage environment during eclogite formation, we would suggest that the extraneous 40Ar*(not produced by in situ radiogenic decay of 40K) in Phengites from Yuka is a locally derived component from the protolith rather than from a late infiltrating fluid.
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40Ar/39Ar geochronological study and the genesis of extraneous 40Ar in Yuka HP/UHP Phengite, North Qaidam, NW China
Earth Science Frontiers, 2014Co-Authors: Hua-ning Qiu, Jan Wijbrans, Fraukje M. Brouwer, Min WangAbstract:Six Phengites from a restrict area in the Yuka terrane of the north Qaidam high/ultrahigh pressure (HP/UHP) metamorphic belt were dated using the 40Ar/39Ar method by laser stepwise heating, in order to investigate their geochronological significances and decipher the genesis of extraneous 40Ar in Phengite. New 40Ar/39Ar age results show that the extraneous 40Ar exists in UHP metamorphic rocks (eclogite and amphibolite) Phengite but not in their country rocks (gneiss and schist) Phengite. High-Si and Mg-rich Phengites from eclogite and Phengite-bearing amphibolite yield disturbed age spectra, anomalously old apparent ages and a large variation in total gas ages (708-534 Ma) and isochron ages (681-513 Ma), indicating the existence of heterogeneous extraneous 40Ar. In contrast, relative low-Si and Fe-rich Phengites from granitic gneiss and schist yield flat age spectra and geologically significant plateau ages (454.4 Ma and 418.0 Ma). The data points constituting the age plateaux yield isochrons with concordantly intercept ages of 456.2±2.3 and 418.1±2.2 Ma, corresponding to initial 40Ar/36Ar ratios of 177.3±151.2 and 287.1±16.9, respectively, implying that there is no extraneous 40Ar inside the minerals. The plateau age of 454.4 Ma recorded by Phengite from granitic gneiss is interpreted as the age of cooling through its retention temperature for argon (ca. 400°C), and responding to the time of the Yuka metamorphic rocks exhumed from upper-mantle depth to middle-lower crustal levels (ca. 10 km in depth) after deep subduction. In contrast, the plateau age of 418 Ma from the schist should correspond to a Later Silurian ductile shearing deformation event. Considering the protoliths characteristics of the eclogite and amphibolite, the extremely high p-T metamorphic condition and aqueous fluid shortage environment during eclogite formation, we would suggest that the extraneous 40Ar*(not produced by in situ radiogenic decay of 40K) in Phengites from Yuka is a locally derived component from the protolith rather than from a late infiltrating fluid.
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40ar 39ar laser probe dating of detrital white micas from cretaceous sedimentary rocks of the eastern alps evidence for variscan high pressure metamorphism and implications for alpine orogeny
Geology, 1996Co-Authors: Hilmar Von Eynatten, Reinhard Gaupp, Jan WijbransAbstract:The detritus of Cretaceous synorogenic sandstones of the northern margin of the Austroalpine microplate contains evidence for a high-pressure metamorphic basement and obducted oceanic crust exposed in early Alpine time. 40 Ar/ 39 Ar laser-probe data of detrital white micas give excellent plateau ages in a narrow range from 320 to 360 Ma. White micas cover the whole range from muscovites up to Phengites (3.04 to 3.48 Si per formula unit). Heavy mineral spectra contain chrome spinel, glaucophane, chloritoid, epidote, and garnet, as well as zircon, tourmaline, and rutile. Glaucophane, chloritoid, and Phengite correlate in their abundance. These minerals also correlate positively with the stable minerals zircon, tourmaline, and rutile derived from continental rock and negatively with chrome spinel, which represents oceanic crustal provenance. We propose that both glaucophane and Phengite come from the same source rock, Variscan high-pressure metamorphic rocks of the Austroalpine basement. The Early Carboniferous age gives constraints for paleogeographic models concerning the collision between Gondwana and Laurussia. Furthermore, detrital glaucophane in the early Alpine sedimentary rocks cannot be used as proof for Cretaceous subduction at the Austroalpine-Penninic plate boundary.
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40Ar/39Ar laser-probe dating of detrital white micas from Cretaceous sedimentary rocks of the Eastern Alps: Evidence for Variscan high-pressure metamorphism and implications for Alpine orogeny
Geology, 1996Co-Authors: Hilmar Von Eynatten, Reinhard Gaupp, Jan WijbransAbstract:The detritus of Cretaceous synorogenic sandstones of the northern margin of the Austroalpine microplate contains evidence for a high-pressure metamorphic basement and obducted oceanic crust exposed in early Alpine time. 40 Ar/ 39 Ar laser-probe data of detrital white micas give excellent plateau ages in a narrow range from 320 to 360 Ma. White micas cover the whole range from muscovites up to Phengites (3.04 to 3.48 Si per formula unit). Heavy mineral spectra contain chrome spinel, glaucophane, chloritoid, epidote, and garnet, as well as zircon, tourmaline, and rutile. Glaucophane, chloritoid, and Phengite correlate in their abundance. These minerals also correlate positively with the stable minerals zircon, tourmaline, and rutile derived from continental rock and negatively with chrome spinel, which represents oceanic crustal provenance. We propose that both glaucophane and Phengite come from the same source rock, Variscan high-pressure metamorphic rocks of the Austroalpine basement. The Early Carboniferous age gives constraints for paleogeographic models concerning the collision between Gondwana and Laurussia. Furthermore, detrital glaucophane in the early Alpine sedimentary rocks cannot be used as proof for Cretaceous subduction at the Austroalpine-Penninic plate boundary.
Karen Ziegler - One of the best experts on this subject based on the ideXlab platform.
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argon oxygen and boron isotopic evidence documenting 40are accumulation in Phengite during water rich high pressure subduction metasomatism of continental crust
Earth and Planetary Science Letters, 2016Co-Authors: Carrie A. Menold, Marty Grove, Natalie E. Sievers, Craig E. Manning, An Yin, Edward D. Young, Karen ZieglerAbstract:Abstract The Luliang Shan area of the North Qaidam high pressure (HP) to ultrahigh pressure (UHP) metamorphic terrane in northwestern China features thick, garnet- and Phengite-rich metasomatic selvages that formed around gneiss-hosted mafic eclogite blocks during HP conditions. Here we present new 40 Ar/ 39 Ar, δ 18 O , and δ 11 B results from a previously studied 30 m, 18 sample traverse that extends from the host gneiss into a representative eclogite block. Previous thermobarometry and new mica-quartz oxygen isotope thermometry from the traverse reveal that the Phengite-rich selvage formed at temperatures similar to those recorded by the eclogites at peak pressure. Quartz and white mica δ 18 O data from the selvage cannot be explained by simple mixing of gneiss and eclogite, and indicate a fluid/rock ratio >1 during regional-scale infiltration of high δ 18 O (ca. 14‰) fluids. Heavy δ 18 O overgrowths of metamorphic zircon over lighter δ 18 O detrital grains indicate that the gneiss was similarly affected. Starkly contrasting boron content and δ 11 B compositions for the host gneiss and the selvage also cannot be explained by local-scale devolatilization of the gneiss to form the selvage. Instead, the boron systematics are best attributed to two distinct phases of fluid infiltration: (1) low-boron selvage Phengite with δ 11 B from −10 to −30‰ grew under HP conditions; and (2) tourmaline and boron-rich muscovite with generally positive δ 11 B crystallized in the host gneiss under subsequent lower pressure epidote–amphibolite facies conditions as the Luliang Shan gneiss terrane was exhumed past shallower portions of the subduction channel. Consistent with observations made worldwide, we were able to identify uptake of excess argon ( 40 Ar E ) in Phengite as a high pressure phenomenon. Phengite 40 Ar/ 39 Ar ages from massive eclogite exceed the ca. 490 Ma zircon U–Pb age of eclogite metamorphism by a factor of 1.5. However, Phengite ages from the more permeable schistose selvage were even older, exceeding the time of eclogite formation by a factor of 1.7. In contrast, lower pressure retrograde muscovite present within the host gneiss and in discrete shear zones cutting the selvage yield 40 Ar/ 39 Ar ages that were younger than the time of HP metamorphism and consistent with regional cooling age patterns. Our observation of high 40 Ar E concentrations in Phengite from schistose rocks infiltrated by regionally extensive fluids at HP conditions runs contrary to widely held expectations. Conventional wisdom dictates that low Phengite/fluid partition coefficients for argon ( D phg/fluid Ar = 10 − 3 to 10 − 5 ) coupled with the dry, closed systems conditions that are widely reported to characterize HP metamorphism of continental crust explains why high concentrations of 40 Ar E partitions are able to accumulate within Phengite. We alternatively propose that Phengite/fluid partition coefficients for argon increase linearly with pressure to values as high as 10 − 2 to allow Phengites to accumulate large amounts of 40 Ar E from aqueous fluids under HP to UHP conditions.
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Argon, oxygen, and boron isotopic evidence documenting 40 Ar E accumulation in Phengite during water-rich high-pressure subduction metasomatism of continental crust
Earth and Planetary Science Letters, 2016Co-Authors: Carrie A. Menold, Marty Grove, Natalie E. Sievers, Craig E. Manning, An Yin, Edward D. Young, Karen ZieglerAbstract:Abstract The Luliang Shan area of the North Qaidam high pressure (HP) to ultrahigh pressure (UHP) metamorphic terrane in northwestern China features thick, garnet- and Phengite-rich metasomatic selvages that formed around gneiss-hosted mafic eclogite blocks during HP conditions. Here we present new 40 Ar/ 39 Ar, δ 18 O , and δ 11 B results from a previously studied 30 m, 18 sample traverse that extends from the host gneiss into a representative eclogite block. Previous thermobarometry and new mica-quartz oxygen isotope thermometry from the traverse reveal that the Phengite-rich selvage formed at temperatures similar to those recorded by the eclogites at peak pressure. Quartz and white mica δ 18 O data from the selvage cannot be explained by simple mixing of gneiss and eclogite, and indicate a fluid/rock ratio >1 during regional-scale infiltration of high δ 18 O (ca. 14‰) fluids. Heavy δ 18 O overgrowths of metamorphic zircon over lighter δ 18 O detrital grains indicate that the gneiss was similarly affected. Starkly contrasting boron content and δ 11 B compositions for the host gneiss and the selvage also cannot be explained by local-scale devolatilization of the gneiss to form the selvage. Instead, the boron systematics are best attributed to two distinct phases of fluid infiltration: (1) low-boron selvage Phengite with δ 11 B from −10 to −30‰ grew under HP conditions; and (2) tourmaline and boron-rich muscovite with generally positive δ 11 B crystallized in the host gneiss under subsequent lower pressure epidote–amphibolite facies conditions as the Luliang Shan gneiss terrane was exhumed past shallower portions of the subduction channel. Consistent with observations made worldwide, we were able to identify uptake of excess argon ( 40 Ar E ) in Phengite as a high pressure phenomenon. Phengite 40 Ar/ 39 Ar ages from massive eclogite exceed the ca. 490 Ma zircon U–Pb age of eclogite metamorphism by a factor of 1.5. However, Phengite ages from the more permeable schistose selvage were even older, exceeding the time of eclogite formation by a factor of 1.7. In contrast, lower pressure retrograde muscovite present within the host gneiss and in discrete shear zones cutting the selvage yield 40 Ar/ 39 Ar ages that were younger than the time of HP metamorphism and consistent with regional cooling age patterns. Our observation of high 40 Ar E concentrations in Phengite from schistose rocks infiltrated by regionally extensive fluids at HP conditions runs contrary to widely held expectations. Conventional wisdom dictates that low Phengite/fluid partition coefficients for argon ( D phg/fluid Ar = 10 − 3 to 10 − 5 ) coupled with the dry, closed systems conditions that are widely reported to characterize HP metamorphism of continental crust explains why high concentrations of 40 Ar E partitions are able to accumulate within Phengite. We alternatively propose that Phengite/fluid partition coefficients for argon increase linearly with pressure to values as high as 10 − 2 to allow Phengites to accumulate large amounts of 40 Ar E from aqueous fluids under HP to UHP conditions.
Philippe Agard - One of the best experts on this subject based on the ideXlab platform.
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Architecture and P-T-deformation-time evolution of the Chinese SW-Tianshan HP/UHP complex: Implications for subduction dynamics
Earth-Science Reviews, 2019Co-Authors: Zhou Tan, Philippe Agard, Patrick Monie, Jun Gao, Timm John, Léa Bayet, Tuo Jiang, Xin-shui Wang, Tao Hong, Bo WanAbstract:We present the first comprehensive P-T-deformation-time and kinematic constraints for HP/UHP eclogite, blueschist and greenschist-facies metavolcano-sedimentary rocks cropping out in the Chinese SW-Tianshan metamorphic complex (within the ~30 km wide N-S Akeyazi-Kebuerte area). We reappraise this HP/UHP “mélange”, which should be divided into three main tectonic units, from north to south, according to their discrepant lithologies and P-T-time-deformation histories. These three units crop out in a tectonic window beneath greenschist-facies metavolcanics. P-T estimates point to 1) UHP-LT conditions around 2.6–2.9 GPa at ~520 °C for the metavolcano-sedimentary rocks of the northern HP/UHP unit, 2) HP-LT conditions of 1.8–2.1 GPa and ~500 °C for the central blueschist horizon and 3) lower-blueschist facies conditions of 1.0–1.5 GPa at ~485 °C for the southern ultramafic-mafic unit. In situ laser probe Ar-Ar age constraints (with textural control) on recrystallized Phengites from the HP/UHP unit cluster within 315 to 325 Ma. Phengite ages from the central blueschist horizon are 10–15 Ma older, at ~325–345 Ma. Laser-ICP-MS U-Pb dating on zircon from the south ultramafic-mafic unit yield ages around ~360 Ma for metamorphic overgrowths. In contrast, step-heating Ar-Ar Phengite age constraints for the greenschist-facies metavolcano-sediments fall within 280–300 Ma. These new field and P-T-time data disclose an episodic exhumation of three main tectonic slices, respectively from ~85 km, ~65 km and ~45 km depths, and a progressive change of metamorphic gradients from ~12 to ~6–7 °C/km, which could reflect a cooling of the subduction system with time. Final juxtaposition at ~20 km was probably achieved around 300 Ma, prior to collision.
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devolatilization history and trace element mobility in deeply subducted sedimentary rocks evidence from western alps hp uhp suites
Chemical Geology, 2013Co-Authors: Gray E Bebout, Philippe Agard, Katsura Kobayashi, Takuya Moriguti, Eizou NakamuraAbstract:Abstract Metapelitic rocks of the Schistes Lustres in the Cottian Alps, Italy (peak metamorphic conditions of 350–500 °C, 1.2–2.0 GPa) and at the UHP Lago di Cignana locality (Valtournenche, Italy; ~ 550 °C, 2.5–3.0 GPa) preserve records of prograde devolatilization in their mineral modes and chemistry, contents of volatiles and fluid-mobile elements (elements relatively mobile in aqueous fluids), and B and N isotope compositions. This suite allows study of prograde devolatilization history, across a wide range in metamorphic grade, in metasedimentary rocks that experienced high-P/T prograde paths similar to those experienced in most modern subduction zones. Across grade, whole-rock samples are in general uniform in their concentrations of relatively fluid-mobile elements N, B, Li, Cs, Ba, and Rb, normalized to the concentrations of the less mobile K 2 O and Al 2 O 3 , showing only hints of loss in several of the highest-grade samples. With increasing grade, ion microprobe analyses of Phengites show subtle decrease in B concentration, uniformity in Ba and Cs concentrations, and increase in Li concentrations, the latter likely due to release from chlorite during its breakdown. In one Cignana sample, Phengite inclusions in garnets are enriched in B relative to matrix Phengite, consistent with either whole-rock B loss after garnet growth or, more likely, closed-system behavior and partitioning of B into paragonite or tourmaline stabilized after garnet growth. In samples with both paragonite and Phengite, paragonite shows relative enrichment in B and Sr, and Phengite is enriched in Cs, Ba, and presumably also N and Rb (the latter showing strong whole-rock correlations with K 2 O). Whole-rock δ 15 N shows a hint of shift to higher values in the highest grade rocks (Cignana) and, accordingly, calculated prograde dehydration histories for appropriate bulk compositions, using the Perple-X database, indicate that significant (~ 20%) dehydration would for some rocks occur over the temperature interval of 450 to 550 °C, largely related to the breakdown of chlorite (and to a lesser extent carpholite). Small amounts of loss of N into these fluids could have resulted in minor shift in δ 15 N, with decrease in whole-rock N concentration masked by heterogeneity inherent with the sedimentary protoliths. Partitioning of Cs and Li (possibly also Rb and Ba) from white micas into H 2 O-rich fluids largely produced by chlorite breakdown could similarly have produced the subtle decreases in the concentrations in these elements noted in several high-grade samples. Neoblastic tourmaline in higher-grade rocks likely sequestered some fraction of the B lost from micas, resulting in a lack of obvious whole-rock B loss to accompany the up-grade trend of decreasing B concentrations in Phengite. This tourmaline shows core-to-rim decrease in δ 11 B consistent with growth during small amounts of progressive B loss from Phengites. Taken together, the whole-rock and SIMS data presented here, and the whole-rock dataset of Busigny et al. (2003) , demonstrate impressive retention, during prograde forearc devolatilization, of elements thought to be relatively fluid-mobile (particularly H, N, B, Li, Ba, and Cs). Retention of these elements in metasedimentary rocks subducted to depths overlapping those beneath arc volcanic fronts (~ 90 km estimated for subsolidus, peak Cignana metamorphism) implies their availability for transfer into arc source regions, in aqueous fluids or silicate melts, or into the mantle to depths beyond subarc regions.
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Interlayer and Si content of Phengite in HP–LT carpholite‐bearing metapelites
Journal of Metamorphic Geology, 2001Co-Authors: Philippe Agard, Olivier Vidal, Bruno GofféAbstract:Phengite occurring along with carpholite±lawsonite and/or chloritoid in HP–LT domains shows not only variable Si–(Mg+Fe) contents, but also variable interlayer contents (IC). To determine whether these chemical variations are coherently related to variation in P–T conditions on a regional scale, c. 100 rock samples were sampled in metapelites metamorphosed at conditions varying from 350 °C, 8 to 12 kbar to 450–500 °C, 18 to 20 kbar (Schistes Lustres complex, franco-italian Western Alps). Based on microstructural and habit criteria, four types of Phengite were differentiated that are related either to the rock mineralogy (carpholite vs chloritoid bearing samples) or correspond to various generations of Phengite occurring in the same rock sample or thin section. Microprobe analyses reveal that each type of Phengite is characterized by a specific composition and that Phengite associated with carpholite has a lower interlayer content than Phengite associated with chloritoid. The successive generations of retrograde Phengite overgrowing carpholite point to a large decrease of interlayer content (c. 0.9–0.7 pfu) and (Fe+Mg) content (c. 0.25–0 pfu) with decreasing P–T conditions. This change is best accounted for by a gradual increase of the pyrophyllite component. In contrast, Phengite from higher-temperature, chloritoid-bearing rock samples shows an almost constant interlayer content (c. 0.9–0.95 pfu) but a larger decrease of (Fe+Mg) content (c. 0.6–0.1 pfu). Hence, (1) the composition of the different Phengite generations occurring (metastably) in the same rock sample may be used to retrieve points in P–T loops and (2) the pyrophyllitic substitution in Phengite is large at low-temperature conditions and cannot be ignored. Thermobarometric estimates based on the Si-content alone will therefore result in pressure over-estimates. We propose a tentative location of the Phengite Si and IC isopleths in P–T space which could allow a direct determination of the P–T conditions in carpholite-bearing rocks. Especially in some carpholite-bearing rocks, new thermodynamic models accounting for tschermak and pyrophyllitic substitution are also required prior to making reliable thermobarometric estimates in HP-LT metapelites.
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interlayer and si content of Phengite in hp lt carpholite bearing metapelites
Journal of Metamorphic Geology, 2001Co-Authors: Philippe Agard, Olivier Vidal, Bruno GofféAbstract:Phengite occurring along with carpholite±lawsonite and/or chloritoid in HP–LT domains shows not only variable Si–(Mg+Fe) contents, but also variable interlayer contents (IC). To determine whether these chemical variations are coherently related to variation in P–T conditions on a regional scale, c. 100 rock samples were sampled in metapelites metamorphosed at conditions varying from 350 °C, 8 to 12 kbar to 450–500 °C, 18 to 20 kbar (Schistes Lustres complex, franco-italian Western Alps). Based on microstructural and habit criteria, four types of Phengite were differentiated that are related either to the rock mineralogy (carpholite vs chloritoid bearing samples) or correspond to various generations of Phengite occurring in the same rock sample or thin section. Microprobe analyses reveal that each type of Phengite is characterized by a specific composition and that Phengite associated with carpholite has a lower interlayer content than Phengite associated with chloritoid. The successive generations of retrograde Phengite overgrowing carpholite point to a large decrease of interlayer content (c. 0.9–0.7 pfu) and (Fe+Mg) content (c. 0.25–0 pfu) with decreasing P–T conditions. This change is best accounted for by a gradual increase of the pyrophyllite component. In contrast, Phengite from higher-temperature, chloritoid-bearing rock samples shows an almost constant interlayer content (c. 0.9–0.95 pfu) but a larger decrease of (Fe+Mg) content (c. 0.6–0.1 pfu). Hence, (1) the composition of the different Phengite generations occurring (metastably) in the same rock sample may be used to retrieve points in P–T loops and (2) the pyrophyllitic substitution in Phengite is large at low-temperature conditions and cannot be ignored. Thermobarometric estimates based on the Si-content alone will therefore result in pressure over-estimates. We propose a tentative location of the Phengite Si and IC isopleths in P–T space which could allow a direct determination of the P–T conditions in carpholite-bearing rocks. Especially in some carpholite-bearing rocks, new thermodynamic models accounting for tschermak and pyrophyllitic substitution are also required prior to making reliable thermobarometric estimates in HP-LT metapelites.
Bo Wan - One of the best experts on this subject based on the ideXlab platform.
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Architecture and P-T-deformation-time evolution of the Chinese SW-Tianshan HP/UHP complex: Implications for subduction dynamics
Earth-Science Reviews, 2019Co-Authors: Zhou Tan, Philippe Agard, Patrick Monie, Jun Gao, Timm John, Léa Bayet, Tuo Jiang, Xin-shui Wang, Tao Hong, Bo WanAbstract:We present the first comprehensive P-T-deformation-time and kinematic constraints for HP/UHP eclogite, blueschist and greenschist-facies metavolcano-sedimentary rocks cropping out in the Chinese SW-Tianshan metamorphic complex (within the ~30 km wide N-S Akeyazi-Kebuerte area). We reappraise this HP/UHP “mélange”, which should be divided into three main tectonic units, from north to south, according to their discrepant lithologies and P-T-time-deformation histories. These three units crop out in a tectonic window beneath greenschist-facies metavolcanics. P-T estimates point to 1) UHP-LT conditions around 2.6–2.9 GPa at ~520 °C for the metavolcano-sedimentary rocks of the northern HP/UHP unit, 2) HP-LT conditions of 1.8–2.1 GPa and ~500 °C for the central blueschist horizon and 3) lower-blueschist facies conditions of 1.0–1.5 GPa at ~485 °C for the southern ultramafic-mafic unit. In situ laser probe Ar-Ar age constraints (with textural control) on recrystallized Phengites from the HP/UHP unit cluster within 315 to 325 Ma. Phengite ages from the central blueschist horizon are 10–15 Ma older, at ~325–345 Ma. Laser-ICP-MS U-Pb dating on zircon from the south ultramafic-mafic unit yield ages around ~360 Ma for metamorphic overgrowths. In contrast, step-heating Ar-Ar Phengite age constraints for the greenschist-facies metavolcano-sediments fall within 280–300 Ma. These new field and P-T-time data disclose an episodic exhumation of three main tectonic slices, respectively from ~85 km, ~65 km and ~45 km depths, and a progressive change of metamorphic gradients from ~12 to ~6–7 °C/km, which could reflect a cooling of the subduction system with time. Final juxtaposition at ~20 km was probably achieved around 300 Ma, prior to collision.
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Architecture and P-T-deformation-time evolution of the Chinese SW-Tianshan HP/UHP complex: Implications for subduction dynamics
Elsevier, 2019Co-Authors: Tan Zhou, Agard Philippe, Monie Patrick, Gao Jun, John Timm, Bayet Léa, Jiang Tuo, Wang Xin-shui, Hong Tao, Bo WanAbstract:International audienceWe present the first comprehensive P-T-deformation-time and kinematic constraints for HP/UHP eclogite, blueschist and greenschist-facies metavolcano-sedimentary rocks cropping out in the Chinese SW-Tianshan metamorphic complex (within the ~30 km wide N-S Akeyazi-Kebuerte area). We reappraise this HP/UHP “mélange”, which should be divided into three main tectonic units, from north to south, according to their discrepant lithologies and P-T-time-deformation histories. These three units crop out in a tectonic window beneath greenschist-facies metavolcanics. P-T estimates point to 1) UHP-LT conditions around 2.6–2.9 GPa at ~520 °C for the metavolcano-sedimentary rocks of the northern HP/UHP unit, 2) HP-LT conditions of 1.8–2.1 GPa and ~500 °C for the central blueschist horizon and 3) lower-blueschist facies conditions of 1.0–1.5 GPa at ~485 °C for the southern ultramafic-mafic unit. In situ laser probe Ar-Ar age constraints (with textural control) on recrystallized Phengites from the HP/UHP unit cluster within 315 to 325 Ma. Phengite ages from the central blueschist horizon are 10–15 Ma older, at ~325–345 Ma. Laser-ICP-MS U-Pb dating on zircon from the south ultramafic-mafic unit yield ages around ~360 Ma for metamorphic overgrowths. In contrast, step-heating Ar-Ar Phengite age constraints for the greenschist-facies metavolcano-sediments fall within 280–300 Ma. These new field and P-T-time data disclose an episodic exhumation of three main tectonic slices, respectively from ~85 km, ~65 km and ~45 km depths, and a progressive change of metamorphic gradients from ~12 to ~6–7 °C/km, which could reflect a cooling of the subduction system with time. Final juxtaposition at ~20 km was probably achieved around 300 Ma, prior to collision
Junfeng Zhang - One of the best experts on this subject based on the ideXlab platform.
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evidence for uhp anatexis in the shuanghe uhp paragneiss from inclusions in clinozoisite garnet and zircon
Journal of Metamorphic Geology, 2020Co-Authors: Jorg Hermann, Shu Zheng, Junfeng ZhangAbstract:The Shuanghe garnet‐bearing paragneiss from the Dabie ultra‐high–pressure (UHP) orogen occurs as an interlayer within partially retrogressed eclogite. A first UHP metamorphic stage at 680°C, 3.8–4.1 GPa is documented by Zr‐in‐rutile temperatures coupled with Phengite inclusions (Si = 3.55) in clinozoisite and grossular‐rich garnet. Relic matrix Phengite and Phengite inclusions in zircon rims display lower Si of 3.42. Combined with garnet compositions and Ti‐in‐zircon temperatures, they provide evidence for a second UHP metamorphic stage at 800–850°C, ~3.8 GPa. Such isobaric heating at UHP conditions has not been documented so far from the adjacent eclogites and other rock types in the Dabie orogen and indicates proximity to the hot, convecting mantle wedge. The dominant mineral assemblage consisting of plagioclase, epidote, biotite, and amphibole provides evidence for widespread retrogression during the exhumation of the UHP paragneiss. Several types of polyphase mineral inclusions were identified. Phengite inclusions hosted by clinozoisite are partially replaced by kyanite and K‐feldspar, whereas inclusions in host garnet consist of relic Phengite, K‐feldspar, and garnet, indicating limited sub‐solidus dehydration of Phengite by the reaction Ph→Kfs+Ky±Grt+fluid. Tightly intergrown K‐feldspar and quartz are preserved as inclusions with sharp boundaries and radial cracks in garnet. Analyses of whole inclusions also show small enrichments in light rare earth elements. These inclusions are interpreted to be derived from melting of an inclusion assemblage consisting of Ph+Coe±Czo. A third type of polyphase inclusion consists of typical nanogranite (Ab+Kfs+Qz±Ep) inclusions in recrystallized metamorphic zircon. Ti‐in‐zircon thermometry and the Si content of Phengite included in these zircon domains indicate that melting occurred at 800–850°C and 3.8–4.0 GPa during isobaric heating at UHP conditions. The partial melting event led to an equilibration of trace elements in garnet, Phengite, and apatite. Using published partition coefficients between these minerals and hydrous granitic melt, the trace element composition of the UHP anatectic melt can be constrained. The melts are characterized by high LILE contents and pronounced relative enrichments of U over Th and Ta over Nb. The REE are below primitive mantle values, likely due to the presence of residual clinozoisite and garnet during partial melting. So far, no major granitic bodies have been found that share the same trace element pattern as the partial melts from the UHP anatexis of the Shuanghe paragneiss.
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An experimental study of dehydration melting of Phengite-bearing eclogite at 1.5–3.0 GPa
Science Bulletin, 2009Co-Authors: Qiang Liu, Zhenmin Jin, Junfeng ZhangAbstract:Dehydration melting experiments were performed on ultrahigh-pressure eclogite from Bixiling in the Dabie orogen at 1.5–3.0 GPa and 800–950°C using piston cylinder apparatus. The results show that (1) eclogite with ∼5% Phengite started to melt at T⩽800–850°C and P = 1.5–2.0 GPa and produced about 3% granitic melt; (2) the products of dehydration melting of Phengite-bearing eclogite vary with temperature and pressure. Fluid released from dehydration of Phengite and zoisite leads to partial melting of eclogite and formation of plagioclase reaction rim around kyanite at pressures of 1.5–2.0 GPa and temperatures of 800–850°C; (3) Phengite reacted with omphacite and quartz and produced oligoclase, kyanite and melt at elevated temperatures. Oligoclase is the primary reaction product produced by partial melting of Phengite in the eclogite; and (4) the dehydration melting of Phengite-bearing eclogite at pressures of 1.5–3.0 GPa and temperatures ⩾900°C results in formation of garnets with higher molar fraction of pyrope (37.67 wt.%–45.94 wt.%). Potassium feldspar and jadeite occur at P = 2.4–3.0 GPa and T⩾900°C, indicating higher pressure and fluid-absent conditions. Our results constrain the solidus for dehydration melting of Phengite-bearing eclogite at pressures of 1.5–3.0 GPa. Combining experimental results with field observations of partial melting in natural eclogites, we concluded that Phengite-bearing eclogites from the Dabie-Sulu orogen were able to partially molten at P = 1.5–2.0 GPa and T = 800–850°C during exhumation. The ultrahigh-high pressure eclogites would have experienced partial melting in association with metamorphic phase transformation under different fluid conditions.