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Stefan M Schmalholz - One of the best experts on this subject based on the ideXlab platform.
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peak alpine metamorphic conditions from staurolite bearing Metapelites in the monte rosa nappe central european alps and geodynamic implications
Journal of Metamorphic Geology, 2021Co-Authors: Joshua David Vaughanhammon, Cindy Luisier, Lukas P Baumgartner, Stefan M SchmalholzAbstract:The tectono-metamorphic evolution of the European Alps is still contentious. The Monte Rosa tectonic unit is a prominent nappe in the Central European Alps and estimates of its peak Alpine pressure (P) and temperature (T) conditions are essential for reconstructing its tectonometamorphic evolution. However, the reported peak Alpine P and T estimates vary considerably between 1.2 and 2.7 GPa and 490 and 640 °C for a variety of lithologies. Here, we show petrology and pseudosection modelling of metapelitic assemblages from the western portions of the Monte Rosa nappe (upper Ayas valley, Italy). We present newly discovered staurolite-chloritoid bearing metapelitic assemblages. These assemblages exhibit an Alpine high-P metamorphic overprint of a former contact-metamorphic mineral assemblage generated by post-Variscan granitic intrusions. Staurolite contains major amounts of Zn (up to 1.0 atoms per formula units), which is currently, in contrast to Fe-and Mg-staurolite end-members, not considered in any thermodynamic database. We employ two end-member mixing models for Zn in staurolite, site mixing and molecular mixing. Both models enlarge the P and T stability range for the observed assemblage, where site mixing has the largest influence of 0.2 GPa and 20 C. Our results for three Metapelite assemblages, with and without staurolite, indicate peak Alpine P of 1.6 ± 0.2 GPa and peak T of 585 20 C. These peak P estimates agree with previously published estimates for metagranites in the nappe, and are in stark contrast with peak P obtained from talc, chloritoid, phengite and quartz bearing lithologies termed 'whiteschists' (>2.2 GPa). Our results confirm a variation of peak Alpine P of 0.6 ± 0.2 GPa between metagranite/Metapelite lithologies and a nearby whiteschist lens (>2.2 GPa) within the metagranite. Field observations indicate that the studied region is structurally coherent and that the whiteschist is not a tectonic slice formed by tectonic melange. We suggest that the consistent peak P for Metapelite and metagranite assemblages represents the regional peak P and that the higher pressure recorded in the whiteschist lens is likely due to dynamic pressure, possibly resulting from tectonic and/or reaction-induced stresses. If the calculated P of 1.6 ± 0.2 GPa represents regional peak Alpine conditions, then the Monte Rosa nappe was exhumed from a significantly shallower depth than previously assumed, based on peak P estimates >2.2 GPa for whiteschist lithologies.
Lifei Zhang - One of the best experts on this subject based on the ideXlab platform.
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petrology and u pb zircon dating of coesite bearing Metapelite from the kebuerte valley western tianshan china
Journal of Asian Earth Sciences, 2013Co-Authors: Xin Yang, Lifei Zhang, Zuolin Tian, Thomas BaderAbstract:Abstract This paper deals with the petrology and U–Pb dating of coesite-bearing garnet–phengite schist from the Kebuerte Valley, Chinese western Tianshan. It mainly consists of porphyroblastic garnet, phengite, quartz and chlorite with minor amounts of paragonite, albite, zoisite and chloritoid. The well preserved coesite inclusions (∼100 μm) in garnet are encircled by a narrow rim of quartz. They were identified by optical microscopy and confirmed by Raman spectroscopy. Using the computer program THERMOCALC, the peak metamorphic conditions of 29 kbar and 565 °C were obtained via garnet isopleth geothermobarometry. The predicted UHP peak mineral assemblage comprises garnet + jadeite + lawsonite + carpholite + coesite + phengite. The Metapelite records prograde quartz–eclogite-facies metamorphism, UHP coesite–eclogite-facies peak metamorphism, and a late greenschist-facies overprint. Phase equilibrium modeling predicts that garnet mainly grew in the mineral assemblages garnet + jadeite + lawsonite + chloritoid + glaucophane + quartz + phengite and garnet + jadeite + lawsonite + carpholite + glaucophane + quartz + phengite. SHRIMP U–Pb zircon dating of the coesite-bearing Metapelite yielded the peak metamorphic age 320.4 ± 3.7 Ma. For the first time, age data of coesite-bearing UHP Metapelite from the Chinese western Tianshan are presented in this paper. They are in accord with published ages obtained from eclogite from other localities in the Chinese western Tianshan and the Kyrgyz South Tianshan and therefore prove a widespread occurrence of UHP metamorphism.
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triassic collision of western tianshan orogenic belt china evidence from shrimp u pb dating of zircon from hp uhp eclogitic rocks
Lithos, 2007Co-Authors: Lifei Zhang, Daniela Rubatto, Biao Song, Samantha Williams, Shuguang Song, David Ellis, J G LiouAbstract:Abstract A newly recognized ultrahigh-pressure (UHP) terrane in the Chinese Western Tianshan orogenic belt contains blueschists, eclogites and Metapelites. This belt extends westward to the “South Tianshan” in Tajikistan, Kyrgyzstan, Kazakhstan and Uzbekistan for more than 2500 km long in central Asia. New ion microprobe (SHRIMP) U–Pb dating of zircon from HP-UHP eclogites and Metapelites indicates Triassic ages for the collision in western Tianshan. Zircon from four eclogites yields magmatic ages of 310–413 Ma in the cores and one Metapelite contained detrital zircon cores as old as 1886 ± 20 Ma. Zircon rims reveal the HP-UHP metamorphic ages of 233 ± 4–226 ± 4.6 Ma. The geochronologial data suggest that a South Tianshan paleo-ocean was developed between the Tarim continent and the Yili-central Tianshan Craton before the Carboniferous (> 310 Ma). During the Permian–Triassic subduction and continent collision, oceanic basalts underwent HP/UHP metamorphism. A new tectonic model for HP-UHP metamorphic rocks of the Chinese Western Tianshan orogenic belt represented by HP-UHP metamorphic eclogitic rocks is proposed in the light of recent paleomagnetic, paleontologic, sedimentary and stratigraphic studies.
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forbidden zone subduction of sediments to 150 km depth the reaction of dolomite to magnesite aragonite in the uhpm Metapelites from western tianshan china
Journal of Metamorphic Geology, 2003Co-Authors: Lifei Zhang, Richard J. Arculus, David J Ellis, Wenbo JiangAbstract:The solid-state reaction magnesite (MgCO3) + calcite (aragonite) (CaCO3) = dolomite (CaMg(CO3)2) has been identified in Metapelites from western Tianshan, China. Petrological studies show that two metamorphic stages are recorded in the Metapelites: (1) the peak mineral assemblage of magnesite and calcite pseudomorphs after aragonite which is only preserved as inclusions within dolomite; and (2) the retrograde glaucophane-chloritoid facies mineral assemblage of glaucophane, chloritoid, dolomite, garnet, paragonite, chlorite and quartz. The peak metamorphic temperatures and pressures are calculated to be 560–600 °C, 4.95–5.07 GPa based on the calcite–dolomite geothermometer and the equilibrium calculation of the reaction dolomite = magnesite + aragonite, respectively. These give direct evidence in UHP metamorphic rocks from Tianshan, China, that carbonate sediments were subducted to greater than 150 km depth. This UHP metamorphism represents a geotherm lower than any previously estimated for subduction metamorphism (< 3.7 °C km−1) and is within what was previously considered a ‘forbidden’ condition within Earth. In terms of the carbon cycle, this demonstrates that carbonate sediments can be subducted to at least 150 km depth without releasing significant CO2 to the overlying mantle wedge.
Cindy Luisier - One of the best experts on this subject based on the ideXlab platform.
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peak alpine metamorphic conditions from staurolite bearing Metapelites in the monte rosa nappe central european alps and geodynamic implications
Journal of Metamorphic Geology, 2021Co-Authors: Joshua David Vaughanhammon, Cindy Luisier, Lukas P Baumgartner, Stefan M SchmalholzAbstract:The tectono-metamorphic evolution of the European Alps is still contentious. The Monte Rosa tectonic unit is a prominent nappe in the Central European Alps and estimates of its peak Alpine pressure (P) and temperature (T) conditions are essential for reconstructing its tectonometamorphic evolution. However, the reported peak Alpine P and T estimates vary considerably between 1.2 and 2.7 GPa and 490 and 640 °C for a variety of lithologies. Here, we show petrology and pseudosection modelling of metapelitic assemblages from the western portions of the Monte Rosa nappe (upper Ayas valley, Italy). We present newly discovered staurolite-chloritoid bearing metapelitic assemblages. These assemblages exhibit an Alpine high-P metamorphic overprint of a former contact-metamorphic mineral assemblage generated by post-Variscan granitic intrusions. Staurolite contains major amounts of Zn (up to 1.0 atoms per formula units), which is currently, in contrast to Fe-and Mg-staurolite end-members, not considered in any thermodynamic database. We employ two end-member mixing models for Zn in staurolite, site mixing and molecular mixing. Both models enlarge the P and T stability range for the observed assemblage, where site mixing has the largest influence of 0.2 GPa and 20 C. Our results for three Metapelite assemblages, with and without staurolite, indicate peak Alpine P of 1.6 ± 0.2 GPa and peak T of 585 20 C. These peak P estimates agree with previously published estimates for metagranites in the nappe, and are in stark contrast with peak P obtained from talc, chloritoid, phengite and quartz bearing lithologies termed 'whiteschists' (>2.2 GPa). Our results confirm a variation of peak Alpine P of 0.6 ± 0.2 GPa between metagranite/Metapelite lithologies and a nearby whiteschist lens (>2.2 GPa) within the metagranite. Field observations indicate that the studied region is structurally coherent and that the whiteschist is not a tectonic slice formed by tectonic melange. We suggest that the consistent peak P for Metapelite and metagranite assemblages represents the regional peak P and that the higher pressure recorded in the whiteschist lens is likely due to dynamic pressure, possibly resulting from tectonic and/or reaction-induced stresses. If the calculated P of 1.6 ± 0.2 GPa represents regional peak Alpine conditions, then the Monte Rosa nappe was exhumed from a significantly shallower depth than previously assumed, based on peak P estimates >2.2 GPa for whiteschist lithologies.
C. De Capitani - One of the best experts on this subject based on the ideXlab platform.
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Phase Relations and Chemical Composition of Phengite and Paragonite in Pelitic Schists During Decompression: a Case Study from the Monte Rosa Nappe and Camughera- Moncucco Unit, Western Alps
Journal of Petrology, 2005Co-Authors: Lukas M. Keller, Rainer Abart, Stefan M. Schmid, C. De CapitaniAbstract:The metamorphic evolution of Metapelites from the eastern part of the Monte Rosa nappe and the Camughera–Moncucco unit, both situated in the upper Penninic units SW of the Simplon line, were investigated using microstructural relationships and equilibrium phase diagrams. The units under consideration experienced preAlpine amphibolite-facies conditions and underwent a complex metamorphic evolution during the Alpine orogeny. Peak pressures during an early Alpine high-pressure stage of 12� 5–16 kbar were similar in the Monte Rosa nappe and Camughera–Moncucco unit. A pronounced thermal gradient is indicated during decompression leading to an amphibolite-facies overprint, as the decompression paths went through the chlorite, biotite and plagioclase stability fields in most of the Monte Rosa nappe, through the staurolite field in the easternmost Monte Rosa nappe and in the Camughera– Moncucco unit, and through the sillimanite field in the easternmost Camughera–Moncucco unit. In high-Al Metapelites the initial formation of staurolite is related to continuous paragonite breakdown and associated formation of biotite. In the course of this reaction phengite becomes successively sodium enriched. In low-Al Metapelites, in contrast, the initial staurolite formation occurs via the continuous breakdown of sodium-rich phengite. In both low- and high-Al Metapelites the largest volume of staurolite is formed during the continuous breakdown of sodium-rich phengite below P–T conditions of about 9� 5 kbar at 600–650 � C. During this reaction phengite becomes successively potassium enriched as sodium from phengite is used to form the albite component in plagioclase. For ‘ normal’ pelitic chemistries, phengite becomes Na enriched during decompression through the breakdown of paragonite along a nearisothermal decompression path. The Na content in phengite reaches its maximum when paragonite is entirely consumed. During further decompression the paragonite component in phengite decreases again because Na is preferentially incorporated into the albite component of plagioclase.
Andrea Giuliani - One of the best experts on this subject based on the ideXlab platform.
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isotopic disequilibrium in migmatitic hornfels of the gennargentu igneous complex sardinia italy records the formation of low 87sr 86sr melts from a mica rich source
Journal of Petrology, 2018Co-Authors: Mario Gaeta, Andrea Giuliani, Tommaso Di Rocco, Vanni Tecchiato, Cristina Perinelli, Vs KamenetskyAbstract:Isotopic disequilibrium is increasingly recognized as a common feature of magmatic systems, but the details of the mechanism(s) underpinning the development of isotopic disequilibrium during partial melting processes are not fully understood. Partial melting of mica-rich lithologies may be predicted to generate melts enriched in radiogenic Sr compared to the bulk protolith compositions due to the typically high Rb/Sr ratio coupled with low melting temperature of mica in crustal rocks. Here we report a puzzling case study where the Sr-isotope composition of the melt fraction (leucosome) of partially molten Metapelites (migmatites) is instead less radiogenic than the restitic component (melanosome). The examined migmatites fringe (∼50 m wide zone) a low-pressure (≤200 MPa), high-temperature (∼1050 °C) quartz-dioritic intrusion, which was emplaced in the Gennargentu Igneous Complex (Sardinia, Italy) at 306 ± 26 Ma (bulk-rock Rb/Sr dating). The migmatites derive from anatexis of the muscovite-rich metapelitic wall-rocks. They include a quartzo-feldspathic leucosome and a melanosome containing cordierite, K-feldspar, plagioclase, biotite, Fe-Ti oxide minerals and both corundum and hercynite. The leucosome has a less radiogenic Sr and more radiogenic Nd isotope composition than the melanosome (87Sr/86Sr (306 Ma) = 0.71068 and 0.71536; eNd(306 Ma) = -6.4 and -9.2, respectively), with bulk migmatite samples having intermediate compositions. The significantly lower content of mica in the migmatites compared to the protolith indicates that muscovite and, to a lesser extent, biotite largely contributed to melt formation. However, the leucosome volume (∼50%) estimated through mass balance calculations is considerably higher than the amount of melt (≤10 vol.%) generally produced by mica-dehydration melting in the crust, suggesting that partial melting was enhanced by an external hydrous fluid. The O-isotope composition of the migmatites is lower than the typical Metapelite values (>10‰) but overlaps with the δ18O range of the quartz-diorites (8.8-9.9‰), suggesting that such a hydrous fluid was released from the quartz-dioritic intrusion. We put forward a model whereby the anatexis temperature conditions (T < 800 °C) favored the preservation of isotopic disequilibrium of micas and plagioclase in the protolith. In this context, the leucosome formed by the preferential melting of less radiogenic plagioclase rather than more radiogenic micas. The melt was then efficiently separated from the melanosome containing restitic biotite and "peritectic K-feldspar and magnetite" derived from mica breakdown. During the anatectic process the quartz-diorite provided not only the heat budget, but also the fluid amount responsible for (i) the hydration of the pelitic country rock, (ii) the increase of melt fraction, and (iii) the higher mobility of anatectic magma.