Multi-stage metamorphic evolution of retrograde eclogite with a granulite-facies overprint in the Zhaigen area of the North Qinling Belt,China |
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| Institution: | 1. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi''an 710069, China;2. College of Earth Science & Engineering, Shandong University of Science and Technology, Qingdao 266590, China;1. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Northern Taibai Str. 229, Xi''an 710069, China;2. Collaborative Innovation Centre of Continental Tectonics, Northwest University, Northern Taibai Str. 229, Xi''an 710069, China;3. School of Earth Science and Resources, China University of Geosciences, Xueyuan Road 29, Beijing 100083, China;1. State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China;2. Department of Geosciences, University of Tuebingen, Tuebingen 72074, Germany;3. State Key Laboratory of Lithosphere Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China |
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| Abstract: | Retrograde eclogite from the central part of the Qinling Complex, Zhaigen area of the North Qinling Belt, was studied using integrated petrology, mineral chemistry, pseudosection modeling, and geochronology. Microstructures and mineral relationships reveal five metamorphic stages and associated mineral assemblages as follows: (1) pre-peak stage M1, which is recorded by the inner cores of garnets together with mineral inclusions of clinopyroxene (Cpx1) + amphibole (Am1) + plagioclase (Pl1) ± quartz ± rutile, occurred under conditions of 760–770 °C and 11.4–14.0 kbar; (2) eclogite-facies stage M2, recorded by garnet cores + relic omphacite (with a high jadeite content up to 31%) + rutile + quartz under conditions of > 16.7 kbar and 679–765 °C; (3) high-pressure granulite-facies stage M3, characterized by clinopyroxene (Cpx2) + plagioclase (Pl2) symplectites under conditions of 14.5–15.6 kbar and 800–850 °C; (4) medium-pressure granulite-facies stage M4, characterized by the growth of plagioclase + orthopyroxene coronas around garnet under conditions of 8.3–10 kbar and 795–855 °C; and (5) retrogressive amphibolite-facies stage M5, which is represented by amphibole (Am3) + plagioclase (Pl3) kelyphitic rims around garnet at conditions of < 4 kbar and < 620 °C. Based on Laser Raman analysis of mineral inclusions, cathodoluminescence images, in situ trace element concentrations from different domains within zircon grains, and LA-ICP-MS and SHRIMP U–Pb dating, the protolith age of the Zhaigen retrograde eclogite is suggested at 786 ± 10 Ma and the eclogite-facies metamorphic age recorded by metamorphic zircon cores is limited within 501–497 Ma. The retrograde zircon rims display ages of 476–447 Ma and 425 Ma that probably reflect the timing of two stages of retrograde metamorphism, respectively. The mineral assemblages, P–T conditions, and zircon U–Pb data define a clockwise P–T–t path for the retrograde eclogite, suggesting that the Neoproterozoic protolith of the retrograde eclogite might evolved into continental subduction and eclogite-facies metamorphism during 501–497 Ma before undergoing retrograde metamorphism during an initial stage of exhumation to middle–upper crust level at 474–447 Ma and subsequent exhumation to shallow upper crust at ~ 420 Ma. |
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