Mylonitized peridotites of Songshugou in the Qinling orogen,central China: A fragment of fossil oceanic lithosphere mantle |
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| Institution: | 1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China;2. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi''an, 710069, China;3. Department of Earth Sciences, University of Adelaide, SA 5005, Australia;4. School of Earth Science and Resources, China University of Geosciences, Beijing, China;1. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, 3, Koptyug ave., Novosibirsk 630090, Russia;2. Novosibirsk State University, 2, Pirogova str., Novosibirsk 630090, Russia;3. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, 3, Koptyug ave., Novosibirsk 630090, Russia;4. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, 1А Favorsky str., Irkutsk 664033, Russia;1. Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering/South China Sea Bioresource Exploitation and Utilization Collaborative Innovation Center, School of Marine Sciences, Sun Yat-Sen University, Guangzhou 510006, China;2. State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;3. Guangdong Ocean University, Zhanjiang 524088, China;1. Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, FL 32611, USA;2. School of Earth Sciences and Resources, China University of Geosciences Beijing, Beijing 100083, PR China;3. Department of Earth Sciences, The University of Adelaide, Adelaide, SA 5005, Australia;4. Department of Earth System Sciences, Yonsei University, Seoul 03722, Republic of Korea;1. Institute of Geophysics, Polish Academy of Sciences, Ks. Janusza 64, PL-01452 Warszawa, Poland;2. Institute of Geological Sciences, Polish Academy of Sciences, Twarda St, PL-00818 Warszawa, Poland;3. Institute of Zoology and Biomedical Research, Jagiellonian University, Gronostajowa St 9, PL-30387 Kraków, Poland;4. Instytut Technologii Elektronowej, Al. Lotników St 32/46, PL-02668 Warszawa, Poland;5. Institute of Biochemistry and Biophysics, Polish Academy of Sciences, Pawińskiego St 5a, PL-02106 Warszawa, Poland;6. Faculty of Earth Sciences, University of Silesia, B?dzińska 60, PL-41200 Sosnowiec, Poland;7. Faculty and Institute of Earth Sciences, University of Iceland, Askja Sturlugata 7, 101 Reykjavík, Iceland;8. Department of Applied Geology, Curtin University, PO Box U1987, WA 6845, Perth, Australia |
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| Abstract: | The Songshugou mylonitized peridotites within the Qinling Group metamorphic rocks in Central China are distributed in the northern part of the Shang-Dan Suture Zone (SDSZ) and contain abundant dunites and harzburgites. The dunites were intensely deformed and mylonitized converting the coarse-grained type to medium- and fine-grained types which contain prominent lenticular structure and relict olivine (Ol) porphyroclasts. Mineralogical and geochemical compositions suggest that the protoliths of the mylonitized peridotites were coarse-grained peridotites of lithospheric mantle origin. The harzburgites occur as enclaves within mylonitic peridotites in the form of lenses or veins. The orthopyroxenes in harzburgites were formed at the expense of Ol and have similar compositions to those of metasomatized harzburgites, characterized by low Al2O3, CaO and Cr2O3 contents. The harzburgites exhibit the gently U-type REE patterns with enriched incompatible elements (Rb, Ba, Sr, Zr and Hf), suggesting the metasomatic origin. The obvious ductile deformation of the large porphyroclastic orthopyroxene (Opx) suggests that the metasomatism occurred before the deformation. Ductile shearing deformation is indicated by the small fold structures and net-style ductile shearing zones within the Songshugou peridotite massif. The process is also result in the alignment of elongated Ol grains from initially coarse-granular via porphyroclastic to fine-granular texture. The relatively low Fo olivine, together with high Al2O3, and CaO contents and the abnormally low total PGE abundance in the fine-grained dunites suggest the ingress of melt/fluid during the mylonitization. The presences of significant amount of amphibole in the peridotites indicate the ingress of hydrous fluids. In general, the Songshugou peridotites have similar compositional characteristics with peridotites of Oman and Troodos ophiolites which are fragments of oceanic lithosphere mantle. One coarse-grained dunite has a TRD age of 875 Ma. Additionally two stages Paleozoic TRD ages are obtained from medium-grained and fine-grained dunites (491 Ma and 550 Ma; 446 Ma and 476 Ma). The broadly coeval nature of mylonitization with progressive metamorphism of surrounding amphibolites suggested that the Songshugou peridotites were generated before the early Paleozoic deformation. Our data, combined with the previous work on the surrounding HP/UHP metamorphic rocks, demonstrate that the Songshugou mylonitized peridotites represent fragments of the Neoproterozoic fossil oceanic lithospheric mantle that experienced extensive deformation during the Early Paleozoic subduction processes. |
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