Re-Os geochronology,O isotopes and mineral geochemistry of the Neoproterozoic Songshugou ultramafic massif in the Qinling Orogenic Belt,China |
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| Institution: | 1. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Northern Taibai Str. 229, Xi''an 710069, China;2. Department of Earth Science, Western University, 1151 Richmond Str., London, Ontario N6A 3K7, Canada;3. Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China;4. College of Geosciences and Engineering, North China University of Water Resources and Electric Power, Zhengzhou 450045, China;1. Department of Geosciences, Biogeology, University of Tübingen, Hölderlinstrasse 12, 72074 Tübingen, Germany;2. Senckenberg Research Centre for Human Evolution and Paleoenvironment, University of Tübingen, Hölderlinstrasse 12, 72076 Tübingen, Germany;3. Instituto de Investigaciones Arqueológicas y Paleontológicas del Cuaternario Pampeano (INCUAPA-CONICET), Universidad Nacional del Centro de la provincia de Buenos Aires, Del Valle 5737, B7400JWI Olavarría, Buenos Aires, Argentina;4. Museo Municipal de Ciencias Naturales Pachamama, Santa Clara del Mar, Argentina;5. Universidad Nacional de Mar del Plata, Argentina;6. Div. Paleontología Vertebrados, Museo de La Plata, Paseo del Bosque, 1900 La Plata, Argentina;7. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja (CRILAR), Provincia de La Rioja, UNLaR, SEGEMAR, UNCa, CONICET, Entre Ríos y Mendoza s/n, 5301 Anillaco, La Rioja, Argentina;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. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Northern Taibai Str. 229, Xi’an 710069, China;2. Collaborative Innovation Center of Continental Tectonics, Northwest University, Northern Taibai Str. 229, Xi’an 710069, China;1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China;2. College of Earth Science, University of Chinese Academy of Sciences, Beijing 10049, China;3. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi''an 710069, China;4. School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China;5. Centre for Tectonics Resources and Exploration, Dept of Earth Sciences, University of Adelaide, SA 5005, Australia |
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| Abstract: | The Qinling Orogenic Belt was formed by subduction and collision between the North and South China Blocks along the Shangdan suture. The Songshugou ultramafic massif located on the northern side of the Shangdan suture provides essential insights into the mantle origin and evolutionary processes during spreading and subduction of the Shangdan oceanic lithosphere. The ultramafic massif comprises harzburgite, coarse- and fine-grained dunites. The spinels from harzburgite exhibit low Cr# and high Mg# numbers, suggesting a mid-ocean ridge peridotite origin, whereas spinels from both coarse- and fine-grained dunites are indicated as resulted from melt-rock reaction due to their systematic higher Cr# and low Mg# numbers. This melt-rock reaction in the dunites is also indicated by the low TiO2 (mostly <0.4 wt%) in the spinel and high Fo (90–92) in olivines. Due to its relatively homogeneous nature in the mantle, oxygen isotopic composition is a sensitive indicator for the petrogenesis and tectonic setting of the Songshugou ultramafic rocks. Based on in-situ oxygen isotope analyses of olivines from twenty-six rock samples, most harzburgites from the Songshugou ultramafic massif show low δ18O values of 4.54–5.30‰, suggesting the olivines are equilibrium with N-MORB magmas and originally formed in a mid-ocean ridge setting. The coarse- and fine-grained dunites exhibit slightly higher olivine δ18O values of 4.69–6.00‰ and 5.00–6.11‰, respectively, suggesting they may have been modified by subduction-related boninitic melt-rock reaction. The δ18O values of olivines systematically increasing from the harzburgites, to coarse-grained dunites and fine-grained dunites may suggest enhancing of melt-rock reaction. The decreasing of Os concentration, 187Re/188Os and 187Os/188Os ratios from harzburgite to dunite suggest an 187Os-enriched, subduction zone melt was responsible for creating the melt channel for melt-rock reactions. Together with the high-temperature ductile deformation microstructures, these isotopic and mineral geochemical features suggest that the harzburgites represent mantle residues after partial melting at mid-ocean ridge or supra-subduction zone, while the dunites were probably resulted from reactions between boninitic melt and harzburgites in a supra-subduction zone. Re-Os geochronology yields a maximum Re depletion model age (TRD) of 805 Ma, constraining the minimum formation age of the harzburgites derived from oceanic mantle. Eight samples of whole rock and chromite yield a Re-Os isochron age of 500 ± 120 Ma, constraining the timing of melt-rock reactions. Combined with the regional geology and our previous investigations, the Songshugou ultramafic rocks favors a mantle origin at mid-ocean ridge before 805 Ma, and were modified by boninitic melt percolations in a SSZ setting at ca. 500 Ma. This long-term tectonic process from spreading to subduction might imply a huge Pan-Tethyan ocean between the Laurasia (e.g., North China Block) and Gondwana (e.g., South China Block) and/or a one-side subduction. |
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