An Early Devonian to Early Carboniferous volcanic arc in North Tianshan,NW China: Geochronological and geochemical evidence from volcanic rocks |
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| Institution: | 1. State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing 102249, China;2. Unconventional Natural Gas Institute, China University of Petroleum, Beijing 102249, China;3. College of Geosciences, China University of Petroleum, Beijing 102249, China;1. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, PR China;2. Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, PR China;1. Institute of Geology and Mineralogy, SB RAS, Koptyuga ave. 3, Novosibirsk 630090, Russia;2. Geology–Geophysical Faculty, Novosibirsk State University, Pirogova St. 2, Novosibirsk 630090, Russia;3. Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro, Tokyo 152-8550, Japan;4. Inst. of Earth Sciences, St.-Petersburg State University, 7/9 University Embankment, St.-Petersburg 199034, Russia;5. Deutsches GeoForschungsZentrum (GFZ), Telegrafenberg, D-14473 Potsdam, Germany;6. Natural Hist. Museum, Centre for Russian and Central EurAsian Mineral Studies (CERCAMS), London SW7 5BD, UK |
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| Abstract: | The Late Paleozoic volcanic and sedimentary rocks are widespread in the North Tianshan along the north margin of the Yili block. They consist of basalt, basaltic andesite, andesite, trachyandesite, dacite, rhyolite, tuff, and tuffaceous sandstone. According to zircon sensitive high-resolution ion microprobe (SHRIMP) dating, the age of the Late Paleozoic volcanic rocks in Tulasu basin in western part of North Tianshan is constrained to be Early Devonian to Early Carboniferous (417–356 Ma), rather than Early Carboniferous as accepted previously. Geochemical characteristics of the Early Devonian to Early Carboniferous volcanic rocks are similar to those of arc volcanic rocks, which suggest that these volcanic rocks could be the major constituents of a continental arc formed by the southward subduction of North Tianshan Oceanic lithosphere. Geochemical studies indicate that the magma source of the volcanic rocks might be the mantle wedge mixed with subduction fluid, which is geochemically enriched than primitive mantle but depleted than E-MORB. The calculation shows that the basalt could be formed by ～10% partial melting of subduction fluid modified mantle wedge. Andesites with high initial 87Sr/86Sr (0.7094–0.7104) and negative εNd(t) (?4.45 to ?4.79) values reveal the contribution of continental crust to its source. The calculation of assimilation–fractional crystallization (AFC) shows that the fractional crystallization process of the basaltic magma, which was accompanied with assimilation by different degree of continental crust, produced andesite (7–9%), dacite (～12%) and rhyolite (>20%). |
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| Keywords: | Volcanic rocks Geochronology Geochemistry Tulasu basin North Tianshan |
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