Geochronology and geochemistry of Late Carboniferous dykes in the Aqishan-Yamansu belt,eastern Tianshan:Evidence for a post-collisional slab breakoff |
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| Authors: | Xiaoping Long Bin Wu Min Sun Chao Yuan Wenjiao Xiao Rui Zuo |
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| Institution: | State Key Laboratory of Continental Dynamics;Department of Earth Sciences;State Key Laboratory of Isotope Geochemistry;State Key Laboratory of Lithospheric Evolution |
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| Abstract: | With aim of providing constraints on the Late Paleozoic tectonic evolution of the southern Central Asian Orogenic Belt（CAOB）,an integrated study was conducted on the geochronological and geochemical data for dioritic,granitic and diabase dykes from the Aqishan-Yamansu belt in the eastern Tianshan,NW China.Zircon U-Pb dating indicates that the dioritic and granitic dykes were both emplaced in the Late Carboniferous（～311 Ma and～315 Ma）.The dioritic dykes show adakitic characteristics and have high Na2 O and positive εHf（t） values（+12 to+17）,which suggest an origin from partial melts of a subducted oceanic slab.The granitic dykes have high SiO2 and K2 O contents and are characterized by en riched light rare earth elements（LREE） and slightly flat heavy rare earth elements（HREE）,with negative Eu and Nb-Ta-Ti anomalies.These dykes are alkali-calcic and show geochemical features of highly fractionated Itype granites.Their positive εHf（t） values（+16 to+17） suggest that they were derived from a juvenile accreted oceanic crustal sou rce.The coeval diabase dykes have low SiO2 and K2 O contents but high TiO2,MgO and Mg#（54-59）.They are enriched in LREE and show characteristics of enriched mid-ocean ridge basalts（E-MORB）.The relatively high Ba/Th,slightly low Th/Ta ratios,and negative Nb-Ta anomalies imply a mantle source metasomatised by slab-derived fluids.Thus,these basic dykes were generated likely by partial melting of the upwelling asthenosphere mantle with a slight influence of slab-derived fluids.Therefore,we suggest that the formation of these Late Carboniferous dykes were triggered by a post-collisional slab breakoff and the Aqishan-Yamansu belt was a continental arc formed by southdipping subduction of the Kangguer oceanic plate. |
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| Keywords: | Central Asian orogenic belt Eastern Tianshan Adakitic Highly fractionated l-Type granite Slab breakoff |
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