首页|Late Early Cretaceous magmatic constraints on the timing of closure of the Bangong-Nujiang Tethyan Ocean, Central Tibet
Late Early Cretaceous magmatic constraints on the timing of closure of the Bangong-Nujiang Tethyan Ocean, Central Tibet
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As one of the most important parts of the eastern Tethys, the evolution of the Bangong-Nujiang Tethyan Ocean (BNTO) has important geological implications for paleogeographic reconstructions of the Tethyan Realm. However, the closure time of the BNTO remains highly debated. Here, we report geochronological, petrological and geochemical data from three newly identified undeformed stitching plutons that intrude a north-south-oriented section of ophiolitic melanges in the middle-eastern parts of the Bangong-Nujiang suture zone (BNSZ), central Tibet. The stitching plutons comprise the Chaqu diorite-granite association, the Jiang Co granite, and the Konglong diorite porphyry. Secondary ion mass spectroscopy (SMIS) zircon U-Pb dating suggests that these three plutons were emplaced at ca. 116-112 Ma. The stitching plutons show heterogeneous Sr-Nd-Hf-O isotopic compositions. The Chaqu diorites have initial ~(87)Sr/~(86)Sr ratios varying from 0.7062 to 0.7064 and e_(Nd)(t) values from -2.0 to -1.6, and slightly high zircon d~(18)O (6.2-6.8± and positive zircon e_(Hf)(t) (+2.6 to +7.4) values. The Chaqu granites show high initial ~(87)Sr/~(86)Sr (0.7064-0.7065) ratios and low e_(Nd)(t) (-2.3 to -2.1) values, and elevated zircon d~(18)O (6.2-7.3± values and variable zircon e_(Hf)(t) (-1.7 to +4.9) values, similar to those of the associated Chaqu diorites. The Jiang Co granites exhibit higher initial ~(87)Sr/~(86)Sr (0.7085-0.7094) ratios and negative e_(Nd)(t) (-8.33 to -8.27) values, and variable zircon d~(18)O (4.8±to 6.0± and e_(Hf)(t) (-6.9 to -2.0) values. The Konglong diorite porphyries have initial ~(87)Sr/~(86)Sr isotopic ratios ranging from 0.7069 to 0.7075 and negative e_(Nd)(t) values ranging from -4.2 to -4.1, and high zircon d~(18)O (9.0±to 9.8± and variable zircon e_(Hf)(t) (-7.3 to +0.7) values. Geochemical evidence indicates that they were derived from different magma sources that included varying proportions of mantle and crustal components. The Chaqu diorites were derived by partial melting of an ancient metasomatized mantle source, and the Chaqu granites were formed by fractional crystallization of the Chaqu dioritic magmas. The Jiang Co granites were generated by partial melting of low d~(18)O high-temperature hydrothermally altered ancient continental crust. The Konglong diorite porphyries were likely generated by partial melting of high-d~(18)O enriched lithospheric mantle. In combination with data from adjacent ophiolites, magmatic rocks, and Cretaceous sedimentary rocks, we suggest that the stitching plutons were generated in a post-collisional setting induced by upwelling of asthenospheric mantle due to slab breakoff beneath the BNSZ. Our data provide an important new upper age limit for the time of collision between the Lhasa and Qiangtang terranes, i.e., the latest closure time for the BNTO.
Stitching plutonClosure timeSlab breakoffBangong-Nujiang Tethyan OceanCentral Tibet
Wan-Long Hu、Qiang Wang、Gong-Jian Tang
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State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
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