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Dong Z.-C.,Ministerial Key Laboratory of Mineralization and Prospecting in Magmatism | Xiao P.-X.,Ministerial Key Laboratory of Mineralization and Prospecting in Magmatism | Gu P.-Y.,Ministerial Key Laboratory of Mineralization and Prospecting in Magmatism | Zhang H.-D.,Ministerial Key Laboratory of Mineralization and Prospecting in Magmatism | And 2 more authors.
Northwestern Geology | Year: 2014

Granodiorite develops in Yanchangbeishan of Lenghu area in Qinghai, which is located in the west segment of the north margin of Qaidam. The samples have higher SiO2 (71. 19%- 71. 81%) and Na2O contents (4. 52%-4. 85%). The alumina saturation average index is 1.02, indicating that the granodiorite belong to weak peraluminous granites. Moreover, these samples are depleted in HFSEs (Nb, Ta) intensively, rich in LILEs (K, Rb, Sr, Th), reflecting the characteristics of island-arc magma. LA-ICP-MS zircon U-Pb dating shows that the formation age of the rock is 265±2Ma. The zircon Hf isotope analysis reveal 176Hf/mHf ratios varying in 0. 282 962 0 to 0. 283 048 7, with high positive eHf (f) (12. 54-15.19) and two-stage Hf model ages (averaged at 367 Ma). In the 176Hf/177Hf-eHf (t) discrimination diagrams, these zircons were plotted nearly on the depleted mantle evolution line, it can be concluded that the rock chiefly derived from the new basaltic lower crust which is originated from depleted mantle. Based on the regional geological background and the geochemical characteristics of the masses, we consider that Yan-granodiorite bears similar characteristics of its primary magma with mafic-ultramaf- ic rocks of Devonian, which were supposed to be originated from partial melting of the overlying basaltic juvenile crust as a result of southward subduction of Zongwulong small oceanic basin. Thereby, it is revealed that the north margin of Qaidam was in volcanic arc or active continental margin tectonic environment during Permian epoch. Source

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