Zircon U-Pb ages, Hf-0 isotopes and whole-rock Sr-Nd-Pb isotopic geochemistry of granitoids in the Jinshajiang suture zone, SW China: Constraints on petrogenesis and tectonic evolution of the Paleo-Tethys Ocean

摘要

The Jinshajiang suture zone, located in the eastern part of the Tethyan tectonic domain, is noticeable for a large-scale distribution of Late Jurassic to Triassic granitoids. These granitoids were genetically related to the evolution of the Paleo-Tethys Ocean. Beiwu, Linong and Lunong granitoids occur in the middle zone of the Jinshajiang Suture Zone, and possess similar geochemical features, indicating that they share a common magma source. SIMS zircon U-Pb dating reveals that the Beiwu, Linong and Lunong granitic intrusions were emplaced at 233.9 ±1.4 Ma (2a), 233.1 ±1.4 Ma (2a) and 231.0 ±1.6 Ma (2a), respectively. All of these granitoids are enriched in abundances of Si (SiO_(2t) = 65.2-73.5 wt.%), and large-ion lithophile elements (LILEs), but depleted in high field strength elements contents (HFSEs, e.g., Nb, Ta, and Ti). In addition, they have low P2O5 contents (0.06-0.11 wt.%), A/CNK values ([molecular Al2O3/(CaO + Na2O + K2O)], mostly < 1.1) and 10,000 Ga/Al ratios (1.7-2.2), consistent with the characteristics of I-type granites. In terms of isotopic compositions, these granitoids have high initial ~(87)Sr/~(86)Sr ratios (0.7078-0.7148), Pb isotopic compositions [(~(206)Pb/~(2040Pb)_t = 18.213-18.598, (~(207)Pb/~(204)Pb)_t= 15.637-15.730 and (~(208)Pb/~(~(204)Pb)_t = 38.323-38.791], zircon δ~(18)O values (7.3‰-9.3‰) and negative ε_(Nd)(t) values ( — 5.1 to —6.7), suggesting they were predominantly derived from the continental crust. Their Nb/Ta ratios (average value = 8.6) are consistent with those of the lower continental crust (LCC). However, variable ε_(Hf)(t) values ( — 8.6 to +2.8) and the occurrences of mafic microgranular enclaves (MMEs) suggest that mantle-derived melts and lower crustal magmas were involved in the generation of these granitoids. Moreover, the high Pb isotopic ratios and elevated zircon δ~(18)O values of these rocks indicate a significant contribution of the upper crustal composition. We propose a model in which the Beiwu, Linong and Lunong granitoids were generated under a late collisional or post-collisional setting. It is possible that this collision was completed before Late Triassic. Decompression induced mantle-derived magmas to be underplated and provided the heat for the anatexis of the crust. Hybrid melts including mantle-derived and the lower crustal magmas were then generated. The hybrid melts thereafter ascended to a shallow depth and resulted in some degree of sedimentary rock assimilation. Such three-component mixing magma source and subsequent fractional crystallization could be responsible for the formation of the Beiwu, Linong and Lunong granitoids.