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 (2 sigma), 233.1 +/- 1.4 Ma (2 sigma) and 231.0 +/- 1.6 Ma (2 sigma), 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 P(2)O(5) contents (0.06-0.11 wt.%), A/CNK values ([molecular Al(2)O(3)/(CaO + Na(2)O + K(2)O)]. 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/(204)Pb)(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 delta(18)O values (7.3 parts per thousand-9.3 parts per thousand) and negative epsilon(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 epsilon(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 delta(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. (C) 2011 Elsevier B.V. All rights reserved.

Publication name	LITHOS　Volume:126 　Issue:3-4 　Pages:248-264　Published:OCT 201
Author(s)	Zhu, Jingjing; Hu, Rui-Zhong; Bi, Xian-Wu; Zhong, Hong; Chen, Heng
Corresponding author	HU Ruizhong  huruizhong@vip.gyig.ac.cn  Chinese Acad Sci, Inst Geochem, State Key Lab Ore Deposit Geochem, Guiyang 550002, Peoples R China

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