Formation mechanism of the Chengba high Sr/Y granodiorite and enclaves in southern Gangdise region, southern Tibet

The Chengba Complex on the southern margin of the Gangdise batholiths in southern Tibet comprises mainly granodio-rite and subordinately dioritic enclaves and leucogabbro. Zircon U-Pb analytical results demonstrate that the granodiorites formed at 29.40±0.18Ma to 29.42±0.25Ma, whereas the fine-grained dioritic enclaves formed at 30.02±0.15Ma. Bulk-rock major and trace el-ement and isotope(Sr-Nd-Hf) analyses indicate that the granodiorites are characterized by relatively high SiO₂(65.2%~66.2%) and K₂O(3.2%~4.0%) but low FeO and MgO, and high Sr(774×10^-6~813×10^-6), low Y(9.9×10^-6~11.2×10^-6), and hence high Sr/Y ratios(>60 and up to 82). In contrast, the dioritic enclaves are relatively low in SiO₂(53%~56.1%) and K₂O(1.5%~3.2%), but high in MgO and FeO with relatively high Na/K ratios(≥2). Both are enriched in LREE and LILE and depleted in HREE and HFSE and have similar relatively high zircon ε_Hf(t) values(+1.1~+6.2) as well as negative whole-rock ε_Nd(t) values(-2.9~-5.9). Combined with data available, the new results obtained by the authors suggest that the Chengba granodiorite and dioritic enclaves were formed within a time span of ca.1Ma. Similar isotopic characteristics imply that they were derived from similar sources. The high Sr/Y ratios in Chengba granodiorite, in contrast to low Sr/Y ratios in the mafic enclaves, possibly do not represent primary magma composition. Instead, the high Sr/Y and high Ba characteristics of the Chengba granodiorite might have resulted from fractional crystallization of parent magmas with low Sr/Y ratios and low Ba content like those mafic enclaves.