Abstract:

A large number of Indosinian granites are widely developed in Guide basin, Qinghai Province, but there is a lack of research on their geochemical characteristics, petrogenesis and origin of granites. The resolution of these problems is very important to resolve formation background of indosinian granites in West Qinling. Therefore, LA-ICP-MS zircon U-Pb dating, whole-rock geochemistry and Lu-Hf isotope analysis of granodiorite from drilling hole ZR2 from Zhacanggou area, Guide basin were selected as research objects. Zircon U-Pb isotopic dating results show that the emplacement age of granodiorite is (225.3 ± 1.3) Ma, which is the characteristic of Late Triassic magmatic activity. Characteristics of major and trace elements show that granodiorite is a weak peraluminous, high-K calc-alkaline series rock, showing moderate Eu negative anomalies (δEu=0.59-0.91), enrichment of large ion lithophile elements (Rb, U) and depletion of high field strength elements (Nb,Ta,Ti). Zircon Hf isotopic composition shows that the εHf(t) values of granodiorite range from -7.70 to 0.22, and the corresponding two-stage model ages (TDM2) range from 1 747 to 1 243 Ma, indicating that the granodiorites in Zhacanggou area of Guide basin are the products of melting and mixing of lower crust and lithospheric mantle. Combined with the previous research results, it is considered that the granodiorite in the study area is a transitional tectonic system from syn-collision to post-collision extension after the closure of A’nimaqing-Mianlue Ocean.

Key words: geochemistry, zircon U-Pb dating, Lu-Hf isotope, granodiorite, Zhacanggou, Guide basin