-Petrogenesis and tectonic implications of Late Cretaceous highly fractionated I−type granites from the Qiangtang block, central Tibet

Abstract

Abstract The tectonic evolution of the Lhasa and Qiangtang collision zone is poorly understood due to a lack of pivotal magmatic records and complex geology in central Tibet. In this study, we present zircon U–Pb ages, whole-rock major and trace elements, and Sr–Nd–Pb isotopic data for the newly discovered Chuburi pluton in the southern Qiangtang block. Zircon U–Pb data reveal that the Chuburi pluton was emplaced during the Late Cretaceous (ca. 73\u202fMa). Bulk-rock analyses show that this intrusion is characterized by high SiO2, Na2O and K2O, but low MgO, CaO and P2O5. The granites are enriched with large-ion lithophile elements and light rare earth elements, with marked Eu anomalies, and there are few heavy rare earth elements and high field strength elements. These geochemical features indicate that the Chuburi pluton is a highly fractionated I-type granite. The nine granitic samples show high (87Sr/86Sr)i ratios (0.7064–0.7089), negative eNd(t) values (−1.94 to −5.59) and have Nd isotopic model ages ranging from 0.76 to 0.97\u202fGa. The initial Pb isotopic ratios of the Chuburi pluton is uniform: (206Pb/204Pb)t\u202f=\u202f18.774–18.884; (207Pb/204Pb)t\u202f=\u202f15.695–15.713; (208Pb/204Pb)t\u202f=\u202f39.362–39.613. Based on geochemical characteristics of the Chuburi pluton, and consideration of the region geological setting, we suggest that the parent magma was generated by magmatic mixing of mantle and lower crust, and then undergoing extensive fractional crystallization with removal of K-feldspar, plagioclase, mafic mineral and minor accessory mineral. The crust beneath the central Tibet may have been significantly thickened following the final Lhasa-Qiangtang amalgamation. We argue the generation of the Chuburi pluton can be interpreted as a result of regional lithospheric delamination.