Late Early-Cretaceous quartz diorite–granodiorite–monzogranite association from the Gaoligong belt, southeastern Tibet Plateau: Chemical variations and geodynamic implications

Abstract

Geochemical variations in granitic rocks may be controlled by their source rocks, melting reactions and subsequent magmatic processes, which resulted from various geodynamic processes related to subduction, collision, or slab break-off. Here we report new LA–ICP-MS zircon U–Pb ages and Hf isotopes, whole-rock chemistry and Sr–Nd isotopes for the late Early Cretaceous quartz diorite, granodiorite and monzogranite in the Gaoligong belt, southeastern Tibet Plateau. The zircon U–Pb dating yield ages of 113.9±1.6, 111.7±0.8, and 112.8±1.7Ma for the quartz diorite, granodiorite, and monzogranite, respectively, which are coeval with bimodal magmatism in the central and northern Lhasa sub-terrane. There are the distinct sources regions for the quartz diorite and granodiorite–monzogranite association. The quartz diorites are sodic, calc-alkaline and have high Mg# (52–54) values. They also have elevated initial 87Sr/86Sr (0.707019 to 0.709176) and low εNd(t) (−5.16 to −7.63), with variable zircon εHf(t) values (+5.65 to −9.02). Zircon chemical data indicate a typical crustal-derived character with high Th (142–1260ppm) and U (106–1082ppm) and moderate U/Yb ratios (0.30 to 2.32) and Y content (705–1888ppm). Those data suggest that the quartz diorites were derived from partial melting of ancient basaltic lower crust by a mantle-derived magma in source region. The granodiorite–monzogranite association has high-K calc-alkaline, weakly peraluminous characters. They show lower Nb/Ta (5.57 to 13.8), CaO/Na2O (0.62 to 1.21), higher Al2O3/TiO2 (24.4 to 44.4) ratios, more evolved whole-rock Sr–Nd and zircon Hf isotopic signatures, all of which suggest derivation from mixed basaltic and metasedimentary source rocks in a deep crustal zone. We propose that the granitic magmatisms at ca. 113–110Ma in the Gaologong belt was triggered by the slab break-off of Bangong–Nujiang Tethyan oceanic lithosphere.