Zircon U–Pb ages and Hf–O isotopes, and whole-rock Sr–Nd isotopes of the Bozhushan granite, Yunnan province, SW China: Constraints on petrogenesis and tectonic setting

Abstract

The Bainiuchang silver-polymetallic ore deposit is a super-large deposit in the western part of the South China tungsten–tin province (or the Nanling tungsten–tin province). The deposit is spatially and temporally associated with the Bozhushan granite pluton. Our new data indicate that the Bozhushan granitoids formed at 86–87Ma. The granitoids are geochemically consistent with A-type granite. The Bozhushan pluton consists predominantly of biotite granite that is characterized by weakly peraluminous to metaluminous compositions and high alkali contents (Na2O+K2O=7.51–9.06wt.%). The granitic rocks are enriched in large-ion lithophile elements (LILE) Rb, Th, U, and K, but relatively depleted in Ba and Sr. In addition, they have high Zr+Nb+Ce+Y contents (310–478ppm) and high 10,000× Ga/Al ratios (2.7–3.1). The temperatures of the parental magmas for the Bozhushan granites are estimated to be 790–842°C based on the zircon saturation thermometer. Isotopically, the Bozhushan granites are characterized by elevated initial 87Sr/86Sr ratios (0.7126–0.7257) and low εNd values (−11.2 to −12.4), and high δ18O values (7.91–9.58‰) and low εHf values (−9.5 to −6.1) for zircon crystals, which indicate a dominant continental crustal source. The two-stage Hf model ages vary from 1.53 to 1.86Ga. The isotopic compositions support the interpretation that the granitic rocks formed by melting of the Meso- and Neoproterozoic metasedimentary basements of the Cathaysia block. These results, together with geological records in the other parts of the western Cathaysia block, suggest that the formation of the Bozhushan A-type granites is related to lithospheric extension and asthenospheric upwelling that are associated with the change of plate motion in Late-Cretaceous.