Geochronology, geochemistry, Sr–Nd–Hf isotopic compositions, and petrogenetic and tectonic implications of Early Cretaceous intrusions associated with the Duolong porphyry–epithermal Cu–Au deposit, central Tibet

Abstract

ABSTRACTThis article reports new zircon laser ablation-multicollector-inductively coupled plasma-mass spectrometry U–Pb and Hf isotope, whole-rock major and trace element, and Sr–Nd isotope data for mineralized and barren intrusions associated with the Duolong porphyry–epithermal copper–(gold) deposit (DPCD, a mining camp containing several individual deposits) in the western Qiangtang Terrane (QT), central Tibet. These data are used to further our understanding of the geological evolution of this region. The mineralized and barren DPCD intrusions are typical I-type granitoids that were synchronously emplaced at ca. 112.6–125.9 Ma. These igneous rocks show arc affinities that are characterized by enrichments in the light rare earth elements (LaN/YbN = 4.08–15.23) and the light ion lithophile elements (Rb, Th, U, K, and Pb), and depletions in the high field strength elements (Nb, Ta, and Ti). They have 87Sr/86Sr(i) values of 0.7046–0.7079, Nd(t) values of –6.0 to +1.1, and two-stage Nd model ages of ca. 823–1410 Ma. Zircons from these intrusive rocks have variable but generally positive εHf(t) values (–2.7 to +13.7) and relatively young zircon Hf crustal model ages of 335–1351 Ma. Combining these data with geochemical data reported in recent studies, we infer that the mineralized and barren DPCD intrusions formed in a continental marginal arc setting and likely originated from a common parental magma that was result of magma mixing of juvenile crust-derived basaltic melts and old lower crust-derived melts. The formation of the DPCD intrusions indicates that the Bangongco–Nujiang oceanic lithosphere was still undergoing northward subduction beneath the western QT at ca. 112.6–125.9 Ma, suggesting in turn that the oceanic basin have not closed completely during the Early Cretaceous. These new data also indicate that the processes that occur during the subduction of oceanic crust in continental marginal arc settings produce and preserve juvenile crustal material, leading to net continental crust vertical growth and thickening.