Zircon U–Pb geochronology and geochemistry of Late Cretaceous–early Eocene granodiorites in the southern Gangdese batholith of Tibet: petrogenesis and implications for geodynamics and Cu ± Au ± Mo mineralization

Abstract

Cu ± Au ± Mo mineralization is found in multiple intrusive suites in the Gangdese belt of southern Tibet (GBST). However, the petrogenesis of these ore-bearing intrusive rocks remains controversial. Here, we report on mineralization-related Late Cretaceous-early Eocene intrusive rocks in the Chikang–Jirong area, southern Gangdese. Zircon U–Pb analyses indicate that the mainly granodioritic Chikang and Jirong plutons were generated in the Late Cretaceous (ca. 92 Ma) and early Eocene (ca. 53 Ma), respectively. They are high-K calc-alkaline suites with high SiO2 (64.8–68.3 wt.%) and Al2O3 (15.1–15.7 wt.%) contents. Chikang granodiorites are characterized by high Sr (835–957 ppm), Sr/Y (118–140), Mg# (58–60), Cr (21.8–36.6 ppm), and Ni (14.3–22.9 ppm), and low Y (6.0–8.1 ppm), Yb (0.54–0.68 ppm) values with negligible Eu anomalies, which are similar to those of typical slab-derived adakites. The Jirong granodiorites have high SiO2 (64.8–65.3 wt.%) and Na2O + K2O (7.19–7.59 wt.%), and low CaO (2.45–3.69 wt.%) contents, Mg# (47–53) and Sr/Y (14–16) values, along with negative Eu and Ba anomalies. Both Chikang and Jirong granodiorites have similar εHf(t) (7.6–13.1) values. The Chikang granodiorites were most probably produced by partial melting of subducted Neo-Tethyan oceanic crust, and the Jirong granodiorites were possibly generated by partial melting of Gangdese juvenile basaltic crust. In combination with the two peak ages (100–80 and 65–41 Ma) of Gangdese magmatism, we suggest that upwelling asthenosphere, triggered by the rollback and subsequent break-off of subducted Neo-Tethyan oceanic lithosphere, provided the heat for partial melting of subducted slab and arc juvenile crust. Taking into account the contemporaneous occurrence of Gangdese magmatism and Cu ± Au ± Mo mineralization, we conclude that the Late Cretaceous–early Eocene magmatic rocks in the GBST may have a significant potential for Cu ± Au ± Mo mineralization.