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

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.