Origin of Miocene Cu-bearing porphyries in the Zhunuo region of the southern Lhasa subterrane: Constraints from geochronology and geochemistry

Abstract

The Zhunuo Cu-bearing porphyries are located in the westernmost part of the Miocene Gangdese porphyry Cu (Mo–Au) deposit belt. Zircon U–Pb dating of the diorite porphyry, K-feldspar granite porphyry, and monzonitic granite porphyry in Zhunuo yielded crystallization ages of 12.5±0.4Ma, 12.3±0.3Ma, and 12.4±0.3Ma, respectively. The diorite porphyry is characterized by low SiO2 (58.61–61.14wt.%) and Th (0.30–0.76ppm) concentrations, low Th/La (0.05–0.1) ratios, and high Mg# (>49) values coupled with low (87Sr/86Sr)i (0.703777–0.703783) and high εNd(t) (+4.07 to +4.90) values. They also have adakite-like affinities, such as low Y (10.5–12.0ppm), and high Sr/Y ratios (61–65). They were probably derived from a thickened juvenile lower continental crust. The K-feldspar granite porphyry probably originated in the middle–upper continental crust because of their high SiO2 (73.59–74.98wt.%) and Th (50.1–52.1ppm) concentrations, high Th/La (1.67–2.10), and low Sr/Y (20.2–20.7) ratios and Mg# (32–38) values, combined with high (87Sr/86Sr)i (0.710921–0.712008), low εNd(t) (−8.47 to −9.26) isotopic compositions and old Nd model ages (1.16–1.25Ga). Their magmas were most likely partial melts of the preserved ancient crust similar to the central Lhasa subterrane. The geochemical characteristics and Sr–Nd isotopic compositions of the monzonitic granite porphyry display trends that lie between those of the diorite porphyry and K-feldspar granite porphyry, and they are therefore likely to be production of hybridization between the above two melts. The ore-bearing diorite porphyry and monzonitic granite porphyry have higher zircon Ce4+/Ce3+ ratios than the ore-barren K-feldspar granite porphyry, indicating a higher oxygen fugacity in the ore-bearing magmas. We suggest that metals were released from the re-melting of arc-related cumulates which formed during lower crustal growth and thickening. This mechanism provides a reasonable explanation for the significant flare-up of mineralization during the Miocene in the Gangdese region. The lower continental crust beneath southern Lhasa subterrane probably was uniformly juvenile but the region to the west of Zhunuo was not mineralized due to input of large ancient crustal materials in the source of these ore-barren adakite-like rocks.