Geochronology and geochemistry of Eocene potassic felsic intrusions in the Nangqian basin, eastern Tibet: Tectonic and metallogenic implications

Abstract

The Jinshajiang–Ailaoshan copper belt is the most significant porphyry copper belt in eastern Tibet. In the northern segment of this belt within the Nangqian basin, which occurs 100km east of the Yulong porphyry copper deposit, several felsic intrusions have been recently discovered. The Yulong porphyry copper deposit is one of the largest porphyry copper deposits in China, and it is associated with peraluminous adakitic rocks formed in a post-collisional setting. The Nangqian felsic intrusions vary from syenite porphyry to monzonite porphyry in rock types. No significant Cu–Au mineralization has been found in the Nangqian felsic intrusions despite extensive exploration in recent years. LA-ICP-MS zircon U–Pb dating reveals that the Nangqian syenite porphyry and monzonite porphyry were emplaced at ~35.6±0.3Ma and from 39.5±0.3 to 37.4±0.3Ma, respectively, similar to the age of the Yulong porphyry copper deposit. The Nangqian felsic intrusions are characterized by metaluminous compositions (A/CNK=0.82–1.01), and they share some common features with shoshonites such as high K2O contents (4.58–5.58wt.%), high K2O/Na2O ratios (0.92–1.28), LREE–LILE enrichments and negative Nb–Ta–Ti–P anomalies, as well as with adakites derived from an eclogite-facies source with high Al2O3 (14.98–15.74wt.%), Sr (954–2190ppm), Sr/Y (68–132) and La/Yb (53–85), and low Y and Yb contents. The Nangqian felsic intrusions have high initial 87Sr/86Sr (0.7050–0.7055), variable εNd(t) (−0.31–1.43) and small variations in (206Pb/204Pb)i (18.68–18.74), (207Pb/204Pb)i (15.53–15.62) and (208Pb/204Pb)i (38.51–38.80). Zircon crystals from both syenite and monzonite porphyries are characterized by positive εHf(t) from 5.2 to 8.5. The results suggest that the syenite and monzonite magmas were differentiated from parental shoshonitic melts by fractional crystallization of olivine, clinopyroxene and minor feldspar. The parent magmas originated from a lithospheric mantle metasomatized by slab-derived fluids or melts during continental subduction. The differences in both sources and depths of partial melting may explain the difference in the extent of Cu–Au mineralization between the Yulong and Nangqian porphyries.