2017 年 51 巻 2 号 p. 123-142
Lamprophyres are widespread in the Xiongcun porphyry copper-gold district in the southern margin of the Lhasa terrane. They represent the latest magmatism and cut all other rock types as well as orebodies in the Xiongcun district. Herein, we report the zircon LA-ICP-MS U-Pb age, whole-rock major and trace elements and Sr-Nd-Pb isotopic compositions of lamprophyre from the Xiongcun district. LA-ICP-MS zircon U-Pb dating yields crystallization ages in Eocene at 47.03 ± 0.61Ma for the Xiongcun lamprophyre. The major element contents of the Xiongcun lamprophyres exhibit high Mg# along with calc-alkaline and ultrapotassic characteristics. The lamprophyres are enriched in LILEs (e.g., K, Rb, and Ba) and LREEs but depleted in HFSEs (e.g., Nb, Ta, and Ti). The Xiongcun lamprophyres are characterized by high (87Sr/86Sr)i ratios ranging from 0.71846 to 0.71858, low εNd(t) values ranging from –14.44 to –15.09, and enriched in radioactive Pb with (206Pb/204Pb)i, (207Pb/204Pb)i and (208Pb/204Pb)i ranging from 18.53–18.97, 15.62–15.75, and 38.75–39.53, respectively. On the basis of the geochemistry of the Xiongcun lamprophyres, we propose that they were derived from the partial melting of an enriched lithospheric mantle source that was metasomatized by fluids released from the subducted Neo-Tethyan oceanic slab and Indian continental crust. This enriched lithospheric mantle source is characterized by phlogopite- and garnet-bearing lherzolite. The break-off of the subducted Neo-Tethyan oceanic slab from the Indian continental lithosphere at 45~50 Ma resulted in asthenospheric upwelling through the slab window. Subsequently, the enriched lithospheric mantle was heated by the underlying hot asthenosphere and partially melted, generating the magma that ascended to form the Xiongcun lamprohyres in an extensional tectonic setting. The parent magmas were fractionated during their ascent, whereas the effect of crustal assimilation on the magma was limited.
