Zircon U-Pb dating and Sr-Nd-Pb-Hf isotopes of the ore associated granite porphyry at the Longgen Pb-Zn deposit, Tibet

Longgen is one typical Pb-Zn skarn deposit form-ed in the Nyaincientanglha metallogenic belt, Western Tibet. Lead-zinc mineralization mainly occurs as vein or lenticular in the external contact belt of skarnization limestone. Granite porphyries yield a weighted average zircon (206)pb/U-238 age of 64.3 +/- 0.7Ma, slightly older than the molybdenite Re -Os isochron age of 61.5 +/- 0.6 Ma, suggesting that the Longgen was formed in the Palaeocene. Granite porphyries show high contents of SiO2, K2O and CaO, low contents of TiO2 and P2O5, and peraluminous high-K calcalkaline of S-type granite, with obvious LREE enrichment and negative Eu anomalies. They have low (Sr-87/Sr-86), ratios of 0.71262 to 0.71482, epsilon Nd(t) values of -6.78 to -6.13, and T-DM2(Nd) ages of 1.37 to 1.42 Ga. Their and (Pb-206/Pb-204)t, (Pb-207/(PB)-P-204)(t) and (Pb-208/Pb-204)(t) values are 18.581-18.735, 15.653-15.713, and 38.932-39.158, respectively. Zircons epsilon Hf(t) values of the granite porphyry range from -4.57 to -2.65, with T-DM2(Hf) ages of 1.30-1.43 Ga. These geochemical and isotopic data imply that the Longgen granite porphyry originated mainly from partial melting of ancient continental crust materials of Gangdese micro-continent accompanied by fractional cryst-allization during the main collision between Indian and Asian. In addition, we conclude that the India Asia continental initial collision started no later than 65 Ma.