Post-collisional ore-bearing adakitic porphyries from Gangdese porphyry copper belt, southern Tibet: Melting of thickened juvenile arc lower crust

Porphyry Cu-Mo deposits, related to the Miocene adakitic porphyries from the Gangdese porphyry copper belt in the southern Tibet, formed in a post-collisional setting. Here, we present the new zircon U-Pb ages, whole-rock chemical, and Sr-Nd and zircon Hf isotopic data for the ore-bearing adakitic porphyries from Gangdese porphyry copper belt. LA-ICP-MS zircon dating for six samples yielded ages ranging from 19 Ma to 14 Ma, indicating they formed in the Miocene. The ore-bearing adakitic porphyries show SiO2 of 61.47-71.67%, K2O of 3.29-4.74%, and high Sr content (394-1106 ppm), high Sr/Y ratios (63-158), and low Y (6.12-10.3 ppm) and heavy rare earth element contents (e.g. Yb=0.52-0.91 ppm). They show steep fractionated REE and flat HREE patterns, and strong enrichment in large ion lithophile elements (Cs, Rb, Ba, Th. and U) and depletion of high field strength elements (Nb), with positive Sr and negative Ti anomalies. There are no linear variations of Ba, La, Sr/Y, Dy/Yb, (Sr-87/Sr-88)(i), and epsilon Nd(t) with increasing SiO2 content. Combined with the zircon positive epsilon Hf(t) values, and wide range of (Sr-87/Sr-88)(i) (0.70559 to 0.70908) and of epsilon Nd(t) (-6.8 to 0) values for all the adakitic samples, they were likely derived from the melting of garnet-bearing amphibolite in the juvenile arc mafic lower crust. Additionally, the adakitic porphyries with low Th/Nb ratios have lower initial Sr-87/Sr-86, and higher epsilon Nd(t) and epsilon Hf( t) values, and those with high Th/Nb show higher initial Sr-87/Sr-88, and lower epsilon Nd(t) and epsilon Hf(t) values. This indicates that the juvenile arc lower crust is derived from the melting of the mantle wedge that was metasomatized by slab fluid or sediment melt during the Neotethyan ocean subduction. Significantly, the juvenile arc lower crust possibly inherited the arc magma characteristics of abundant in F, Cl, and Cu and high oxidation state, which contributed to the porphyry Cu-Mo deposits in the Gangdese porphyry copper belt. (C) 2011 Elsevier B.V. All rights reserved.