The discovery and geological significance of the Eocene adakitic rocks in Paxia,Renbu,Xizang

The emphasis in the present paper is placed on LA-ICP-MS zircon U-Pb dating and whole-rock lithogeochemical analysis of quartz diorite cropped out in the middle part of the Gangdise tectonic belt. The sampled quartz diorite yields a weighted average age of 49.5 ±1.7 Ma (n=9, MSWD=2.0) dated by using LA-ICP-MS zircon U-Pb dating method. Geochemical data suggest that the quartz diorite belongs to the metaluminous high-K cala-alkaline series rocks, with the total REE contents (ΣREE) ranging from 110.14×10^-6 to 143.36×10^-6, LREE/HREE ratios ranging from 7.09 to 10.23, and δEu values ranging from 0.93 to 1.14. These rocks are characteric of the enrichment of the large ion lithophile elements (LILE) U, Th, K, Ba and Rb, depletion of the high field strength elements (HFSE) Ta, Nb and Ti, and high Sr contents and low Y contents, which represent typical features of adakitic rocks. The authors in the present paper contend that the adakitic quartz diorite in the study area may originate from a mixed magmatic origin including the lower crust and mantle wedge, with the residual phases of rutile and hornblende and without the plagioclase phase, and with the fractional crystallization during the emplacement of the magmas. The quartz diorite pluton may record the mixed crust-mantle magmatic origin associated with the mantle upwelling triggered by the broken subducting plate in the middle part of the Gangdise tectonic belt at about 50 Ma.