Origin of the Dexing Cu-bearing porphyries, SE China: elemental and Sr-Nd-Pb-Hf isotopic constraints

摘要

The Dexing porphyry copper deposit, part of the circum-Pacific porphyry copper ore belt, is the largest porphyry copper deposit in China. We present new LA-ICP-MS zircon U-Pb and molybdenite Re-Os dating, bulk-rock elemental and Sr-Nd-Pb isotopic as well as in situ zircon Hf isotopic geochemistry for these ore-bearing porphyries, in an attempt to better constrain their petrogenesis. LA-ICP-MS zircon U-Pb dating shows that the Dexing porphyries were emplaced in the early Middle Jurassic (～171 Ma);;molybdenite Re-Os dating indicates that the associated Cu-Mo mineralization was contemporaneous (～171 Ma) with the igneous intrusion. The rocks are mainly high-K calc-alkaline and show adakitic affinities, including high Sr and low Y and Yb contents, high Sr/Y and La/Yb ratios, and high Mg# (higher than pure crustal melts). These porphyries have initial ~(87)Sr/~(86)Sr ratios of 0.7044-0.7047, ε_(Nd){T) values of -1.5 to +0.6, and ε_(Hf)(T) (in situ zircon) values of +2.6 to +4.6. They show unusually radiogenic Pb isotopic compositions with initial ~(206)Pb/~(204)Pb ratios up to 18.41 and ~(207)Pb/~(204)Pb up to 15.61. These isotopic compositions are distinctly different from either Pacific MORB or Yangtze lower crust but are similar to the subducting sediments in the western Pacific trenches. Detailed elemental and isotopic data suggest that the Dexing porphyries were emplaced in a continental arc setting coupled with westward subduction of the palaeo-Pacific plate. Partial melting involved the subducted slab (mainly the overlying sediments), with generated melts interacting with the lithospheric mantle wedge, thereby forming the investigated high-K calc-alkaline porphyry magmas.