Petrogenesis and geodynamic setting of granitoids at Machangqing Cu–Mo (Au) deposit, western Yangtze craton, southwestern China: constraints from zircon U–Pb and molybdenite Re–Os geochronology, Lu–Hf isotopes, and geochemistry

Abstract

The Machangqing Cu–Mo (Au) deposit is located in the central part of the Jinshajiang – Red River belt in the Sanjiang orogen, which lies across the Qiangtang terrane and western Yangtze craton, southwestern China. Zircon U–Pb dating constrains that the granite porphyry and porphyritic granite emplacements occurred at 35.92 ± 0.31 Ma and 34.92 ± 0.31 Ma, respectively. The Re–Os model ages of molybdenite are 34.94 ± 0.38 Ma. The new ages presented here, along with previously published data in the region, define a short duration of potassic magmatism and mineralization from 37 Ma to 34 Ma in the Jinshajiang – Red River belt. Zircon Ce4+/Ce3+ values of the porphyritic granite and granite porphyry vary from 50.32 to 1579.20 (averaging 481.01) and 33.18 to 1511.80 (averaging 452.98), respectively, and the log(fo2) values vary from –6.66 to −23.86 and −9.88 to −25.18, respectively, which plot within the range of the fayalite–magnetite–quartz buffer curve to the magnetite–hematite buffer curve, indicating an oxidized magma source, which may have facilitated the Cu–Au enrichment. Zircons from granitoids show εHf(t) values ranging from −0.75 to +2.33 and crustal model ages between 0.9 and 1.1 Ga. The features of Lu–Hf isotopes and wide range of Mg#, Cr, and Ni contents imply that the magmas of the Machangqing granitoids were probably derived from partial melting of juvenile lower crust and mixed with some mantle melts. Combined with the features of the Machangqing granitoids, the following evolution process are concluded. During the Cenozoic, the India–Asia continental collision triggered upwelling of hot asthenosphere and underplating of the thickened juvenile lower crust, which caused the formation of mafic and felsic magmas. Those magmas ascended, mixed, crystallized, and formed Machangqing ore-bearing granitoids in an intracontinental extension setting.