Zircon Hf-O isotope and magma oxidation state evidence for the origin of Early Cretaceous granitoids and porphyry Mo mineralization in the Tongbai-Hong'an-Dabie orogens, Eastern China

The Tongbai-Hong'an-Dabie orogens constitute the east segment of the Qinling-Dabie orogenic belt, which host several large molybdenum deposits, with a proven reserve of over 8 million tons metal Mo. In this study, we present an integrated study of in situ zircon U-Pb dating and Hf-O isotopes, in combination with magma oxidation state estimation for the Early Cretaceous granitoids in the Tongbai-Hong'an-Dabie regions. These granitoids can be divided into three groups on the basis of zircon U-Pb ages, including early-stage ore-related granitoids from Mushan, Xiaofan and Wangwan deposits (143-136 Ma), ore-barren granites (136-130 Ma), and late-stage ore-related granitoids from Qian'echong, Bao'anzhai, Tangjiaping and Shapinggou deposits (130-110 Ma). Early-stage ore-barren granitoids have the lowest average epsilon(Hf)(t) and delta O-18 values of -27.1 +/- 1.6 and 4.25 +/- 0.63 parts per thousand as well as the oldest two-stage Hf model ages (T-DM2) of similar to 2.8 Ga, which are roughly comparable with the inferred Dabie lower crust, indicating that they were mainly derived from the partial melting of the lower crust. Ore-forming granitoids in two stages show similar two-stage Hf model ages from 2.2 to 2.3 Ga which differ from any inferred layer of crusts, while they exhibit different zircon Hf-O isotope and oxygen fugacity evolutionary trends. Granitoids display decreasing average epsilon(Hf)(t) and delta O-18 values from -17.1 +/- 1.9 to -27.1 +/- 1.6, and 5.71 +/- 0.50% to 4.25 +/- 0.63%, and increasing average logfO(2) values from -17.4 +/- 4.5 to 6.7 +/- 3.8 with a decrease in age from 143 to 130 Ma. In addition, relict zircon cores with Paleoproterozic and Archean ages were observed with epsilon(Hf)(t) and delta O-18 values analogous to those of the Kongling Complex. These characteristics suggest that the early-stage granitoids were possibly produced by the remelting of the subducted lower crust mixed with the Kongling Complex, with the involvement of reducing Mo-rich black shales in the Yangtze Block that seemingly account for the lower oxygen fugacity of ore-forming granitoids with younger two-stage Hf model ages and lead to the formation of those early-stage Mo deposits. As the increasing contributions of the lower crust in the magma source, the metallogenic gap developed from 136 to 130 Ma in the Tongbai-Hong'an-Dabie orogens. Zircon Hf-O isotopic compositions of ore-related granitoids increase simultaneously up to -14.2 +/- 4.3 and 5.82 +/- 0.33% respectively at a lower slope after 130 Ma. Part of zircon delta O-18 values is slightly higher than 6.5%, implying the involvement of supercrustal materials. Meanwhile, magma oxygen fugacity exhibits an abrupt drop at ca. 130 Ma in the Qian'echong deposit with the lowest average logfO(2) value (-19.3 +/- 4.5) and Ce4+/Ce3+ ratios (1-138), then rises quickly up to a maximum average logfO(2) value of -7.3 +/- 2.3. As such, these granitoids were likely generated by mixing of a blended crustal magma, derived from the lower crust and supercrustal materials, and the mantle-derived component which also contributes to the late-stage Mo mineralization. We propose that not only the highly oxidized magma but also the continuous change of oxygen fugacity in the deep magma chamber may have positive effects on the formation of porphyry Mo deposits. Given the significant geochemical change at ca. 130 Ma instead of 125 Ma, we agree that the intensive magmatism and mineralization in the Tongbai-Hong'an-Dabie orogens are closely associated with the delamination of the thickened lower crust of the Yangtze Block, which caused the asthenosphere upwelling that provided the heat source and led to partial melting of overlaying lower crust.

Automated summary

The study reveals that the early granitoids in the Tongbai-Hong'an-Dabie region were mainly produced by partial melting of the lower crust, involving a mixture of the subducted lower crust and the Kongling Complex. With time, ore-forming granitoids show increasing zircon Hf-O isotopic compositions and oxygen fugacity trends, potentially indicating the sources and background for the formation of early-stage Mo deposits.