Early paleozoic granodioritic plutons in the Shedong W–Mo ore district, Guangxi, southern China: Products of re-melting of middle Proterozoic crust due to magma underplating

Abstract

The Shedong W–Mo ore district in the south–central Dayaoshan Uplift of Guangxi, southern China hosts the Baoshan and Pingtoubei deposits, both of which occur in granodioritic plutons. Zircon U–Pb dating of granodiorites and its mafic microgranular enclaves (MMEs) in the Baoshan deposit yielded ages of 439.8±3.2 and 441.1±2.2Ma, respectively. Granodiorites have moderate SiO2 (54.5–63.0wt.%) and high Al2O3 (15.4–17.8wt.%) contents, wide variations in major element ratios, significant rare earth element fractionation, and small negative Eu anomalies. They are rich in Th, U, Zr, and Hf, and depleted in Ba, Nb, and Ti. Their initial 87Sr/86Sr, εNd(t), and εHf(t) values are in the range of 0.7086–0.7091, −5.2 to −6.6 and −6.3 to +1.6, respectively. Rounded or lenticular MMEs have relatively low silica and high mafic components, depletion in Eu, Sr, and Zr, and marked negative Eu anomalies. Rb/Sr and Nb/Ta ratios, and εNd(t) and εHf(t) values of the MMEs are higher than those of host granodiorites, indicating a different magmatic source. Zircon U–Pb dating of the unexposed granodiorite porphyry in the Pingtoubei deposit yielded an age of 440.0±1.7Ma. The granodiorite porphyries have high SiO2 and low K2O, FeOT, and MgO contents, with similar trace element features to the granodiorites at the Baoshan deposit, although the former has small negative Eu anomalies. Its initial 87Sr/86Sr values range from 0.7162 to 0.7173, εNd(t) values from −8.7 to −12.3, and εHf(t) values from −7.8 to +1.3, indicative of a crustal source. Nd and Hf two-stage model ages of the granodiorites, MMEs, and granodiorite porphyries have a narrow range between 1.3 and 2.2Ga. We propose that the granodiorites and MMEs at the Baoshan deposit were produced through re-melting of middle Proterozoic crust as a result of underplating of mantle-derived magmas in a transitional compression-to-extension tectonic setting. Mantle-derived magmas provided the heat and material for the formation of the granodiorites and MMEs.