Geochemistry and Hf-Nd isotope characteristics and forming processes of the Yuntoujie granites associated with W-Mo deposit, Guangxi, South China

摘要

The Yuntoujie W-Mo deposit is so far the only Indosinian granite-related deposit with an element association of W-Mo in the Nanling ore belt, South China. The granites in the Yuntoujie ore field consist of the Yuntoujie batholith and late-stage granites. The W-Mo mineralization is associated closely with the late-stage granites. Here we report major and trace element, Sr-Nd isotope and zircon Hf isotope compositions of the Yuntoujie batholith and the late-stage granites. The Yuntoujie batholith and the late-stage granites have high SiO2 (70.77-74.86 wt.%) and alkaline (K2O + Na2O = 6.27-8.27 wt.%) concentrations and are characterized as peraluminous (A/CNK > 1.1, A/NK = 1.30-1.43). The Yuntoujie batholith and late-stage granites feature negative correlation between Th, Y and Rb, no positive correlation between P2O5 and SiO2 and high A/CNK values > 1.1, suggesting that they could be classified as S-type granites. The Yuntoujie batholith and the late-stage granites might be sourced from the partial melting of Proterozoic crust without contribution from juvenile crust, as evidenced from the low Nd-143/Nd-144 values of 0.511942-0.512477, the epsilon(Nd)(t) values of -8.52 to -11.76 (falling in the field of Nd isotope evolution for the Proterozoic crust), and the zircon epsilon(Hf)(t) values of -7.19 to -14.89 with the T-DM2 ranging from 1713 Ma to 2187 Ma. The major and trace element compositions of the Yuntoujie batholith and late-stage granites, especially the low CaO/Na2O and CaO/(MgO + FeOT) ratios and the high Rb/Ba and Rb/Sr ratios, further indicate that their sources are predominantly argillaceous sedimentary. The trace element characteristics, such as the positive correlation between La/Yb and La and between Ba and Sr and negative correlation between Ba and Rb, suggest that the late-stage granites are the product of highly fractionated melts of the Yuntoujie magma chamber. Moreover, 1) the tetrad effect observed in the REE patterns for the late-stage tourmaline muscovite monzogranite porphyry and 2) the abundant fluid inclusions in their quartz phenocrysts imply that the processes of the volatile phase exsolution and the fluid-melt reaction had taken place in the evolved water-saturated magma. These geochemical features, together with the fact that the Yuntoujie magma chamber had survived for > 10 Myr, suggest that the peraluminous magma experienced a prolonged period of crystallization differentiation, which plays a key role in the formation of W-Mo mineralization associated with granites. From the observations that the granites in the Yuntoujie district were formed about 20-10 Ma later than the peak age of the Indosinian collisions and that they all fall in the post-CEG area in the tectonomagmatic discrimination diagrams, we proposed that the Yuntoujie granites were formed in the post-collisional environment (C) 2016 Elsevier B.V. All rights reserved.