Petrogenesis of Jurassic tungsten-bearing granites in the Nanling Range, South China: Evidence from whole-rock geochemistry and zircon U–Pb and Hf–O isotopes

Abstract

The Nanling Range (NLR) is the largest tungsten metallogenic province in China and perhaps in the world. The tungsten mineralization is believed to be related to Jurassic granitic magmatism. However, the petrogenesis of these granites and their relation to the tungsten mineralization are still debated. Whole-rock geochemical and Sr–Nd–Hf isotopic data and zircon in situ U–Pb ages and Hf–O isotopes are reported for W-bearing granitic intrusions from the southern Jiangxi Province in the NLR, in order to constrain their magmatic sources and petrogenesis. The NLR granites include biotite granites, two-mica granites and garnet muscovite granites. SIMS and LA-ICPMS U–Pb dating of zircons and monazites give emplacement ages of 161–154Ma for these rocks. The granites are metaluminous to strongly peraluminous with high SiO2 (>72.3wt.%) and high K2O (>3.7wt.%). Petrographic and geochemical features show that they are highly fractionated I-type granites. The biotite granites are enriched in light rare earth elements (LREEs) relative to heavy REEs, have weakly negative Eu anomalies and are depleted in Nb, Ba, P and Ti. In contrast, the two-mica and garnet-bearing muscovite granites have tetrad-type REE patterns with strongly negative Eu anomalies and are extremely depleted in Ba, Nb, Sr, P and Ti. Magmatic garnets are mainly almandine and spessartine, and have low-Mn cores and high-Mn rims. Their (Y+HREE) contents are high and generally decrease from core (1.2wt.%) to rim (average=4955ppm). All of these granites are characterized by variable whole-rock initial 87Sr/86Sr (0.7053–0.8000), εNd(t) (−12.6 to −9.4) and εHf(t) (−12.3 to −8.5), as well as variable zircon εHf(t) and δ18O, with values of −16.3 to −7.4 and 7.6 to 10.0‰, respectively. They contain abundant zircon xenocrysts and xenoliths of micaceous schist. All of these features are consistent with a process of crystal fractionation of crustally-derived magmas coupled with strong assimilation of sedimentary rocks. The assimilation of meta-sedimentary materials increased the concentrations of tungsten, alumina and manganese in the magmas, whereas high degrees of crystal fractionation further enriched them in the evolved granitic rocks. The W-enriched magmatic fluids formed the tungsten-bearing quartz veins, revealing the genetic relationship between tungsten mineralization and emplacement of garnet-bearing two-mica or muscovite granites in the NLR.