Neoproterozoic tourmaline-bearing peraluminous granitoids in the western Jiangnan Orogen, South China: Geochemistry, petrogenesis and tectonic implications

Abstract

Neoproterozoic granitoids are widespread in the Jiangnan Orogen in South China that was formed by assemblage between the Yangtze and Cathaysia Blocks. The peraluminous S-type granitoids from the Guibei region in the western Jiangnan Orogen are composed of quartz, alkali feldspar and plagioclase with subordinate biotite and muscovite. They show large variable FeO/MgO (1.9–12.8) and (K2O + Na2O)/CaO (2–84) ratios, indicating the rocks experienced fractional crystallization. Their εNd values (−9.31 to −5.45) and two-stage Nd model ages (1.83–2.34 Ga) suggest that the Guibei granitoids were derived from a mature source that was mainly composed of recycled supracrustal materials. The tourmalines, occurring as disseminated and nodular grains, show high Na + K (0.56–0.77 apfu), low Ca (0.01–0.08 apfu) and X-site vacancy (0.18–0.51 apfu), and belong to the alkali group. Their high Fe (1.64–2.28 apfu), low Mg (0.14–0.80 apfu) and Mg/(Mg + Fe) (0.016–0.325) suggest that the minerals are dominated by schorl end member in the schorl-dravite solid solution series. The tourmalines have constant and strong negative δ11B values (−13.4‰ to −9.5‰), and the nodular tourmalines are isotopically lighter (δ11B = −10.0‰ to −13.4‰) than the disseminated tourmalines (δ11B = −9.5‰ to −12.5‰). The disseminated tourmalines are suggested to have been crystallized in melts at the early magmatic stage, whereas the nodular tourmalines formed in fluid-rich immiscible melts at the late magmatic stage. The boron isotopic variations within or among the tourmaline generations are mainly controlled by Rayleigh fractionation. In combined with the regional geology, the peraluminous granitoids in this study are considered to be partial melts of the supracrustal rocks in the post-orogenic setting.