Neoproterozoic granitoids in South China: crustal melting above a mantle plume at ca. 825 Ma?

Abstract

Neoproterozoic pre-rifting granitoids are widespread in the Yangtze craton, South China. Mineralogical, petrographic and geochemical characteristics indicate that there are two types of peraluminous, S-type granitoids—muscovite-bearing leucogranites (MPG) and cordierite-bearing granodiorites (CPG), and two types of I-type granitoids—K-rich calc-alkaline granitoids (KCG) and tonalite–trondhjemite–granodiorite (TTG). Sm–Nd isotopic data suggest that all were generated by partial melting of various crustal rocks without appreciable involvement of new mantle-derived magmas, i.e. pelitic and psammitic sources for MPG and CPG, and tonalitic to granodioritic and amphibolite sources for KCG and TTG, respectively. Despite pronounced geochemical and isotopic heterogeneity of these granitoids, and their vast areal extent, new SHRIMP U–Pb zircon ages, together with previous U–Pb data, indicate that they formed within short time interval of ∼5 Ma at ca. 825–820 Ma. The granitoids were essentially coeval with ∼825 Ma mafic/ultramafic intrusions in South China. The granitoids exhibit no temporal or spatial zonation, hence no simple genetic relationship can be established between the different granitoid types and geodynamic environments such as subduction, continental collision, and post-collisional relaxation, as suggested previously. Instead, the coeval intrusion of such mixed types of granitoids and mafic/ultramafic rocks over an area of >1000km×700 km suggests that these rocks are more likely to have resulted from extensive crustal anatexis caused by conductive heating above a mantle plume beneath South China at ∼825 Ma. This interpretation has significant implications to better understanding of geodynamic environments in which the granitoids formed, i.e. granitoids can form not only in various environments during different stages of the Wilson cycle but also above mantle plumes.