Petrogenesis of mid-Neoproterozoic (ca. 750 Ma) mafic and felsic intrusions in the Ailao Shan–Red River belt: Geochemical constraints on the paleogeographic position of the South China block

Abstract

Abundant mid-Neoproterozoic magmatic rocks are exposed in the western Yangtze block, a part of the South China block. Currently, based on contrasting interpretations of their origin, two competing reconstruction models, the internal model and the external model, have been formulated to constrain the paleogeographic position of the South China block in Rodinia. In this study, we examined 17 representative samples from three intrusions (Kuchahe, Leidashu, and Qizanmi) within the Ailao Shan–Red River belt, located in the southwestern Yangtze block. Laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon U-Pb dating suggests that they were emplaced during the mid-Neoproterozoic (ca. 750 Ma). The Kuchahe and Leidashu intrusions share geochemical characteristics of S-type and A2-type rocks, respectively. Elemental and isotopic data suggest that the Kuchahe intrusion was derived by biotite-dehydration melting of a metapelitic source, whereas the Leidashu intrusion was produced by partial melting of earlier Neoproterozoic granodiorites in the middle crust (∼15 km), with mixing of basaltic magmas. The Qizanmi intrusion is composed of hornblende gabbro and was likely derived by partial melting of a shallow and enriched lithospheric mantle source modified by slab-released fluids, with accumulation of plagioclase and fractionation of mafic minerals. The three intrusions were emplaced in a back-arc setting, in response to the roll-back of a subducted slab rather than mantle plume activity. The mid-Neoproterozoic (ca. 750 Ma) subduction process argues against the South China block occupying an internal setting in Rodinia, which was finally assembled before 0.9 Ga.