New geochronological and geochemical data of the Longsheng mafic-ultramafic suite in northern Guangxi, China, and their implications in Rodinia breakup

Abstract

The Longsheng mafic-ultramafic suite consists of more than 100 mafic and ultramafic intrusions and are extensively distributed within the Neoproterozoic Danzhou Group along the Jiangnan Orogenic Belt in northern Guangxi, China. The suite has been a hotly debated subject in the context of emplacement age, source of magmas, and tectonic setting for several decades. In this article, we present newly obtained LA-ICP-MS zircon U-Pb age data, whole-rock major and trace elemental data, and Sr-Nd and zircon Lu-Hf isotopic data. These data support a middle Neoproterozoic intrusive emplacement mechanism as a result of asthenospheric underplating and subsequent lithospheric thinning, extension, and rifting. This integrated process might well correspond to the early stage of the Rodinia breakup. For example, five mafic-ultramafic rock samples yielded an emplacement age of 774\u2009±\u20092 Ma. The samples contain a large number of inherited zircon xenocrysts dated at 821\u2009±\u20093 Ma, 1.4–2.1 Ga, and 2.3–2.8 Ga, indicative of crustal contamination. The mafic-ultramafic rocks are characterized by calc-alkaline to shoshonitic associations; their Zr/Nb, Th/Yb, La/Nb, Ba/La ratios, trace, and REE patterns show an OIB affinity. Their initial 87Sr/86Sr values range from 0.697721 to 0.708579 with an average of 0.702352. They have relatively low initial 143Nd/144Nd values of 0.511530–0.511859 with an average of 0.511661. Sixteen magmatic zircon grains from diabase samples show 176Hf/177Hf values of 0.281326–0.282638, εHf(t) values of −\u200934.48–11.89, and TDM2 of 925–3819 Ma. Their Nb/U and Th/Ta ratios support crustal contamination as well. It is concluded that the Longsheng mafic-ultramafic suite was originated from partial melting of the enriched upper mantle (EM-I type) due to asthenospheric underplating and lithospheric extension during the incipient Rodinia breakup, and that the magmas were contaminated with crustal material during ascent.