Neoproterozoic high-Mg basalts formed by melting of ambient mantle in South China

The southeastern Yangtze Block, South China, contains voluminous late Neoproterozoic felsic plutonic rocks and minor mafic-ultramafic volcanic rocks. Some ca. 830 Ma lavas hosted in the Neoproterozoic strata have pillow structures and micro-spinifex textures and thus previously were considered as komatiitic basalts derived from a mantle plume. These rocks have high MgO (10.1-16.3 wt%), variable CaO (6.57-11.2 wt%), restricted Al2O3 (11.2-13.4 wt%), and low TiO2 (0.50-0.61 wt%), P2O5 (0.03-0.04 wt%) and incompatible trace elements. They also have negative ɛNd (−5.4 to −2.9), and high 207Pb/204Pb (15.71-15.76) and initial 187Os/188Os ratios (0.1100-0.1931), indicating crustal contamination. The most primitive samples show arc-like trace elemental features which are characterized by enrichment of LREE and LILE relative to HFSE in the primitive-mantle-normalized trace-element diagrams, suggesting derivation from enriched mantle sources. PRIMELT2 modeling shows that their primary magmas were produced by about 30% partial melting of a fertile peridotite source having a mantle potential temperature of 1440-1500 °C. Such a potential temperature is in agreement with the estimates for ambient mantle during the Neoproterozoic. These evidence suggest that the high-Mg basalts are similar to boninites and probably be part of an ophiolite assemblage. They were generated by melting of a hot ambient mantle source in relation to slab subduction during the amalgamation of the Yangtze and Cathaysia Blocks.