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

Abstract

The southeastern Yangtze Block, South China, contains voluminous late Neoproterozoic felsic plutonic rocks and minor mafic–ultramafic volcanic rocks. Some ca. 830Ma 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 (10.1–16.3wt%), variable CaO (6.57–11.2wt%), restricted Al2O3 (11.2–13.4wt%), and low TiO2 (0.50–0.61wt%), P2O5 (0.03–0.04wt%) 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.