Neoproterozoic boninite-series rocks in South China: A depleted mantle source modified by sediment-derived melt

Abstract

A series of Neoproterozoic boninitic pillow lavas are hosted in Early Neoproterozoic strata along the southeastern margin of the Yangtze Block. These lavas record a period of subduction initiation during the secular evolution of South China. In this work we present analytical results from three units of pillow lavas (the Zhangyuan, Shexian and Lushan units) that form a common liquid line of descent with the previously reported 830-Ma boninites in the region, suggesting that they were produced by differentiation from similar parental melts. MELTS calculations show that major element compositions require fractionation of olivine + clinopyroxene + plagioclase + spinel from primary boninitic melts to have been accompanied by variable amounts of crustal contamination. The pillow lavas from the three localities show similar rare earth element patterns and arc-like primitive mantle normalized trace element patterns. They have high ^(206)Pb/^(204)Pb (18.41–19.33), ^(207)Pb/^(204)Pb (15.63–15.79) and ^(208)Pb/^(204)Pb ratios (38.78–40.26). Their ε_(Nd) values decrease from the Lushan lavas (+ 0.7 to + 1.9) to the Shexian lavas (− 0.5 to − 1.1) to the Zhangyuan lavas (− 4.0 to − 1.0), reaching values lower than the upper-level crustal contaminants and therefore indicating progressive enrichment of their sources by sediment melts. Compared with nearby but older (850 Ma) MORB-like tholeiitic basalts, which yield mantle potential temperature below 1400 °C and H_2O contents nearly 0.13 wt.%, the primary magmas of the boninite series rocks in this study were formed under higher pressures (3.5–1.0 GPa) and mantle potential temperatures (1440–1500 °C) with high H_2O contents (3420 to 4830 ppm), clear slab-derived sedimentary melt input, and harzburgite residues. The introduction of slab-derived water and sedimentary melt components into the mantle wedge at such high pressures, together with high temperatures and high degrees of melting, indicate that within a time span of only 20 million years after subduction initiation (850 to 830 Ma), the slab subducting under the southeastern margin of the Yangtze Block achieved substantial penetration to depth and interacted with hot overlying mantle, before subduction was abruptly ended by the amalgamation of the Yangtze and Cathaysia blocks.

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