Petrogenesis of high-alumina basalts: Implications for magmatic processes associated with the opening of the South China Sea

Abstract

Results of International Ocean Discovery Program (IODP) Expeditions on the northern continental margin of the South China Sea (SCS) have nullified the early notion that the SCS is a magma-poor margin. However, there are continuing debates on how the rapid transition from continent to ocean is supported by geochemical and petrologic data, and whether such a process was related to plate subduction or mantle plume activities. Here we present the results of bulk-rock and plagioclase phenocryst geochemistry of the basalts generated during the early spreading stage (initial and steady ocean stages) of SCS basin extension, which were drilled at IODP Sites U1500 and U1503, respectively. We combined these findings with published data from the late spreading stage of the SCS basin, which provides an excellent opportunity to examine the magmatic processes associated with the evolution of the SCS basin. Our results revealed that the SCS basalts generally exhibit higher Al contents than global Mid-Ocean Ridge Basalts, particularly at lower MgO contents, which is similar to the characteristics of modern arc basalts. Nevertheless, their origins are site-specific and complex. For instance, some of the Site U1503 basalts originated through fractional crystallization of water-rich parental magmas with the lowest degree of melting, while the basalts from the other sites, including Site U1500, were primarily generated by the accumulation of plagioclase. More specifically, the water-rich magmas from Site U1503 displayed strong subduction signals in terms of trace elements, and their correlations with Al2O3 content suggest that they are the product of partial melting of the mantle wedge, closely related to the northward subduction of proto-SCS. This subduction had a significant effect on triggering the opening of the SCS, rather than a mantle plume. Moreover, the Site U1500 basalts exhibit a significant increase in plagioclase An values with the increase of host magma Al contents, indicating that the plagioclase floatation mechanism cannot account for the origin of their plagioclase accumulation. Therefore, we propose that the high abundance of plagioclase in Site U1500 basalts requires rapid ascent of magma, supporting a rapid rifting and strong magmatism during the initial opening of the SCS.