Petrogenesis of Cenozoic submarine basalts from the South China Sea spreading ridge: Constraints from geochemistry and Al-in-olivine thermometry
The South China Sea (SCS) is a typical case of a tectonic plate having undergone a complete Wilson cycle. Here we report new major- and trace-element compositions of bulk-rock samples and volcanic glasses, combined with mineral chemistry of phenocrysts (i.e., olivine, pyroxene, and plagioclase) and of olivine-hosted spinel inclusions from three International Ocean Drilling Program drill cores, i.e., Sites U1431, U1433, and U1434, in the SCS spreading ridge. The bulk-rock samples are all tholeiitic in composition and show relative depletion in light rare-earth elements (LREEs) and enrichment in heavy rare-earth elements (HREEs), resembling the pattern of mid-ocean ridge basalt (MORB). Nevertheless, the volcanic glasses are relatively enriched in alkalis and LREEs and depleted in HREEs and exhibit positive anomalies in Nb, Ta, and Ti as well as a positive Nb relative to La and Th, suggesting ocean island basalt (OIB)-type geochemical features related to a mantle plume. Complex zoned textures in the Ca-rich plagioclase, Ca-deficient augite, and olivine phenocrysts are suggestive of fractional crystallization and magma-mixing processes, resulting in a change in bulk-rock composition. On the basis of distinct thermometers, we determined crystallization pressure and temperature for the SCS Cenozoic volcanic rocks to be 2.6–4.9kbar (bulk-rock composition), 2.0–6.5kbar (volcanic glasses), and 3.2–5.6kbar (clinopyroxene-liquid) and 1,196–1,312°C (clinopyroxene-liquid) 1,250–1,346°C (olivine-liquid), and 1,178–1,260°C (plagioclase), respectively. The application of an Al-in-olivine thermometer provided mantle potential temperatures varying within 1,295–1,419°C, a range much higher than that of other primitive MORBs but comparable to that of Icelandic OIB. Mantle peridotite did not serve as the sole source of the SCS basalts; the melt involved 18.5% eclogite (dense, recycled oceanic crust) and 46.1% garnet pyroxenite (produced by the reaction between the peridotite melt and recycled oceanic crust). These findings are consistent with the geochemical features of the late Cenozoic volcanic rocks in Hainan and surrounding areas associated with a mantle plume. Our results support a mantle-plume–ridge interaction model.