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.

南海（South China Sea, SCS）是经历完整威尔逊旋回（Wilson cycle）的构造板块典型案例。本文报道了采自南海扩张脊上3个国际大洋钻探计划（International Ocean Drilling Program, IODP）钻孔（即U1431、U1433和U1434站位）的全岩样品与火山玻璃的主量、微量元素组成，以及斑晶（橄榄石、辉石与斜长石）和橄榄石包裹尖晶石的矿物化学数据。全岩样品均属拉斑玄武岩（tholeiitic）成分，表现出轻稀土元素（light rare-earth elements, LREEs）相对亏损、重稀土元素（heavy rare-earth elements, HREEs）相对富集的特征，其配分模式与洋中脊玄武岩（mid-ocean ridge basalt, MORB）相似。然而，火山玻璃相对富集碱金属与轻稀土元素，亏损重稀土元素，并在Nb、Ta、Ti处呈现正异常，且Nb相对于La和Th表现为正异常，暗示其具有与地幔柱相关的洋岛玄武岩（ocean island basalt, OIB）型地球化学特征。富钙斜长石、贫钙透辉石及橄榄石斑晶中的复杂环带结构，指示了分离结晶与岩浆混合作用过程，该过程导致了全岩成分的变化。基于不同的温压计，本文确定了南海新生代火山岩的结晶压力与温度：全岩组分对应2.6–4.9千巴（kbar），火山玻璃对应2.0–6.5千巴，单斜辉石-熔体体系对应3.2–5.6千巴；结晶温度方面，单斜辉石-熔体体系为1196–1312℃，橄榄石-熔体体系为1250–1346℃，斜长石体系为1178–1260℃。利用Al-in-橄榄石温压计得到的地幔潜在温度介于1295–1419℃之间，该范围远高于其他原始洋中脊玄武岩，但与冰岛洋岛玄武岩的地幔潜在温度相当。南海玄武岩的源区并非仅由地幔橄榄岩构成；其熔体包含18.5%的榴辉岩（致密的再循环洋壳）以及46.1%的石榴石辉石岩（由橄榄岩熔体与再循环洋壳反应形成）。上述发现与海南岛及周边地区晚新生代火山岩的地球化学特征一致，该类火山岩与地幔柱活动相关。本研究结果支持地幔柱-洋中脊相互作用模型。