Abstract
Cenozoic volcanic rocks were recently discovered during full-coring kilometer-scale major scientific drilling in the Xisha Islands, northwestern South China Sea. A systematic mineralogical study of these samples was performed for this paper. The results show that the volcanic rock samples are basaltic pyroclastic. The major elements demonstrate that the clinopyroxenes are diopside and fassaite, which contain high Al2O3 (5.33–11.2 wt. %), TiO2 (2.13–4.78 wt. %) and CaO (22.5–23.7 wt. %). Clinopyroxenes have high REE abundances (104–215 ppm) and are strongly enriched in LREE (LREE/HREE = 3.56–5.14, La/YbN = 2.61–5.1). Large-ion lithophile elements show depleted characteristics. Nb/Ta shows obvious fractionation features: Nb is lightly enriched, relative to primitive mantle, but Ta is heavily depleted, relative to primitive mantle. The parental magma of the basaltic pyroclastic rocks belongs to a silica-undersaturated alkaline series, characterized by a high temperature, low pressure, and low oxygen fugacity. The AlIV content increases with decreasing Si concentration. The Si-unsaturated state causes Si-Al isomorphic replacement during the formation of clinopyroxene. The electric charge imbalance caused by the replacement of Si by Al is mainly compensated by Fe3+. The clinopyroxene discrimination diagrams show that the parental magma formed in an intraplate tectonic setting environment.
Highlights
As one of the largest marginal seas of Eastern Asia, the South China Sea (SCS) is located at the intersection of the Eurasian Plate, the Pacific Plate, and the Indian Ocean-Australian Plate
The clinopyroxenes from Well CK-2 have high ratios of AlIV /AlVI, ranging from 5 to 105. Combining this information with mineralogical structural features, we suggest that the clinopyroxenes from Well CK-2 belong to the fourth type and are crystallized from basaltic magma at low pressure
We suggest that the clinopyroxenes from Well CK-2 belong to the fourth type and are crystallized from basaltic magma at low pressure
Summary
As one of the largest marginal seas of Eastern Asia, the South China Sea (SCS) is located at the intersection of the Eurasian Plate, the Pacific Plate, and the Indian Ocean-Australian Plate. Clinopyroxene composition mainly depends on primary magma characteristics, the crystallization environment, and the tectonic setting of magmatism [25,26,27,28,29,30,31,32]. The mineral chemistry of relict clinopyroxene in igneous rocks can reflect the characteristics of the primary magma well and has been widely used to study the nature of the original magma and complicated processes that have affected the lithospheric mantle [21,35,36,37,38,39,40,41,42]. Well CK-2 was analyzed to explore the origin of the clinopyroxene and primary magma It is of great significance for understanding the characteristics of the Cenozoic basalts in the SCS and the formation and evolution of basaltic magma. The results will provide new evidence for the comprehensive understanding of the characteristics of Cenozoic magmatism in the SCS and the northern margin
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