Mineral chemistry of clinopyroxene in pyroclastic rocks of the Xisha Islands and their geological significance

Li Shiying; Yu Kefu; Zhang Yu; Tao Shichen; Yan Hongqiang

doi:10.3969/j.issn.0253-4193.2019.07.006

Volume 41 Issue 7

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Li Shiying，Yu Kefu，Zhang Yu, et al. Mineral chemistry of clinopyroxene in pyroclastic rocks of the Xisha Islands and their geological significance[J]. Haiyang Xuebao，2019, 41(7)：65–76，doi:10.3969/j.issn.0253−4193.2019.07.006

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Mineral chemistry of clinopyroxene in pyroclastic rocks of the Xisha Islands and their geological significance

doi: 10.3969/j.issn.0253-4193.2019.07.006

Li Shiying^{1, 2
,},
Yu Kefu^{1, 3, 4, 5
,
,},
Zhang Yu^{3, 4, 5},
Tao Shichen¹,
Yan Hongqiang¹

1.
Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China
2.
University of Chinese Academy of Science, Beijing 100049, China
3.
Guangxi Laboratory on the Study of Coral Reefs in the South China Sea, Guangxi University, Nanning 530004, China
4.
Coral Reef Research Center of China, Guangxi University, Nanning 530004, China
5.
School of Marine Sciences, Guangxi University, Nanning 530004, China

Received Date: 2018-03-08
Rev Recd Date: 2019-01-01

Available Online: 2021-04-21

Publish Date: 2019-07-25

Abstract

Abstract

The composition of clinopyroxene in pyroclastic rocks at the bottom of the Xisha Islands was determined in detail by electron microprobe analysis. The results show that the clinopyroxenes are mostly rich in calcium and have a zonal structure. Ca, Fe and Ti concentrations increased from the core to the outer layers, reflecting the normal sequence of magmatic crystallization. The chemical characteristics of the pyroxene, including low Si, high Al (SiO₂=41.40%–48.44%, Al₂O₃=5.54%–10.20%) and even higher Al^IV concentrations, coincide with those of basic magma. The main element data of the clinopyroxene show that the concentration of monoclinic Ca is high. The Ca/(Ca+Mg+Fe) ratio is between 46.1% and 51.4%. The large amount of high-calcium clinopyroxene may be attributed to the high concentration of Ca in magma. Combining this with the earthquake and tectonic data of the Xisha sea area, we speculate that the basement of the coral reefs of Chenhang Island is a flat-topped seamount composed of basaltic volcanic clastic rocks. Further, we infer that its formation involved the passing of magma through the lithospheric layer of the fault and its eruption in the seabed of the Xisha Islands. The volcanic clastic material is thus formed by accumulation and consolidation, and the original rock of the volcanic clastic rock is an intraplate alkaline basalt.
- Xisha Islands,
- pyroclastic rock,
- clinopyroxene,
- mineral chemistry,
- coral reef basement

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