LA-ICP-MS Analysis of Clinopyroxenes in Basaltic Pyroclastic Rocks from the Xisha Islands, Northwestern South China Sea

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 basalt...

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Bibliographic Details
Published in:Minerals (Basel) 2018-12, Vol.8 (12), p.575
Main Authors: Zhang, Yu, Yu, Kefu, Qian, Handong
Format: Article
Language:English
Subjects:
Aluminum
Calcium
Cenozoic
clinopyroxene
Composition
Crystallization
Diagenesis
Earth
Electric charge
Gasoline
Geochemistry
Geology
Igneous rocks
Iron
Islands
LA-ICP-MS
Lava
Low pressure
Magma
Magnesium
mineral chemistry
Mineral composition
Minerals
Petrogenesis
Petrology
Plate tectonics
Pressure
Silicon
South China Sea
Spreading
Tomography
Volcanic activity
Volcanism
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Summary: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.
ISSN:2075-163X
2075-163X
DOI:10.3390/min8120575