西沙群岛基底火山碎屑岩中单斜辉石的矿物化学特征及其地质意义

本文利用电子探针对南海西沙群岛琛航岛珊瑚礁底部火山碎屑岩中的单斜辉石矿物的化学特征进行了研究。结果表明，该单斜辉石属于富钙透辉石，部分有正环带结构，从核部到边部Ca、Fe、Ti的含量逐渐增加，是岩浆正常结晶顺序的反映，说明该区域的岩浆演化是向着富Ca、Fe、Ti方向发展的。主量元素数据显示，单斜辉石具有低Si高Al的特征(SiO₂=41.40%～48.44%，Al₂O₃=5.54%～10.20%)，且AlⅣ含量较高，说明母岩浆为不饱和碱性岩浆系列；此外，单斜辉石Ca含量偏高，Ca/(Ca+Mg+Fe)值在46.1%～51.4%之间，推测是母岩浆的高Ca含量导致了大量高钙辉石的产出。结合西沙海域的地震和构造资料，推测琛航岛珊瑚礁的基底是玄武质火山碎屑岩组成的平顶状海山，系岩浆穿过断裂发育的岩石圈层在西沙群岛的海底喷发，随后火山碎屑物质经过堆积、固结作用而形成；该火山碎屑岩的原岩为板内碱性玄武岩。

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

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.