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长江中下游地区繁昌火山盆地在早白垩世先后发育3个喷发旋回:中分村旋回、赤沙旋回和蝌蚪山旋回。其中蝌蚪山旋回下段以凝灰质粉砂岩为主,中段为玄武岩和流纹质凝灰岩互层,上段以流纹岩为主。蝌蚪山玄武岩中的辉石既存在斑晶中也存在于基质中,这两种辉石在成分上有明显的差异,从斑晶辉石的核部到边部再到基质的核部,化学成分由富镁向富铁演化,显示出拉斑玄武岩系列的特点,辉石成因的判别图解表明其寄主岩浆为板内拉斑玄武岩,但其化学成分的变化趋势又表明岩浆在结晶分异的过程中向着碱性系列演化。根据单斜辉石与熔体平衡原理计算出玄武岩形成时的温度约为1 100℃,压力约为400MPa,相应的深度约为15km,证明岩浆在地壳中有过短暂的停留,并在上地壳部位发生分离结晶作用后喷出地表快速冷凝形成蝌蚪山玄武岩。
The Fanchang volcanic basin in the middle and lower reaches of the Yangtze River developed three eruption cycles successively in the Early Cretaceous: middle sub-village cycle, Red sand cycle and tadpole cycle. The lower part of the Tadpoles mountain cycle is mainly tuff siltstone, the middle part is interbedded with basaltic rhyolitic tuff, and the upper part is mainly rhyolite. The pyroxenes in the tadpoles mountain basalts exist both in the presence of plaque and in the matrix. Both pyroxenes are significantly different in composition from the nucleus to the edge of the plagioclase pyrites to the core of the matrix. The chemical The composition evolves from magnesium rich to iron rich, showing the characteristics of the tholeiitic basalt series. The discrimination diagram of the pyroxene causes shows that the host magma is intraplate internal tholeiitic basalts, but the variation tendency of the chemical composition indicates that the magma Process toward the alkaline series evolution. Based on the principle of clinopyroxene and melt equilibrium, the temperature of basalts is about 1,100 ℃ and the pressure is about 400 MPa. The corresponding depth is about 15 km. It proves that the magma had a short stay in the crust, Part of the role of crystallization occurred after the rapid discharge of condensation spray to form tadpoles mountain basalts.