云南峨眉山高钛和低钛玄武岩的岩石成因

Petrogenesis of high-Ti and low-Ti basalts in Emeishan, Yunnan, China

峨眉山玄武岩主要的岩石类型是低钛玄武岩和高钛玄武岩,并有少量的苦橄岩。它们不同程度地富集大离子亲石元素和轻稀土元素,相对亏损重稀土元素,稀土元素分馏明显或比较明显,相容元素(Co、V、Cr、Ni)显著亏损。低钛玄武岩浆受到陆壳物质的明显混染,高钛玄武岩浆也受到混染,但混染程度弱于低钛玄武岩浆。同化混染对Sr同位素和大离子亲石元素的影响程度大于对稀土元素和Nd同位素的影响程度。混染物是下地壳变质岩,也有少量上地壳物质。未受混染的样品具有适度亏损的Nd、Sr同位素。高钛玄武岩在岩浆演化过程中主要分离结晶相/堆晶相是单斜辉石,并有少量的斜长石。低钛玄武岩中,单斜辉石和斜长石的分离结晶作用是最主要的因素。低钛玄武岩的主体部分是在尖晶石稳定域与石榴石稳定域之间的过渡带熔融的;高钛玄武岩的主体部分是在石榴石稳定域内熔融的,极少部分是在尖晶石稳定域内熔融的。

The most main rock type of the Emeishan basalts are low-Ti basalts and high-Ti basalts, and has the few picrites. They enrich in LILE and LREE at different degrees, and deplete relatively to HREE, REE fractionate evidently, compatible elements (Co, V, Cr, Ni) deplete significantly. Low-Ti basalt magma was obviously contaminated by continent crust, in a larger extent than high-Ti basalt magma. And influences of contamination on Sr isotope and LILE are more evident than those on Nd isotope and REE. Nd and Sr isotope prove that contamination materials are mainly metamorphic rocks of lower continent crust, but also have minor rocks of upper continent crust. That the samples were not contaminated shows Nd and Sr isotopic compositions of moderately depleted mantle. Clinopyroxenes are main fractional crystallization phases/accumulation phases, plagioclases are minor in differentiation processes of high-Mg tholeiitic magma. However, clinopyroxenes and plagioclases are main fractional crystallization phases/accumulation phases of high-Mg transitional magma. Main body part of low-Ti basalts melted around the spinel-garnet transition zone;but main body part of high-Ti basalts melted within the garnet stability field,extremely few are partial melting within the spinel stability field.