岩浆分离结晶作用的微量元素广义初始比值计算方法

本文提出了一种如何从分异的派生岩浆追溯和反演演化前的原生岩浆的微量元素初始比值的方法。据分离结晶方程logaC_1~q=k·loga C_1~p b,得广义初始比值:C_0~q/(C_0~p)~k=a~b。它与已公认的原生岩浆的判别参数如~(87)Sr/~(86)初始比、δ~(18)O‰、SI、Mg′值及微量元素丰度等有极好的一致性。并将这种方法应用于两个实例,得出大宁花岗闪长岩及其暗色微粒包体分属壳源和幔源;五大连池富钾火山岩的原生岩浆为白榴苦橄碧玄岩浆,而其中的超镁铁质岩包体则是分离结晶的火成堆晶体。

AN ALGORITHM FOR CALCULATING GENERAL INITIAL RATIOS OF TRACE ELEMENTS IN THE PRIMARY MAGMA FROM ITS DERIVATIVES FORMED BY FRACTIONAL CRYSTALLIZATION

Fractional crystallization has been proved by more and more evidence to widely take place in the magmatic processes of different compositions, cont- roiling the petrogenesis and mineralisation and accounting for the systematic variation of major and trace elements in the rock bodies. In this paper, it is for the first time that the author presents how to trace back and calculate the general initial ratios in the primary magma by use of the analytical data of trace element abundances in the evolved secondary magmas.According to the fractional equation: log_aC_1~q=klog_aC_1~p b We obtain the general initial ratio: C_0~(?)/(C_0~p)~k=a~b where C_0~p and C_0~q are the initial concentrations of trace elements p and q in the primary magma, and k and b are the slope and intercept of the logarithmic linear equation and could be caeulated through linear regression.The calculated results(Table 1 and 3) have shown that the general initial ratios are well consistent with the widely accepted discriminating parameters such as Si and Mg values, as well as trace element abundances.Two application examples are given, and the following conclusions could be drawn:(1)Darting granodiorite batholith in Guangxi has very similar general initial ratios with those of North America Shale Composite, illustrating that it was derived from the melting of sedimentary source rocks, but that the darkcoloured microgranular enclaves, almost having the same general initial ratios with those of average basalt,were solidified through fractional crystallization from the mantle-derived basaltic magma.(2) The primary magma for the potassium-rieh volcanic roeks (mainly leucite basanite, occasionally leucite-picritic basanite) in the vicinity of Wudalianchi, Northeastern China, are leueite-bearing picritie basanitie magma; whereas the ultramafic inelusions within them,predominantly being lherzolite, websterite, dunite and clinopyroxenite(wehrlite), are the igneous cumulates formed by fractional crystallization of the primary magma at the depth of about 45 to 70 kilometers in the upper mantle indicated by pyroxene thermobarometers.