几种镁铁矿物平衡共生的成分标志

本文以晋冀内蒙边界地区变质岩中主要镁铁质造岩矿物辉石,闪石为例,阐明镁铁矿物平衡共生的一条普遍规律,即平衡共生的正辉石,钙闪石之间有相同的镁铁比,其分配系数等于或接近于1.引用冀东,吉南,泰山和南极凯西站等地的实例,论证了此规律同时存在于黑云母与钙闪石,黑云母与正辉石之间,也存在于辉长岩和花岗岩等岩浆岩的橄榄石和正辉石,角闪石与黑云母等矿物对之间。这是判断镁铁矿物是否平衡的成分标志。

COMPOSITION INDICATOR IN SOME EQUILIBRIUM COEXISTING MAFIC MINERALS OF HIGH GRADE METAMORPHIC ROCKS

Numerous analyses of minerals in high grade metamorphic rocks indicate that, there is a regular relationship among the equilibrium coexisting mafic minerals. It is well known that the tie-lines joining the composition expressed in term of their Wo, En and Fs contents of many coexisting ortho-and clino-pyroxene pairs intersect the CaSi03-MgSiO:joining at approximately Wo₇₀-W₈₀. The author emphasizes the composition relationship of other mafic minerals such as orthopyroxene (Opx), Ca-amphibole (Hb) and biotite (Bi). The probe data of minerals show the Mg-Fe relationships regular between Opx and Hb, Opx and Bi, Hb and Bi, and among Opx, Hb and Bi. The Mg/(Mg+Fe) of minerals are used to show the Mg-Fe ratio. It is very clear that there are similar Mg/(Mg+Fe) ratios in coexisting Opx, Hb and Bi, and the K_D^(Opx-Hb), K_D^(Opx-Bi) and K_D^(Hb-Bi) are all close to 1.These samples are collected from many different locations: N. Shanxi, Inner Mongolia, E. Hebei, S. Julin, and Shandong of China, and Casey Station, Antarctic. It is sufficient to prove that this kind of mineral composition relationship is of universal significance in high grade metamorphic rocks. If Opx, Hb and Bi are equilibrium coexisting they have to have the similar Mg/Fe ratio and the K_D will be very close to 1.If their Mg/ Fe ratios are very different and their K_D are departed from 1 too far, we can say that those minerals are unequilibrated. It is necessary to emphasize that this is a universal law. The similar Mg / Fe and the K_D approaching 1 are the indicator of Opx, Hb and Bi in the equilibrium coexistiong. Present study shows the mafic minerals of North China granulite facies belt are very close to ideal solid solution, and the granulites of North China are reached to equilibrium.