超镁铁质捕虏体中共存矿物的Cr~#在岩石成因中的意义

PETROGENETIC SIGNIFICANCE OF Cr~# IN COEXISTING MINERALS OF ULTRAMAFIC XENOLITHS

中国东部新生代玄武质岩石中的超镁铁质捕虏体,主要包括五种类型:石榴石二辉橄榄岩(±少量尖晶石)、尖晶石二辉橄榄岩、尖晶石方辉橄榄岩、辉石岩和巨晶矿物。不同类型捕虏体间的共存矿物存在系统的化学成分变化。尤以Al_2O_3、Cr_2O_3变化明显。本文提出捕虏体中共存矿物的Cr~#[100Cr/(Al+Cr)]可作为分类和岩石成因的重要标志。并将五种捕虏体划分为三种地幔成因类型:饱满的或原始的地幔(石榴石二辉橄榄岩和尖晶石二辉橄榄岩),亏损的或残余的地幔(尖晶石方辉橄榄岩),地幔条件下熔浆分离的产物(辉石岩和巨晶矿物)。

Ultramafic xenoliths of garnet lherzolites (±rare spinel), spinel lherzolites,spinel harzburgites, pyroxenites and pyroxene megacrysts were collected from the Cenozoic basaltic rocks in all parts of eastern China. From their Cr~# in coexisting minerals all the xenoliths may be divided into three types representing respectively:a fertile or more primitive mantle (garnet lherzolite and spinel lherzolite), a refractory or more depleted mantle (spinel harzburgite), and inclusions derived from crystallization at mantle pressure (pyroxenites and megacrysts). There are significant differences between these types in mineral composition. We have come to the conclusion that Cr~# in coexisting minerals is a significant indicator of the xenolith type. The increase of Cr~# in coexisting minerals reflects an increase in the degree of partial melting in the mantle.