一个辉长苏长岩包体的矿物及其成因研究

THE MINERALOGY AND ORIGIN OF A GABBRO-NORITE COGNATE INCLUSION

本文论述的玄武岩中辉长苏长岩包体的暗色矿物可分三组:Ⅰ组的普通辉石含钙低(Wo 27.8),晶出于1301℃。Ⅱ组的普通辉石形成于约1000℃。含大量(≥20%)形态多样斜方辉石出溶片晶,且具有(110)出溶。第Ⅲ组普通辉石亦形成于约1000℃。根据矿物结构及成分,可以认为一些暗色矿物集合体是尖晶石二辉橄榄岩的碎块,进入岩浆后与高温岩浆反应生成辉石Ⅱ,同时岩浆局部镁含量得以提高,成为辉长苏长岩成分。包体矿物所反映的包体形成及岩浆运移的过程是:岩浆上升至100km,辉石Ⅰ晶出。岩浆携带着捕获的地慢岩碎块上升至70—80km的岩浆房作长时间的停留,辉石Ⅱ在此出溶大量片晶,同时岩浆进行高压分异,最后形成包体。

The pyroxenes occurring in a gabbro-norite cognate inclusion from Niutoushan, Fujian Province can be divided into three groups. The augites of Group Ⅰ are low in CaO content (Wo 27.8), which crystallized at 1301℃. Augites of Group Ⅱ contain more than 20% exsolution lamellae of orthopyroxene of various shapes. Both Groups Ⅱ and Ⅲ were formed at～1000℃. As viewed from mineral structure and composition, it is supposed that some aggregates of pyroxene are the fragments of spinel-lherzolite. When the fragments of mantle rocks were trapped, they reacted with the magma at high temperature, resulting in pyroxenes of Group Ⅱ. Meanwhile, the MgO content of magma was partly enhanced, leading to a gabbro-norite composition. The information on the formation of the inclusion and the movement of the host magma developed from the minerals hosted shows that when the magma moved upwards to 100km, pyroxenes of Group Ⅰ began crystallizing. In moving further upwards, the fragments of mantle rock were trapped. When the host magma suspended in the chamber at a 70—80kin depth for a long time, Group Ⅱ would exsolute lamellae of various shapes. At the same time, the magma experienced differentiation at high pressure. Finally, the gabbro-norite inclusion was formed at the time the magma moved upwards to a depth of<30km and plagioclases crystallized.