Petrogenesis of charnockites from western Junggar, Xinjiang, China

Abstract

Charnockites occur as giant enclaves or pendants(?) in the Miaoergou Batholith, Western Junggar region. These rocks are composed of ortho- and clinopyroxenes, fayalite, perthite, antiperthite, and quartz, and are characterized by positive εNd values (+4.8+5.9). Mineral assemblage, texture and field occurrence indicate an igneous origin for the charnockites.Temperature and pressure estimations give700800 ℃and 45 kb crystallization conditions.The charnockites enclose small surmicaceous enclaves and large banded angular xenoliths. The former are considered as restites, while the latter are equivalent to the country pyroclastic rocks. The positive but lowerεNd values (+2.6+3.5) for the restites require that the juvenile lower crust was the dominant source for charnockitic magma, and parental magma of the charnockites from this source was mixed with some melt from depleted mantle, before crystallization. The voluminous alkaline granites in the region were probably from the same magma source, but crystallized at upper crust depth.

紫苏花岗岩呈岩体形式侵入于石炭纪地层中,又为碱长花岗岩侵入.紫苏花岗岩由斜方辉石、单斜辉石、铁橄榄石、条纹长石、石英组成,以正εNd 为特征(εNd=+4.8~+5.8).矿物组成、结构和野外产状表明:这些紫苏花岗岩是火成成因的.温压计算估测其结晶条件是T=700~800 ℃, P=420~575 kPa.紫苏花岗岩中有小的富云母包体和巨大的条带状捕虏体.前者被认为是残留体,后者与其围岩-火山碎屑岩相当.残留体中较低的εNd 正值(εNd=+2.6~+3.5)要求新生的下部地壳是紫苏花岗岩的主要源岩:来自这些源岩的紫苏花岗岩的母岩浆在结晶前与一些来自于亏损地幔中的融体混合.该地区体积巨大的碱性花岗岩可能也来自相同的岩浆源,只不过它是在较浅的地壳深度上结晶的.关键词:新疆;西准噶尔;紫苏花岗岩;富云母包体;碱性花岗岩紫苏花岗岩呈岩体形式侵入于石炭纪地层中,又为碱长花岗岩侵入.紫苏花岗岩由斜方辉石、单斜辉石、铁橄榄石、条纹长石、石英组成,以正εNd 为特征(εNd=+4.8~+5.8).矿物组成、结构和野外产状表明:这些紫苏花岗岩是火成成因的.温压计算估测其结晶条件是T=700~800 ℃, P=420~575 kPa.紫苏花岗岩中有小的富云母包体和巨大的条带状捕虏体.前者被认为是残留体,后者与其围岩-火山碎屑岩相当.残留体中较低的εNd 正值(εNd=+2.6~+3.5)要求新生的下部地壳是紫苏花岗岩的主要源岩:来自这些源岩的紫苏花岗岩的母岩浆在结晶前与一些来自于亏损地幔中的融体混合.该地区体积巨大的碱性花岗岩可能也来自相同的岩浆源,只不过它是在较浅的地壳深度上结晶的.