阿尔泰山南缘中泥盆世苦橄岩中单斜辉石的成分特征及其地质意义  

Mineral chemistry and geological significance of clinopyroxenes from Middle Devonian picrite on the southern margin of the Altay Mountain

作　　者：蔡劲宏[1] 杜杨松[1] 李顺庭[1] 

机构地区：[1]中国地质大学地质过程与矿产资源国家重点实验室

出　　处：《岩石矿物学杂志》2007年第2期141-146,共6页Acta Petrologica et Mineralogica

基　　金：国家自然科学基金资助项目(40572047);国家305项目(2001BA609A-07-02);教育部新世纪优秀人才支持计划(NCET-040728)

摘　　要：岛弧苦橄岩非常稀少,但在阿尔泰山南缘却分布有世界上罕见的中泥盆世厚大苦橄岩层。本文对该地区苦橄岩及其共生的玄武岩中单斜辉石的矿物化学成分进行了电子探针分析,结果表明,苦橄岩中的单斜辉石全部为透辉石,玄武岩中的单斜辉石种属有普通辉石和透辉石两类,但玄武岩中的单斜辉石成分呈连续演化关系。根据单斜辉石-熔体平衡原理计算了苦橄岩中单斜辉石斑晶形成的温度和压力,其变化范围主要集中在1350～1387℃和16.6～21.3kPa,相应的深度为55～70km,大致代表了苦橄岩岩浆房的深度。与此不同的是,玄武岩中单斜辉石斑晶结晶的深度较苦橄岩浅,为37～41km,暗示玄武岩浆是在相对低压条件下分离结晶后形成的演化岩浆。基于单斜辉石的成分和演化,确定了该地区火山岩属亚碱性系列,其构造背景为岛弧环境。Picrite in island arcs has been rarely reported. However, several thick layers of picrite, which were rarely found in the world, were recognized in the Middle Devonian strata on the southern margin of the Altay Mountain. This paper presents new data of electronic microprobe analyses on clinopyroxenes from the layers of picrite and associated basalt in this area. It is shown that the clinopyroxenes from the picrite are exclusively diopside, whereas those from the basalt are diopside and augite, suggesting a continuous variation in composition. The crystallization temperature, pressure and depth of clinopyroxene phenocrysts from the picrite and basalt were calculated on the basis of the clinopyroxene-melt equilibrium principle. The temperature of mineral crystallization in depth, which represents the condition of picritic magma chamber, is estimated to be in the range of 1 350-1 387℃, and the corresponding pressure and depth are 16.6-21.3 kPa and 55-70 km, respectively. In contrast, the depth of mineral crystallization in the shallow part, which represents the condition of basaltic magma chamber, is in the range of 37-41 km, indicating that the basaltic magma might have been an evolved magma formed by fractionation crystallization of primary magma under lower pressure. Moreover, the mineral chemistry of the clinopyroxenes shows that the volcanic rocks in this area belong to subalkaline series and were probably formed in an island arc setting.

关 键 词：单斜辉石 矿物化学 苦橄岩 中泥盆世 阿尔泰 

分 类 号：P578.954[天文地球—矿物学] P588.146[天文地球—地质学]