阿尔泰南缘中泥盆世北塔山组火山岩中单斜辉石的矿物学研究及其地质意义  

Mineralogy of Clinopyroxene from the Midddle Devonian Volcanic Rocks in the South Margin of the Altay Mountains and Its Geological Significances

作　　者：苏慧敏[1] 张东阳[1] 艾羽[1] 张招崇[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,北京100083

出　　处：《地质学报》2008年第11期1602-1612,共11页Acta Geologica Sinica

基　　金：国家自然科学基金项目(编号40772045;40572047);国家305项目(编号2001BA609A-07-02);111计划(编号B07011);长江学者和创新团队发展计划资助的成果

摘　　要：阿尔泰南缘晚古生代火山岩十分发育,其中中泥盆世北塔山组为一套以中基性火山岩为主的火山—沉积岩系,并且其下部发育少见的苦橄岩和富辉橄玄岩。北塔山组富辉橄玄岩和玄武岩中存在着三种类型的单斜辉石:大斑晶辉石、小斑晶辉石和基质辉石。这三种类型的辉石在成分上有明显的差异,从大斑晶辉石—小斑晶辉石核部—小斑晶辉石边部和基质辉石,成分由富镁、钙向富铁方向演化,显示出拉斑玄武岩系列的特点。根据单斜辉石与熔体平衡原理计算了富辉橄玄岩和玄武岩中斑晶相辉石形成的温度和压力,前者变化范围主要集中在1345～1366℃和1.75～2.05GPa,其深度相当于58～67km,平均地温梯度为20～24℃/km,该区这种异常高的地温梯度可能是导致本区形成斑岩铜矿的主要原因之一;后者的温度和压力都相对较低,分别为1129～1191℃和1.19～1.60GPa,而相应的深度为39～53km,暗示着玄武岩是在相对低温低压的环境中形成的演化岩浆岩。单斜辉石的成分指示了其构造背景为岛弧环境。In the south margin of the Altai Mountains are widely-developed the Late Paleozoic volcanic rocks, in which the Middle Devonian Beitashan Formation（Fm. ） consists of a series of volcanic-sedimentary rock predominated by intermediate-basic volcanic rocks with rare picrite and ankaramite at the base. Three different kinds of clinopyroxenes were found in ankaramite and basalts, big phenocryst, small phenocrysts and microcrystals respectively. The three kinds of clinopyroxenes show distinct differences in compositions. The compositions display a change trend, from Mg and Ca-enrichment towards Feenrichment from big phenocryst-the core of small phenocryst-the rim of small phenocryst and microcrystals, showing a typical characterisitc of tholeiite series. The crystallization temperature and pressure of clinopyroxene phenocrysts from the ankaramite and basalt were calculated according to the clinopyroxene-melt equilibrium principle. The temperatures and pressure of ankaramite are 1345-1366℃ and 1.75-2.05GPa, respectively, with average geothermal gradient of 20-24℃/km, suggesting that the anomalous heat or geothermal gradient in this region may be one of the key reasons to produce the porphyry copper deposits in the region. In contrast, basalts formed at a relatively low crystallization temperature （1129-1191℃）, and lowe pressure （1.19-1.60GPa） at depth of 39-53km, suggesting that the basalt formed under a condition of lower temperature and pressure. In addition, the compositions of the clinopyroxene are consistent with those in arc lavas.

关 键 词：单斜辉石 矿物化学 富辉橄玄岩 玄武岩 中泥盆世 阿尔泰 

分 类 号：P588.14[天文地球—岩石学]