西藏雅鲁藏布江缝合带西段普兰蛇绿岩地幔橄榄岩矿物学研究和成因探讨  

Mineralogical study and the origin discussion of Purang ophiolite peridotites,western part of Yarlung-Zangbo Suture Zone(YZSZ),Southern Tibet

作　　者：周文达[1,2] 杨经绥[2] 赵军红[1,3] 熊发挥[2] 马昌前[1,3] 徐向珍[2] 梁凤华[2] 刘飞[2] 

机构地区：[1]中国地质大学地球科学学院,武汉430074 [2]大陆构造与动力学国家重点实验室地幔研究中心,中国地质科学院地质研究所,北京100037 [3]地质过程与矿产资源国家重点实验室,武汉430074

出　　处：《岩石学报》2014年第8期2185-2203,共19页Acta Petrologica Sinica

基　　金：国家行业专项(SinoProbe05-02);国家自然科学基金重点项目(409303)、国家自然科学基金创新群体项目(41221061);中国地质调查局工作项目(1212011121263)联合资助

摘　　要：西藏普兰蛇绿岩以出露面积约600km2的特大型地幔橄榄岩体而引人注目。我们在普兰岩体的东段,完成了一条近垂直走向并穿过岩体的长约10km的地质剖面,其目的是探讨地幔橄榄岩体岩相的变化及其成因。研究表明,岩体主体为方辉橄榄岩,分布于岩体内部,二辉橄榄岩呈条带状,分布于岩体边部,方辉橄榄岩和二辉橄榄岩出露面积之比约为4∶1。剖面显示,二辉橄榄岩向方辉橄榄岩岩相渐变,Ol和Sp含量增加,Opx和Cpx减少,橄榄石Fo值和NiO含量也呈逐渐增加的特征。斜方辉石主要为顽火辉石（En=85～0）,Mg#值变化于88～92之间,Al2O3含量0.89%～5.16%。单斜辉石包括顽透辉石和透辉石,以低铝（Al2O3=1.16%～6.02%）,高镁（Mg#值为90～94）为特征。二辉橄榄岩的铬尖晶石Cr#值在19～32之间,低于方辉橄榄岩的Cr#值（25～72）,两者之间呈连续变化。另一方面,方辉橄榄岩的各矿物百分含量、成分特征及部分熔融程度在岩体东部变化较大。结合前人成果,认为岩体中部分方辉橄榄岩不仅仅为单一的地幔残余,而可能经历了后期流体/熔体交代作用。依据尖晶石-橄榄石/单斜辉石矿物化学成分,估算出二辉橄榄岩是地幔源区经历约5%～12%部分熔融作用形成,而方辉橄榄岩则最终经历了约12%～32%部分熔融作用。研究结果表明,从二辉橄榄岩到方辉橄榄岩的演变,起因于部分熔融的差别,没有证据表明是受构造背景变化制约,因此,简单地用铬尖晶石等矿物成分的变化判断地幔橄榄岩的产出构造背景的方法值得商榷。The Purang ophiolite is characterized by containing a large mantle peridotite massif ca. 600km2 in area. In order to discuss the lithofacies variation and origin, a transect （AB） about 10km across the ophiolite which located in the east of the Purang mantle peridotite massif is sampled. The mantle peridotite is composed dominantly of harzburgite which mainly distribute within the peridotite massif, and the banded lherzolites mainly distribute in the slide of the peridotite massif, and the ratio of the exposure area of harzburgites and lherzolites is about 4 ： 1. The tranaect also shows that the lherzolites change to the harzburgites gradually, the contents of O1 and Sp increase, the contents of Opx and Cpx decrease, and the Fo and NiO contents of olivine in the peridotites increase gradually from lherzolite to harzburgite. The orthopyroxene porphyroblasts are enstatite （ En = 85. 1 ~ 90. 1 ）, with Mg# of 88 to 92. 0 and A1203 contents of 0. 89% to 5. 16% , whereas the clinopyroxenes are endiopside and diopside with low A1203 （1.16% -6. 02% and high Mg# （90 ～ 94）. Spinels in the lherzolites have much lower CrY（ 19 ～ 32） than those in the harzburgites （25 -72）. On the other hand, the modal mineralogy, mineral compositions and the degree of partial melting change both gradually and abruptly within the harzburgites across the AB transect, indicating that a part of the harzburgites are not only the simple residues of partial melting but maybe experienced later fluids/melts metasomatism. On the basis of the spinel-olivine-elinopyroxene mineral chemistry, we calculate that the lherzolites are the residues of 5% ～ 12% partial melting of the mantle source, whereas the harzburgites are formed by 12% ～ 32% partial melting. The research results demonstrate that the evolution from lherzolites to the harzburgites owing to the partial melting differences, there is little evidence that the evolution is controlled by the variation of the tectonic setting. Thus, the method distinguishing the tectoni

关 键 词：地幔橄榄岩 矿物学 普兰蛇绿岩 雅鲁藏布江缝合带 

分 类 号：P575[天文地球—矿物学] P588.125[天文地球—地质学]