西藏日喀则地区白朗蛇绿岩中石榴辉石岩的岩石地球化学、年代学及其构造意义  

Geochemistry and geochronology of Bailang garnet pyroxenite within Xigaze area in Tibet,and its tectonic significance

作　　者：赵佳楠[1,2] 许志琴[2] 梁凤华[2] 

机构地区：[1]中国地质大学地球科学与资源学院,北京100083 [2]中国地质科学院地质研究所、大陆构造与动力学国家重点实验室,北京100037

出　　处：《岩石学报》2015年第12期3687-3700,共14页Acta Petrologica Sinica

基　　金：中国地质调查局地质调查项目(1212010818035、12120114022601、12120115026801);国家自然科学基金创新研究群体项目(41221061)联合资助

摘　　要：白朗石榴辉石岩位于西藏南部雅鲁藏布江蛇绿岩带上,地处于日喀则地区白朗县境内,因其具有重要的大地构造位置和意义,得到了国内外诸多学者关注。国内外学者对该岩体的研究有助于深入剖析雅鲁藏布江蛇绿岩带的就位机制。本文以白朗石榴辉石岩为研究对象,其野外产出为构造岩块,围岩为没有变形特征的蛇纹石化地幔橄榄岩,早白垩世昂仁组和紫红色硅质岩也没有构造变质现象,且石榴辉石岩与蛇纹石化地幔橄榄岩间界限截然。通过对其进行岩石地球化学和年代学的研究,并结合大地构造动力学观点,认为西藏日喀则地区石榴辉石岩属钙碱性超基性岩,形成于无流体无水的稳定物理化学环境中,略亏损高场强元素Th、Ta、Nb等,锆石LA-ICP-MS U-Pb年龄为149.0±3.1Ma,且锆石εHf(t)平均值为+17.7,说明原始岩浆主要源于亏损地幔,且没有发生岩浆混染。总结:本文确定白朗石榴辉石岩原始岩浆来源于上地幔,并且该岩石的形成与印度板块和欧亚板块俯冲有关,但在该岩石形成过程中印度板块和欧亚板块没有发生陆陆碰撞。Bailang garnet pyroxenite is located in Yarlung-Zangbo ophiolite belt of southern Tibet within the Xigaze region. Many scholars are interested in it as a result of its important tectonic location and significance. Studying it is helpful to research emplacement mechanism of the whole ophiolite belt. Field occurrence of Bailang garnet pyroxenite is a tectonic block,surrounded by serpentinized mantle peridotite. The Early Cretaceous Angren Formation and purple red silicolites get no deformation and strong tectonic foliation without undergoing metamorphism. In the field, there is a clear boundary between serpentinized mantle peridotite and garnet pyroxenite. Furthermore, Bailang garnet pyroxenite is studied in the laboratory with the approaches to petro-geochemistry,geochronology and tectonic geodynamics. Based on petro-geochemical analysis,Bailang garnet pyroxenite is formed in the stable physical and chemical environment without fluid,and it is depleted in high-field strength elements such as Th,Ta and Nb. Tested by LA-ICP-MS,its magmatic zircon U-Pb age is at least 149. 0 ± 3. 1Ma. Moreover the average value of zircon εHf（ t） is ＋ 17. 7,indicating that its primitive magma is mainly derived from depleted mantle without magma mixing. According to trace elements studying,a conclusion is made that its primary magma originated from upper mantle. In addition,the forming of this rock is related to subduction between Indian plate and Eurasian plate. It＇s confirmed that forming position is located in upper mantle and primitive magma originates from N-MORB. It suggests that Neo-Tethys Ocean had been closed and continental collision did not substantially occur. The same to say,since 149 Ma,Indian plate and Eurasian plate are still moving with opposite direction. In terms of geochronological and tectonic geodynamical analysis,the above explanation is quite reasonable.

关 键 词：白朗石榴辉石岩 岩石学 地球化学 年代学 大地构造动力学 

分 类 号：P588.125[天文地球—岩石学] P597.3[天文地球—地质学]