藏西北戈木日榴辉岩岩石学特征及其构造意义  

Petrological characteristics of the eclogite from Gemuri area,northwestern Tibet,and tectonic implications

作　　者：苑婷媛 赵中宝[2] 曾庆高 刘焰[2] 

机构地区：[1]东华理工大学地球科学学院,南昌330013 [2]中国地质科学院地质研究所,北京100037 [3]西藏自治区区域地质调查大队,堆龙德庆851400

出　　处：《岩石学报》2016年第12期3729-3742,共14页Acta Petrologica Sinica

基　　金：中国地质调查局青藏专项项目(12120115026901)资助

摘　　要：戈木日榴辉岩产出于羌塘中部龙木错-双湖古板块缝合带内,是羌塘中部高压变质岩带的重要组成部分,前人多认为其系洋壳冷俯冲的产物。该榴辉岩的矿物组合可分为四期:第一期为含Fe^(3+)的钙钠闪石+钠云母+钠长石+绿泥石+石英+富含LREE、Th和U的褐帘石+榍石±富Mn的石榴子石。富Mg的石榴子石+贫Fe的绿辉石+不含微量元素的黝帘石+石英+钠云母+金红石+黄铁矿则为第二期矿物组合。第三期为退变质组合,包括钠长石、绿泥石、榍石和阳起石。第四期为磁铁矿、新生绿帘石和针状钛铁矿。热力学半定量模拟表明,石榴子石核部形成于T=644℃,P=13.0kbar,lgfO_2=-16.3^-16.4,而石榴子石幔部则形成于T=695℃,P=16.6kbar,lgfO_2=-19.0^-19.5,表明该榴辉岩属中温榴辉岩,而不是前人所言的冷榴辉岩,这反映了羌塘中部古特提斯洋盆的构造演化历史比较复杂,不仅有冷俯冲的洋壳,还有中温俯冲的洋壳。该榴辉岩在进变质过程中经历了还原作用,早期含Fe^(3+)的钙钠闪石和褐帘石等形成于高氧逸度之下的含水矿物,转变为形成于低氧逸度条件下的石榴子石和贫Fe的绿辉石、黝帘石等矿物,释放出水和轻REE、Th、U、Ba等微量元素。折返阶段新生成的矿物均不含有这些微量元素,表明早期阶段释放的微量元素很可能离开了俯冲板片,随流体进入上覆地幔楔。本文的研究进一步证实了进变质与还原作用共同促进了物质从俯冲板片中活化、转移至上覆地幔楔,这有助于理解具有独特地球化学特征的岛弧岩浆的起源。Occurring in the Longmu Co-Shuanghu suture zone, the Geamri eclogite is an important component of the central Qiangtang high-pressure metamorphic belt, which was previously regarded as a consequence of cold subduction of the Palaeo-Tethyan Oceanic crust. Mineral assemblages of the Gemuri eclogite can be divided into four stages. The first stage consists of Fe3 ~ -bearing Ca- Na amphibole, paragonite, albite, chlorite, quartz, allanite enriched in LREE, Th and U, sphene and Mn-rich garnet core. The mineral assemblage of the second stage is composed of Mg-rich garnet, Fe-poor omphicite, trace-element-free clinozoisite, quartz, paragonite, futile and pyrite. The third stage includes the Fe-rich garnet occurring in the rims of garnet porphyroblast, albite, chlorite, sphene and actinolite. The last stage is composed of magnetite, trace-element-free epidote and needlelike ilmenite. Semiquantitatively thermodynamic modeling has revealed that the core garnet was formed at T = 644℃, P = 13.0kbar, lgfO2 = - 16. 3 - - 16.4, while the mantle garnet was formed at T = 695℃, P = 16. 6kbar, lgfO2 = - 19. 0 - 19.5, indicating that the Gemuri eclogite is an intermediate-temperature eclogite rather than a cold eclogite as previously thought. In combination with previous studies, our work further suggests that tectonic evolution of the Palaeo-Tethyan Ocean is more complex than previously thought. Not only cold, but also intermediate-temperature oceanic subduction occurred. The Gemuri eclogite has suffered reduction in the prograde stage. The early Fe3 ~ -bearing Ca-Na amphiboles, a11anites enriched in LREE, Th and U, as well as other water-bearing minerals formed at high oxygen fugacity are transformed into garnets, Fe-poor omphacites and trace-element-free clinozoisite, releasing water, REE, Th, U, and Ba in the subduction channel. However, none of the neogenie minerals in the exhumation stage contain these trace elements, suggesting that the trace elements were likely released from the subducted slabs, and then transferred to

关 键 词：榴辉岩 西藏西北部 变质演化 还原反应 微量元素迁移 

分 类 号：P588.348[天文地球—岩石学]