南迦巴瓦峰地区雅鲁藏布蛇绿混杂岩带矿物学特征及时代

The mineralogy and ages of the Yarlung Zangbo ophiolitic mélange zone in the Namjagbarwa area,Xizang

关键词:  南迦巴瓦  雅鲁藏布蛇绿岩带  矿物成分  时代  西藏

Key words:  Namjagbarwa  Yarlung Zangbo ophiolitic mélange zone  mineral composition  times  Xizang

摘要:

雅鲁藏布蛇绿混杂带在南迦巴瓦峰地区呈弧形连续分布,主要岩石类型有石英岩和云母石英片岩、绿片岩、斜长角闪岩、变质辉长岩、变余辉绿岩等。主要造岩矿物中,角闪石属富镁的镁角闪石、镁闪石、韭闪角闪石、浅闪角闪石等;单斜辉石为富镁的类型,成分相当于透辉石;橄榄石为典型的镁橄榄石;斜长石在角闪岩类中为中长石,是变质成因的。利用地质温压计估算出的变质温压条件分别为:t=500~650℃,p=0.75~0.8GPa,相当于角闪岩相变质作用,发生在47~47Ma左右。晚期经历了低温、高压变质作用,形成高压型的多硅白云母,时代为27~20Ma,与该区结合带边界阿尼桥断层和米林-鲁朗断层发生强烈走滑有关。地壳加速隆升、剥蚀时期形成的多硅白云母为中、低压型。

Abstract:

The Yarlung Zangbo ophiolitic mélange zone is aligned as an arc along the Yarlung Zangbo Grand Canyon in the Namjagbarwa area,Xizang. The lithologies there consist mainly of quartizite,mica quartz schist,green schist,amphibolite,metagabbro and blastodiabase. The major rock-forming minerals are composed of:(1) amphibole including Mg-rich magnesio-amphibole,magnesio-cummingtonite,pargasite and edenite;(2) clinopyroxene including Mg-rich species corresponding to diopside in composition;(3) olivine including representative forsterite,and (4) plagioclase dominated by andesine of metamorphic origin in amphibolites. The temperatures and pressures for the metamorphism processes estimated from the geothermometer and geobarometer are:t=500 -650℃ and p=0.75-0.8 Gp,corresponding to those for the amphibolite facies dated back to ca. 47-43 Ma. The later low-temperature and high-pressure metamorphism resulted in the formation of the high pressure-type lepidomorphite dated back to ca. 27-20 Ma,relative to the intense strike-slipping of the Aniqiao fault and Mainling-Lunang fault at the boundary of the Yarlung Zangbo ophiolitic mélange zone. The rapid uplifting and erosion of the crust in the study area permitted the formation of lepidomorphite of medium-to low-pressure types.