班怒带西段达如错斜长角闪岩变质温压条件及地质意义

Metamorphic temperature and pressure of the Daru Co amphibolite in western Bangong-Nujiang Suture Zone and their geological significance

班公湖—怒江缝合带是中特提斯构造演化的重要构造带之一,目前班公湖—怒江洋盆的俯冲演化过程存在诸多争议,尤其是俯冲过程中的变质作用鲜有报道。本文通过对班公湖—怒江缝合带西段的达如错斜长角闪岩开展角闪石和斜长石矿物的显微岩相学分析和原位EPMA测试工作,查明岩石的矿物组合为辉石+角闪石+斜长石。利用相平衡模拟得出的斜长角闪岩变质温度660℃~750℃,压力2~6 kbar,对应角闪岩相的温压条件,与角闪石全铝压力计(3.7~5.4 kbar)及斜长角闪石温度计(615℃~881℃)计算结果一致。综合前人研究成果,本文认为达如错地区的斜长角闪岩原岩形成于侏罗纪晚期洋内俯冲引起的弧后盆地扩张环境,角闪岩相变质作用的发生与早白垩世班怒洋壳俯冲过程中的板片底板变质有关,班公湖—怒江洋盆西段达如错地区的闭合时间为103.7~100.1 Ma。

Bangong-Nujiang Suture Zone(BNSZ)is an important structural zone to study the tectonic evolution of middle Tethys.In previous studies,there were great disputes on the subduction process of Bangong-Nujiang Tethyan Ocean(BNTO),especially few reports on metamorphism in the subduction process.In this paper,in-situ EPMA testing of amphibole and plagioclase were carried out on amphibolites in Daru Co,western BNSZ.Microscopic observation shows that amphibolite consists of pyroxene+hornblende+plagioclase,with alteration features,and the metamorphic temperature(660℃~750℃)and pressure(2~6 kbar)of amphibolite obtained by phase equilibrium simulation are likely to reflect the temperature and pressure of amphibolite facies,which is consistent with the results of Al-in-Hornblende barometry(3.7~5.4 kbar)and plagioclase-amphibole thermometer(615℃~881℃).Combined with previous studies,it is considered that the amphibolite protolith in Daru Co area was formed in the back-arc basin spreading caused by intra-oceanic subduction in the Late Jurassic.Amphibolite metamorphism may be related to metamorphism sole caused by the subduction of the ocean crust in Early Cretaceous,where the complete closure of the BNTO and subsequent slab break-off led to the tectonic return of the subducted slab.The closure time of the BNTO in the Daru Co is 103.7~100.1 Ma.