青藏高原冈底斯带南部的紫苏花岗岩:安第斯型造山作用的证据  

Petrology and geochronology of the charnockite from the southern Gangdese belt, Tibet: Evidence for the Andean-type orogen.

作　　者：张泽明[1,2] 王金丽[1] 董昕[1] 赵国春[3] 于飞[4] 王伟[4] 刘峰[4] 耿官升[1] 

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]中国地质大学,地质过程与矿产资源国家重点实验室,武汉430074 [3]香港大学地球科学系 [4]中国地质大学地球科学学院,武汉430074

出　　处：《岩石学报》2009年第7期1707-1720,共14页Acta Petrologica Sinica

基　　金：中国地质调查局地质调查工作项目(1212010918012);国家自然科学基金项目(40772049)资助

摘　　要：青藏高原南部白垩纪形成的冈底斯岩基是新特提斯大洋岩石圈向北部的欧亚大陆之下俯冲形成的安第斯型会聚板块边缘的重要组成部分。已经有研究证明,冈底斯岩基主要由钙碱性和埃达克质的花岗闪长岩组成,但本文报道了在冈底斯带东南缘产出的紫苏花岗岩。这种岩石主要由中长石、微斜长石、顽火辉石、透辉石、石英、富钛的黑云母和角闪石组成。在化学成分上,其铁镁指数FeO/(FeO+MgO)=0.47～0.50,钙碱指数MALI为-3.97～+0.11,铝饱和指数ASI为0.83～0.92,是典型的镁质、钙碱性、准铝质紫苏花岗岩。计算出的结晶温度为850～950℃,压力>0.4GPa,形成深度为13～15km。紫苏花岗岩中的锆石为岩浆结晶成因,两个样品的U-Pb定年分别给出了87Ma的相同加权平均年龄。岩石学和地球化学特征表明,冈底斯带南缘晚白垩纪的紫苏花岗岩形成在典型的大陆岩浆弧环境。紫苏花岗岩和伴生高温麻粒岩的存在为冈底斯带在晚中生代经历的安第斯型造山作用提供了更加确凿的证据。The Gangdese batholith emplaced in southern Tibet in the Cretaceous has been widely regarded as the major component of an Andean-type convergent margin resulting from the northward subduction of the Neo-Tethyan oceanic lithosphere beneath Asia. While the Gangdese batholith consists predominantly of calcalkaline and adackitic ganodiorites, we have recognized a suite of charnockites in the eastern part of the Gangdese belt. They consist of andesine, microcline, enstatite, diopside and Ti-rich biotite, with FeO/(FeO + MgO) ratios of 0.47 similar to 0.50, MALI of -3.97 similar to +0.11, and ASI of 0.83 similar to 0.92, suggesting a typical characteristics of magnesian, alkali-calcic and metaluminous granitic rocks. The estimated temperature and pressure conditions for the crystallization of charnockites are 850 similar to 950 degrees C and >0.4GPa, and the speculated formation depth is below 13 similar to 15km. Zircons from the charnockites are typical of magmatic origin, and give the weighted mean ages of 87Ma. This demonstrates that the charnockites may have formed in a continental magmatic are. The presence of charnockites and associated high-temperature granulites in the southern Gangdese indicates that the Gangdese belt has been subjected to an Andean-type orogenic event in the Late Cretaceous.

关 键 词：冈底斯岩基 紫苏花岗岩 岩石学 锆石U-PB定年 安第斯型造山作用 青藏高原 

分 类 号：P588.121[天文地球—岩石学] P597.3[天文地球—地质学]