喜马拉雅淡色花岗岩比以前认为的更热

Abstract

<p>喜马拉雅造山带新生代淡色花岗岩是纯地壳熔体的典型代表. 它们主要是由变沉积岩在缺自由流体的条件 下通过白云母脱水部分熔融产生, 其熔融温度一般低于800°C. 本研究利用3种独立的方法限定淡色花岗岩的初始 研究温度, 其结果一致表明, 淡色花岗岩形成于相对高的温度条件(>800°C). 首先, 通过对比变泥质岩来源的熔体与 喜马拉雅淡色花岗岩间的成分, 发现高温(820~900°C)熔体的A/CNK(Al2O3/(CaO+Na2O+K2O), mol)值与大部分花 岗岩的一致(1.0~1.3), 而低温(750~800°C)熔体则具有更高的A/CNK值(1.3~1.4). 除此之外, 高温熔体、低温熔体和 花岗岩在其他主量元素组成上并无显著差别. 其次, 本研究数据和文献数据都证实, 喜马拉雅淡色花岗岩的锆石饱 和温度低于相应样品中很大一部分的锆石Ti温度值. 这一观测结果表明, 初始岩浆并没有达到锆石饱和, 而锆石的 饱和结晶主要通过贫Zr矿物的结晶导致. 很多样品的锆石Ti温度最大值均超过了800°C. 最后, 对两件代表性二云 母花岗岩样品的相平衡模拟计算显示, 5 kbar(1 bar=105 Pa)结晶条件下的液相线温度均约为800°C, 且液相线温度 随结晶压力的增加而增加. 因此, 如果淡色花岗岩岩浆起源于更深的地壳层位(8~10 kbar), 它们的形成温度应更高. 综上, 我们认为, 喜马拉雅淡色花岗岩的初始岩浆温度比以前认为的更高. 通过地壳深熔产生的岩浆作用有助于开 启高喜马拉雅结晶岩系的折返和藏南拆离系和主中央逆冲断层的活动<br></p>

Cenozoic leucogranites in the Himalayan orogenic belt which was developed in response to the collision of Indian and Asian plates at ca. 60 Ma are generally accepted as typical examples of low-temperature and pure crustal melts. The leucogranites provide a window to decipher the nature of anatectic sources and physicochemical conditions of partial melting at depth. However, problems are encountered in determining the temperature of crustal anatexis for the leucogranite magmatism. Most of them were previously considered as originating from fluid-absent partial melting of metasedimentary rocks by muscovite dehydration at temperature lower than 800°C. Cenozoic Himalayan leucogranites have been generally regarded as low-temperature magmas (<800°C), and previous models mostly considered that they were produced by intracrustal heating through shearing heating associated with the movements of the Main Central Thrust and the South Tibetan Detachment System, decompression melting caused by the exhumation of the Higher