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榴辉岩的两种变质演化轨迹和俯冲大陆地壳的差异折返-——以柴北缘都兰超高压地体为例

宋述光1,Niu Yaoling2,张立飞1   

  1. 1. 北京大学 造山带和地壳演化教育部重点实验室,地质学系,北京 100871 2. Department of Earth Sciences, Durham University, Durham DH1 3LE, UK
  • 收稿日期:2007-09-20 修回日期:2007-09-20 出版日期:2007-09-20 发布日期:2007-09-20

Two Different Metamorphic Paths of Eclogites and Differential Exhumation of Subducted Continental Crust: A Case Study of the Dulan UHP Terrane in the North Qaidam UHP Belt

SONG Shu-guang1, NIU Yao-ling2, ZHANG Li-fei1   

  1. 1. MOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University,Beijing 100871, China; 2. Department of Earth Sciences, Durham University, Durham DH1 3LE, UK
  • Received:2007-09-20 Revised:2007-09-20 Online:2007-09-20 Published:2007-09-20

摘要: 都兰榴辉岩地体位于柴北缘—南阿尔金超高压变质带的东端,是唯一确定含柯石英的超高压变质地体,约700 km,其特点是含有两个特征不同的变质亚带,并经历了不同的折返过程。柯石英假像和温压计算表明两带榴辉岩峰期变质的压力都在柯石英的稳定域(2.8~3.3 GPa),但它们退化变质的p–T 轨迹具有明显不同的特征。北带榴辉岩经历了两个阶段的折返:早期从地幔深度快速折返到中部地壳层次,伴随岩石的等温降压,并发生角闪岩相退化变质;晚期抬升到地壳浅部。都兰南带榴辉岩折返过程中经历了高压麻粒岩相变质的改造,高压麻粒岩阶段的p–T条件为p=1.9~2.0 GPa,T=873~948℃, 并进一步经历了角闪岩相退化变质,说明都兰南带榴辉岩折返速率较慢,发生了壳幔过渡带(或加厚的深部地壳)层次的强烈热松弛。这种热松弛发生在许多大陆俯冲带的超高压岩石的折返过程中,并且是榴辉岩发生深熔作用的主要机制。都兰两个变质带不同的变质演化轨迹反映了俯冲的大陆地壳具有差异折返的特征。

Abstract: The Dulan eclogite-bearing terrane is the only coesite-bearing ultrahigh-pressure (UHP) metamorphic terrane so far recognized in the 700 km-long North Qaidam-Altun eclogite belt. The key feature of the Dulan terrane is that it consists of two sub-belts, the north Dulan belt (NDB) and the south Dulan belt (SDB) with different exhumation processes and histories. Coesite pseudomorphs in garnet and omphacite, together with p-T calculation, suggest that the peak metamorphic stage of eclogite in both belts is well within the stability field of coesite (p =2.8-3.3 GPa). The retrograde p-T paths are much different between the north and the south belts. The p-T path of the NDB eclogites suggests that the NDB eclogites underwent two stages of exhumation, an earlier rapid exhumation from a mantle depth to a middle crust level with a near-isothermal decompression, and a later uplift to the Earth’s surface. The SDB eclogites, on the other hand, were strongly overprinted by a high-pressure granulite-facies metamorphism under conditions of p =1.9~2.0 GPa and T =873~948℃, followed by temperature and pressure decrease to the amphibolitefacies. The process of temperature increase with decreasing pressure from UHP eclogite to high-pressure granulite suggests that the SDB eclogites exhumed at low speed and underwent strong thermal relaxation at a level of crustmantle transition or thickened lower crust. This thermal relaxation may occur in most UHP metamorphic terranes and may be the major mechanism for eclogite anatexis. The different metamorphic evolutional paths of the two sub-belts signify the differential exhumation histories of the subducted continental crust.