摘要
北祁连造山带是早古生代大洋板块"冷"俯冲的典型地区,形成了一系列高压低温变质的岩石组合。基性榴辉岩是俯冲洋壳变质的产物,根据其结构构造特征可以划分为(1)粒状榴辉岩,峰期变质矿物组合为Grt+Omp+Phn+Rut±Lws,硬柱石呈包裹体和假象分布于石榴石内,变质温压条件T=465～508℃,p=2.30～2.60GPa。(2)片状榴辉岩,岩石具片状构造,其共生矿物组合Omp+Czo/Ep+Phn+Gln构成透入性面理S_1和线理L_1,以及宏观和显微同斜褶皱F_1。其峰期变质温压条件为T=466～510℃,P=1.9～2.2GPa。(3)蓝片岩相退化变质榴辉岩,蓝片岩相变质矿物Gln+Ep+Phn±Ab强烈交代榴辉岩相矿物,并发生强烈D_2期变形作用,岩石形成明显的透入性面理S_2和线理L_2,以及同斜剪切褶皱F_2。蓝闪石和多硅白云母线理的统计揭示D_2期变形以斜向走滑剪切为主,与D_1期变形的榴辉岩的运动学特征大体相似。D_2同变质的温压条件为T=422～487℃,P=1.15～1.37GPa。高压变质带晚期的绿片岩相叠加和D_3期变形,形成透入性面理S_3和线理L_3,其运动特征为近于垂直构造线的逆冲剪切。结合榴辉岩变形特征,变质温压条件和同位素年代学资料,我们认为退变质的强弱与变形程度有密切关系。峰期变质后的榴辉岩经过早期绿帘石榴辉岩相到蓝片岩相斜向走滑剪切,和晚期绿片岩相逆冲,在泥盆纪早期快速折返的出露地表。
The North Qilian Mountains is a typical Early Paleozoic orogenic belt where the 'cold' oceanic subduction belt occurs with rock assemblages characterized by high-pressure low-temperature blueschist and eclogite. According to the structure, three types of eclogites have been recognized. (1) The granular eclogite stands for peak stage metamoprhism with paragenetic mineral assemblage of Grt + Omp + Phn + Rut +/- Lws. Lawsonite and its pseudomorph retain as inclusions in garnet, and P-T conditions are P = 2.30 similar to 2. 60GPa and T = 465 similar to 508 degrees C. (2) The schistose eclogite consists of mineral assemblage of Grt + Omp + Czo/Ep + Phn + Gln with penetrative foliation S, lineation L, and macro-/micro isoclinal folds F, P-T calculation yields conditions of P = 1.9 similar to 2.2Gpa and T = 466 similar to 510 degrees C. (3) The blueschist-overprinted eclogite is characterized by strong retrogression of eclogite facies minerals into blueschist facies minerals GIn + Ep + Phn +/- Ab with penetrative foliation S(2) lineation L(2) and isoclinal shearing folds F(2) under P-T conditions of 1.15 similar to 1.37GPa and 422 similar to 487 degrees C. The lineation statistics from glaucophane and phengite Shows that D(2) deformation displays kinematic character of oblique strike-slip movement. The later overprinting of greenschist metamorphism appears with penetrative foliation S(3) and lineation L(3) Statistics of L(3) show vertical thrusting movement in perpendicular to the trending of high-pressure tectonic slices. Combining macro-and micro-scope structures, P-T conditions and isotopic geochronology data, we concluded that the grade of retrogression is associated with shearing deformation. Eclogites have undergone early oblique strike-slip deforruation under epidote-eclogite facies to blueschist-facies conditions, and finally thrust to the Surface before Early Devonian.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2009年第9期2235-2246,共12页
Acta Petrologica Sinica
基金
国家自然科学基金(40825007,40821002,40773012)
全国博士学位论文作者专项资金(200531)
北京大学“校长基金”资助
