| 中国西部柴北缘—阿尔金的超高压变质榴辉岩及其原岩性质探讨 |
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| 引用本文： | 杨经绥,张建新,孟繁聪,史仁灯,吴才来,许志琴,李海兵,陈松永.中国西部柴北缘—阿尔金的超高压变质榴辉岩及其原岩性质探讨[J].地学前缘,2003,10(3):291-314. |
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| 作者姓名： | 杨经绥 张建新 孟繁聪 史仁灯 吴才来 许志琴 李海兵 陈松永 |
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| 作者单位： | 中国地质科学院,地质研究所,大陆动力学实验室,北京,100037 |
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| 基金项目： | 国土资源部科技攻关项目，国家地质调查项目，国家自然科学基金，20010201，，49732070，，， |
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| 摘 要： | 中国西部祁连山柴北缘地区和南阿尔金地区存在一条被阿尔金断裂错开 4 0 0km ,但构造上相连的早古生代超高压变质带。通过对柴北缘地区大柴旦、锡铁山、都兰和南阿尔金地区且末一带榴辉岩的岩石地球化学研究 ,发现榴辉岩原岩主要由玄武岩和苦橄岩两类岩石组成 ,进一步分为高Ti型 (w(TiO2 ) =2 %～ 5 % ) ,中Ti型 (1%～ 2 % )和低Ti型 (<1% ) 3种类型 ,识别出榴辉岩的原岩类型有洋脊玄武岩、岛弧拉斑玄武岩和洋岛玄武岩类等产在不同环境的岩石类型。榴辉岩的Nd同位素组成与现代洋脊玄武岩类相似 ,ε(Nd ,0 )主要为正值 ,少量为轻微负值 ,表明榴辉岩的原岩曾是海底玄武岩 ,并且经过了消减俯冲作用 ,混入了部分的地壳物质。榴辉岩的超高压变质年龄为 5 0 0～ 4 4 0Ma,原岩年龄分别为 80 0～ 75 0Ma和～ 10 0 0Ma。研究表明 ,柴北缘滩涧山群中存在两套时代不同的基性超基性岩 ,一套为产在绿梁山的新元古代时期形成的蛇绿岩组合 ,新获得的年龄值为 (76 8±39)Ma(Rb Sr)和 (780± 2 2 )Ma(Sm Nd) ,另一套主要为产在赛什腾山的晚寒武世岛弧火山岩 ,形成时代约在 5 15～ 4 86Ma。榴辉岩的岩石化学成分和Nd同位素组成 ,以及 80 0～ 75 0Ma的原岩时代与其中的新元古代基性岩类可以对比。初步认为它们是同一套岩石?
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| 关 键 词： | 超高压变质带 榴辉岩 基性火山岩 柴北缘 青藏高原 |
| 文章编号： | 1005-2321(2003)03-0291-24 |
| 修稿时间： | 2003年5月12日 |
ULTRAHIGH PRESSURE ECLOGITES OF THE NORTH QAIDAM AND ALTUN MOUNTAINS,NW CHINA AND THEIR PROTOLITHS |
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| YANG Jing-sui,ZHANG Jian-xin,MENG Fan-cong,SHI Ren-deng,WU Cai-lai,XU Zhi-qin,LI Hai-bing,CHEN Song-yong.ULTRAHIGH PRESSURE ECLOGITES OF THE NORTH QAIDAM AND ALTUN MOUNTAINS,NW CHINA AND THEIR PROTOLITHS[J].Earth Science Frontiers,2003,10(3):291-314. |
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| Authors: | YANG Jing-sui ZHANG Jian-xin MENG Fan-cong SHI Ren-deng WU Cai-lai XU Zhi-qin LI Hai-bing CHEN Song-yong |
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| Abstract: | An Early Paleozoic ultrahigh pressure metamorphic belt exists in the Qilian-Altun Mountains and is offset about 400 km westward by the Altyn Tagh strike-slip fault. Eclogites in the belt consist of major basaltic and minor picritic rock types and can be subdivided into three groups: high w(TiO_2)(2%～5%), medium w(TiO_2)(1%～2%) and low w(TiO_2)(<1%). Geochemical evidence shows that the protoliths of the eclogite were basaltic rocks from different tectonic environments, including mid-ocean ridge basalt, island arc basalt and ocean island basalt. The Nd isotopes of these rocks are similar to those of MORB, characterized by mainly positive and minor negative ε(Nd,0) values, providing further evidence that the eclogite protoliths were ocean floor basalts, to which minor crustal components were added during subduction to form UHP metamorphic rocks. Geochronological data indicate that the UHPM occurred about 500～440 Ma, and that there were two eclogite protoliths with ages of 800～750 Ma and ～1 000 Ma. Geochemically and isotopically, the eclogites are similar to basaltic rocks in Luliangshan, North Daqaidam Mountains, which have Sr and Nd ages of (768 ± 39) Ma and (780±22) Ma. The age and composition of these volcanic rocks suggest that they were one of the protoliths for the eclogites. A later series of calc-alkaline island arc volcanic rocks were formed at about 500 Ma, essentially contemporaneously with the UHP metamorphism. Based on the available geochronological and geochemical data, we suggest the following tectonic model for the evolution of the UHP belt in the North Qaidam Mountains. At about 1 000 Ma the area was amalgamated to form the Rodinian continent, which contained some oceanic volcanic rocks, possibly as ophiolitic fragments. This part of Rodinia was then rifted at about 800～750 Ma to form an oceanic basin with a variety of MORB and ocean island basalts. Closure of this ocean basin produced the Neoproterozoic ophiolites and granitic gneisses in the North Qaidam Mountains. In the Early Paleozoic, a new Qilian ocean basin formed and subduction of this oceanic lithosphere formed the island arc volcanic rocks at about 500 Ma and the subduction-related granites. Once the oceanic lithosphere was consumed, continent-continent collision led to deep subduction of continental crust, which was tectonically mixed with the eclogites. |
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| Keywords: | UHPM belt eclogite basalt North Qaidam Tibetan plateau |
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