北祁连蛇绿岩的特征、形成环境及其构造意义

THE CHARACTERISTICS OF NORTH QILIAN OPHIOLITES, FORMING SETTINGS AND THEIR TECTONIC SIGNIFICANCE

文中总结了北祁连蛇绿岩的特征，指出北祁连蛇绿岩大多具有ＭＯＲＢ的性质，有玻安岩产出，形成在弧后和岛弧环境。北祁连蛇绿岩大多侵位在岛弧增生楔或活动陆缘地体之上，蛇绿岩属于科迪勒拉型，早古生代的北祁连造山带属于科迪勒拉型造山带。部分蛇绿岩之上整合产出一套沉积－火山岩系，称为蛇绿岩的上覆岩系。指出蛇绿岩及其上覆岩系的枕状熔岩分别来自不同的源区，具有不同的构造意义。还讨论了北祁连早古生代板块构造格局，认为北祁连洋盆属于古亚洲洋的一部分，可能曾经是一个较大规模的洋盆。献中通常把它当成增生或俯冲杂岩带的一部分来看待〔１３，１６－１７〕；大岔大坂蛇绿岩带，其向两侧的延伸情况不清楚；九个泉（或塔墩沟）蛇绿岩带，向东可连到景泰县老虎山蛇绿岩，有人认为，向西可与榆树沟蛇绿岩相连〔２０〕。早先认为，北祁连存在新元古代、中寒武和早－中奥陶世三个时代的蛇绿岩〔２，１１〕，经过多年研究，目前大多数同意蛇绿岩主要是晚寒武－奥陶纪的〔１３，１６〕。图１北祁连早古生代蛇绿岩分布图１．前寒武纪基底；２．俯冲杂岩带；３．蛇绿岩。图中数字：１．九个泉；２．大岔大坂；３．边马沟；４．玉石沟；５．小八宝；６．百经寺；７．老虎山；８．榆树沟山２北祁连几?

The North Qilian ophiolites, with the characteristics mainly of MORB and boninite, were mostly formed at back arc and island arc environment and emplaced over island arc accretion wedge and active continental margin. So, It is possible that the North Qilian ophiolites similar to that of Cordilleran type by geochemical and emplaced characteristics. In North Qilian ophiolite belt, a volcanic sedimentary sequence lies conformably on the top of the ophiolite section, as in the Laohushan and Jiugequan area. The pillow lava at the top of the ophiolite is originated from the magma chamber beneath the spreading ridge, while the pillow lava in the overlying rock sequence is formed from partial melting of the depleted asthenospheric mantle off ridge, so they have different tectonic significance respectively. In the paper, the framework of plate tectonic of early Paleozoic North Qilian, and the periods and scale of North Qilian ocean basin were disccused. It can also be inferred that the North Qilian oceanic basin was a part of the Paleo Asian oceanic, and the scale of the North Qilian ocean basin was probably very large, and similar to that of today's Pacific ocean.