北山地区早古生代板块构造特征

位于甘肃省西北边界和内蒙古自治区西端的北山地区,早古生代大地构造单元由塔里木板块东段北缘和北侧贝加尔期分裂出来的旱山微板块组成,其间被石板井-小黄山蛇绿混杂岩带所分隔。在漫长的构造演化进程中发育有蛇绿岩带。同时,经历了大西洋型、安第斯型(?)和西太平洋型大陆边缘的演化阶段,陆壳增厚,地壳成熟度增加,由大洋地壳和过渡型地壳向大陆型地壳转化。晚古生代初,全区进入板内活动时期。

EARLY PALEOZOIC PLATE TECTONICS IN BEISHAN AREA

Early Paleozoic tectonic unit in Beishan area consists of the northern margin of the astern end of Tarim plate and the Hanshan micro-plate in its northern flank. These two plates-were separated by an ophiolitic melange belt. All of them had undergone the tectonic evolution of continental margins of Atlantic, Andes and West Pacific ocean types.The continental margin was composed of open continental shelf and continental slope from Early Cambrian to Early Ordovician. On the southern flank of the continental shelf a single shallow-sea basin was a pulled apart up warping zone. Its northern flank, Shibanjin-Xiaohuangshan, is an ocean basin. The north of the ocean basin is the Mingshui-Hanshan micro-continent or micro-plate. This area is situated in a tensile geodynamical environment, and shows the characteristics of passive margin of Atlantic type.During middle and late of Early Ordovician the continental crust on the southern frings of the shallow-sea basin was pulled apart and formed a primary narrow rift ocean basin. In middle Ordovician, this area enters a evolution period, during which an extension and a sub-duction existed simultaneously. The continental margin of the southern flank extened to the end of the passive margin. On its northern side, a marginal volcanic zone that is a calci-alkali series was formed.In Late Ordovician-Silurian, the ocean crust double subducted southward, and the Heng-luanshan-Xichanjing marginal basin into an interarc basin, in which Hongliuhe-Niujuanzi be-hindarc basin and Baiyunshan-East Qiyishan volcanic arc had formed and associated with activity of granitic magma.At the beginning of Devonian, the Tarim plate came into collision with the Hanshan micro-plate finally, the ocean basin closed, forming the suture belt of the Mingshui-Shibanjin-Xiaohuangshan. There were volcanic molasse basins on both sides of this erogenic zone. The Beishan area entered an active period of intra-plate from Late Paleozoic.