阿拉善地块南缘早古生代大地构造特征和演化

Early Paleozoic Tectonic Features and Evalutions in the Southern Margin of Alaxa Massif

本文依据早古生代阿拉善地块南缘各地段基本地质特征的差异将其划分为3个构造带:北祁连北缘火山—岩浆岛弧带,走廊弧后盆地区和北大山—龙首山板内花岗岩带。通过对北祁连北缘下奥陶统火山岩系的研究,认为它们属于岛弧环境中的钙碱质火山岩系列。通过对走廊地区岩石建造组合的研究,认为走廊地区经历了从寒武纪离散型大陆边缘到奥陶纪发育在大陆地壳上的弧后盆地类型的转变。通过对本区花岗岩的研究,提出在阿拉善地块南缘存在两种不同成因类型的花岗岩:板缘岛弧同熔型和板内重熔改造型,并讨论了它们与区域构造的关系和可能的成因机制。最后,分4个发展阶段讨论了阿拉善地块南缘早古生代的构造演化。

This paper discusses early Paleozoic tectonic features and evalutions in the southern margin of Alaxa Block from several aspects: tectonic pattern, volcanic features, petrographic formation associations, basin types and granite series. According to differences of essential geological features, the southern margin of Alaxa Block is divided into three tectonic zones: volcanic-magnatic are in the northern area of North Qilian, the backarc basin in the Hexi Corridor and the intraplate granite zone in the Longshoushan-Beidashan mountains. Through studying lower Ordovician volcanics in the northern areas of North Qilian, we consider that they belong to calc-alkalic series in the island-arc environment. We consider that the Corridor region underwent transformation from divergent continental margin during the Cambrian to backarc basin on continental crust during the Ordovician on the basis of study of petrograghic formation associations. By studying granites in this region, we suggest that there are two different genetical types of granitic seriess: con solute-type series in the island-are of plate margin and transformation-type in the interior of the continental plate. Their genesis and relationship with regional tectonics are also discussed. Finally, tectonic evalutions in this region are discussed in the light of four different stages.