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

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.