西秦岭北带侏罗纪火山岩构造环境

THE TECTONIC SETTING OF JURASSIC VOLCANIC ROCKS IN THE NORTH ZONE OF WESTERN QINLING

本文依据西秦岭北带2处侏罗纪火山岩分布特征、剖面结构、岩石组合、岩相、喷发类型、岩石地球化学及与周边各地质体接触关系等实际资料综合分析,认为其形成的构造环境为陆内断陷盆地,喷发类型为裂隙式和中心式,岩相为溢流相和火山通道相,推测岩浆来自壳源,为元古界秦岭群中深变质岩部分熔融产物。侏罗纪该区以浅层次断陷(伸展)作用为主,侏罗纪末再次发生陆内造山作用,具有脆性断层逆冲作用特征。

Volcanic rocks have close relationship with the tectonic evolution of orogenic belt. There are Jurassic volcanic rocks developed in two separate sites in the north zone of western Qinling. According to their characteristics of distribution, section structure, rock assemblage, lithofacies, and contact relationship with neighbouring stratums, it can be spectulated that these volcanic rocks formed in continental rift basin with fissure or central type eruption, and the magma might source from crust formed by the partial melting of hypometamorphic rocks in Paleoproterozoic Qinling group. Their lithofacies are effusive or volcanic vent facies. During Jurassic, shallow fault depression （extension） mechanism controlled the tectonic evolution of this area. In the end of Jurassic, intra-continental orogenesis reactivated with brittle thrust faults developed.