吉林延边和龙地区泉水村组火山岩锆石U—Pb定年、地球化学特征及其地质意义

Zircon U—Pb dating,geochemistry and geological significance of volcanic rocks in Quanshuicun Formation of Yanbian Helong area,Jilin province

通过对延边地区泉水村组火山岩的岩相学、年代学以及地球化学研究来约束该地区的构造演化。泉水村组火山岩分为两段,上段为流纹岩、流纹质凝灰岩、流纹质角砾岩,下段为安山岩、安山质角砾岩。整体为两个喷发旋回,呈爆发相-喷溢相的持续。锆石U—Pb测年表明流纹质凝灰岩形成年龄为(115. 5±5. 3) Ma,为早白垩世晚期,其地球化学特征表明原始岩浆来源于地壳的部分熔融;安山岩地球化学特征显示其岩浆源区是地幔楔由于遭受俯冲流体交代作用从而发生部分熔融,同时有较多地壳物质的混染,表现出活动大陆边缘构造背景下岩浆岩的特征。结合其他证据推断古太平洋板块至少在早中侏罗世就已经向吉黑东部开始俯冲,晚侏罗世—早白垩世早期俯冲作用消减,早白垩世晚期又发生强烈俯冲。

Petrography,chronology and geochemistry of volcanic rocks in Quanshuicun Formation in Yanbian area have been used to constrain the tectonic evolution in this area. The volcanic rocks in Quanshuicun Formation are divided into two sections,of which the upper section comprises rhyolite,rhyolitic tuff and rhyolite breccia,whereas the lower section consists of andesite and andesitic breccia. There are two eruption cycles running from eruption facies to flooding facies. LA-ICP-MS zircon U-Pb dating indicates that the formation age of the rhyolitic tuff in Quanshuicun Formation is( 115 ± 5. 3) Ma,which is the late stage of Early Cretaceous,and its geochemical characters indicate that the primary magma could be mainly derived from partial melting of the crust. The geochemical characters of andesite indicate that the magma is derived from partial melting of the mantle wedge resulted from the metasomatism of the subduction fluid and is mixed with crustal materials at the same time,showing thecharacteristics of magmatic rocks formed under the active continental margin. Combined with other evidence,it is concluded that the paleo-Pacific plate,subducted to the eastern region of Jilin and Heilongjiang in the Early-Middle Jurassic,and the subduction was weakened in Late Jurassic-Early Cretaceous,but resumed again in the late stage of Early Cretaceous.