阿拉善地块北缘朱拉扎嘎毛道晚二叠世火山岩的发现

研究区位于阿拉善地块北部的朱拉扎嘎毛道地区,根据前人研究,该区还未发现火山岩出露。通过对朱拉扎嘎毛道地区的岩石样品野外观察和室内薄片鉴定,确定该区存在球粒流纹岩。该套球粒流纹岩玻璃质含量较高,约75%,玻璃质呈球粒状,有脱玻化现象,其中还包含大量自形程度较高的长石颗粒,可能表明球粒流纹岩在喷发之前捕获有围岩成分。锆石U-Pb年龄表明,该套球粒流纹岩的主要喷发时间为晚二叠世。根据Lu-Hf同位素组成,U-Pb年龄为240～325Ma的锆石,εHf(t)全为负值(-39.2～-13.0),且模式年龄远远大于U-Pb年龄,表明形成这套球粒流纹岩的岩浆来源于古老地壳再循环。而研究区位于华北板块西部块体的北缘,所以该套球粒流纹岩是由华北板块西部块体北缘的古老地壳物质再循环产生。

Discovery of the Late Permian volcanics in the Zhulazhagamaodao,northern margin of the Alxa Block

According to the previous studies,there are not volcanic outcrops being found from Zhulazhagamaodao,in the north of the Alxa block.On the basis of field observation and microscopic petrographic thin section studies,spherulitic rhyolite had been found from this region.The content of the glassy materials in the spherulitic rhyolite is as high as 75%(vol),and spherulitic structures are prominant.The devitrification phenomena can be observed in the thin section and a lot of feldspars phenocrysts are wrapped within the glassy materials.It indicates that there must be a lot of minerals from the surrounding rocks trapped into the magma before the spherulitic rhyolite eruption.U-Pb ages reveal that the eruption of spherulitic rhyolite occurred in the late Permian.Analyzing the Lu-Hf isotopic compositions,the εHf(t) values of the zircons with the ages of 240-325 Ma are less than zero(-39.2——13.0) and the model ages are far larger than the U-Pb ages,so it suggests that the spherulitic rhyolite was originated from the recycling of the ancient crust.Combined with the sampling location,we infer that the ancient crustal materials were from the northern margin of the western block of North China plate.