Abstract:

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 240325 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.

Key words:  Alxa Block, Late Permian, spherulitic rhyolite, U-Pb age, Hf isotope