Petrogenesis of the Late Cretaceous Rhyolites in the Changtang Basin, Northern Guangdong: Constraints from Geochemistry and Sr-Nd-Pb-Hf-O Isotopes

Abstract

Changtang rhyolite is the acidic end-member of asymmetric bimodal volcanic rock association, which is composed mainly of amounts of rhyolite and minor basalt. It is considered to be the youngest Yanshanian rhyolite in southern China (SHRIMP age of 96.8 Ma), but the petrological and geochemical characteristics are, as yet unclear. In this paper, the focus is on the whole rock major elements, Sr-Nd-Pb-O isotopes and zircon Hf isotopes. Rhyolite belongs to weakly sub-alkaline peraluminous rock. It is enriched in rare earth elements with strong light and heavy REE fractionation with obvious europium depletions. Rhyolite has low (87Sr/86Sr)i, hight εNd(t), low (206Pb/204Pb)i, (207Pb/204Pb)i and (208Pb/204Pb)i values. The zircon has high Hf isotopes and δ18O values. Whole rock TDM2(Hf) values are similar to TDM2(Nd), indicating the derivation of juvenile lower crust. Combined with regional geological data, it is suggested that the Changtang rhyolite was formed by melting of Neoproterozoic lower crust which was derived from EMⅡ mantle source mixed with minor crustal materials. The primary magma experienced fractional crystallization during eruption. Petrogenesis of early stage of the Late Cretaceous rhyolite in South China provides the time evidence for crustal extensional thinness to normal thickness.

Keywords

• rhyolite
• early late cretaceous
• sr-nd-pb-hf-o isotopes
• young lower crust
• changtang basins