Th-rich zircon from peralka line A-type granite: Mineralogical features and petrological implications

Abstract

The Taohuadao, Qingtian and Laoshan granites are three typical late Yanshannian peralkaline granitic plutons in the coastal area, eastern China. In this paper, internal structures and chemical compositions of zircon from these A-type granites were investigated by electron microprobe. It is shown that zircon grains are mainly composed of two distinctly separated parts. One is rich in Th (ThO2 >1 wt%, and ThO2/UO2>2), and attains ThO2 up to 10.1 wt%; such value exceeds the dissolution limit of Th in the zircon structure (ThO2=5.5±.5 wt%) determined in previous experiment. On the other hand, the other part is poor in Th (ThO2<l wt%), but contains many thorite micro-inclusions with sieved texture. By comparison, it is also implied that zircon in aluminous A-type granites is characterized by low content of ThO2 (<1 wt%), ThO2/UO2 <2 and absence of thorite inclusion. Based on mineralogical features, one is tempted to assume that the Th-rich zircon is formed during the early crystallization of deep-sourced, high-temperature and Th-enriched A-type granitic magma. Such zircon is then subjected to late dissolution owing to accumulation of fluids at the end of magmatic evolution of A-type granite. Recrystallization finally leads to formation of sieved low-Th zircon with thorite micro-inclusions, which may coexist with remnants of Th-rich zircons. The Th-rich zircon may be considered to be one of characteristic accessory minerals of peralkaline A-type granites.