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The Taohuadao, Qingtian and Laoshan granites are three typical late Yanshannian peralkaline granitic plu- tons 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 dis- tinctly 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±2.5 wt%) determined in previous experiment. On the other hand, the other part is poor in Th (ThO2<1 wt%), but contains many thorite micro-inclusions with sieved tex- ture. By comparison, it is also implied that zircon in alumi- nous 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 crystalli- zation 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 fi- nally 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.
The Taohuadao, Qingtian and Laoshan granites are three typical late Yanshannian peralkaline granitic plu- tons 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 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 ± 2.5 wt%) determined in previous experiment. On the other hand, the other part is poor in Th (ThO2 <1 wt%), but contains many thorite micro-inclusions with sieved tex - ture. By comparison, it is also implied that zircon in alumi- nous 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 crysta lli- zation of deep-sourced, high-temperature and Th-enriched A-type granitic magma. Such zircon is then subjected to late dissolution due to accumulation of fluids at the end of magmatic evolution of A-type granite. Recrystallization fi- nally 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 the characteristic accessory minerals of peralkaline A-type granites.