Chronology and meta-sedimentary genesis for gneissic alkaline granites in Donghai county,Jiangsu province
Xia Bin1, Li Jianfeng1,2, Zhang Yuquan1, Liu Liwen3, He Guansheng4, Wang Hong1, Wang Min1, Yang Zhiqing5
1. Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Ceochemistry, Chinese Academy of Sciences, Guangzhou 510640;
2. Graduate School, Chinese Academy of Sciences, Beijing 100039;
3. No. 719 Geological Team, Guangdong Bureau of Geological Exploration, Zhaoqing, Guangdong 526020;
4. Jiangxi Bureau of Exploration, Nuclear Industrial Geology Bureau, Nanchang County, Jiangxi 330029;
5. Beijing SHRIMP Centre, Chinese Academy of Geological Sciences, Beijing 100037
Donghai region is tectonically located in the southwestern margin of the Su-Lu Orogenic Belt,where the gneissic alkaline granites were outcropped.This paper made an integrated study of petrology,mineralogy and geochemistry for the Moshan,Hushan and Fangshan gneissic alkaline granites in Donghai.It was shown that these rocks should belong to A-type granite.These plutons were characterized by sedimentary layers,the orthogonal intersection of the elongate direction and polysynthetic twin in the plagioclase,the thick arrangement of zircons in the thin section and the clastic old cores of the metamorphic zircons in the cathodeluminescence images.The allochthonous and polygenetic characteristics of the old cores of the metamorphic zircons suggested the protolith of the gneissic alkaline granite in Donghai being sedimentary rocks,rather than magmatite.The 206Pb-238U mean ages of 224.7±9.4 Ma and 212±3.5 Ma (or 209.4±2.5 Ma) for the rims of metamorphic zircons indicated that the metamorphic time of the granite should be the Indosinian epoch,correspond to Late Trassic,which were also the diagenetic time of these granites.The 206Pb-238U ages for the old cores ranged from 866 Ma to 248 Ma,which indicated that the formation time of the protolith of the these granites should not earlier than Late Paleozoic.