Petrogenesis of the volcanic rocks in Fanchang basin,the Middle-Lower Yangtze River Belt: Zircon Hf-O isotopic constraints

Fanchang basin is one of the volcanic basins in the Middle-Lower Yangtze River Belt. Studies on the volcanic rocks in the basin are still weak,so this article carried out zircon Hf-O isotopes and U-Pb ages analysis on rhyolite from Zhongfencun Formation,trachyandesite from Chisha Formation and rhyolite from Kedoushan Formation in order to discuss petrogenesis of the volcanic rocks in Fanchang basin. The results of zircon LA-ICPMS U-Pb dating are 132. 1 ± 1. 5Ma( Zhongfencun Formation),129. 1± 1. 8Ma( Chisha Formation) and 129. 5 ± 3. 3Ma( Kedoushan Formation). The rang of zircon eHf( t) values are- 8. 2 ~- 5. 8,- 6. 0 ~- 3. 4,- 7. 4 ~- 2. 2 and the rang of zircon δ18O values are 6. 3 ‰ ~ 7. 9 ‰,7. 0 ‰ ~ 7. 8 ‰,6. 6 ‰ ~ 8. 2 ‰ for rhyolite from Zhongfencun Formation,trachyandesite from Chisha Formation and rhyolite from Kedoushan,respectively. From Zhongfencun Formation to Chisha Formation and Kedoushan Formation,the values of eHf( t) and δ18O are slightly increased.Comprehensive researches suggest that sources of the volcanic rocks from the Zhongfencun Formation,Chisha Formation and Kedoushan Formation are similar. They were formed by the mixing of minor magma derived from enriched lithospheric mantle and major magma derived from Neoproterozoic juvenile crust. From 148 ~ 133 Ma to 132 ~ 126 Ma,the region had experienced a tectonic transition from a subducting regime to a lithosphere extensional regime, and the tectonic transition happened at about 132 Ma.