ZIRCON GEOCHRONOLOGY FOR SEDIMENTARY ROCKS OF NANXIONG GROUP IN CHANGTIAN AREA, NORTHEAST GUANGDONG: Geological Implication

Changtian Basin is one of significant energy basins in Guangdong Province. To learn the geological structure, sedimentary material characteristics and tracer sandstone provenance in detail, microscopic identification, electron probe microanalysis(EPMA)and laser ablation inductively coupled plasma mass spectrometer(LA-ICP-MS)are used to systematically study the typical sandstones of Nanxiong Group in Changtian Basin on the basis of field geological survey. The results show that the Upper Cretaceous Nanxiong Upper Subgroup is mainly composed of(carbonaceous)lithic quartz sandstone, followed by calcareous sandstone, (argillaceous)siltstone, fine sandstone, conglomerate and greywacke, generally developed with obvious secondary changes and metallization. The analysis of detrital zircons from Nanxiong Group reveals that most of the zircons in the samples have perfect oscillatory bands with Th/U values usually greater than 0.4, indicating the zircons are mainly of magmatic crystallization origin, with a few metamorphic zircons of complex origin. The U-Pb concordant ages can be roughly divided into five groups: 2506-1666 Ma, 1815-941 Ma, 510-434 Ma, 308-234 Ma, and 172-99 Ma, which correspond to the known tectonic events in northeast Guangdong, reflecting that the tectono-magmatic activities in the study area are mainly controlled by the magmatic activities in eastern China, with the characteristics of episodic development and evolution. Based on the systematic analysis of lithography, zircon U-Pb age and REE characteristics of clastic rocks from Nanxiong Group, and comparison with the possible provenance, it is concluded that the detrital materials were mainly derived from Mesozoic magmatic intrusions in the western margin of Changtian Basin.