The Characteristics And Geological Significance Of Cenozoic Trachyte In The Sanshui Basin,Guangdong

The Sanshui basin is located in the northern part of the South China Sea and is one of the nearest basin of the east areas in China.Compared with the other Cenozoic volcanic basins,the eruption area of the Sanshui basin is large,and there is a complete record of volcano-sedimentary sequence.The volcanic activities are just in the critical period of the cracking process of the South China Sea and the continental margin.In particular,with the most powerful eruption,the stage X,erupting in the middle period of the Huachong formation,is dominated by the trachyte.In this paper,the Cenozoic trachyte from Zoumaying,Libianshan,Yigang,Shiling and Xiqiaoshan of the Sanshui basin are analyzed by Petrology,Geochronology,Geochemistry and Mineralogy.Combined with field reconnaissance and related geological data,the genetic type and tectonic environment are discussed.The fresh surface color of trachyte is gray,grayish-green and grayish-purple.They are massive and rough structures.The phenocrysts are dominated by alkaline feldspar,and the darker minerals such as pyroxene and amphibole are fewer.With the characteristics of rich alkalinity,Al and low Ti,Fe,the trachyte is intermediate volcanic rocks and belongs to shoshonite series,it also perform enrichment in HFSE such as Th,U,Ta,Nb,Zr,Hf et al.and is negative anomaly in Ba,Sr,P,Eu,Ti.In addition,with relatively high content and large range of REE,chondrite-normalized REE pattern suggests that the trachyte is significantly enriched in LREE.The above shows that the trachyte has relatively low pressure forming condition.The trachyte can be found in the middle of the western half part of the basin and Xiqiaoshan.In this paper,the erupting period of the trachyte in Xiqiaoshan,Zoumaying,Libianshan,Shiling and Yigang in the Sanshui basin is determined for the first time,it shows that the eruption of the XQS trachyte can be at least divided into 2 stages,the eruption of SL,YG and LBS trachyte is mainly in 48~52Ma and the eruption of ZMY trachyte can be at least divided into 3 stages.The eruption time is postponed to the late Oligocene which is the time of the eruption of South China Sea.This shows that along with the expansion of the South China Sea,there are still volcanic activities.At the same time,it is determined that the earliest eruption time of the ZMY was early Paleocene,thus,the age is more connected to sedimentary sequence in the research area and is determined the initial period of the rift valley activities.The analysis of volcanic rocks shows that the rocks display bimodal features(basalt-trachyte/ rhyolite).It is found that the trachyte is formed by fractional crystallization of the mantle-derived basaltic rocks,and produced in the environment of continental plate,they has the similar geochemical characteristics with the trachyte of the same origin in the Tianchi Lake of the Changbai Mountains,Saudi Arabia,Kenya rift and Antarctica.