The Lithological Characteristics And Tectonic Setting Of Cenozoic Trachyte In The Sanshui Basin

The Sanshui basin is the only one of the many basins in the Land south China that is closest to the sea basin in the South China Sea and has large-scale volcanic eruption in the early Cenozoic,which has a distinctive feature in the regional tectonic evolution.This article selects the trachyte of mesofacial rocks in Sanshui basin as the basic research object.This paper studies the petrology,geochemistry and mineral chemistry of Cenozoic trachyte from Zoumaying,Libianshan,Yigang,Shiling and Xiqiaoshan areas.Discussing the genetic types,magma formation and evolution and tectonic background of trachyte based on the research results of predecessors and the project team.The fresh surface color of trachyte is gray-green-burgundy.They are massive and rough structures.The phenocrysts are dominated by alkaline feldspar,and the darker minerals such as pyroxene and amphibole are fewer.Dark minerals such as pyroxene and amphibole contain in small or no amount according to the collection site of trachyte.The rhyolite with porphyritic structure can be seen in the samples of Libianshan and Xiqiaoshan.In addition to alkaline feldspar,a small amount of quartz can be found in the porphyry and matrix of rhyolite.However,the samples collected by Zoumaying all have coarse surface structure,and the porphyry crystal is all alkaline feldspar.In the low-pressure environment,the basalt is mixed with a small amount of crustal material through high degree of crystallization differentiation,forming the trachyte.Among,the evolution degree of Zoumaying trachyte is lower than that of Libianshan and Xiqiaoshan trachyte.The magmatic evolution model of Sanshui basin was established according to the distribution characteristics of volcanic rocks and magmatic conditions.Under the background of the regional tectonic tension,the deep mantle was partially melted to form basaltic magma,which gradually evolved into coarse surface magma with high water content,low density and high viscosity through crystal differentiation.A magma reservoir was formed,when the coarse magma rose to a shallow crust of about 2.3-3.8km,which hindered the rise of basaltic magma at the bottom.However,only in the cracks of the trachyte magma chamber or in the periphery of the trachyte magma chamber,a small amount of basaltic magma reached the surface and ejected basalt.The volcanic activity in the basin was dominated by the eruption of trachyte rocks and the amount of basalt was relatively small.The eruption time span of trachyte in Sanshui basin is large,which can be divided into three stages.The volcanic activity of phase II(55-50Ma)was the most intense volcanic activity,with intense outbursts of trachyte.In space,the eruption system is distributed along the north-south axis,and the two main eruptive centers of trachyte(the area of Zoumaying-Libianshan and Xiqiaoshan)are located at both ends of the axis,while basalt rocks are mostly distributed in the middle section.The basalt in the basin belongs to the mantle upwelling product of intraplate spreading and splitting,and the trachyte are derived from the crystallization of basaltic magma.The large-scale volcanic activity is directly related to the mantle activity through the basic magma,which presents the basic characteristics of the volcanic continental rift,and shows that the passive rift valley becomes the active rift valley,which is the potential center of continental disintegration.Therefore,the Sanshui rift of early Cenozoic is likely to be a branch of the entire continental margin that died in the process of fragmentation and has no direct connection with the subsequent expansion of the South China sea.