The Record Of Cenozoic Magmatism In Sanshui Basin And Its Relationship With The Early Tectonic Evolution Stage Of The South China Sea

As the only active tectonic belt with large-scale early Cenozoic magmatism in South China,the Sanshui Basin plays a potential role of“perspective window”that monitors the early evolution of the South China Sea before and during the rupture of the South China Continent.The author applied petrology,mineralogy,geochronology,and elemental and isotope geochemistry-based methods to systematically study the relationship between volcanic rock assemblages,their temporal and spatial distributions,source characteristics,magmatic processes,basin properties,and their relationships with the evolution of the South China Sea.The research has reached the following important conclusions.The Cenozoic volcanic rocks in the Sanshui area are categorized into three types:basalt,trachyte,and rhyolite.The author identified 12 volcanic eruptions,each beginning with basic magma eruption and ending with an alkaline or acidic magma eruption,leading to the formation of a basalt-trachyte or basalt-rheolite assemblages.However,this association is only manifested in the cyclicality of the magmatic processes in the time sequences.Spatially,they do not appear in the same rock mass,and each has its own clear distribution range:basalt is concentrated in the eastern part of the basin,and trachyte and rhyolite are concentrated in the western part of the basin.In this study,the chronological data of the 29.27±1.52Ma basalt and 28.25±1.14Ma rhyolite in the study area were obtained for the first time,thus the bimodal volcanic eruption model was formally correlated with the South China Sea expansion period.According to the newly constructed volcanic chronological framework of 66.04–28.25 Ma,it can be observed that the volcanic activity in the Sanshui Basin peaked in middle Eocene and continued to the early stage of the expansion of the South China Sea Basin.The pyroxene group minerals in the basalt of the study area comprise mostly monoclinic pyroxene,reflecting the alkaline environment inside the plate.However,orthorhombic pyroxene,reflecting the porphyry magma,is occasionally observed in the basalt.Furthermore,the orthorhombic pyroxene has a sieve-like reaction edge,indicating that a mixture of alkaline magma and porphyry magma exists during its formation.The volcanic rocks in the study area generally exhibit a relatively high degree of alkalinity.The tectonic setting is determined as the extensional environment,and the characteristics of the volcanic rocks are similar to those of the volcanic rocks in intracontinental rift.Geochemical analysis indicates that trachyte,rhyolite,and basalt are genetically related,and the zircon?_Hf?t?of the medium-acidic rock is 8.1–15.5?mean 13?,relatively close to that of the depleted mantle?DM?,indicating that it contains the magma from the mantle.In addition,some zircons are relatively old?>180 Ma?,indicating that the continental crust material participated in the diagenesis process.The presence of inert gases and Sr–Nd–Pb isotopes indicate that the source region may be affected by the oceanic crust in the southern hemisphere.The basalt experienced the differential crystallization of olivine and monoclinic pyroxene during the evolution of magma.The trachyte and rhyolite experienced the differential crystallization of monoclinic pyroxene and plagioclase.The volcanic rocks in the Sanshui Basin can provide clues for research regarding the early evolution of the South China Sea from continental rupture to rift expansion.In the period before 66 Ma,regional extension began;since then,the volcanic activity continued until the early expansion of the South China Sea Basin?approximately 28Ma?,which can be considered as a mirror reflecting the evolution of the South China Sea from the intracontinental rift to the intercontinental rift.However,the basin has not reached the stage of rift maturity that is indicated by the presence of a large number of basalts;further,the scale of magma eruption has shrunk severely,reflecting the limitations of the basin development.The rifting of South China Sea,Red Sea,and Atlantic Ocean Basin seem to have a high degree of similarity in the early stages;however,they appear to“choose”different development paths later.Additionally,this is reflected in the volcanic rock records in the study area.