Geochronology And Geochemical Characteristics Of Metabasic Igneous Rocks In Southeastern And Northeastern Guangxi And Its Tectonic Significance

The South China Block(SCB)is composed of the Yangtze Block and the Cathaysia Block with complex tectonic events.The SCB has experienced multiple-stages tectonic superimposed transformation and magmatic intrusion and destruction since Phanerozoic.At the same time,it is covered by thick sedimentary rocks,with a relatively small exposed area,which also makes many important geological problems about the junction zone become complicated and confusing.Especially at the intersection of the southwestern Yangtze Block and the Cathaysian Block,there are divergent opinions on some key geological issues.Such as the division of two major blocks,the nature and closure time of intra-plate basins,the dynamic mechanism and driving forces of collision orogeny,and the process of tectonic evolution.Through research on petrology,zircon U-Pb dating and main and trace geochemistry of the basic rocks in the Yunkai area in Southeastern and Northeastern Guangxi,the following understandings are obtained based on previous research results:The basic igneous rocks in Nuodong in Cenxi area and Luchuan area of the Yunkai area in Southeastern Guangxi have the ages ranging from 487 Ma to 428 Ma and were formed in the Early Paleozoic.The diagenetic ages of the two areas in the Early Paleozoic represent an Early Paleozoic magmatic-tectonic thermal event.The diagenetic ages of the chloritization garnet biotite schist in the Daning area of Northeastern Guangxi and the biotite schist in the Jundong area are 131～110 Ma,with a formation age of the Late Yanshanian period.It is speculated to be a product of the third stage of the Yanshanian period of the SCB.The MesoarchaeanNeoarchean zircons reveal that there might be an unexposed Archean crystalline basement in the deep part of the area.The whole-rock geochemical analysis results show that the samples are enriched in large ion lithophile elements(LILEs)such as K,Rb,Ba,Sr,etc.and light rare earth elements(LREE),and depleted in high field strength elements(HFSE)such as Nb,Ta,Ti,etc.,showing characteristics similar to island arc basalt(IAB),and formed in the depleted mantle.Based on previous studies on the Yangtze and Cathaysia Blocks,the age of the Early Paleozoic basic rocks(487～428 Ma)in this paper is close to the diagenetic age of the granitic rocks in Eastern SCB,indicating an internal dynamic relationship.In addition,previous studies have shown that the formation of large-scale granite in the SCB is the result of crustal deep melting under an intra-continental background.This indicates that during the post collisionalextensional stage,stress release might promote the uplift of the lithospheric thermal interface,leading to thermal disturbance of the ancient enriched lithospheric mantle and local melting of the enriched mantle.This study provides important clues for understanding the plate tectonics and intra-continental tectonic evolution of the SCB,and for revealing the complex evolution process that it has experienced.However,the time limit of lithospheric extension for the Mesozoic Yanshanian(131～110 Ma)samples can be restricted by the geochemical characteristics of intermediate-acid rocks of different ages,which reflects that the SCB is in a multi-period and long-term extensional background.Therefore,it is speculated that the samples in this study were formed by the melting of ancient subductional metasomatic mantle wedges,and the basic rocks in the study area may have formed during the post collisional lithospheric extensional stage.