Study On The Evolution Of Archean-Paleoproterozoic Crystalline Basement In The Western Margin Of Yangtze Block

The study of the early evolution process of the Yangtze plate is of great significance to the tectonic evolution and tectonic events in China and the world.Due to the extensive Middle-Neoproterozoic magmatic events in the study area,the study of magmatism in the Archean eon is relatively weak,which limits the understanding of the tectonic evolution history of the Yangtze plate in the Archean-Paleoproterozoic era.Based on field geological survey,lithography,geochemistry,U-Pb chronology of SIMS zircon and O-Hf isotopes in situ,this paper discusses the petrogenesis,tectonic evolution characteristics and geological significance of the land-based dykes in the salt margin of the western margin of the Yangtze block.In this paper,the neoarchean basement age was found for the first time in the analysis and testing of the Huangtian mafic rocks in the Yanbian area of the western margin of Yangtze Block.The U-Pb isotope of SIMS zircon was used to determine the Huangtian mafic-gabbro of the Yanbian group.The upper intersection of the HT-13 rock was 2512±81Ma Neoarchean and the lower intersection was 829±21Ma Neoproterozoic.In the middle of HT-14 rock,the age of the upper intersection is2519±9.9Ma of Neoarchean,and the age of the lower intersection is 834±35Ma of neoproterozoic.The age of the upper intersection represents the formation and emplacement of the gabbro,which indicates the presence of the Archean strata in the study area.The ages of the lower intersections containing the accretive edges of metamorphic zircons obtained from some Huangtian rocks were 829±21Ma and834±35Ma,respectively,indicating that metamorphism occurred in some aridonic rocks in the Neoproterozoic,and its geological significance remains to be further clarified.The whole rock geochemistry of Huangtian mafic rocks is similar to N-MORB,and the differences in the distribution patterns of main elements,trace elements and rare earth elements further indicate that they may come from the products of different evolution stages of the parent rock magma.The initial value of zircon ¹⁷⁶Hf/¹⁷⁷Hfin gabbro was calculated to be 0.28124?0.28264,corresponding?Hf?t?=-10.3?7.7,with an average value of about 2.9.The model age of zircon in the first-stage deficit mantle Hf model was around 800Ma in Neoproterozoic,T_DM1 was 1076?1741Ma,and that of the Archean model was around 2500Ma,2587?2752Ma.The T_DM2 of the second-stage deficit mantle Hf model with an age of about 800Ma in Neoproterozoic was1205?2311Ma,and that of the Archean model with an age of about 2500Ma was2671?2972Ma,which was slightly greater than the diagenetic age.These data indicate that the Neoarchean-Paleoproterozoic gabbro is derived from the depleted mantle,indicating that there is a depleted mantle under the western margin of the neoarchean Yangtze.Combined with a large number of Archean-Proterozoic detrital zircons in the western margin of the Yangtze block,it provides new evidence for the existence of an Archean-Proterozoic double layer crystal basement in the western margin of the Yangtze block.Through the comparative analysis of crustal evolution between the salt frontier Huangtian mafic rocks and other Archean regions of the Yangtze craton,it is found that the Archean Yangtze craton has obvious heterogeneity in structure and is probably formed by the superposition of several Archean terrains.At the same time,the tectonic evolution process of the western margin of the Yangtze block before the convergence of the Columbia supercontinent is speculated,which provides a new basis for the formation process of the Yangtze craton.