Geochemical Characteristics,Petrogenesis And Tectonic Significance Of Triassic Volcanic Rocks In Gongjue Of Jiangda-Weixi Continental Margin Arc

A large amount of petrological evidence of westward subduction in the Permian-Triassic period of the Jinshajiang Paleo-Tethyan ocean is preserved in the Jiangda-Weixi continental margin arc.In order to understand the evolution process of the Jinshajiang Paleo-Tethyan ocean,predecessors have done a lot of work on the Jiangda-Weixi continental margin arc and its adjacent areas,and made a series of progress.However,there are still disputes on the precise formation age,petrogenesis and tectonic significance of Triassic volcanic rocks in the northern part of the continental margin arc,which greatly limits the understanding of the tectonic evolution process of the northern part of Jinshajiang Paleo-Tethyan ocean.In this paper,the Triassic volcanic rocks exposed on the Jiangda-Weixi continental margin arc and the edge of the Changdu block in the Gongjue area are selected as the research object,and the detailed geological mapping,petrology,mineralogy,zircon U-Pb age,whole rock geochemistry and Sr-Nd-Hf isotopes are systematically carried out to limit their formation age,explore their petrogenesis and magma source area,and elaborate their geotectonic significance,reveal the Triassic evolution process of the northern part of Jinshajiang Paleo-Tethyan ocean.The main understandings are as follows:(1)Field geological survey and petrological research show that the volcanic rocks in Gongjue section of Jiangda-Weixi continental margin arc are mainly distributed in Lower Triassic Pushuiqiao Formation(T₁p),Lower-Middle Triassic Malasongduo Formation(T_1-2m)and Middle-Upper Triassic Dongka formation(T_2-3dk).Pushuiqiao Formation(T₁p)and Dongka Formation(T_2-3dk)are exposed on the continental margin arc.Pushuiqiao Formation is a basalt-andesite-andesitic tuff combination.The top of the formation is in unconformity or fault contact with sedimentary strata such as Dongdu Formation(T₁dd)and Gongyelong Formation(T₂g).Dongka Formation is mainly composed of basaltic andesite-trachyandesite intercalated with sandstone-siltstone,which is unconformably covered on the limestone of gongyelongFormation.Malasongduo Formation is exposed at the western edge of the continental margin arc and borders the Changdu block.It is mainly composed of dacite-rhyolite assemblage,and is in unconformity or fault contact with the underlying stratum,Aoqu Formation(C₂a)and overlying stratum,Jiapila Formation(T₃j).(2)Zircon U-Pb age of the Pushuiqiao Formation andesite in Wadui area,Gongjue County is 251±2.4 Ma,indicating that the andesites were formed in the end of Late Permian to the beginning of Early Triassic.The rocks are rich in Al,Na and K,and relatively enriched in large ion lithophile elements(LILE)Rb and Th,depleted in Sr elements,and depleted in high field intensity elements(HFSE)Nb,Ta and Ti,with corresponding?_Hf(t)values ranging from+7.24 to+15.55.The volcanic rocks of the Pushuiqiao Formation are derived from the lithospheric mantle source affected by subduction fluid metasomatism.The volcanic rocks of Pushuiqiao Formation are derived from the lithospheric mantle source area affected by the metasomatism of oceanic crust subduction fluid or sediment.They are the product of the westward subduction of Jinshajiang Paleotethys Ocean from the end of Late Permian to the beginning of Early Triassic.(3)Zircon U-Pb ages of dacite-rhyolite in Malasongduo Formation range from 250Ma to 245Ma,and were formed in Early-Middle Triassic.It is rich in Si and K,poor in Na and Mg,relatively rich in large ion lithophile elements such as Rb and Th,relatively deficient in Sr elements,relatively deficient in high field strength elements such as Nb and Ta.The corresponding?_Hf(t)values range from-22.50 to+5.50,most of which are less than zero,indicating that the Jinshajiang Paleo-Tethyan ocean was in the transition stage of ocean-continent subduction to continent-continent collision during Early-Middle Triassic.(4)Zircon U-Pb ages of dacite of Dongka Formation in Zeba area range from 238Ma to 236Ma,which were formed in Late Triassic.Gray green inclusions can be seen in the basalt-andesite of Dongka Formation in Wadui village.They are rich in Al,Na,K and medium in Mg content.The contents of Sr,Zr,Hf and other elements change greatly,corresponding to?_Hf(t)values ranging from-1.44 to+3.16.These inclusions are formed by assimilation and mixing of lithosphere mantle formed by subduction oceanic crust fluid metasomation and partially melted lower crust material.It indicates that the Jinshajiang Paleo-Tethyen ocean was in the transitional stage of continental collision compression to post-collision extension at the beginning of the Late Triassic.(5)According to the data of this paper,combined with the regional geological data and previous studies,it is considered that the Jinshajiang Paleo-Tethyan ocean entered the ocean continent subduction stage at the end of the Early Permian and continued to the Early Triassic(about 251Ma),forming continental margin arc rocks association represented by the basites in the Pushuiqiao Formation.In the Early-Middle Triassic,the rocks assemblage represented by Malasongduo Formation acid rocks proved that the northern segment of Jinshajiang Paleo-Tethys began land-land collision in the Early-Middle Triassic (250Ma?245Ma),and entered the conversion stage of ocean-land subduction to land-land collision,until the ocean crust subduction in the Middle Triassic(about 240Ma)was completely completed.At the beginning of the Late Triassic,the northern part of the Jinshajiang Paleo-Tethys entered the transformation stage from continental collision compression to post collision extension(240Ma?238Ma),forming the basalt andesite assemblage of the Dongka Formation.After that,it entered the post-collision extension stage with a duration of 10?15Ma,forming representative rock assemblages such as bimodal volcanic rocks(235Ma?225Ma)of Dongka Formation.Finally,the northern segment of the Paleo-Tethys of the Jinshajiang entered the late-collision orogenic stage,forming a continental molasse formation represented by the Jiapila Formation,which unconformably covered the Jiangda-Weixi continental margin arc,announcing the end of the tectonic evolution of the Paleo-Tethys and entering the intracontinental orogenic stage.