Geochemical Characteristics And Tectonic Evolution Study Of Volcanic Rocks In The Northern Section Of Jomda-Weixi Continental Margin Arc

The information of ocean subduction and arc-continent collision process is recorded by the magmatism developed in the Jinshajiang junction and its sides,which is the key to invert the Jinshajiang arc basin structure evolution.The Jomda-Deqin-Weixi continental arc is located between the Jinshajiang junction and Changdu-Lanping-Simao.There have been many studies on the continental arc,however,the heated debates about the genesis and age of the volcanic rocks in the middle-north of Jomda-Deqin-Weixi continental arc still exist.In this paper,the volcanic rocks were selected as the object of study in the middle-north of Jomda-Deqin-Weixi continental arc,the detailed field geology survey and indoor mineralogy and petrology research have been done.In addition,we founded middle carboniferous arc volcanic rocks in the northern of Jomda.The research of arc volcanic rock is significant to restrict the subduction process of the oceanic plate.We establish the space-time evolution framework of the Triassic volcanic rocks in the middle-north of Jomda-Deqin-Weixi continental arc,discuss the tectonic environment change of volcanic rocks,reveal the significance of volcanic rocks for oceanic crust subduction of jinshajiang by LA ICP-MS or SHRIMP zircon U-Pb dating,Hf isotopic dating,zircon dating,main and trace element analysis of the whole rock and Sr-Nd isotopic tracing.Based on the regional geological research results,this paper discusses the tectonic evolution pattern of the Jomda-Deqin-Weixi continental arc,and provides important information for the the evolution process of jinshajiang ocean basin.The paper has obtained the following main opinions:(1)The Pantiange group rhyolite zircon U-Pb age is 247±1.3 Ma in the Weixi area,the Cuiyibi group rhyolite zircon U-Pb age is 244±1.3 Ma,showing that Jomda-Weixi continental arc mainly developes in the early Triassic.(2)The Cuiyibi group bi-model volcanic geochemical characteristics: basalt is Calcium potassium alkaline series,it is enriched LREE,Ba?Th?U and other large ion lithophile elements are enriched,and depleted in the high field strength elements Nb,Ta,Ti,the geochemical characteristics are similar to the arc igneous rock;The main acidic volcanic rocks are rhyolite and dacite.They are enriched LREE,are enriched LILE,the high field strength elements Nb ? Ta are enriched.Sr,Ti have obvious negative anomaly characteristics.The Pantiangei group rhyolite:Rb,Ba and other large ionic lithophile elements are enriched,and losses Nb,Ta,Zr,Hf and other high field strength elements.Sr has obvious negative anomalies and Ti has negative anomalies.The strong positive Pb anomaly,the strong Sr,P and Ti negative anomaly,clearly show that the Pantiange group rhyolite has the properties of both board and arc.(3)The new late Middle Carboniferous island arc volcanic rocks are founded in the Jomda area,which are dominated by basalts and andesites.Zircon U-Pb age result shows they were formed at 332±3.3 Ma.They show obviously differentiation between LREE and HREE.They are enriched in strong incompatible trace elements,such as Th and U,and depleted in high field strength element,such as Nb and Ta.The zircons from andesite have positive ?Hf(t)values(+11.0~+15.6),which indicates that the magma source might be the enriched lithosphere mantle.This paper considers that late Middle Carboniferous volcanic rocks were formed at the island arc tectonic enviorment,which is related to subduction of Jinshajiang Tethys ocean.which provides direct volcanic evidence for the western subduction in the Middle Permian.(4)Jinshajiang ocean basin begans to subducte in the Middle Carboniferous,and began the arc-land collision in Early-Middle Triassic.The volcanic rocks were formed in the Carboniferous and Weixi areas in Jomda area respectively.