Genetic Types And Tectonic Significance Of Volcanic Rocks Of Sanhuashi Group On The Northwest Margin Of Yangtze Craton

The Proterozoic basement volcanic-sedimentary metamorphic rocks are exposed on the northwestern margin of the Yangtze craton.The study of metamorphic basement is helpful to understand the early continental crust growth and tectonic evolution history of the Yangtze Craton.Currently,the research on the Proterozoic volcanic-sedimentary metamorphic rocks is still relatively lacking.This paper systematically studies the field geology,petrology,geochemistry and zircon U-Pb geochronology,Hf and O isotopic compositions of the Sanhuashi Group volcanic rock strata exposed in the Hannan uplift area of the northwestern margin of the Yangtze craton,and explores the volcanic diagenesis age,genetic type and tectonic setting.The main understandings are as follows:?1?The volcanic rock assemblages of Sanhuashi Group are mainly andesite and dacite,with the characteristics of low TiO₂,high K₂O/Na₂O and Al₂O₃,belonging mainly to the calc-alkaline series.Trace elements are characterized by strong enrichment of light rare earth elements,large ion lithophile elements Ba,K and Rb,relative depletion of heavy rare earth elements,high field strength elements Nb,Ti and Ta,and weak Eu negative anomaly.The discriminant diagram analysis of the tectonic environment show that the volcanic rocks have typical island arc volcanic rock composition characteristics,indicating that the volcanic rocks were formed in the continental margin arc environment.?2?The zircon U-Pb dating results show that the diagenetic ages of the two dacite rocks are 1780.5±3.3 Ma and 1781.2±3.4 Ma,respectively,representing the diagenetic ages of volcanic rocks of the Sanhuashi Group,revealing that the formation time of the volcanic-sedimentary strata was early Mesoproterozoic.The metamorphic age of some zircon is 840 Ma,indicating that the volcanic-sedimentary rocks have undergone the transformation of the tectonic thermal events during the Neoproterozoic Rodinia supercontinent convergence period.?3?The in-situ Hf isotope composition of the dacite rock shows that the?_Hf?t?values are all negative,and the two-stage mode age is much higher than the zircon crystal age,suggesting that the source region is an ancient crustal material.The in-situ O isotopic composition of the zircons shows that the average value of?¹⁸O is 6.12,which is higher than that of typical mantle zircons,suggesting that the source region are mixed with crustal materials and have the property of mixing source region between subducted oceanic plates and continental marginal crust.?4?According to the comprehensive analysis,the andesite and dacite of Sanhuashi Group are not the products of crystallization differentiation of homologous magma.The andesitic magma may be formed in the partial melting of mantle wedge caused by plate dehydration during oceanic plate subduction,and it may be contaminated with crust during its emplacement process.The dacite magma may be formed in the partial melting of the ancient continental crust on the upper mantle wedge caused by the thermal action of the deep-sourced magma.?5?Previous studies have suggested that the Yangtze craton underwent a strong arc-land collision orogenic activity of 2.0-1.8 Ga,and¹.85 Ga,collision background changed to extensional system.This study shows that there are still arc-land collision events in the northwestern margin of the Yangtze craton in the 1.78 Ga,and the volcanic rocks of Sanhuashi Group are the products of the growth stage of the Columbia supercontinent.