The Geological Characteristics Of Late Triassic Volcanism In Haibangou,West Qinling

The paper is based on the project of of I:50,000 Yongning Town(I48E013017),Liangdang County(I48E013018),Taiyangsi(I48E012018)geological surveys in Gansu Province.A new set of volcanic-pyroclastic rock bounded by faults was newly divided out from the originally planned Shujiaba Group in the Haibangou area of Liangdang County,Gansu Province,West QinlingAccording to Field research and rock mineralogy show that the newly-painted volcanic-pyroclastic rock combination is mainly composed of andesite tuff,dacite,rhyolite,and intermediate acid tuff,partially exposed acidic volcanic breccia,andesite and basaltic andesite.The volcano-sedimentation rhythm from bottom to top is:(1)Acidic volcanic breccia,Intermediate acid tuff with Iron ore body?(2)Dacite,Rhyolite with Andesite porphyrite and Iron ore body Limestone on top?(3)Interlayer between Inganite tuff and Andesite tuff with Basaltic andesite?(4)Andesite tuff,Anshan tuff sandstone,reflecting the characteristics of central volcanism.LA-1CP-MS zircon U-Pb dating results show that the rhyolite(18HG-1)206Pb/238U weighted average age is(204±2)Ma,and the dacite(18HG-2)206Pb/238U weighted average age is(209±3)Ma.This indicates that the newly-filled volcanic-pyroclastic rock series was formed in the Late Triassic.Based on rock assemblage and isotopic dating results,combined with regional geological comparisons,the volcanic-pyroclastic rock series was divided from the Shujiaba Group(D2-3S)and placed under the Huari Formation(T3hr).The study results reveal that the Late Triassic volcanic rocks in the Western Qinling region can extend from Xinghai County in Qinghai Province along Guide County,Tongren County,Hezuo city and Dangchang city to Liangdang County in Gansu Province,intermittent extension about 680 kilometers.Rock geochemical analysis shows that the Huari Formation volcanic rocks have the characteristics of calc-alkaline series.According to the comprehensive judgment,the Huari Formation volcanic rocks were formed in the intra-plate rift environment,and were reformed by the late right-slide sliding action.The iron deposits in the study area were formed in the mid-late volcanism,and the ore bodies mainly exist near the crater,which is obviously controlled by the volcanic factor.The iron ore body is produced in the two layers of intermediate-acid volcanic lava in the overflow phase and intermediate-acid volcanic breccia in the eruption phase,which is equivalent to the volcanic hydrothermal-deposited Longqishan-type and high-temperature gas-liquid metasomatous-filled Aoshan-type deposits in the metallogenic model of porphyry type iron deposits.According to 1:50,000 ground high-precision magnetic survey anomalies,1:50,000 abnormal measurement of water sediments,regional fault structure,volcanic rocks' distribution range of Huari Formation and known minerals,the study area is divided into three gold metallogenic prediction areas:Taiyangsi-huanglugou,Maatigou-Madigou,Niangniangmiao-Shuimogou,and two iron metallogenic prediction areas of Hongmiaozi-Angou,Pingliang-Haibangou.Based on the above results,it is preliminarily believed that the geological structure evolution is different between the East Qinling and West Qinling Mountains in the Indosinian period.The East Qinling Mountains mainly exhibited intro-continental orogeny.The West Qinling experienced strong intro-continental orogeny,Later occurred intraplate extension and intracontinental overburdening.The dynamic mechanism and geological significance of the geological structure evolution need further study in the Indosinian period between the eastern and western Qinling Mountains.