ZIRCON U-Pb GEOCHRONOLOGY,WHOLE ROCK GEOCHEMISTRY AND GEOLOGICAL IMPLICATION OF THE NEOPROTEROZOIC LONGTANHE FORMATION TUFFS IN THE CHENGKOU AREA,NORTHERN MARGIN OF THE YANGTZE PLATE
The volcano-sedimentary sequence of the Neoproterozoic Longtanhe Formation around the Chengkou-Fangxian fault on the northern margin of the Yangtze Plate is considered to be the product of the convergence and fragmentation of the Neoproterozoic Rodinia supercontinent.This formation records sedimentary,magmatic,and tectonic events in this area.Therefore,studying Longtanhe Formation tuffs can shed light on the tectonic evolution of the northern margin of the Yangtze Plate during the Neoprotero-zoic.In this paper,zircon SHRIMP U-Pb geochronology,whole rock geochemistry are used to study the Longtanhe Formation tuffs in the Chengkou area.The analytical results show that the zircons in the tuffs have a typical ring structure,with Th/U ratios greater than 0.4,indicating magmatic origins.The weigh-ted average zircon U-Pb ages obtained are 871.0±25.0 Ma,819.0±8.1 Ma,813.0±11.0 Ma,and 798.0±15.0 Ma,suggesting that the Longtanhe Formation tuffs in the Chengkou area were volcanic products of magmatic activity in the Middle to Late Qingbaikou Period.Geochemical analysis of the tuffs shows that the rocks are enriched in light rare earth elements,with a flat distribution of heavy rare earth elements and strong differentiation between light and heavy rare earth elements.The spider diagram of standardized trace elements in the primitive mantle shows obvious enrichment of large ion lithophile elements(LILEs)such as Rb,Ba,Th,U,and K,and strong depletion of high field strength elements(HFSEs)such as Nb,Ta,and Ti,consistent with the geochemical characteristics of arc magmas.The Zr/TiO2 and Nb/Y diagrams show that the rock type is consistent with an active continental margin association,and the Th/Tb and Ta/Yb diagrams also show strong continental affinity,reflecting that the Longtanhe Formation tuffs may have formed in a subduction-related magmatic arc setting.Comprehensive analysis of existing geological,geochemical,and chronological data suggests that the northern margin of the Yangtze Plate was still undergoing subduction from 871 to 798 Ma in the context of the Neoproterozoic breakup of the Rodinia supercontinent.However,the impact of the Rodinia breakup on the tectonic activity of the north-ern margin of the Yangtze Plate was relatively weak at that time.
Neoproterozoicnorthern margin of Yangtze PlateLongtanhe Formationtuffsubductionconstructive significance
万方数据
维普
