Geochemistry And Zircon U-Pb Geochronology Of Bijigou Bimodal Volcanic Rock In The Northwestern Edge Of The Yangtze Plate

This thesis carried out detailed field work for the Neoproterozoic magmatic rocks that outcrop in the northwestern margin of the Yangtze block and discovered a set of inter-bedded basic-felsic bimodal volcanic rocks in the Bijigou Fe-Ti-V deposit.Through comprehensive studies of geology,in situ zircon U-Pb age dating and Hf-O isotopes,whole-rock geochemistry on the volcanic sequence,this thesis draws achievements as follows:(1)The basic-felsic volcanic rocks occur as a 1200m-wide tectonic slice in the eastern part of the main orebody of the Bijigou Fe-Ti-V deposit in Sangxi Town,Yang County,Shanxi Province.It is in contact with the underlying gabbroic massif by fault and consisted with basalt-basaltic andesite and rhyolite,showing affinity of the typical bimodal volcanic rock.(2)Petrological and geochemical features reveal that Bijigou bimodal volcanic rocks can be divided into two eruption cycles:a basalt-dominated lower cycle and a rhyolite-dominated upper cycle.Each cycle is composed of basalt-basaltic andesite and rhyolite,showing the features of broad bimodal volcanic rock with minor magmas mixing.(3)The volcanic rocks from lower cycle have relatively low ?REE concentrations(73.25-243.6 ?g/g).The basalts show a nearly flat REE pattern with La/Yb of 1.2-2.8,while the rhyolites possess a distinguish difference with a right inclined REE pattern with La/Yb of 7.5-11.9 and Nb,Ta,Sr,and Ti enrichment in nearly-flat trace element spider diagram.The volcanic rocks from the upper cycle consist of basalts-andesite basalts and rhyolites.They show considerable high REE concentrations(?REE= 223.4-275.8 ?g/g)relative to rocks from the lower cycle.They display consistently right inclined REE patterns with La/Yb of 5.3-17.3 and Nb,Ta,Sr and Ti depletion in the trace element spider diagram(4)LA-ICP-MS U-Pb dating shows that a basalt(15BJ2-56)from the lower cycle gives a zircon age of 825.5±3.1 Ma,and a rhyolite sample(15BJ2-46)from the upper cycle yields a mean ages of 825.7 ±3.0 Ma,respectively.(5)Comprehensive researches reveal that rocks from the lower cycle of the Bijigou bimodal volcanic rocks are the alternative eruption products of basaltic melts from enriched mantle and felsic magma from partial melting of lower crust.Rocks of the upper cycle are the products of intermittent eruption from the mama chamber where the mantle-derived basic magma and crust-remelted felsic magma underwent extensively magma mixing.The volcanic sequence are formed in the initial stage of Rodinia breakup,accompanied by the mantle plume-induced asthenospheric upwelling and the uplift and extension of lithosphere.