Geochemical Characteristics And Petrogensis Of The Zhashui Pluton In The South Qinling Block

Numerous Triassic granitoid plutons exposed in the South Qinling block,as a key carrier for the final collision of the Qinling orogenic belt,might effectively make up for the lack of records of Early Mesozoic ultrahigh pressure metamorphism and hence can be an important object for research of the formation process of the Qinling orogenic belt.Based on detailed field investigation and petrology,in this study,the Zhashui pluton has been chosen as the research object and zircon isotopic geochronology,lithological and isotopic geochemistry were taken for the purpose on petrogensis of this pluton.In order to compare and summarize magma sources and tectonic setting,previously reported results have been also adequately collected.Field investigations indicate that the predominant rock type of the Zhashui pluton is monzonitic granite,with minor syenogranite.Zircon U-Pb isotopic dating shows that the monzonitic granite rocks were emplaced in 214～208 Ma,while the syenogranite rocks were emplaced in 220～217 Ma.Both of them have characteristics of metaluminous to weak peraluminous and geochemical signatures of adakitic rock and belong to the high-K calc-alkaline series.They have similar characteristics in trace and rare earth elemental compositions,such as enrichment in large-ion lithophile elements and depletion in high-field-strength elements.Sr-Nd-Hf isotopic compositions of Monzonitic granite and syenogranite rocks and their magma sources were mainly composed of Neoproterozoic and Early Paleozoic juvenile crusts existing in South Qinling.Prominent contribution of detrital material caused a phenomenon of decoupling in Nd-Hf isotopic compositions.The melt temperatures of monzonitic granite and syenogranite rocks of the Zhashui pluton are placed at 710～754℃ and 755～777℃,respectively,but the magmas have similar temperatures and pressures during their emplacement.When summarizing information of source material and tectonic setting of the Triassic granitoid plutons in South Qinling,processes relating to subduction of the Mianlue Ocean and subsequent continental subduction,mixture of crust and mantle triggered by asthenosphere upwelling,and melting of the crust,can be more effectively recognized.