Geochemical And Chronological Characteristics Of Gejiu Jiasha Gabbro-Monzonite And Its Geological Significance

The Yanshanian magmatic rocks were widespread in Gejiu area,including the Jiasha gabbro and monzonite.Although numerous studies have been conducted on these medium-basic rocks,the magma source,petrogenesis and the geological significance of these rocks are still under controversial.This paper put focus on the Jiasha gabbro-monzonite in west section of Gejiu orefield using the whole-rock geochemistry,Sr-Nd-Pb isotopic geochemistry,and zircon U-Pb geochronology methods.The results and conclusions are as follows:(1)Zircon U-Pb results show that the Jiasha gabbro and monzonite have ages of 82.02?85.96 Ma,and belong to part of the large-scale magmatism in the Late Mesozoic in western South China.(2)The Jiasha gabbro and monzonite exhibit high-K and alkali-rich characteristics,and have similar trace and rare earth element contents with a right-leaning rare earth element distribution pattern and weakly negative Eu anomalies.In the diagram of primitive mantle-normalized diagram,these rocks enriched in Rb,K,Th,U,La,Pr,Nd,Sm,Gd,and depleted in high field strength elements(Nb,Ta,P,Zr,Ti)and Ba and Sr.(3)The Jiasha gabbro and monzonite have similar Sr-Nd-Pb isotopic results,the characteristics of Sr-Nd-Pb isotopes show that the Jiasha gabbro-monzonite was derived from partial melting of the enriched mantle,accompanied by a certain degree of crustal contamination during the magma intrusion.(4)The Jiasha gabbro and monzonite formed in the Yanshanian lithospheric extension tectonic environment.The extension and thinning of the lithosphere led to upwelling and underplating of the asthenosphere accompanied by partial melting of the enriched lithospheric mantle.During the ascend of magma,crystallization differentiation and a certain of crustal contamination occurred.