Geochronology and geochemistry of Permian basalts in western Guangxi Province,Southwest China:Evidence for plume-lithosphere interaction

摘要

The Emeishan flood basalts are part of an important large igneous province(LIP)along the western margin of the Yangtze Block,Southwest China.Mafic rocks interlayered with Permian sedimentary rocks in western Guangxi Province,on the southeastern margin of the Yangtze Block,have SHRIMP zircon U-Pb weighted mean ages of 259.6±5.9 Ma and 259.1±4.0 Ma,identical to mafic plutons associated with the Emeishan basalts.The basaltic rocks,which are SiO2-poor,and FeO-,TiO2-and P2O5-rich,geochemically resemble the Emeishan high-Ti basalts.These rocks are enriched in large ion lithophile elements(LILE)and light rare earth elements(LREE),and have(Nb/La)n=0.75-0.94 and(Hf/Sm)n=0.78-0.90.They are characterized by moderate ~(86)Sr/~(87)Sr(i)ratios(0.704922-0.705804),intermediate epsilon_(Nd)(t)values(-0.23 to 1.50),high ~(206)Pb/~(204)Pbr ratios(18.77-19.67)and prominent positive DELTA 8/4(67.7-82.7)and DELTA 7/4(0.96-8.19)values.These geochemical signatures are also comparable to the Emeishan high-Ti basalts and the associated high-Ti intrusions in SW China,and might be attributed to low degrees of melting of a garnet-bearing mantle source.The elemental and isotopic compositions suggest that the magma source reservoir may have involved HIMU-and EM1-components,indicative of plume-lithosphere interaction at the periphery of the plume.We propose that these basalts in western Guangxi Province are spatially and temporally associated with the Emeishan LP and that the Emeishan flood basalts extend over a broader region than previously thought.