The Geochemical Difference Of Mantle-derived Dykes:Defining The Western Boundary Of Yangtze Block And Cathaysia Block

Mantle-derived magmatic rock is an effective petrologic probe to reveal the compositions of deep continental lithosphere.Nine mafic to intermediate dyke samples in the Guangxi Province and western Hunan Province have been studied in this study.Zircon U-Pb dating results indicate that these dykes intruded at four stages of-260 Ma,175-150Ma,128-88Ma and 33-36 Ma.Petrologic,geochemistry and Nd-isotope compositions indicate that these mafic-intermediate dykes can be divided into the high REE and low ?REE groups.High ?REE samples,collected from the Pingle,Mashan and Luocheng,are characterized by abundant biotite/phlogopite phenocrysts,high E REE contents(E REE>400ppm)and significant Eu,Nb-Ta and Ti anomalies.This group samples also have higher Th/Ta,Th/Tb and Rb/Sr ratios,and low Nb/La,Ce/Pb and Ti/Y ratios than the low REE group,and enriched in Nd-isotope compositions with ?Nd(t)<-4.66.The low REE group samples,collected from the Pingnan,Guigang and Jiangyong regions,are characterized by the present of pyroxene or/and olivine phenocrysts,low ?REE contents(<255ppm),and little Eu,Nb-Ta and Ti anomalies.They have high Nd-isotope compositions with ?Nd(t)>-1.9.These characteristics suggest that the primitive magma of the high ? REE dykes originated from strongly enriched mantle that experienced the metasomatism of crustal-derived fluids or melts.Whereas the parental magma of the low ? REE dykes derived from weakly enriched mantle that suffered the metasomatism of mantle-derived melts.Comparisons with available geochemical data of mafic rocks in South China suggest that the high E REE dykes,collected from the northern Guangxi Province,show similar geochemical features to those in the southwestern Yangtze Block,whereas the low ?REE dykes in the central and southern Guangxi and western Hunan provinces exhibit geochemical characteristics similar to those in the western Cathaysia Block.Therefore,the NE-trending boundary between the two areas with different mantle compositions probably represents the southwestern extending of the suture between the Yangtze and Cathaysia blocks.In addition,two diabase samples in Pingxiang and Dahua are also studied in this study.They show the age,geochemistry and the Nd isotopics similar to the basalts in the Emeishan large igneous province.It suggests that these diabases are part of the Emeishan mantle plume,and consequently expanding the outer band of the Emeishan mantle plume to the whole west of Guangxi province.