Geochemistry of Neoproterozoic Mafic-Ultramafic Rocks in Zaibian,SE Guizhou Province,China

The NeoProterozoic mafic-ultramafic rocks have been found in Zaibian area by geologic mapping.The mafic-ultramafic rocks are belonging to calc-alkaline;the minerals compositions are mainly feldspar,pyroxene and chlorite of the mafic-ultramafic rocks.The major elements have the characteristics of high contents of Na2O(5.04%-6.64%),CaO(0.76%-6.67%) and MgO(6.79%-7.88%),low contents of TiO2(0.30%~ 0.4%),and poor contents of K2O(0.10%-0.29%) and P2O5(0.04%-0.05%).The ΣREE is relatively high with 83.37×10-6-164.97 ×10-6,and the variety values of LREE/HREE and(La/Yb)N are relatively large,indicating that the fractionation between light rare earth elements and heavy rare earth elements is existent.However,the(La/Sm)N and(Gd/Yb)N are relatively less,with 1.81-3.19 and 1.22-1.53,respectively,reflecting that the fractionation of light rare earth elements and heavy rare earth elements is feebly.The Zaibian mafic-ultramafic rocks are LREE enriched,and have no evident Eu,with Ce abnormity,but have evident Nb,and anomaly negative Ta.The trace elements,Y/Nb,and Zr/Nb imply that the mafic-ultramafic rocks maybe originated from the Transition-Enriched-Mantle.The results indicate that the mafic-ultramafic rocks maybe formed in the inner-plate at about 800 Ma,and maybe related with the mantle plume which makes the Rodinia breakup.