Geochemistry of Late-Cenozoic basalts from Leigiong area: The origin of EM2 and the contribution from sub-continental lithosphere mantle

The major-, trace-element and Sr-Nd-Pb isotopic ratios of 21 Late-Cenozoic basalts from Leiqiong area were determined by wet chemical method, ICP-MS and MC-IPCMS, respetively. Normative mineral calculation suggested that Leiqiong basalts were dominated by quartz tholeiites with subsidiary olivine tholeiites and alkaline basalts. The trace elements and isotopic composition of these basalts resembled OIBs. In addition, the incompatible elements abundance increased with increasing SiO2-undersaturation. The chemical and isotopic compositions of most basalts might be inherited from their mantle source except that sample R4-1 showed characteristics of crustal contamination. The positive correlation between Sr-87/Sr-86 and Pb-206/Pb-204 as well as negative correlation between Nd-143/Nd-144 and Pb-206/Pb-204 implied a mixing of a depleted asthenospheric mantle and an EM2 composition ( existing in the sub-continental lithosphere mantle beneath this region). The even more depleted Sr and Nd isotope of mantle xenoliths from Leiqiong than their host basalts indicated that the spinel peridotite mantle represented by the xenoliths could never produce these basaltic rocks. The La/Yb-Sm/Yb system of Leiqiong basalts required that the melting occurred in the garnet peridotite and in turn implied the EM2 composition should be derived from the lower lithosphere mantle ( garnet phase stable field). The geochemical diversity of Leiqiong basalts could be interpreted as follows; The low degree of melt (7% similar to 9%) from upwelling asthenospheric mantle and garnet peridotite mantle produced the alkaline basalts and olivine tholeiites while relatively higher degree of melt (10% similar to 20%) from asthenospheric mantle and a higher proportion of garnet peridotite mantle made the quartz tholeiites.