PETROLOGY AND GEOCHEMISTRY OF MANTLE PERIDOTITE XENOLITHS FROM SE CHINA

Geochemical data on Type I spinel peridotite and garnet peridotite xenoliths in Cenozoic basalts from SE China demonstrate that the lithospheric mantle under this region is heterogeneous. The depletion and enrichment shown by these peridotite xenoliths are not related to their locations as suggested earlier. Samples from individual localities, at the continental margin or the continental interior, show large variational ranges from depleted harzburgite to fertile lherzolite. The measured Nd and Sr isotopic compositions of clinopyroxene separates range from epsilon(Nd) 4.9 to 16.0 and from Sr-87/Sr-86 0.70256 to 0.70407, respectively. The depleted signatures of Sr and Nd isotopic compositions and major-element contents (low CaO and Al2O3) in most xenoliths require an old depletion event, probably mid-Proterozoic, and the enrichment of LREE in the depleted peridotites implies a young metasomatic event shortly before Cenozoic magmatism. Major-element compositions of the peridotite xenoliths are controlled largely by the degree of partial melting, and the extra-fertile peridotites (high CaO and Al2O3) are probably the products of interaction between peridotites and a basaltic component. The equilibrium P-T conditions, determined from coexisting mineral phases, indicate that these xenoliths equilibrated over a wide P-T range, from 770 to 1250 degrees C and from 10 to 27 kbar. Calculated oxgen fugacities for most spinel peridotites range from near the FMQ buffer to similar to 2.5 log units below. The late-stage metasomatism did not change the redox state in the upper mantle.