TEXTURE-TEMPERATURE-GEOCHEMISTRY RELATIONSHIPS IN THE UPPER-MANTLE ASREVEALED FROM SPINEL PERIDOTITE XENOLITHS FROM WANGQING, NE CHINA

Spinel peridotite xenoliths from Wangqing, NE China, exhibit correlate d variations in texture, temperature, geochemistry and radiogenic isot opes. Protogranular and transitional peridotites are less refractory t han equigranular samples. which are predominantly hazburgites and clin opyroxene-poor lterzolites. Deformed harzburgites and lherzolites are variably enriched in light rare earth elements (LREE), and Ce/Yb-MgO v ariations indicate that these olivine-rich rocks have been preferentia lly infiltrated by metasomatic agents. Clinopyroxenes from the fertile protogranular and transitional lherzolites have high Nd-143/Nd-144 (e psilon(Nd) = 10-18) and low Sr-87/ Sr-86 (0.7020-0.7036) consistent wi th long-term time-integrated depletions (>1.34 Gy) By contrast, clinop yroxenes in the majority of the equigranular samples have higher Sr-87 /Sr-86 (>0.7035) and less radiogenic Nd-143/Nd-144 (epsilon(Nd) <4.5) values. Mineral chemistry shows intra-grain and inter-grain heterogene ity, and the degree of heterogeneity decreases from protogranular to e quigranular peridotites. Thermometric calculation further reveals a co oling event from >1200 degrees C to 835-930 degrees C at which all sam ples last equilibrated. Integration of these data suggests that the Wa ngqing xenoliths may represent fragments of the lithosphere that has b een isolated from the asthenosphere for a long time (>1.34 Gy). This a ged lithosphere was locally altered by asthenosphere-derived fluids, w hich in turn enhanced grain boundary migration and recrystallization, giving rise to coupled textural and geochemical variations. During the thermal erasion of the lithosphere that took place since the late Mes ozoic, the base of lithosphere represented by the precursor rocks of t he Wangqing peridotites became rheologically similar to the thermal bo undary layer such that it was able to intrude with the asthenosphere d iapirically the rigid uppermost lithospheric mantle at a time just bef ore tile host basalt eruption.