Thermal and petrological structure of the lithosphere beneath Hannuoba, Sino-Korean Craton, China: evidence from xenoliths

Deep-seated xenoliths entrained in the Hannuoba basalts of the northern Sin o-Korean Craton include mafic and felsic granulites, mantle wall-rock from spinel- and garnet-spinel peridotite facies, and basaltic crystallisation p roducts from the spinel-pyroxenite and garnet-pyroxenite stability fields. The mineral compositions of the xenoliths have been used to estimate temper atures and, when possible, pressures of equilibration. and to construct a g eothermal framework to interpret the upper mantle and lower crustal rock-ty pe sequences for the region. The xenolith-derived paleogeotherm is constrai ned in the depth interval of 45-65 km and like others from areas of young b asalt magmatism. is elevated and strongly convex toward the temperature axi s. Two-pyroxene granulites give the lowest temperatures and garnet pyroxeni tes the highest. while the spinel lherzolites fall between these two groups . The present-day Moho beneath the Hannuoba area is defined at 42 km by sei smic data, and coincides with the deepest occurrence of granulite. Above th is boundary, there is a lower crust-upper mantle transition zone about 10-k m thick, in which spinel lherzolites and mafic granulites (with variable pl agioclase contents) are intermixed. It is inferred that this underplating h as resulted in a lowering of the original pre-Cenozoic Moho (then coincidin g with the crust-mantle boundary, CMB) from about 30 km to its present-day position and was due to intrusions of basaltic magmas that displaced perido tite mantle wall-rock and equilibrated to mafic granulites. Trace element p atterns of the diopsides (analysed by laser ablation-ICPMS) from the Cr-dio pside series spinel lherzolites and associated layered xenoliths (spinel lh erzolites and pyroxenites) indicate 3 fertile uppermost mantle with moderat e depletion by low degrees of partial melting and little evidence of metaso matic activity. The similarity in major and trace element composition?; of the minerals in both rock types suggests that the layered ultramafic xenoli ths formed by mantle deformation processes (metamorphic segregation), rathe r than by melt veining or metasomatism. (C) 2001 Elsevier Science B.V. All rights reserved.