Geochemical significance of a garnet lherzolite from the Dahongshan kimberlite, Yangtze Craton, southern China

The Dahongshan kimberlite field in Hubei Province, China, is situated in th e northern margin of the Proterozoic Yangtze Craton. The kimberiites erupte d in the Paleozoic (ca. 326 Ma), some of which are associated with barren l amproites. An unusually fresh garnet lherzolite collected from this kimberl ite field provides an opportunity to directly study the nature of the Paleo zoic lithospheric mantle beneath the Yangtze Craton. The lherzolite is mode rately depleted in major element compositions (2.66wt% Al2O3, 2.82wt%n CaO, and 90.9 Mg#), with a forsterite content in olivine around 90.5%. The rare earth element characteristics of the whole rock [(La/Sm)(N) = 13 and (Eu/Y b)(N) = 0.48] and of the constituent minerals such as clinopyroxene [(La/Yb )(N) = 61] reveal that at least two processes were recorded in the lherzoli te: depletion followed by enrichment. Depletion process was derived from lo w degree of partial melting, as demonstrated by relatively high NiO content s (0.50%) in olivines and PPGE-enriched mantle-normalized PGE pattern of th e whole rock. The enrichments in LREE and LILE are attributed to melt infil tration that must be an event prior to the kimberlite eruption as constrain ed by trace element and Sr-Nd isotopic data. The Rb-Sr and Sm-Nd data const rained an age of about 510 Ma, which is much older than the age of kimberli te eruption, but consistent with the eruption time of diamond-bearing lampr oites on the centre of the Craton. The stable Gt-Opx-Cpx mineral assemblage gives a T-P estimation of 1042 deg reesC and 34 kbar, corresponding to 110 km depth, well within the graphite stability field. The geotherm inferred from this lherzolite was much higher than the typical cratonic geotherm (40 mW/m(2)), but still lower than the oceanic geotherm. Thus, our study and previous data demonstrate that the li thospheric mantle beneath the Yangtze craton was less depleted and was hott er in Paleozoic than the lithospheric mantle beneath typical old Cratons, i .e., the Kaapvaal, the Siberia, and the North China.