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

抄録

The Dahongshan kimberlite field in Hubei Province, China, is situated in the northern margin of the Proterozoic Yangtze Craton. The kimberlites erupted in the Paleozoic (ca. 326 Ma), some of which are associated with barren lamproites. An unusually fresh garnet lherzolite collected from this kimberlite field provides an opportunity to directly study the nature of the Paleozoic lithospheric mantle beneath the Yangtze Craton. The lherzolite is moderately depleted in major element compositions (2.66wt% Al₂O₃, 2.82wt% 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/Yb)_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 lherzolite: depletion followed by enrichment. Depletion process was derived from low degree of partial melting, as demonstrated by relatively high NiO contents (0.50%) in olivines and PPGE-enriched mantle-normalized PGE pattern of the whole rock. The enrichments in LREE and LILE are attributed to melt infiltration that must be an event prior to the kimberlite eruption as constrained by trace element and Sr-Nd isotopic data. The Rb-Sr and Sm-Nd data constrained an age of about 510 Ma, which is much older than the age of kimberlite eruption, but consistent with the eruption time of diamond-bearing lamproites on the centre of the Craton. The stable Gt-Opx-Cpx mineral assemblage gives a T-P estimation of 1042°C 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/m²), but still lower than the oceanic geotherm. Thus, our study and previous data demonstrate that the lithospheric mantle beneath the Yangtze craton was less depleted and was hotter in Paleozoic than the lithospheric mantle beneath typical old Cratons, i.e., the Kaapvaal, the Siberia, and the North China.