Abstract:

       Li contents and isotopic compositions of minerals in the peridotite xenoliths hosted by Cenozoic basalts from Penglai,
Shandong Province have been measured using ion probe. The results show that the Li abundance ranges from 2.0 to 10.4×10-6,2.8 to 23.8×10-6 and 3.3 to 59.7×10-6 in olivine (ol), orthopyroxene (opx) and clinopyroxene (cpx), respectively, and the δ7Li values varies from-28.6 ‰ to +0.5 ‰, -12.7 ‰ to +12.8‰ and -41.5 ‰ to +22.6 ‰ in ol, opx and cpx, respectively. Both the Li contents and isotopic ratios deviate strongly from the normal mantle range and the equilibrated trend. These extreme disequilibria imply that the strong Li enrichments were induced by strong mantle metasomatic events. The high Li and δ7Li enrichments of pyroxene reflected the nature of the recent mantle metasomatism that fed them, whereas the moderate Li enrichment and strong δ7Li depletion in the olivines may record a relatively ancient low-δ7Li signature. The reproduction of the measured profiles and the decoupling of Li enrichment from the LREE-depleted pattern in cpx suggest that both enrichment processes were relatively recent and transient. Accordingly, we conclude that the“ pristine” recycled components with anomalous Li composition existed in the upper mantle beneath the eastern North China Craton (NCC). The fractionation of Li isotopic compositions in the recycled components was linked to the subduction of the Pacific plate beneath East Asia since the Mesozoic, and the recycled components were reactivated recently to feed these anomalous Li enrichments as recorded by the Penglai peridotite xenoliths.

Key words: North China Craton, mantle peridotite, lithium isotopes, mantle metasomatism, recycled material