Transformation Of The Lithospheric Mantle Through Peridotite-melt Interaction

The North China Craton(NCC)was suffered markedly modification and destruction since the Phanesozoic,therefore,it is an ideal place to study the evolution of lithospheric mantle.The ancient typical cratonic(refractory,cold and thick)lithospheric mantle beneath the eastern part of the NCC has been transformed to present-day "oceanic"-like(fertile,hot and thin)lithospheric mantle.Mantle-derived xenoliths carried to the surface by basalts and kimberlites provide direct samples to study the composition,texture and evolution of the lithospheric mantle.Thus,this paper focuses on the mantle peridotite xenoliths and garnet pyroxenite xenoliths respectively hosted by the Cenozoic basalts from the Weichang and Hannuoba regions in the northern NCC,and reports a detailed petrological study and an in-situ major and trace element analysis in constituent minerals,as well as an especial analysis for in-situ Sr isotope ratios in clinopyroxenes,in order to deciphere the multiple-stage modification of the lithospheric mantle beneath the northern NCC.And it is the first time to identify an important role of the asthenosphere-lithosphere interaction played in the evolution of lithospheric mantle in terms of the in-situ Sr isotopic composition of clinopyroxenes.The Weichang peridotite xenoliths are composed of spinel-facies lherzolite and harzburgite,and can be subdivided into two groups by their petrology and mineral chemistry.Group I peridotites display porphyroclastic texture,and their cores of olivine have slightly higher forsterite contents(Fo)than the rims(90.6-91.2 vs.89.8-90.8).Clinopyroxenes exhibit uniform LREE-depleted patterns and their cores have slightly higher 87Sr/86Sr ratios than the rims(0.7025-0.7043 vs.0.7025-0.7038).These geochemical features suggest that the Group I peridotites were weakly affected by the recent asthenospheric melts.In contrast,Group II peridotites show resorption texture,partial orthopyroxenes are surrounded by melt pockets,which are mineral aggragetes of fine-grained olivine,clinopyroxene and orthopyroxene.Their cores of olivine have higher forsterite contents(Fo)than the rims(86.9-91.3 vs.76.9-90.6),and their cores of clinopyroxene have higher 87Sr/86Sr ratios than the rims(0.7035-0.7049 vs.0.7020-0.7046).The clinopyroxenes are characterized by enrichments in LREE,though they have different REE patterns(LREE-enriched,convex-upward and spoon-shaped).The LREE-enriched clinopyroxenes have the highest(La/Yb)N and lowest Ti/Eu and fall into the zone of carbonatite metasomatism;Spoon-shaped REE pattern clinopyroxenes show an increase in LREE,Ba,Th and U contents from the cores to the rims,and partial cores have LREE-depleted patterns.These features indicate that the Group II peridotites witnessed obvious metasomatism by recent asthenosphere-derived silicate and carbonatite melts or their mixture.Because LREE-depleted(i.e.(La/Yb)N<1)clinopyroxenes were slightly affected by the recent metasomatism,they can represent the nature of the lithospheric mantle before the recent metasomatism.Compared with the Mesozoic NCC lithospheric mantle,the heterogeneous and lower 87Sr/86Sr ratios(0.7025-0.7049)of the LREE-depleted clinopyroxenes reveal that the ancient lithospheric mantle could have been modified by asthenospheric melts before the recent metasomatic event.Thus,author concludes that the Cenozoic lithospheric mantle beneath the Weichang region underwent multiple-stage modification through asthenosphere-lithosphere interaction,i.e.lithospheric mantle refertilization.The Hannuoba garnet pyroxenite xenoliths are of garnet websterite with more or less olivine,and show coarse-grained texture.Some garnets have anhedral spinel inclusions,and their rims as well as the cracks of the garnet were hybridized.The rims of olivines have lower Fo and NiO contents,but higher CaO and MnO contents than the cores.Their compositional features are similar to those of olivines from the peridotite xenoliths.Orthopyroxenes,clinopyroxenes and garnets have higher Mg#than their counterparts in the pyroxenite cumulate,but their compositions are silimar to those from reactive garnet pyroxenites.Thus,the Hannuoba garnet pyroxenite could be formed by the reaction between the lherzolite and Al-rich silicic melt.During the reaction,pyroxenes and garnets could have grown at the expense of olivines.Clinopyroxenes display convex-upward REE partterns,suggesting equilibrium with the reacted melt.Fresh zones in the garnets show strongly LREE-depleted partterns.The compositions of the reactive melt calculated by the partition coefficient between the clinopyroxene and melt exhibit high Ce/Pb?Ti/Y?Nb/La,but low Sr/Y ratios,which is consistent with the silicic melt derived from the partial melting of the subducted slab after the dehydration.Combined with the previous research,author conclude that the Hannuoba garnet pyroxenites could be formed through the reaction of the lherzolite with silicic melt derived from subducted Paleo-Asian oceanic slab,which plays an important role in the transformation of the lithospheric mantle beneath the Hannuoba zone.Intergrated with the previous research on the Mesozoic and Cenozoic mantle-derived volcanism and intrusions as well as their-borne mantle xenoliths from the NCC,it suggests that the lithospheric mantle beneath the NCC mainly undergone two stage refertilization:the first stage is suffered from a reaction between the peridotite and crust-derived melt/fluid prior to early Mesozoic,which lead to an enrichment of Sr-Nd isotopic composition in lithospheric mantle;The second stage is involved in a reaction between the peridotite and asthenosphere-derived melt/fluid during the late Mesozoic-Cenozoic,which lead to the depletion of Sr-Nd isotopic composition.The in-situ Sr isotopic compositions of clinopyroxenes from the Weichang peridotite xenoliths in this study further certify the presence of the mantle refertilization resulted from the infiltration of asthenospheric melts into the lithospheric mantle.Peridotite reaction with melts/fluids of different origins changed the nature of the lithospheric mantle beneath the NCC.Therefore,peridotite-melts/fluids reactions could be an interior mechanism for the transformation of the lithospheric mantle beneath the NCC.