Geochemical Characteristics Of Mantle Xenoliths From Changle:Complex Mantle Metasomatism Of Cenozoic Lithospheric Mantle In Eastern North China Craton

Peridotite-melt interaction is an important way of the compositional transformation of the mantle,and thus it is also a key point for the destruction of lithospheric mantle in the eastern block of North China Craton?NCC?.We focused on a suit of mantle xenoliths,including lherzolite,wehrlite,pyroxenite and clinopyroxene?Cpx?,entrained in the Cenozoic basalts from the eastern block of the NCC.Based on the in-situ characteristics of major and trace elements of olivine?Ol?,orthopyroxene?Opx?,Cpx and spinel?Sp?,and Sr isotopic compositions of Cpx and carbonate inclusion in Ol,suggesting that the lithospheric mantle of the eastern block of NCC have experienced multi-episode complex mantle metasomatism.The following several conclusions were made in this study:1.Multi-episode silicate metasomatism and the origins of silicic meltsAccording to the petrological and in-situ major and trace element geochemical of Cpx,lherzolits were classified into two types: the Cpx in the type-1 lherzolites have higher Mg#?> 92?,while the Cpx in the type-2 lherzolites have lower Mg#?<92?.The Cpx cores in the type-2 lherzolites show different extent of enrichments in large ion lithophile elements?LILE?and light rare earth elements?LREE?but depleted in high field-strength elements?HFSE?,and also show a positive correlation between?La/Yb?N and Ti/Eu ratios and plot in the area of silicate metasomatism.These features have reflected the silicate metasomatism recording by the Cpx cores in the type-2 lherzolites at first stage.In addition,cores of these Cpx have relatively high 87Sr/86 Sr ratios?0.7033-0.7035?as same as the carbonate inclusions.This reflects that the silicic melt not only enrichments in carbonate substance but also deriving from the recycled oceanic crust.What's more,Cpx of the pyroxene in Changle show slightly enrichments in LILE and LREE and high Ti/Eu ratios and relatively high 87Sr/86 Sr ratios?0.7033-0.7037?similar to the Cpx cores in the type-2 lherzolites,indicating that this pyroxene could be the product of the reaction of the CO₂-rich silicic melt with the type-2 lherzolites at first stage.However,compare with the Cpx core in the type-2 lherzolites and pyroxenes,the Cpx xenocryst in the Changle basalts have obviously the lower content of trace elements and weak positive Eu anomalies and indistinctively negative Pb anomalies.It also shows slightly enrichments in LILE,LREE and high Ti/Eu ratios?3394-4705?,indicating that another silicate metasomatism was recorded by this Cpx xenocryst.In addition,the Cpx shows slightly high 87Sr/86 Sr ratios?0.7035-0.7037?and combine with the character of weak positive Eu anomalies and indistinctively negative Pb anomalies imply that the silicic melt may be relative with the partial melting of gabbroic lower oceanic crust.Therefore,multi-episode reactions of peridotites with melts from different sources lead not only to these peridotites enrichments in trace elements but also to the transformation from lherzolites to pyroxene.2.Multi-episode carbonatite metasomatism and the origins of carbonatitic meltsCompare with the Cpx cores of the type-2 lherzolites,the Cpx rims show the high content of LREE,Sr and Cu.From the Cpx cores to the rims,they plot the trend of carbonatite metasomatism in the Ti/Eu-?La/Yb?N diagram.These characters indicate that the type-2 lherzolites were modified by a small amount of carbonaitic melts at the late stage.In addition,low-87Sr/86 Sr ratios?⁰.7024?of Cpx rims in the type-2 lherzolites suggest the carbonatitic melt could be derived from partial melting of the carbonated mantle.Cpx in the type-1 lherzolites are strong enriched in LILE and LREE but depleted in HFSE and heavy rare earth elements?HREE?,and show high Ca/Al,Zr/Hf and?La/Yb?N ratios but low Ti/Eu ratios.These features indicate that these Cpx were crystallized from a carbonatitic melt.Compare with the Cpx rims in the type-2 lherzolites,these Cpx show slightly high 87Sr/86 Sr ratios?⁰.7030?and significantly high?La/Yb?N ratios?16.4-40.5?,indicating that two episode of carbonatite metasomatism were recorded by two types of lherzolites.Such carbonatitic melt with such slightly high Sr isotopic composition may be originated from the reaction of CO₂-rich silicic melt with peridotites due to consumption of silicic component at first stage.Furthermore,a lot of second-generation Cpx,Ol and apatite and carbonate inclusions were found in wehrlite of this study.These features reflect the wehrlite could be originated from the reaction of the carbonatitic melt with peridotites.Furthermore,the Cpx in the wehrlite also have strong enrichments in LILE,LREE but depleted in HFSE and HREE and the slightly high 87Sr/86 Sr ratios? 0.7030?as same as the Cpx in the type-1 lherzolite,indicating that this wehrlite could be the product of the reaction between the type-1 lherzolites and the evolutive carbonatitic melts from the CO₂-rich silicate melt.In summary,carbonatitic melts could be formed by reaction of CO₂-rich silicic melt cause to consumption of silicic component as well as the partial melting of carbonated mantle.In addition,the reaction of the carbonatitic melt with mantle also leads to the mantle enrichments in trace elements,and to formed wehrlite due to the dissolution of Opx and precipitation of Ol and Cpx.3.Multi-episode complex mantle metasomatism impact on the re-enrichment of lithospheric mantle in the eastern block of the NCC.Mantle xenoliths of this study record of complex mantle metasomatism.The reaction of CO₂-rich silicic melt derived from the recycled oceanic crust with the mantle will ultimately transform into the carbonatitic melt with enriched incompatible elements due to the consumption of silicic component.However,the reaction of peridotites with melts with different properties results in different mineral compositional changes.The pyroxene could be formed by the reaction of the CO₂-rich silicic melt with lherzolites at first stage,and the lherzolite will be ultimately transformed into wehrlite due to the reaction of such evolution carbonatitic melt with mantle peridotites at the later stage.In addition,these complex mantle metasomatisms lead the chemical composition of the mantle beneath the eastern block of the NCC d changing from depleted and refractory to enriched and fertile cause of in different extent of enrichments in trace elements.