Crust-mantle Interaction In The Lithospheric Mantle Beneath Northeastern China Constrained By In-situ Analyses

Crust-mantle interaction is an important process of materials exchange between crust and mantle.And the recycling of crustal materials into the mantle will significantly affect the physical and chemical properties of the mantle,including the petrographic,geochemical,and rheological characteristics of the mantle and the stability of the lithospheric mantle structure.The crustal materials can enter the mantle by subduction or delamination,and stabilize as the form of solid phase or melt/fluids due to the partial melting,and further react with mantle rocks to achieve the deep cycle of the crust materials.The compositions of mantle rock are controlled by partial melting and mantle metasomatism.Peridotites underwent partial melting are generally depleted in basaltic elements relative to the fertile asthenospheric mantle,while the study of mantle xenoliths reflect that the lithospheric mantle has complex compositions.The influence of mantle metasomatism on the mantle rocks is controlled by the properties and sources of the metasomatic agents,especially the melt/fluid derived from the crustal materials due to the complex types of sediments and different partial melting conditions.The northeastern China is located at a multi-plate subduction section,which has been affected by the subduction of the Paleo-Asian Ocean to the north since Paleozoic and the subduction of the Pacific Ocean to the east since Mesozoic.Previous studies have shown that the lithospheric mantle beneath the northeastern China records the multiple effects of the above subduction plate and upwelling of the asthenosphere,but the specific impact of the subducted plates,especially the impact of the subducted Paleo-Asian Ocean on the lithospheric mantle in different positions from the north to the south in the northeastern China is not clear yet.In-situ analytic technology has great advantages in tracing the multi-stage melt/fluid activities recorded by mantle rocks.It can reveal the melt/fluid activities of different stages and sources through the element and isotopic compositional zonation.This work reveals that the lithospheric mantle beneath the northeastern China has undergone multi-stage and multi-source of melt/fluid metasomatism through detailed petrological and in-situ major and trace elements and Sr isotopic composition of the mantle xenoliths captured by the Cenozoic basalts in Huinan,Yitong,and Keluo.In addition,the source and properties of metasomatic agents and their influence on the composition and oxygen fugacity of the lithospheric mantle are also limited.And we get the following conclusions:（1）The lithospheric mantle beneath the northeastern China is heterogeneous.Olivines in the ultramafic xenoliths captured by the Cenozoic basalts in Huinan,Yitong,and Keluo show chemical compositions different from those of igneous olivines.Except that some xenoliths in Yitong show low Mg#and Ni content,olivines in other xenoliths have high Mg#and Ni content,but all of these olivines are featured by low Ca O,Al₂O₃,and Ti O₂contents,which is consistent with the chemical composition of mantle-derived olivine,indicating that these ultramafic xenoliths are derived from the lithospheric mantle.There is no garnet or garnet-related mineral assemblages and structures are not found in mantle xenoliths in Huinan,Yitong,and Keluo,and spinel can be observed in most samples,indicating that these mantle xenoliths come from spinel lithospheric mantle at a relatively shallow depth.The mantle xenoliths in Huinan are mainly composed of harzburgite,lherzolite,and contain a small amount of dunite,pyroxenite,and wehrlite.No newly-formed metasomatic mineral is obtained.The spongy texture is common in the wehrlite,and consists of clinopyroxene and olivine,without melt glass.The proportion of harzburgite in the Yitong xenolith is lower than those of the Huinan xenolith.The mantle xenoliths in Yitong are mainly composed of lherzolite and wehrlite and contain a small amount of harzburgite and pyroxenite.The spongy texture can be obtained in half of the Yitong mantle xenoliths,and consists of clinopyroxene,olivine,apatite,Fe-Ti oxidize,and glass.The composition of glass has a trend of evolution to form feldspar.The mantle xenoliths in Keluo are mainly composed of lherzolite and pyroxenite and contain a small amount of harzburgite.Phlogopite are present.No spongy texture can be obtained.As a whole,the differences in petrological features of these mantle xenoliths from the south to the north of northeastern China reflect that the subcontinental lithospheric mantle is heterogeneous.（2）Multi-stage and multi-sources of melt/fluid metasomatism recorded by lithospheric mantle beneath northeastern China.In addition to spatial heterogeneity in petrological features,in-situ microanalysis of mantle xenoliths in Huinan,Yitong,and Keluo shows that some clinopyroxenes in these mantle xenoliths show complicated chemical zonation with element contents and Sr isotopes.These compositional zonations are difficult to be explained by simple infiltration of host basalt,which reflects the influence of multi-stage and multi-source melt/fluid metasomatism.The proportions of olivine and/or orthopyroxene of some Huinan mantle xenoliths show a trend of decrease,indicating the consumption of these minerals during mantle metasomatism.Trace elements of clinopyroxene varied in a large range.Clinopyroxenes of some harzburgite and dunite varied from high Ti/Eu and ⁸⁷Sr/⁸⁶Sr to low Ti/Eu and⁸⁷Sr/⁸⁶Sr,and clinopyroxene from one harzburgite retained the highest ⁸⁷Sr/⁸⁶Sr（0.7065）,indicating early-stage metasomatism of silicate melt derived from the subducted slab,such as the Paleo-Asian Ocean.The variation trend of clinopyroxenes in harzburgite,dunite,and wehrlite xenoliths in the plot of Ti/Eu-（La/Yb）_Nindicates the overprinting of subsequent carbonatite metasomatism.The low⁸⁷Sr/⁸⁶Sr ratio（～0.7035）of carbonatite melts suggests an origin of the asthenosphere mantle.⁸⁷Sr/⁸⁶Sr ratios of clinopyroxenes in some pyroxenite and dunite xenoliths increase with increasing U/Zr and Ti/Eu ratios,while in some harzburgites,they decrease with increasing U/Zr and Ti/Eu ratios.These features together suggest a later metasomatic event caused by fluid-bearing silicate melts with a moderately high ⁸⁷Sr/⁸⁶Sr ratio（～0.7048）which were suggested to be related to the subduction of the Paleo-Pacific Oceanic slab.The proportion of wehrlite in the Yitong mantle xenoliths is higher than those of the Huinan mantle xenolith,reflecting the consumption of orthopyroxene by silicate-undersaturated melt.Clinopyroxene in the granular Yitong mantle xenolith show increased Ti/Eu and meanwhile increased（La/Yb）_N,and some clinopyroxene retained the higher ⁸⁷Sr/⁸⁶Sr（0.7060）in the core than the rim,indicating early-stage silicate metasomatism with high⁸⁷Sr/⁸⁶Sr,which is similar to the harzburgite in Huinan.We speculate that these silicate melt could be derived from the subducted Paleo-Asian Ocean.Ti/Eu in clinopyroxene in some granular and spongy-texture Yitong mantle xenolith decrease with the increase of（La/Yb）_Nand Zr/Hf,and⁸⁷Sr/⁸⁶Sr（～0.7040）of the clinopyroxene with lowest Ti/Eu is similar to those of wehrlite in Huinan.While abundant glass can be observed in the spongy rim,indicating that the metasomatic agent is not carbonatite but carbonatite-bearing melt derived from the asthenosphere.⁸⁷Sr/⁸⁶Sr in clinopyroxene in some spongy-texture Yitong mantle xenoliths are correlated with Fe O content.⁸⁷Sr/⁸⁶Sr increase with the increase of Fe O content in some samples,while decreases with the increase of Fe O content in some other samples,indicating the metasomatism of Fe O-rich melt with medium ⁸⁷Sr/⁸⁶Sr.Excluding the samples metasomatized by carbonatite-bearing melt and meanwhile Fe O-rich melt,the ⁸⁷Sr/⁸⁶Sr of clinopyroxene with high Fe O ranges from 0.7030 to 0.7038,which is higher than the depleted mantle.Besides,clinopyroxene with high Fe O content show increased Ti/Eu,Y/Yb,and Gd/Yb,which is similar to Fe-rich melts derived from the partial melting of eclogite oceanic crust.Based on the similar Sr isotopic compositions between these Yitong mantle xenoliths and mantle xenoliths captured by the Cenozoic basalts in the eastern North China Craton,we speculated that these Fe O-rich melt could be derived from the subducted Pacific Ocean.Phlogopite and abundant CO₂,H₂O,carbonate,phosphate,and sulfate are observed in the Keluo mantle xenolith.Ti/Eu in clinopyroxene decrease with the increase of（La/Yb）_N,while Zr/Hf changes limitedly.In addition,clinopyroxene in the Keluo mantle xenoliths without phlogopite are featured by lower Ce/Pb.These features are consistent with the metasomatism related to fluid.⁸⁷Sr/⁸⁶Sr in clinopyroxene are correlated with Sr content,Ti/Eu,and（La/Yb）_Nwell,and can be up to 0.7063.Combined with geological location and ⁸⁷Sr/⁸⁶Sr of mantle xenoliths from Huinan and Yitong,we speculated that metasomatic fluids recorded by these Keluo mantle xenoliths are derived from the subducted Paleo-Asian Ocean.Results of petrography and in-situ microanalysis show that the lithospheric mantle beneath Huinan recorded at least three types of melt/fluid metasomatism,including silicate melt with high Sr isotopic composition from the subducted Paleo-Asian Ocean,carbonatite melt from the asthenosphere mantle,and fluid-bearing silicate melt from the subducted Paleo-Pacific in a later stage.The lithospheric mantle beneath Yitong also recorded at least three types of melt/fluid metasomatism,including silicate melts with high Sr isotopic composition from the subducted Paleo-Asian Ocean,carbonate-bearing silicate melt from the asthenosphere mantle,and Fe O-rich silicate melt from the subducted Paleo-Pacific Ocean.The lithospheric mantle beneath Keluo records the fluid metasomatism of high Sr isotopic composition from the subducted Paleo-Asian Ocean.From the perspective of spatial distribution,from Keluo in the north to Yitong and Huinan in the south,the contribution of the subducted Paleo-Asian Ocean derived melt/fluid to the lithospheric mantle is overprinted by the late asthenospheric mantle or the subducted paleo-Pacific Ocean,especially in Huinan and Yitong,the traces of subducted Paleo-Asian Ocean are reserved only in the core of some samples since gradually affected by the subduction of the Paleo-Pacific plate.The combination of the in-situ analyses of mineral element content and isotopic composition with the macro scale of geological background reveals the multi-stage and multi-source melt/fluid metasomatism of the lithospheric mantle beneath the northeastern China.（3）The oxygen fugacity of the lithospheric mantle can be influenced by crust-mantle interaction.The calculation of oxygen fugacity of mantle xenoliths shows that the oxygen fugacity of lithospheric mantle beneath the eastern China is highly heterogeneous,and shows no correlation with Mg#of olivine and Cr#of spinel.These features show that different degrees of partial melting are difficult to explain such a large change of oxygen fugacity in the lithospheric mantle beneath the eastern China.The lithospheric mantle of Huinan,Yitong,and Keluo in the northeastern China has higher oxygen fugacity than those of mid-ocean ridge basalt,abyssal peridotite,peridotite massifs,and continental lithospheric mantle xenoliths captured by the Cenozoic basalts in the eastern North China Craton.In particular,the mantle xenoliths in Keluo have significantly higher oxygen fugacity（logf O₂）than those of continental settings（ΔFMQ）_Ballhaus=FMQ+0.51-FMQ+1.88),which is close to the mantle rocks in the subduction setting.There are also abundant inclusions with oxidizing materials such as CO₂,sulfate,phosphate,carbonate,and H₂O in the mantle xenolith of Keluo.In addition,the lithospheric mantle beneath the northern North China Craton and Xing-Meng orogenic belt which has been modified by the subduction of the Paleo-Asian Ocean show similar characteristics of high oxygen fugacity,which indicates that the recycled crust materials could play an important role in changing the redox state of the lithospheric mantle.In summary,the lithospheric mantle beneath the northeastern China has been modified by multi-stage and multi-source melt/fluid derived from the asthenosphere,the subducted Paleo-Asian Ocean,and the subducted Pacific Ocean,resulting in the petrological and geochemical heterogeneity.The nature of metasomatic agents and degrees of metasomatism in different areas are different.The lithospheric mantle beneath Keluo recorded fluid-related metasomatism in the mantle,while those of Yitong and Huinan are overprinted by the subducted Paleo-Pacific Ocean,and the traces of the subducted Paleo-Asian Ocean are reserved only in the core of some samples.Besides,the oxygen fugacity of the Keluo lithospheric mantle is higher and similar to those of mantle rocks in the subduction setting,while the oxygen fugacity of the lithospheric mantle beneath Huinan and Yitong is lower than those of Keluo but similar to each other,indicating the different influences of crust-derived melt/fluid on the redox state of the lithospheric mantle.The crust-mantle interaction will not only result in the heterogeneity of the lithospheric mantle but also play an important role in changing the redox state of the lithospheric mantle.Thus,this work provides in-situ micro geochemical evidence for the influence of crust-mantle interaction on lithospheric mantle composition and redox state and further complements the relevant research on the influence of the Paleo-Asian Ocean subduction on the lithospheric mantle in northeastern China.