Mantle Nature And Crust-mantle Interaction Of Peridotites In The Dabie Orogenic Belt

Lithospheric mantle is the key link between the shallow crust and the deep mantle.It is also the most important place for material storage and energy exchange in the earth.It plays an important role in the interaction between the crust and mantle,the growth and differentiation of the crust,and the formation of mineral resources.Mantle peridotites and mantle metasomatites in massif are important carriers to record the deep mantle information of subduction zone.In this study,we take the Maowu ultramafic massif which is a representative rock mass of the Dabie UHP orogenic belt as a new breakthrough,exploring the complex crust-mantle interaction of the lithospheric mantle.Combining the research of petrology,geochemistry and geochronology,aim to:(1)investigate the mantle nature and petrogenesis of the southern margin of the North China Craton(NCC);(2)identify the complex mantle-wedge metasomatism in multiple periods and depths;(3)comprehend the distribution and transfer process of mantle-wedge water in continental margin.Specifically,the following conclusions can be drawn:(1)The Maowu harzburgites underwent complex mantle processes:the evidences from petrographic characteristics.According to the features of petrography,the evolution of Maowu harzburgites is divided into 4 stages(M1-M4):M1 is the spinel phase in the shallow mantle,orthopyroxene(opx1),spinel(Spl)and early olivine(Ol1)were wrapped as fine-grained inclusions by orthopyroxene porphyroblast(Opx2).M2 represents the mineral association of peak metamorphism,including coarse-grained Ol2 and Opx2 porphyroblast that is balanced with garnet.And there are residual Ol1 inclusions in the interior of the garnet porphyroblast.M3 is the exsolution of a large amount of clinopyroxene and Cr-spinel(Cr-Spl)from the orthopyroxene porphyroblast.There is a radial symplektite in the edge of the garnet phenocrysts,composed of Opx+Cpx+Cr-Spl.M4 is the hydrated minerals,such as Ti-clinohumite,phlogopite and dolomite veinlets that grow along the matrix fractures in the advanced stage.(2)Harzburgites and dunites:fragments from the NCC mantle wedge.Harzburgites and dunites are refractory with high Mg#(as high as 92.4)and Ni O(0.253?0.323 wt.%)content,as well as low Al2O3(0.08?0.76 wt.%)and Ca O(0.05?0.32wt.%)contents.Apparently,the Maowu peridotites are similar to the M-type peridotites in the Dabie-Sulu orogenic belt,but distinct from the C-type peridotites.Olivines with high Fo values(0.92?0.93),Ni O(0.316?0.493 wt.%)and Mn O(0.028?0.191 wt.%)contents are different from those of cumulates and replaced peridotites.These results show that Maowu harzburgites and dunites are the fragments of the subcontinental lithospheric mantle wedge of the NCC,and experienced?40%partial melting.(3)Garnet pyroxenite veins:products of interaction between refractory peridotite and Si-rich melts.There are inclusions of olivine in garnet porphyroblast of the garnet pyroxenites.The Mg#and Ni is higher to 83?91 and 963?2353 ppm respectively.The PGE content and distribution pattern of garnet pyroxenites are similar with harzburgites.Meanwhile,the enrichment of incompatible elements such as LREE and LILE indicates garnet pyroxenite experienced the crust-derived melt/fluid metasomatism by subduction block.Therefore,the Maowu garnet pyroxenites are the products of the interaction between mantle-wedge peridotite and crust-derived Si-rich melt.(4)Zircons from peridotites reveal the long-term metasomatism from sediments carried by Paleo-Tethyan slab.There are in-situ zircons with a wide range of U-Pb ages(1892±23 Ma to 199±4 Ma)in harzburgites and pyroxenites.And the core-mantle-edge structure with quartz and plagioclase inclusions in the residual magmatic zircons has a complex Proterozoic U-Pb age distribution.Besides,the U-Pb ages of metasomatic zircons is from early Paleozoic to Triassic.As the influences of fluid dissolution and modification,the core was dissolved into dark,while the mantle and edge were covered with garnet,orthopyroxene,olivine,and dolomite inclusions.There is no obvious structural characteristic in the metamorphic zircons.And the U-Pb ages are fasten on 227±6 Ma.These complex U-Pb ages propose that silicate melts introduced the Proterozoic detrital zircons from the crust-derived sediments into the cratonic mantle wedge by strong metasomatism in the early Paleozoic(?450 Ma).Metasomatic zircon U-Pb ages in harzburgites and pyroxenites range from421±8 Ma to 256±6 Ma,with high Th,U and HREE contents,and positive?_Hf(t)(+0.6?+8.2),as well as Ce and Eu anomalies.The carbonate mineral inclusions in zircon and the dolomite and magnesite veinlet in peridotite matrix indicate that there is a perennial carbonate metasomatism and oxidation in the pyroxenite-veined mantle wedge accompanied by long-term subduction of the Paleo-Tethyan slab.In addition,the Triassic ages(?227±6 Ma),depletion of HREE,and negative?_Hf(t)(-13.4?-2.9)of metamorphic zircon record the UHP metamorphic event of the continental-continental collision between the NCC and Yangtze block.The evidence suggests that there is a long-term metasomatism in the mantle wedge of the craton from oceanic subduction to continental collision,and the mantle wedge at the margin of the craton represents an important reservoir and filter,regulating the circulation of metals and volatile elements in the subduction zone.(5)Influence of garnet pyroxenite water content on water transport and distribution heterogeneity in mantle wedgeThe Maowu garnet pyroxenites are divided into garnet-poor pyroxenites(garnet content<20 wt.%)and garnet-rich pyroxenites(garnet content>60 wt.%)as the different melt/rock ratios.The Mg#value and Ni content are high(maximum is 91and 2353 ppm,respectively)in garnet-poor pyroxenites,which manifest it formed in low melt/rock ratio.While the garnet-rich pyroxenites are enrich in LREE,LILE and other incompatible elements that represents the crust-derived melt.The water content and H2O/Ce ratio of garnet-poor pyroxenites are 89?142 ppm and 7?506,respectively,which is similar to the MORB source areas;the garnet-rich pyroxenites have water content of 266?609ppm and H2O/Ce ratio of 84?648,which is higher than the EM and OIB source areas and similar to the inferred HIMU and lower edge of the arc magma mantle source areas.Therefore,the garnet-rich pyroxenites with high water content and H2O/Ce ratio are closer to the water content of the melt,while the garnet-poor pyroxenite with the lowest water content and high Mg#value stores less water due to the buffering effect of the country rock peridotites.The comparison of water contents of different types of pyroxenites show that pyroxenites formed in different melt/rock ratios are formed by the segregation process of melts.They carry different degrees of water into the mantle wedge during the formation process.The water transfer is different,which makes the water distribution of the mantle wedge uneven.The crust-derived melt/fluid from subducted slab reacts with mantle wedge peridotite of overlying continental lithosphere at the slab-mantle interface to form pyroxenite with different mineral composition.A large amount of water can be stored in these mantle metasomatic rocks to form mantle water reservoir with high water content.Finally,the hydration of the mantle wedge will affect the migration of water to the deep mantle and the water cycle of the earth.(6)Carbonate metasomatism at different depths recorded by dunites.Carbonate minerals with different occurrence are found in the dunites.The first type is the magnesite megacryst formed by the metasomatic melts captured by the dunites,and immiscible melts form inclusions such as calcite and diopsides.The second type is the reaction of dolomitic melt with orthopyroxene at 2.5 GPa to form magnesite and clinopyroxene.The third type is calcite veinlet reacts with magnesite to produce dolomite at the edge of vein.The clinopyroxenes in the Maowu dunites are characterized by high Ca O content and Mg#value,low content of Al2O3 and Ti O2,high(La/Yb)N,low Ti/Eu,and have enrichment in Th,T,Sr(up to 2273 ppm),LREE and high ⁸⁷Sr/⁸⁶Sr ratio(0.7079?0.7097),which indicates that the clinopyroxenes in the Maowu dunites are formed by carbonate metasomatism.Carbonate minerals have low?¹³C_V-PDB(-19.7?-15.6‰)and high?¹⁸O_V-SMOW(18.1?28.1‰),indicating that the metasomatic agent is mainly composed of sedimentary carbonate and organic carbon.Therefore,the carbonate minerals in the Maowu dunites were formed before the subduction/collision of the continental block.The oceanic slab carried the sedimentary carbonate and organic carbon into the mantle wedge and formed carbonate melt after oxidation and decomposition to metasomatize the mantle wedge peridotite.Carbonate melt generated from subducted oceanic crust and metasomatized mantle-wedge periodtites.The carbonate sediments are wrapped by dunites to form the carbonate melt sac.The dolomitic melts react with orthopyroxenes to form magnesites.Finally,the CO2-rich fluid formed by the dehydration and melting of continental plate subduction,carbon migrated in various forms in the subduction zone.The surface carbonate and organic carbon migrated to the deep mantle wedge through subduction zone,and then existed in peridotite in the form of carbonate minerals,and finally returned to the surface,realizing the complex carbon cycle between the sedimentary carbon reservoir and the mantle wedge.