Petrological And Geochemical Study Of Diamond-bearing Mafic-ultramafic Rocks In The Southeastern Margin Of The North China Craton

Diamonds have extensive applications in technological research fields such as defense,aerospace,and scientific exploration,and gem-quality diamonds also possess significant economic value.Moreover,diamonds and their host rocks are crucial research objects in understanding the composition,properties,and circulation of deepseated materials in the Earth’s crust,mantle,and core,serving as important "probes"and "windows" for studying the magma mantle source and its evolution,and helping to reveal the mechanism of the evolution of the lithospheric mantle.Investigating diamonds and their host rocks has both significant scientific and economic implications.In the southeast margin of the North China Craton,there are three primary diamond deposits with different periods,types,and origins,as well as diamond-bearing rocks.This thesis selects the diamond deposits in Langan,Anhui Province;Mengyin,Shandong Province,and Xuzhou,Jiangsu Province as research objects,and conducts systematic mineralogical,petrological,isotopic,and geochemical research on the diamond-bearing rocks to identify their rock formation and mantle source properties,determine the tectonic evolution history of the same period,and reconstruct the lithosphere and mantle evolution processes of the southeast margin of the North China Craton.Finally,based on comprehensive research on all diamond-bearing rocks,primary diamonds,and diamond indicator minerals;this paper analyzes and summarizes three models of diamond deposit genesis,including the mantle plumerelated high-pressure diamond deposit model,deep-mantle derived magma capturetype diamond deposit genesis model,and basic rock secondary transport diamond genesis model.These models provide beneficial supplements for the diamond formation and mineralization models.The Anhui Langgan diabase-type diamond deposit was formed in the Neoproterozoic period.Based on the petrological and geochemical analysis of the mafic-ultramafic rocks from the Langgan deposit and other magmatic rocks exposed in the North China Craton,Congo Craton,and Sao Francisco Craton,three types of maficultramafic rocks were identified:high Ti-1,high Ti-2,and low Ti.According to the SrNd-Hf-O isotopic features and mantle potential temperature,it is believed that the primary magma for the high Ti-1 samples originated from the edge of the mantle plume mixed with a small amount of lithospheric mantle at the spinel-garnet transition zone during the early stage of mantle plume activity.The upwelling high-temperature magma mixed more lithospheric mantle components in the mantle plume axis,forming the primary magma of low Ti samples in the spinel stability field.The thicker lithospheric mantle outside the mantle plume limited the partial melting process to the garnet stability field,resulting in the formation of the primary magma of high Ti-2 samples.Based on the diamonds,zircons,garnets,quartz,and phosphates found in the Langgan diamond deposit,a mantle plume-related high-pressure diamond deposit model was proposed in this study.The subduction materials stayed at the root of the lithosphere and provided a carbon source.When the mantle plume rose,it was blocked by the thick lithospheric root,which created a high-pressure and medium-temperature condition that favored the formation and preservation of diamonds.The mafic-ultramafic magma formed by the melting of the lithosphere and mantle plume captured the newly formed diamonds and transported them to the surface,finally forming the diamond deposit.The Mengyin diamond deposit in Shandong Province was formed in the Paleozoic period.The diamond-bearing kimberlites exhibits a slight enrichment-depletion characteristics of Sr-Nd isotopic composition,indicating the influence of subcontinental lithospheric and subducted oceanic materials on the kimberlite source region.Based on the data presented in this study and previous research,it is suggested that the addition of fluids/melts derived from the subducted oceanic crust triggered low-degree partial melting of the carbonated asthenosphere and the lowermost lithospheric mantle root,resulting in the formation of Ordovician kimberlites in the eastern North China Craton.For the formation of deep-mantle derived-magma diamond deposits,a stable,ancient crystalline basement is required to provide formation and preservation space for diamonds,followed by deep-mantle derived mafic-ultramafic magmas that capture diamonds from the lithospheric mantle root and transport them to the shallow crust.Finally,deep faults and deep tectonic activities provide a rapid ascent pathway for the diamond-bearing magmas.Two primary diamonds have also been found in the Mesozoic basic-ultrabasic rock belt in Xuzhou,Jiangsu Province.Petrological and geochemical studies were conducted on the lamprophyre and kimberlite rocks in the belt,revealing significant heterogeneity in the lithospheric mantle of the southeastern margin of the NCC.The southern lithospheric mantle was affected by fluid derived from the subduction of the palaeoPacific plate in the period of Jurassic and the deep subduction of the South China Block in the period of Triassic.Multiple episodes of interaction have led to the extreme enrichment of incompatible elements in the lithospheric mantle in this region.The major element characteristics of mantle olivine also show that the lithospheric mantle in eastern part of NCC had underwent an uneven process of erosion and thinning in early Cretaceous.The remaining thick lithospheric mantle underwent extremely lowdegree partial melting,forming the Group Ⅱ kimberlite in the North China Craton.The southeastern margin of the North China Craton has lost the conditions for diamond formation and preservation in the Mesozoic period.Combined with the highly similar characteristics of diamonds found in Xuzhou,Mengyin,and Fuxian,as well as the 476 Ma age founded in the Xuzhou mafic-ultramafic rock belt,it is inferred that the maficultramafic magma in the Xuhuai area may have captured early-formed diamondbearing rocks and transported them to the shallow surface,finally proposing a secondary transport-type diamond production model.In summary,this research selected three diamond deposits in Langan,Mengyin and Xuzhou as the research objects.Through systematic mineralogy,petrology,isotope geochronology and geochemistry studies of diamond-bearing rocks,the genesis of the rocks and properties of their mantle sources were elucidated.The tectonic evolution history during that period was determined,enabling restoration of the lithospheric mantle evolution process at the southeastern margin of North China Craton and providing insights into the genesis mechanisms for these three diamond deposits.