Study On The Mechanism Of Massive Mantle Rock Accumulation In Hannuoba Alkaline Basalt

Plenty of mantle peridotite xenoliths are included in the Hannuoba alkaline basalts,and these peridotite xenoliths are accumulated at the same location.But there is no systematic genetic mechanism for this geological phenomenon.In this paper,the olivine xenocrysts in the alkaline basalts of Hannuoba are further studied through petrographic observation,electron microprobe(EPMA),electron backscatter diffraction(EBSD),insitu trace element analysis and in-situ Fe-Mg isotope analysis.The aim is to discuss the accumulation mechanism of massive mantle rock in alkaline basalt of Hannuoba.The olivine xenocrysts of Hannuoba have obvious chemical zonings.From core to rim,the Fo value of the olivine xenocrysts gradually decreases,which is expressed as the composition of peridot at the core and mantle gradually evolves into the composition of hyalosiderite at the rim of olivine xenocrysts.Meanwhile,the rim composition of xenocrysts is the same as the composition of olivine phenocryst in alkaline basalt.And the Fo value of the olivine xenocrysts has a good linear relationship with their major element,indicating that only the exchange of elements occurs between the olivine xenocrysts with the basaltic magma,but no new minerals are produced.According the in-situ Fe-Mg isotope profiles,it can be seen that from the core to the rim,the ?56Fe of the olivine xenocrysts decreases first and then increases,while ?26Mg increases first and then decreases slightly,and the ?56Fe and ?26Mg at the core are higher than the rim.Overall,?56Fe and ?26Mg also shows a negative correlation,indicating that only simple Fe-Mg interdiffusion occurs after the olivine xenocrysts enter the basaltic melt.Therefore,the residence timescales of olivine xenocrysts in magma can be well constrained by the element diffusion of olivine.We have investigated the residence timescales of olivine xenocrysts in basaltic magmas based on diffusional modeling of the Mg-Fe zonings and NIi zonings in the xenocrysts.Because the olivine xenocrysts in alkaline basalt are formed by mantle xenoliths falling into basaltic magma during migration,so the longest timescales we simulate is actually the shortest timescales for mantle peridotite xenoliths to react with basaltic melts.Therefore,the residence timescales of mantle peridotite xenoliths in the alkaline basaltic magma is more than ten years.Therefore,the accumulation mechanism of massive mantle peridotite in Hannuoba alkaline basalts is the lava lake model,which roughly goes through three stages.First,the basaltic magma carries a large amount of mantle peridotite xenolith in the process of ascending After reaching the surface,the mantle peridotite xenoliths flow along with the basaltic magmas,and migrates to the low-lying terrain,gradually accumulates at the low-lying terrain,forming a lava lake.Subsequently,the mantle peridotite xenolith gradually sinks and accumulates bottom of the lava lake under the action of gravity.Finally,the basaltic magma continues erupting,covering the previously deposited mantle peridotite xenoliths As a result,a large amount of mantle peridotite xenoliths are trapped by alkaline basalts in the lower part,and the overlying rocks are also alkaline basalts.