Olivine Crystal Populations Of Cenozonic Alkaline Basalts In Yangyuan:Types,Origin And Significances

The alkaline basalts in Yangyuan which contain abundant olivine“phenocrysts”are the typical small-volume continental intraplate OIB-type basalts.The composition of olivine is often used to discuss the evolution of the host magma and source lithology,but the specific source and origin of olivine is not yet clear.Therefore,with the olivine as the research object in this paper,the author carried out a series of studies about their types,origin and significances.The following are major achievement obtained in the thesis:1.Based on the characteristics of the petrography and mineral chemistry and crystal size distribution in olivine,the Yangyuan alkaline basalts contain a variety of multiple olivine crystal populations.The first type is a large number of(about3.9-8.6%)an-to subhedral form and embayment margin and high-Fo,high-Ni,low-Ca(0.32-0.39 wt.%Ni O,0.02-0.11 wt.%CaO,at Fo_85.8-92.7 in the core)of olivine,whose size is about 0.01-0.24 mm and the all olivine crystal size distribution in the sample tends to curved concave upward F-CSD pattern,we propose the olivines are derived from the disggregation of mantle-derived peridotitic xenoliths or mantle olivine xenocrysts.The second type is a few(less than 1%)eu-to subhedral and low-Fo,low-Ni,high-Ca(0.17-0.20 wt.%Ni O,0.18-0.26 wt.%CaO,at Fo₈₆ in the core)of olivine,whose size is from 0.05 to 0.56 mm and the CSD graph presents the L-CSD pattern.Combining with the morphological textures and mineral chemistry and the comparison of the olivine features in the reaction of orthopyroxene xenocrysts,the olivines are probably not magmatic crystals with crystallization in the host magma itself,rather than periectic crystals which formed by Si-poor alkaline basaltic magma with orthopyroxene due to the periectic reation.The third types is a very small number of anhedral,reverse zoning and low-Fo,low-Ni,low-Ca(0.24 wt.%Ni O,0.09 wt.%CaO,at Fo_73.3 in the core)of olivine,which may be derived from cumulate rocks in the lithosphere or from co-magmatic cumulates in the magma systems.2.The paper had used Fe-Mg diffusion chronometry in olivine xenocrysts to characterize timescales of crystal storage and transport to the eruption.Diffusion modeling using the greyscale intensity of backscattered electron images as a proxy for olivine composition reveals the most probable Fe-Mg diffusion timescale for the Yangyuan alkaline basalts is from several houors to a dozen days time.It shows that the relative small-volume alkaline basalts with a lot of mantle olivine xenocrysts have a rapid ascent rate(0.07-0.36m/s).In addition,the author had a preliminary understanding of the deep evolutionary process of the Cenozoic alkaline basaltic magmas in Yangyuan,and established a magmatic ascent model.3.The thesis points out the opinion that deep magmatic processes probably make the difference of the olivine“phenocrysts”component,such as fractional crystallization,periectic reation,diffusive re-equilibration.Therefore,when making use of the olivine component to define or trace the magmatic source lithology,magmatic evolution,petrogenetics or tempesrature and pressure conditions,we must first analyze the origin of olivine“phenocrysts”in detail.