Petrogenesis And Temperature And Pressure Conditions Of Jidong Harzburgite In Inner Mongolia

In this paper,the harzburgite containing chromite in Jidong of Hegenshan is studied.By performing petrography,zircon U-Pb geochronology,major and trace elements of whole-rock geochemistry as well as the major and trace elemental analysis of single minerals on typical samples,age of sample formation,nature of magma source area and tectonic environment are identified,and further discussed the Late Paleozoic magma-tectonic evolutionary history of the study area in light of previous studies.The harzburgite in Jidong consists mainly of olivine and orthorhombic pyroxene,with minor amounts of clinopyroxene and chrome spinel.The clinopyroxene exists as isolated crystal or in the symbiotic form of chrome spinel-clinopyroxene-orthorhombic pyroxene.Zircon U-Pb geochronology shows that the age of the harzburgite in Jidong of Hegenshan is concentrated between 321 and 351 Ma,with peak ages of 325 Ma and 351 Ma and a weighted average age of 333±14 Ma.Combined with the test results of the zircon U-Pb geochronology(342±5.5 Ma)of the overlying gabbro sample,it indicates that harzburgite in Hegenshan was formed in the Late Carboniferous.Trace elements analysis of zircon indicated that the samples have typical characteristics of magma-derived continental environmental zircons.A total of twenty samples of harzburgite collected in the study area were tested for whole-rock geochemistry analysis.The analytical results show that the harzburgite in Hegenshan has high burn loss(11.92-14.49%),low Si O₂ content(34.70-36.92 wt.%),low potassium content(K₂O?0.12 wt.%)and low calcium content(0.010.18 wt.%),Mg O content of 39.5-41.71 wt.%,Al₂O₃ content of 0.32-1.61 wt.%and TFe₂O₃ content is5.23-9.34 wt.%.The mineral size and density of each layer of the Hegenshan ophiolite increase with the increase of depth.Olivine,chromite and other minerals with higher density occurred at the bottom,while pyroxene feldspar is in the shallow part,showing obvious gravity differentiation characteristics.The parent magma of harzburgite may be formed by the primary magma of mantle peridotite equilibrium,which underwent weak fractional crystallization of gabbro components.Harzburgites are characterized by high Zr and Hf contents,enrichment of Th and U,as well as the presence of captured zircon,suggesting that the harzburgites may have been contaminated by crustal materials during their ascent.However,the positive correlation between Ce/Pb and La/Nb is not obvious,which indicates that the crustal contamination of lherzolite is not strong.The harzburgite is moderately to highly deficient(?20%)and shows a high Fo content in olivine(90.48-91.66),with a smaller range of Fo values showing typical characteristics of minerals in melt-residue mantle peridotite.The light rare-earth element deficit of orthorhombic pyroxene is severe and the light and heavy rare-earth differentiation of clinopyroxene are very obvious(Ce/Yb)_N ratio of0.09-0.12),indicating that the clinopyroxene and orthorhombic pyroxene in harzburgite in Jidong have undergone modification of aquatic fluids.The spinel has moderate Cr^#values,which is the characteristic of the secondary mineral,chrome spinel in mantle peridotite,while high Cr^#(0.58-0.60)and low Mg^#(0.49-0.57)are the typical characteristics of a high degree of melting.According to the calculations of the equilibrium symbiosis of diabase and monoclinic pyroxene grains in harzburgite in Jidong,the crystallization temperature is 963-1106°C,with an average value of 1034°C and a crystallization pressure of 0.42-0.9 Gpa;the crystallization temperature of chrome spinel is 1380-1389°C,with an average value of1385°C;the crystallization pressure is 2.97-0.9 Gpa.3.02 Gpa;indicated the mantle origin of the harzburgite in Jidong.Geochemical simulations indicate that the harzburgite in Hegenshan represents a remnant of approximately 18-22%of the original mantle after partial melting.In summary,chromite in harzburgite in Jidong,Hegenshan may have undergone decompressional melting.These petrographic and geochemical features,as well as the synthesis of previous research results in this area,indicate that the harzburgite in Hegenshan was formed in the Late Carboniferous,and that the source is a lossy mantle peridotite that has undergone about 18-22%partial melting.It is inferred to have formed from the upwelling of mantle material in the stretching environment.