Abstract:Located in Xilingol League of central Inner Mongolia, Hegenshan high-Al podiform chromitites has Cr# values of 47.8~54.9 and Mg# values of 64.1~73.7. The spinel-hosted inclusions from the chromitites principally consist of silicates which comprise olivine, orthopyroxene, clinopyroxene, pargasite and albite. Based on the shapes, mineral assemblages and distribution, the silicate mineral inclusions can be divided into three types. The type 1 mineral inclusions are monomineralic inclusions which are consisting of olivine and clinopyroxene. The type 2 mineral inclusions are compound inclusions which are characterized by the coexistence of orthopyroxene and clinopyroxene. Both the type 1 and type 2 mineral inclusions which are xenocrysts included by the host spinel have rounded edges and are scattered in spinel grains. The type 3 mineral inclusions, concentrated at the nucleus of host spinel, are polygonal and generally include many different mineral types. The type 3 mineral inclusions are the result of the trapped melt by the host spinel, indicating they are melt inclusions. Temperatures (1148~1254℃) and pressures (4.9~12.9 kbar) based on monomineralic clinopyroxene inclusions and the composite inclusions consisting of clinopyroxene and othorpyroxene suggest a shallow forming environment (16 km~43 km) for the chromitites, which could be the upper mantle. The compositions of melt inclusions which include albite and pargasite show that the parental melt is rich in H2O, Na, and Si. The calculated Al2O3, TiO2 and (FeO/MgO) of the parental melt corresponding to the Hegenshan podiform chromitites are 15.4%~16.3%, 0.3%~0.9% and 0.6~1.1, respectively, which resemble low-Ti tholeiitic melt. Based on spinel and olivine inclusions, we calculated the composition of primitive melt for the Hegenshan podiform chromitites, which has a high MgO content of 19.8%. The differences in composition between parental melt and primitive melt suggest an evolving process for primitive melt from high Mg to low Mg. This evolutionis probably caused by melt-rock reaction and melt mixing. Due to the discovery of diamonds in Hegenshan podiform chromitites, we believe that the genesis of Hegenshan podiform chromitites experienced the deep mantel and finally formed in shallow mantle. The formation in shallow mantle involves two processes: (1) the reaction between the melt and harzburgites; (2) mixing of evolved melt and primitive melt.