Mineralogical characteristics and constraints of formation conditions of garnet in kimberlite in Shizhuang area, southern He'nan Province

Shizhuang area in southern He nan located in the south margin of north China platform, with kimberlite.In order to determine the types of garnet and their relationship with diamond, 40 selected garnet mineral single crystals were analyzed by electron probe micro-area.The results show that the cations of group A in the general formula A₃B₂ SiO₄₃ of the garnet mineral group are mainly occupied by Mg²⁺, Fe²⁺ and a small amount of Ca²⁺, the cations of group B are mainly occupied by Al³⁺ and a small amount of Fe³⁺, Cr³⁺ and Mn³⁺.The garnet in the area is mainly composed of Alm - Pry - Gro series.According to the mineral chemical characteristics, the Mg-Al garnet in the area is eclogite type, which is characterized by titanium-calcium, low-chromium and iron-rich.It belongs to Grutter' s G4-G5 group(containing titanium and calcium)Mg-Al-Fe-Al garnet, and has the same characteristics as kimberlite, diamond inclusion or garnet in group A eclogite.Supersilicon garnet is found in the area, and its formation pressure ranges from 10.2 GPa to 10.9 GPa, with a depth of more than 250 km.