Abstract: In order to investigate the very low-grade metamorphism of the Upper Paleozoic group in Heilongjiang Province, the crystallinity, polytype and b dimension of illites from the argillaceous rocks have been measured by XRD technique. The results showed that the illite crystallinity (Kübler index) ranged mainly from 0.31 to 0.77 °Δ2θ, while the illite (muscovite) b dimension ranged mainly from 8.983 to 9.011 A and the illite (muscovite) polytype presented three types of 1M_d, 1M_d+2M₁ and 2M₁. The mineralogical parameters of illites in above indicated that the argillaceous rock from the upper Paleozoic Group in the area belongs to the category from late diagenesis to very low-grade metamorphism. Moreover, there is a negative correlation between crystallinity and b dimension of illites, which shows crystallinity decreases (temperature increases) with the b dimension increases(pressure increases). It is suggested that the illites formed in the normal geological background from diagenesis to metamorphism related to burial depth, which reveals that both diagenesis and metamorphism of the Upper Paleozoic Group in the area belong to an unitary and continuous process in stead of two independent and unrelated geological event. Combining the regional geological background and achievements in research of illites in Inner Mongolia and Jilin Province, it can be included that the main part of the Upper Paleozoic Group in Northeast China did not undergo a massive greenschist facies regional metamorphism. Some greenschist facies(or higher degree) metamorphic rocks in parts of the area should be resulted from dynamic or contact metamorphism.

Key words: illite crystallinity, illite polytype, illite b dimension, very low-grade metamorphism, mineralogy, Upper Paleozoic Group, Heilongjiang Province