Abstract:

Some of the Late Paleozoic strata in the middle Inner Mongolia developed zeoliteand/or prehnitepumpellyite facies contact metamorphism due to the intrusion of Mesozoic magmas. Studies of clay minerals, illite crystallinity and vitrinite reflectance of typical profile indicate that (1) illite and chlorite are the dominant clay minerals, interlayered illite and smectite (I/S) or smectite are present in very few samples, and in most cases interlayered illite and chlorite (I/C) are rare; that (2) illite crystallinity ranges from 0.22 to 0.40; and that (3) the vitrinite reflectance values (Ro) are in the range of 1.5—4.0. The results suggest that only very lowgrade contact metamorphism rather than largescale greenschist facies regional metamorphism developed in the limited areas affected by the magmatic intrusion, and this lowgrade metamorphism affected only sedimentary strata and oil resources formed before the Late Permian.

Key words:  very lowgrade metamorphism, illite crystallinity, clay minerals