Retarded illite crystallinity caused by stress-induced sub-grain boundaries in illite

An XRD-TEM study was carried out on low-grade metapelites (Transantarctic M ountains) to determine the cause of apparent inconsistences in grade as det ermined by IC and independent geologic relations. The illite crystallinity (IC) data indicate that the three units investigated (BT, RBT, MS) were aff ected by very low- (IC = 0.24 degrees Delta 2 theta in BT) to low-grade met amorphism (0.19 degrees Delta 2 theta in MS). In all three samples, mica cr ystals are of a size typical of the epizone, but the mean size increases fr om BT to RBT and MS, due to the increasing strain features from BT to MS. T hese results indicate that strain-induced reduction in crystal size was ret ained in BT samples (with anomalously high IC values). Microtextures in RBT and MS (with smaller IC) samples suggest a recovery of sub-grain boundarie s. A decrease in crystal size may occur with increasing grade where strain rates are high relative to the rate of recrystallization.