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

An XRD-TEM study was carried out on low-grade metapelites (Transantarctic Mountains) to determine the cause of apparent inconsistences in grade as determined by IC and independent geologic relations. The illite crystallinity (IC) data indicate that the three units investigated (BT, RBT, MS) were affected by very low- (IC = 0.24°Δ2θ in BT) to low-grade metamorphism (0.19°Δ2θ in MS). In all three samples, mica crystals are of a size typical of the epizone, but the mean size increases from BT to RBT and MS, due to the increasing strain features from BT to MS. These results indicate that strain-induced reduction in crystal size was retained in BT samples (with anomalously high IC values). Microtextures in RBT and MS (with smaller IC) samples suggest a recovery of sub-grain boundaries. A decrease in crystal size may occur with increasing grade where strain rates are high relative to the rate of recrystallization. An XRD-TEM study was carried out on low-grade metapelites (Transantarctic Mountains) to determine the cause of apparent inconsistences in grade as determined by IC and independent geologic relations. The illite crystallinity (IC) data indicate that the three units investigated (BT, RBT, MS) were affected by very low- (IC = 0.24°Δ2θ in BT) to low-grade metamorphism (0.19°Δ2θ in MS). In all three samples, mica crystals are of a size typical of the epizone, but the mean size increases from BT to RBT and MS, due to the increasing strain features from BT to MS. These results indicate that strain-induced reduction in crystal size was retained in BT samples (with anomalously high IC values). Microtextures in RBT and MS (with smaller IC) samples suggest a recovery of sub-grain boundaries. A decrease in crystal size may occur with increasing grade where strain rates are high relative to the rate of recrystallization.