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Retarded illite crystallinity caused by stressinduced sub-grain boundaries in illite

Published online by Cambridge University Press:  09 July 2018

G. Giorgetti ,
I. Memmi  and
D. R. Peacor
G. Giorgetti*
Affiliation:
Department of Geological Sciences, The University of Siena, Via Laterina, 8, 53100 Siena, Italy Department of Geological Sciences, University of Michigan, Ann Arbor MI 48109-1063, USA
I. Memmi
Affiliation:
Department of Geological Sciences, The University of Siena, Via Laterina, 8, 53100 Siena, Italy
D. R. Peacor
Affiliation:
Department of Geological Sciences, University of Michigan, Ann Arbor MI 48109-1063, USA
*
*E-mail: [email protected]
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Abstract

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.

Keywords

illite crystallinity (IC)stress-induced boundariesTEMAntarctica
Type
Research Article
Information
Clay Minerals , Volume 35 , Issue 4 , September 2000 , pp. 693 - 708
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2000

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