Abstract

Core-mantle structures in mineral grains have typically been interpreted as having formed during crystal plastic deformation. Optical, scanning electron, and transmission electron microscope investigations of what appear to be core- mantle structures in amphibole from the Cheyenne belt, southeastern Wyoming, indicate that subgrainlike structures (herein "subgrains") formed as a result of cataclastic rather than crystal plastic deformation. Optical and scanning electron microscope evidence to support a cataclastic origin include (1) the angular shape of subgrains with boundaries commonly parallel to (110) cleavage; (2) the wide variety of subgrain sizes; (3) the compositional zonation of the amphibole subgrains with actinolitic hornblende interiors overgrown by magnesio- hornblende; and (4) the observation that some of the undulatory extinction in porphyroclast cores is due to slip along cleavage planes. Whether dislocations observed in the amphibole grains were formed prior to cataclastic deformation (work hardening) or formed in response to crack propagation is unknown. The implication of these observations is that core-mantle structures in other minerals may, at least in some cases, have also formed during cataclastic deformation rather than during recovery and/or recrystallization. Therefore, optical identification of core-mantle structures is not a priori evidence for a crystal plastic deformation mechanism.

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