Shear zones in a pegmatite: A study of albite-mica-quartz deformation

Abstract

A tectonic foliation of variable intensity is associated with closely spaced discontinuous shear zones in a pegmatite deformed under amphibolite grade conditions at I Mondei, Italian Alps. This hetrogeneous foliation is an alternation of quartz, albite, biotite and/or muscovite layers together with albite-mica mixtures. The foliation is defined by the alignment of micas and the elongation of quartz and albite grain aggregates. An examination of this foliation using optical and electron microscopes reveals significant microstructural differences between individual minerals and within individual layers. Microstructures in deformed albite can be related to both strain and the presence of other phases such as mica. In the pure albite regions there is a ubiquitous development of a coarse polygonal microstructure and abundant peristerite. The recrystallized albites recognised in the mica rich areas have grain sizes appreciably smaller than adjacent mica-poor areas and have a restricted development of peristerite. In areas dominated by muscovite, grains are gently bent, fractured and partly recrystallized with the preservation of relic pegmatitic albite grains, whereas, biotite is deformed into kink-like structures and has been modified by extensive new grain nucleation. Recrystallized quartz crystals have undergone abnormal grain growth, have a subgrain structure and a strong c-axis preferred orientation perpendicular to the foliation. The microstructural observations suggest that varying mechanical properties and the distribution of individual minerals control the deformation within the shear zones.