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Book Sections Year : 2015

Pegmatite mylonites: origin and significance

Denis Gapais
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Pegmatites generally behave as resistant objects compared to their host rocks during solid-state deformation. They usually show folding or boudinage within their host rocks, features observed from low-grade to high-grade deformations, up to upper-amphibolite metamorphic conditions. Here we describe mylonitic pegmatite veins emplaced within S-C fabric-bearing two-mica granites. Pegmatites consist mainly of feldspar and quartz ribbons, with remnants of coarse-grained Kfs phenocrysts. Quartz ribbons are made of large grains elongated in the shearing direction, with evidence of dynamic recrystallization involving grain boundary migration. Ribbons show very strong lattice-preferred orientations of c-axes at low angles to the stretching lineation. From these, we propose that quartz ribbons are formed by oriented growth during syn-shearing crystallization of the pegmatite. Subsequent syn-shearing solid-state deformation involved grain size reduction accompanied by prism slip. The occurrence of pegmatite mylonites within granitoid intrusions should be considered as a univocal argument for synkinematic emplacement.
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insu-01121077 , version 1 (27-02-2015)



Denis Gapais, Awa Louan A. Brem Moundi. Pegmatite mylonites: origin and significance. G. F. ZELLMER, M. EDMONDS and S. M. STRAUB (Eds.). Rock Deformation from Field, Experiments and Theory: A Volume in Honour of Ernie Rutter, 409 (1), pp.167-173, 2015, geological society special publication, ⟨10.1144/SP409.7⟩. ⟨insu-01121077⟩
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