Figure F37. Photomicrographs showing that the evolution of brown amphibole gabbro during deformation and metamorphism is complex and involves several stages. AC. Sample 209-1271B-14R-1, 2529 cm (cross polarized light [XPL]: blue filter). (A) Ductile deformation. Amphibole porphyroclast with wavy extinction (a(p)) represents a microboudin that has been syntectonically recrystallized to fine-grained amphibole neoblasts (a(n)) along its margins (field of view [FOV] = 5.5 mm; image 1271B_058). (B) Amphibole porphyroclasts (a(p)) have been partially replaced by amphibole neoblasts (a(n)). The amphibole is surrounded by bands of former plagioclase neoblasts that have been statically altered to sericite (sc(n)) (FOV = 5 mm; image 1271B_059). (C) The outline of a "ghost" plagioclase porphyroclast replaced by sericite (sc(p)) can be recognized in this sample. The original polysynthetic twinning of the plagioclase porphyroclasts (white arrows) is pseudomorphed by sericite with different orientations. Former plagioclase neoblasts are pseudomorphically replaced by sericite aggregates (sc(n); red arrows) (FOV = 2.75 mm; image 1271B_060). D, E. Sample 209-1271B-11R-1, 4446 cm (XPL: blue + dark gray filters; FOV = 1.4 mm). (D) Intergrowth of fibrous amphibole (a2) and chlorite after former high-temperature amphibole. Fibrous amphibole and chlorite are stretched and deformed along shear bands (image 1271B_049). (E) Amphibole-chlorite schists after gabbroic protolith. The secondary fibrous amphibole (a2), sericite (sc), and chlorite (ch) are stretched and banded, defining the schistosity (image 1271B_048). F. Fold of fibrous amphibole (a2) in a amphibole-chlorite schists after gabbro. Chlorite (ch) crystallized in the pressure shadow of the inner part of the fold. The matrix is composed of fine fibrous aggregates of secondary amphibole (a2) and chlorite (a2) (may by replacing former high-temperature amphibole) alternating with dark bands of sericite and hydrogrossular (sc and hgr) (may by replacing former plagioclase) (Sample 209-1271B-11R-1, 5557 cm) (plane polarized light: blue + dark gray filters; FOV = 5.5 mm; image 1271B_053).
Next Figure | Table of Contents