Deformation-Induced and Reaction-Enhanced Permeability in Metabasic Gneisses, Iona, Scotland: Controls and Scales of Retrograde Fluid Movement

Figure 2

Field photographs of sampled exposures of gneisses. (a) Partially retrogressed metabasic gneiss blocks exposed in beach sand. Looking directly down on the exposure. Top of photograph represents a southeast direction. Elongate thin block towards the top of the view contains pale green 5 cm wide epidosite (marked by green arrow, shown in (b)). Blue arrow points towards the part of the exposure shown in (c); yellow arrow points to the view shown in (d). Positions of thin section samples F, G, H, and I are shown. Other thin sectioned samples are from adjacent blocks. (b) View of epidosite exposure looking towards the southwest with sand removed from the gully; (c) dark metabasic amphibole-rich gneisses with unmineralized (u) and mineralized (m) fractures, the latter is filled with pale green epidote and separates pervasively altered epidote-rich gneiss (eg) from relatively unaltered plagioclase-rich gneiss (pg); (d) metabasic gneiss with coarse pegmatite veins and hydrothermal veins filled with epidote and central zone of quartz. Gneiss in the central lower part of the view shows extensive alteration of plagioclase to green epidote and has an irregular boundary with the unaltered gneiss to the right of the view. Plagioclase in the pegmatite also locally shows extensive alteration to epidote (arrowed); (e) local alteration of feldspar-rich pegmatite to green epidote and associated epidote replacement of plagioclase in the host gneiss. Note the spatial restriction of alteration of the pegmatite by an epidote-filled cataclasite; (f) K-feldspar-bearing granitic gneiss with epidosite-filled fractures and limited pervasive retrogression of the host gneiss; (g) sheets of metabasic and granitic gneiss showing pervasive epidotization of plagioclase in the former and more limited alteration of feldspars restricted to the margins of the granitic gneiss; (h) epidote veins within pseudotachylite (ps) and highly fractured granitic gneiss (gg) with no obvious marginal alteration of either host. Hand lens for scale.

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