Tectonothermal evolution of the Neoproterozoic Grampian and Appin groups, southwestern Monadhliath Mountains, Scotland

Abstract

Metasedimentary rocks of the Neoproterozoic Grampian and Appin groups exposed in the Glen Roy district (Monadhliath Mountains, Scotland) were strongly folded during an initial progressive tectonothermal event of the Caledonian orogeny. The attitude and style of the first folds varies from open, upright NE-trending structures in the west (e.g. the Appin Synform) to recumbent NW-facing nappes in the east (e.g. the Treig Synform). These early nappes occur only south of the Corrieyairack Complex and their amplitude decreases towards the NW. Ductility contrasts induced sliding such as that locally recorded along the Grampian-Appin group lithostratigraphical boundary. Subsequently the early structures were deformed by tight, upright ‘D2’ folds. NW-thrusting along the Eilrig Shear Zone, during ‘D2’, is interpreted as occurring in response to a transfer of bulk transport to deeper structural levels relative to ‘D1’. Prograde metamorphism accompanied deformation, with the thermal peak in the Glen Roy district (T = 500–600 °C; P = 7.0–8.0 kbar) coinciding with ‘D2’ folding. It is suggested that the regional variation in the style of deformation was controlled, at least in part, by pre-existing major structures, most notably fundamental, transverse basement lineaments. Temporal changes in the style of deformation occurred in response to changing crustal architecture during progressive deformation. The study demonstrates the diachronous nature of deformation and also illustrates how its is possible to preserve areas with simple deformation histories in the centre of a major orogenic belt.

Chemical and textural equilibration of garnet during amphibolite facies metamorphism: The influence of coupled dissolution–reprecipitation

“…Primary chlorite is absent from any of the rocks examined in this study or by Phillips et al. (). Although some investigations have reported chlorite‐bearing assemblages in the general vicinity (Farber et al., ), the textural descriptions and images presented of these rocks are suggestive of retrograde chlorite after biotite.…”

“…Relatively large (1–4 mm) abundant garnet porphyroblasts occur in a schistose matrix that is dominated by strongly aligned fine grained (~50–100 μm) muscovite, together with quartz and plagioclase (Figure a). Progressive deformation has formed an early S1 foliation and a dominant S2 foliation (Phillips et al., ). In all of the samples from this location, a third deformation phase takes the form of open minor folds of the penetrative S2 cleavage.…”

“…Regional metamorphism of the Glen Roy rocks was probably due to the Grampian Event, c. 480 Ma, in the early phase of the Caledonian Orogeny (Dempster et al, 2002;Oliver, Chen, Buchwaldt, & Hegner, 2000), although the extent of earlier Precambrian metamorphism in the Dalradian block is uncertain (Dempster & Jess, 2015). Previous studies have documented the structural evolution of the area and the timing of porphyroblast growth relative to these deformation phases (Phillips & Key, 1992;Phillips et al, 1994). Peak metamorphic conditions are estimated at 500-600°C, 0.7-0.8 GPa (Phillips et al, 1994;Richardson & Powell, 1976), and the lack of staurolite in these rocks is attributed to the scarcity of primary chlorite (Phillips et al, 1994).…”

“…Previous studies have documented the structural evolution of the area and the timing of porphyroblast growth relative to these deformation phases (Phillips & Key, 1992;Phillips et al, 1994). Peak metamorphic conditions are estimated at 500-600°C, 0.7-0.8 GPa (Phillips et al, 1994;Richardson & Powell, 1976), and the lack of staurolite in these rocks is attributed to the scarcity of primary chlorite (Phillips et al, 1994). The absence of chloritoid from the vast majority of Dalradian pelites in lower grade assemblages, and specifically within the Leven schists from the Glen Roy, area has been noted, either as a matrix phase or as inclusions within other porphyroblasts (Phillips & Key, 1992).…”

“…Farber, Caddick, & Timm, 2014) indicate that the rocks plot below the garnetchlorite tie line in AFM projections, this together with the relatively high modal proportion of biotite to garnet indicates that chloritoid is probably not involved in the formation of staurolite. In addition, given the estimated relatively high pressures of Barrovian metamorphism in this location (Phillips et al, 1994), it is probable that chloritoid breakdown would occur at lower temperatures than the garnet chlorite breakdown reaction (Droop & Harte, 1995). On this basis, we would predict that staurolite-bearing assemblages should contain most textural and chemical evidence of garnet dissolution, and based on AFM phase relationships contain more Mg-rich biotite and garnet compositions than the staurolite-free assemblages (Thompson, 1957).…”