Emplacement and thermal metamorphism associated with the post-orogenic Strath Ossian Pluton, Grampian Highlands, Scotland

Abstract

The multiphase Strath Ossian Pluton was intruded into metasedimentary rocks of the Neoproterozoic Grampian and Appin groups (Grampian Highlands, Scotland) during Silurian or early Devonian times. Emplacement followed the main ductile tectono-thermal history of the area and took place during post-orogenic regional uplift and cooling. Early emplacement of dioritic magma in the northern part of the pluton resulted in migmatization of its immediate country rocks with the generation of new ductile structures. The main granodiorite was then emplaced with magma migrating towards the southeast where wall-rock stoping took place. Elsewhere the pluton created its own space with little stoping or veining. Thermal metamorphism caused by granodiorite emplacement resulted in the progressive development of the assemblage quartz -I-plagioclase + biotite + cordierite + andalusite + K-feldspar in the metapelitic country rocks. Six prograde mineral assemblage zones are identified in the aureole. Final emplacement of a marginal porphyritic microgranite was accompanied by the release of alkaline fluids into the thermal aureole. This produced sillimanite (fibrolite) in association with hydrous phases such as chlorite and white mica. The development of andalusite and cordierite-bearing assemblages is estimated to have occurred at temperatures of 650±50°C at an estimated pressure of 3.2 + 0.5 kbars. An approximately isobaric temperature change of 300 ±50 °C across the width of the main aureole is deduced. The migmatisation close to the plutons margins took place at temperatures of about 700 °C. An estimated depth of emplacement of about 11 km is obtained for the Strath Ossian Pluton. This implies considerable regional uplift both prior to, and immediately after its emplacement. Thus it has been estimated that at the peak of regional metamorphism, probably during the Ordovician Period, the country rocks were at a depth of 15 to 18.5 km, whereas the early Devonian dykes of the Etive dyke swarm, which cut the Strath Ossian Pluton, were emplaced at, or near surface.

Thermodynamic modelling of the reaction muscovite+cordierite→Al2SiO5+biotite+quartz+ H2O: constraints from natural assemblages and implications for the metapelitic petrogenetic grid

“…Textural evidence (see below) supports this interpretation. Other localities that indicate a similar isograd sequence include the Strath Ossian aureole (Key et al ., 1993), in which andalusite is developed upgrade of the assemblage Ms + Crd + Bt + Qtz, and the regional contact metamorphic setting near Whitehead Harbour, Nova Scotia (Raeside et al ., 1988).…”

A new ‘microstructural mapping’ methodology for the identification, analysis and interpretation of polyphase deformation within subglacial sediments

“…and Key 1992) defined by alternating quartz (white to grey) and mica (predominantly muscovite, blue to yellow-green colour) domains; (b) Poikiloblast of garnet wrapped by a well-developed schistosity (defined by muscovite (pink to yellow-green) within a amphibolite facies garnet-biotite-mica schist (Neoproterozoic Leven Schist, Glen Roy, Scotland), note the presence of lenticular pressure shadows composed of quartz immediately adjacent to the garnet; (c) Rotated, "snowball" garnet porphyroblast developed within a partially annealed, micaceous quartz mylonite (Eilrig Shear Zone, near Fort Augustus, Scotland; see Phillips et al, 1993); (d) Anhedral andalusite (pale grey) porphyroblast overgrowing an S2 crenulation cleavage within a contact metamorphosed garnet-biotite-mica-schist (Strath Ossian, Scotland). The andalusite developed as a result of thermal or contact metamorphism associated with the emplacement of the Strath Ossian granite pluton; see Key et al, 1993). Also note the development of a biotite (red-brown) reaction rim on the relict garnet porphyroblast due to the break down of this mineral during this contact metamorphic event; (e) Well-developed crenulation cleavage developed within a middle to upper greenschist facies chlorite-muscovite-schist (Neoproterozoic Southern Highlands Group, Scotland).…”

The Glas Bheinn Appinitic Complex, Glen Roy: a model for foliation development during emplacement

“…Thermal metamorphism associated with this post-orogenic granitic intrusion resulted in the development of andalusite-and cordierite-bearing assemblages in its aureole at estimated temperatures of 650 ± 50°C and pressures of 3.2 ± 0.5 kbar (Key et al 1993). Intrusion of the Strath Ossian Pluton post-dated emplacement of the appinitic suite in Glen Roy.…”