P–T conditions of Grenville-age eclogite facies metamorphism and amphibolite facies retrogression of the Glenelg–Attadale Inlier, NW Scotland

Abstract

Peak and retrograde P-T conditions of Grenville-age eclogites from the Glenelg-Attadale Inlier of the northwest Highlands of Scotland are presented. Peak conditions are estimated as c. 20 kbar and 750-780 • C, in broad agreement with previous work. The eclogites subsequently followed a steep decompression path to c. 13 kbar and 650-700 • C during amphibolite facies retrogression. Peak eclogite facies metamorphism occurred > 1080 Ma and retrogression at c. 995 Ma, suggesting fairly sluggish uplift rates of < 0.3 km/Ma and cooling rates of < 1.25 • C/Ma, when compared with other parts of the Grenville orogeny and/or modern orogens. However, current poor constraints on the timing of peak metamorphism mean that these rates cannot be used to interpret the geodynamic evolution of this part of the orogen. The P-T-t data, together with petrology and the field relationships between the basement rocks of the Glenelg-Attadale Inlier and the overlying Moine Supergroup, mean that it is difficult to support the currently held view that an unconformable relationship exists between the two. It is suggested that more data are required in order to re-interpret the Neoproterozic tectonic evolution of the northwest Highlands of Scotland.

Meso-Neoproterozoic Grenville-Sveconorwegian intracontinental orogen: history, tectonics, geodynamics

“…В тесной временнóй и пространственной связи с ГСНО находятся приуроченные к юго-западному и северо-восточному ограничениям провинции Грен-вилл комплексы эклогитов субдукционного типа [Carlson et al, 2007;Mosher et al, 2008;Sanders et al, 1984;Sanders, 1988;Storey et al, 2005] и осадочно-вулканогенный комплекс рифта Кьюиноу-Мидкон-тинент [Van Schmus et al, 1982;Vervoort et al, 2007] (рис. 1).…”

“…Высокобарные породы на северо-восточном фланге сектора исследованы на Северо-Западном плоскогорье Шотландии в восточной части текто-нического окна Гленелг-Аттадейл [Sanders et al, 1984;Sanders, 1988;Storey et al, 2005; и ссылки там же]. С тоналит-трондьемит-гранодиоритовыми гнейсами ассоциированы многочисленные включе-ния кианит-и омфацитсодержащих гнейсов, мафи-товых эклогитов и марганцовистых метаосадков.…”

Multiple high-pressure metamorphic events and crustal telescoping in the NW Highlands of Scotland

“…It has been directly dated using U-Pb geochronology of titanite growing only within fabrics within the shear zone at 669 AE 31 Ma (Storey et al 2004). However, retrograde reactions within eclogites to amphibolite facies during decompression from c. 20 kbar to 13 kbar within the Eastern Unit (Storey et al 2005) can be correlated with fabrics within the Barnhill Shear Zone, and this retrogression has been dated by U-Pb zircon at 995 AE 8 Ma (Brewer et al 2003). This remains an unresolved problem, but movement is bracketed to the period c. 995-670 Ma.…”

A field guide to the Glenelg-Attadale Inlier, NW Scotland, with emphasis on the Precambrian high-pressure metamorphic history and subsequent retrogression

“…Where not retrogressed they contain garnet, omphacite, plagioclase, quartz, kyanite, rutile, biotite and scapolite (Sanders 1989;Storey et al 2005) (Fig. 3e).…”

“…3e). From this assemblage Storey et al (2005) calculated peak P-T conditions of c. 20 kbar and 750 °C. Typically the omphacite has been replaced by amphibole, and garnet is commonly partially replaced by biotite.…”