Synopsis

The Portsoy Gabbro was emplaced into Argyll Group Dalradian metasediments as part of a suite of mafic intrusives that outcrop around the low-P/high-T Buchan metamorphic domain in NE Scotland. This paper presents new field data together with geobarometry and calculated pseudosections for pelites and metabasites, and geochronological data from the Portsoy Shear Zone, on the western boundary of the Buchan domain, which constrain the relative and absolute timing of deformation, metamorphism and magmatism. Textural and cross-cutting relationships indicate the D2 event, magmatism and Buchan metamorphism overlapped. Pelitic schists containing all three Al2SiO5 polymorphs occur structurally beneath the Portsoy Gabbro. The sequence of aluminosilicate growth in these rocks is andalusite–sillimanite–kyanite, implying an anti-clockwise P–T–t path. This study indicates that the andalusite to sillimanite transformation took place during D2, synchronous with the intrusion of the Portsoy Gabbro, and was followed by kyanite growth after D2. The late growth of kyanite indicates that, at least initially, cooling was not associated with uplift. Magmatic zircons from the Portsoy Gabbro yield a U–Pb age of 471.3 ± 0.6 Ma. A U–Pb zircon age from an undeformed pegmatite that cuts the S2 foliation in the gabbro indicates D2 shearing had largely terminated by ~ 471 Ma. We demonstrate, using a metabasite pseudosection, that the titanite formed during deformation as the gabbros cooled. The cooling rate for the gabbro from 1000°C to 620°C was rapid – at least 49°C Ma−1 – and, allowing that the Portsoy Shear Zone was not an uplift structure, was caused by a change in the thermal regime.

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