The age, geological character and structural setting of quartz-pyrite veins in the Assynt Terrane, Lewisian Complex, NW Scotland.

Abstract

A set of previously unrecognised quartz-pyrite veins are present in the Assynt Terrane of the mainland Lewisian Complex, NW Scotland. The veins cross-cut the Badcallian and Inverian fabrics, and the Scourie Dykes. The veins have been reworked by Laxfordian deformation fabrics (ca. 1.8 Ga) and later brittle faults of various ages. Fieldwork analyses suggest that the veins are a multi-modal system of tensile/hybrid fractures which are locally influenced by the existing foliation of the gneisses. They are inferred to have formed during regional NW-SE extension, an orientation that is almost orthogonal to the NE-SW extension direction associated with the intrusion of the Scourie Dykes. Microstructures within the quartz veins suggest that overprinting Laxfordian events reached maximum temperatures of 500°C under moderate strain rates, while pervasive ductile deformation was restricted mainly to the Canisp Shear Zone and was succeeded by brittle deformation as the temperature decreased but strain rates remained high within the shear zone. Re-Os dating of the pyrite within the quartz veins gives an age of 2259±61 Ma, placing the emplacement of the veins after the oldest dates for the Scourie Dykes (2420, 2400 & 2375 Ma) but before the youngest ages (1990 Ma). Sulphur isotope analysis suggests that the pyrite is of primitive mantle origin and may have been either stripped from the crust by fluid circulation or was associated with the intrusion of the Scourie dykes. The presence of the quartz-pyrite veins in both the Assynt and Gruinard Terranes suggest they were amalgamated during or prior to Inverian deformation while the absence of the veins in the Rhiconich Terrane is consistent with the suggestion that this terrane was not amalgamated until the Laxfordian Orogeny. The emplacement of the veins may linked to the formation and/or amalgamation of the Loch Maree Group supracrustal sequence.