Structural characteristics and Re–Os dating of quartz-pyrite veins in the Lewisian Gneiss Complex, NW Scotland: Evidence of an Early Paleoproterozoic hydrothermal regime during terrane amalgamation

Abstract

In the Archaean basement rocks of the Assynt and Gruinard terranes of the mainland Lewisian Complex in NW Scotland, a regional suite of quartz-pyrite veins cross-cut regional Palaeoproterozoic (Badcallian, ca. 2700Ma; Inverian, ca. 2480Ma) fabrics and associated Scourie dykes. The quartz veins are overprinted by amphibolite-greenschist facies Laxfordian deformation fabrics (ca. 1760Ma) and later brittle faults. The hydrothermal mineral veins comprise a multimodal system of tensile/hybrid hydraulic fractures which are inferred to have formed during a regional phase of NW-SE extension. The almost orthogonal orientation of the quartz veins (NE-SW) to the Scourie dykes (NW-SE) are incompatible and must result from distinct paleostress regimes suggesting they are related to different tectonic events. This hypothesis is supported by Rhenium–Osmium dating of pyrite that yields an age of 2249±77Ma, placing the vein-hosted mineralisation event after the oldest published dates for the Scourie Dykes (2420Ma), but before the youngest ages (1990Ma). Sulphur isotope analysis suggests that the sulphur associated with the pyrite is isotopically indistinguishable from primitive mantle. The presence of the ca. 2250Ma quartz-pyrite veins in both the Assynt and Gruinard terranes confirms that these crustal units were amalgamated during or prior to Inverian deformation. The absence of the veins in the Rhiconich Terrane is consistent with the suggestion that it was not finally amalgamated to the Assynt Terrane until the Laxfordian.