Rhyacian Paleoproterozoic Hydrothermal Activity From Scotland

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

Authors:

Vernon et al

Abstract:

In the Archaean basement rocks of the Assynt and Gruinardterranes of the mainland Lewisian Complex in NW Scotland, a regional suite of quartz-pyrite veins cross-cut regional Palaeoproterozoic (Badcallian,ca.2700 Ma;Inverian, ca. 2480 Ma) fabrics and associatedScourie dykes. The quartz veins are overprinted by amphibolite-greenschistfaciesLaxfordian deformation fabrics (ca. 1760 Ma) 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 theScouriedykes (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 pyritethat yieldsan age of 2249 ± 77 Ma, placing the vein-hosted mineralisation event after the oldest published dates for the Scourie Dykes (2420 Ma), but before the youngest ages (1990 Ma). Sulphur isotope analysis suggests that the sulphur associated with the pyrite is isotopicallyindistinguishable from primitive mantle. The presence of the ca. 2250 Ma quartz-pyrite veins in both the Assynt and Gruinard terranesconfirms that these crustal units were amalgamated during or prior to Inverian deformation. The absence of the veins in the RhiconichTerrane is consistent with the suggestion that it wasnot finally amalgamated to the AssyntTerrane until the Laxfordian.