Geochronological constraints on Paleoproterozoic crustal evolution and regional correlations of the northern Outer Hebridean Lewisian complex, Scotland

Abstract

Ion-microprobe U–Th–Pb geochronological data are presented for four samples from Paleoproterozoic belts in the Lewisian of the northern Outer Hebrides, north-west Scotland. Two of these samples, a tonalite sheet associated with the South Harris igneous complex, and a psammite from the Leverburgh metasupracrustal belt, South Harris, yield zircons with a dominant ca. 1.87 Ga age. These are interpreted as the igneous crystallisation age for the tonalite and the source rock for the psammite, and their age concordance suggests that the latter was developed in an arc basin sequence, derived largely from contemporaneous igneous rocks, and buried during collision, which resulted in documented >1.83 Ga high-grade metamorphism. A diorite from the Paleoproterozoic shear zone at the northern tip of Lewis has a probable 2.7–2.8 Ga protolith age, although its zircons have strongly been affected by Pb-loss during later events culminating in development of low Th/U overgrowths at ca. 1.86 Ga. Zircons from a tonalite from Berneray in the Sound of Harris yield an Archean crystallisation age of ca. 2.83 Ga, with no indication of later disturbance, thus providing a southern limit to the region affected by Paleoproterozoic tectonothermal events. The Paleoproterozoic arc in South Harris represents a major tectonic boundary (active margin) in the Lewisian of the Outer Hebrides, possibly correlated with the Laxford or Gairloch shear zones of the mainland Lewisian. Contrasts in the flanking region geology and geochronology, possibly reflecting lateral heterogeneities, may be introduced by major thrusts and/or extensional faults (e.g. the Outer Isles fault) developed between the shear zones. On a broader regional scale, evidence for a magmatic arc in the Lewisian is consistent with the tectonic style of other ca. 1.9 Ga Paleoproterozoic collisional orogens throughout Laurentia–Fennoscandia, suggesting a reappraisal of the formerly proposed intracratonic evolution of the Lewisian at this time.