Introduction Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol105.Issue2-4.2001:

Precambrian Research 105 2001 227 – 245 Geochronological constraints on Paleoproterozoic crustal evolution and regional correlations of the northern Outer Hebridean Lewisian complex, Scotland Martin J. Whitehouse a, , David Bridgwater b,1 a Swedish Museum of Natural History, Box 50 007 , SE- 104 05 Stockholm, Sweden b Geological Museum, Copenhagen Uni6ersity, DK- 1350 Copenhagen, Denmark Received 12 January 1999; accepted 23 June 1999 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 ThU 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 andor 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. © 2001 Elsevier Science B.V. All rights reserved. Keywords : Lewisian; Paleoproterozoic; Absolute age U – Pb zircon; Laurentia – Fennoscandia www.elsevier.comlocateprecamres Corresponding author. Fax: + 46-8-51954031. E-mail address : martin.whitehousenrm.se M.J. White- house. 1 Deceased.

1. Introduction

The Lewisian complex is exposed as the fore- land to the Caledonian orogen in north-west 0301-926801 - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 0 0 0 0 1 1 3 - 3 mainland Britain, and throughout the ca. 200 km length of the Outer Hebrides archipelago. Broadly similar, late-Archean tonalite – trondhjemite – gra- nodiorite TTG suite grey gneisses dominate these major outcrops, together with Paleoprotero- zoic mafic intrusions Scourie dykes and related units, and substantial post-Scourie dyke struc- tural reworking and granite intrusion. On a re- gional basis, these same broad features suggest correlations to the Archean cratons of Laurentia East Greenland and North Atlantic cratons and Fennoscandia or Baltica; Karelian craton, with their respective Paleoproterozoic reworking Nag- suggtoquidian and Svecokarelian and crust gener- ation Ketilidian and Svecofennian events. The Outer Hebridean Lewisian has not at- tracted the same intensive geochemical and geochronological investigations that have been carried out on the mainland over the past three decades, although there is a wealth of literature on structural aspects of the widespread Pale- oproterozoic events see summaries by Fettes et al., 1992; Park et al., 1994. As a result, attempts to correlate the two main outcrops of the Lewisian remain speculative e.g. Coward and Park, 1987. Recent geochronological and isotope geochemical studies of the mainland Lewisian Whitehouse, 1989; Kinny and Friend, 1997; Whitehouse et al., 1997a and the Outer Hebrides Whitehouse, 1990a; Cliff et al., 1998 provide a better database for such correlations, both within the Lewisian and in a wider regional context. In this paper, we present new U – Pb zircon geochronology ion-microprobe for four rocks from key localities in the northern Outer Hebrides which, together with a synthesis of existing iso- topic and geochronological data, permit a better constrained evaluation of the Paleoproterozoic evolution and possible correlations.

2. The Outer Hebridean Lewisian complex