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.