Palaeoproterozoic Evolution of the Outer Hebridean Lewisian Complex, Northwest Scotland: Constraints from Ion Microprobe Zircon Geochronology

Martin Whitehouse (martin.whitehouse@nrm.se)1 &

David Bridgwater2

1 Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden

2 Geological Museum, Copenhagen University, Copenhagen, Denmark

The late Archaean Lewisian Complex of north-west Scotland was extensively reworked during the Palaeoproterozoic (locally termed Laxfordian). Correlation with Palaeoproterozoic collisional orogens throughout the Laurentia - Fennoscandia region (e.g. Whitehouse et al., 1997), however, is complicated by a paucity of appropriate ages in the Lewisian literature. In one of the earliest detailed studies of the Lewisian of the Outer Hebrides, Dearnley (1962) recognised a post-Scourie dyke (i.e. < 2.0 - 2.4 Ga) granulite facies event which he termed "early" Laxfordian, followed by "late" Laxfordian retrogression and formation of granite - migmatite complexes which are particularly well-developed in north Harris and south-west Lewis (Myers, 1971). The latter are dated at ca. 1.7 Ga (van Breemen et al., 1971) and are broadly correlated with granites of the mainland Lewisian in the type Laxfordian (Loch Laxford). Recently, granulite facies metamorphism in South Harris has been constrained to > 1.827 ± 0.016 Ga from an Sm-Nd mineral isochron (Cliff et al., 1998). To date, however, geochronological confirmation of the regional nature of early Laxfordian events has been based upon relatively imprecise and model dependent Sm-Nd whole-rock model ages (Whitehouse, 1990a) or Pb-isotope evolution arguments (Whitehouse, 1990b), with other chronometers (e.g. Ar-Ar, Cliff et al., 1998) responding to late Laxfordian events.

Ion-microprobe U-Th-Pb data are presented from a number of localities throughout the northern Outer Hebrides. Cathodoluminescence imaging (CL) of zircons from a diorite at the Butt of Lewis reveals a complex polyphase history starting at ca. 2.85 Ga. Most grains have concordant, low-Th/U euhedral tip overgrowths (metamorphic?) which yield a weighted average 207Pb/206Pb age of 1860 ± 10 Ma (2). Zircons from a tonalite at Borve, South Harris show oscillatory zoning in CL and crystallised from a melt at 1881 ± 13 Ma (2, weighted average 207Pb/206Pb age), with rare late Archaean cores suggesting some interaction with older continental material. Detrital grains from a Langavat supracrustal sample yield concordant 207Pb/206Pb ages in the range 1.83 - 2.15 Ga, with a strong peak at 1874 ± 6 Ma (2, weighted average of 13 analyses). These data provide the first unambiguous evidence for regional ca. 1.87 Ga events in the northern Outer Hebrides, with development of a calc-alkaline continental marginal arc and contemporaneous, probably arc derived, sedimentation, only a few tens of Ma prior to granulite facies metamorphism.

Whitehouse MJ, Bridgwater D & Park RG, Terra Nova, 9, 260-263, (1997).

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van Breemen O, Aftalion M & Pidgeon RT, Scott J. Geol., 7, 139-152, (1971).

Cliff RA, Rex DC & Guise PG, Precamb. Res, 91, 401-418, (1998).

Whitehouse MJ, Scott J. Geol., 26, 131-136, (1990a).

Whitehouse MJ, Isotope. Geosci., 86, 1-20, (1990b).

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