Fernando Corfu á Alan Crane Desmond Moser á Graeme Rogers U-Pb zircon systematics at Gruinard Bay, northwest Scotland: implications for the early orogenic evolution of the Lewisian complex Received: 23 October 1997 / Accepted: 20 July 1998 Abstract The Gruinard Bay area of the mainland Lewisian complex comprises a metamorphosed suite of Archaean trondhjemites and minor granites enclosing remnants of older tonalitic gneiss and ma®c to ultra- ma®c enclaves. The U-Pb zircon dating yields ages of 2731 14 Ma and 2728 2 Ma for two trondhjemite and 2732 4 Ma for one granite sample, also revealing the presence of large amounts of inherited xenocrystic zircons. Although the region has been pervasively overprinted by retrogressive events in amphibolite to greenschist facies, the textural relations between biotite, hornblende, quartz and titaniferous minerals indicate that these minerals are pseudomorphs of pyroxene and high-Ti amphibole formed in hornblende-granulite fa- cies. Structural relations link this metamorphism to a steep northeast-trending fabric coeval with the intrusion of the trondhjemites, dated at 2730 Ma. Dating of zir- con in amphibolite and tonalite enclaves yields complex internal isotopic relations with apparent ages ranging from 2825 to 2740 Ma. This age range re¯ects new growth during the 2730 Ma metamorphic/metasomatic events, superimposed on older zircon phases which in- clude combinations of xenocrystic cores, and magmatic and/or metamorphic growth phases whose mode of formation cannot clearly be resolved by imaging tech- niques (e.g. cathodoluminescence) alone. A pegmatitic vein that escaped the D 3 strain and related isotopic disturbances yields a precise age of 2792 2 Ma, which constrains to some degree the earliest orogenic events in the area. Age relationships displayed in the central block at Scourie±Badcall, and in the Gruinard Bay area indi- cate that petrogenetic events in both areas were com- parable about 2800 Ma and that both areas underwent trondhjemitic magmatism about 2730 Ma. In contrast, at Gruinard Bay there is no isotopic evidence for a pe- riod of high-grade metamorphism and magmatism at 2490±2480 Ma that drastically aected the Scourie block indicating that at this stage the two regions occupied dierent levels of the crust. Introduction Following the pioneering geological mapping by Peach et al. (1907) and by Sutton and Watson (1951), Giletti et al. (1961) were the ®rst to apply geochronological techniques to the study of the Lewisian complex in northern Scotland. Their work demonstrated an Arch- aean age for the complex, and set the stage for numerous subsequent isotopic studies on the origin and evolution of the high-grade gneisses, the timing and mode of metamorphism and the Proterozoic overprints (e.g. Evans 1965; Moorbath et al. 1969; Pidgeon and Bowes 1972; Evans and Lambert 1974; Chapman 1979; Ham- ilton et al. 1979; Humphries and Cli 1982; Heaman and Tarney 1989; Whitehouse 1988, 1989; Waters et al. 1990; Cohen et al. 1991; Burton et al. 1994; Corfu et al. 1994; Friend and Kinny 1995; Whitehouse et al. 1996, 1997; Zhu et al. 1997a,b; Kinny and Friend 1997). Many of these geochronological studies broke new ground in developing and testing advanced techniques, and gen- erated valuable but often controversial results, whose interpretation remains the object of intense debate. Critical problems of Lewisian geology concern the va- Contrib Mineral Petrol (1998) 133: 329±345 Ó Springer-Verlag 1998 F. Corfu (&) á D. Moser 1 Jack Satterly Laboratory, Royal Ontario Museum, 100 Queen's Park, Toronto, ON, Canada M5S 2C6 A. Crane Department of Geology and Petroleum Geology, University of Aberdeen, Meston Building, King's College, Aberdeen AB24 3UE, UK G. Rogers Isotope Geosciences Unit, Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 0QF, UK Present address: 1 Department of Geology and Geophysics, University of Utah, 135S, 1460 E, Salt Lake City, UT, 84112-0111, USA Editorial responsibility: I. Parsons