U-PB ISOTOPIC EVIDENCE FOR THE ACCRETION OF DIFFERENT CRUSTAL BLOCKS TO FORM THE LEWISIAN COMPLEX OF NORTHWEST SCOTLAND

Single zircon and titanite U-Pb SHRIMP data presented for tonalite-tro ndhjemite-granodiorite (TTG) suits gneisses and an ultramafic rock fro m the northern and central regions of the Lewisian Complex of northwes t Scotland, show that protolith ages of tonalitic gneisses in the nort hern region (2800-2840 Ma) art significantly younger than those in the central region (2960-3030 Ma). Further evidence of a major (2490-2480 Ma) metamorphic event in the central region is documented by a metamo rphic zircon associated with a granulite facies ultramafic body. A dio ritic gneiss from the northern region has also been dated at c. 2680 M a. The northern region therefore does not comprise reworked central re gion rocks and consequently the old models for the evolution of the Le wisian which were based upon this concept need replacing. It is instea d proposed that two distinct crustal blocks, now the northern and cent ral regions, were tectonically juxtaposed along a boundary correspondi ng to the Laxford Front. Juxtaposition would appear to have occurred i n Proterozoic times, as it must have postdated the 2490-2480 Ma (?Inve rian) metamorphism recorded only in the central region, and the emplac ement of granite sheets restricted to the northern side of the boundar y. The first recorded event common to both regions is resetting of tit anite ages associated with c. 1750 Ma Laxfordian amphibolite facies me tamorphism. Zircon inheritance in rocks of both regions is scarce. Wit hin one zircon from the northern region a c. 3550 Ma core was found. T his represents the oldest known material from the region.