The Loch Maree Group: Palaeoproterozoic subduction-accretion complex in the Lewisian of NW Scotland

The Lewisian complex of northwest (NW) Scotland has long been correlated wi th intercontinental Palaeoproterozoic belts of the North Atlantic region bu t uncertainty about the age and origin of the supracrustal rocks of the Loc h Maree Group (LMG) and the apparent lack of subduction-related intrusive r ocks have precluded interpretations of a similar tectonic setting for the L ewisian. We present integrated field, geochemical and geochronological data that resolve both issues and are consistent with an intercontinental setti ng. The LMG is made up of two components, one oceanic (plateau basalts or p rimitive arcs, plus associated abyssal sediments, ferruginous hydrothermal deposits, and platform carbonates) and the other continental (deltaic flysc h, greywacke shale). The metasediments have geochemical characteristics tha t imply a source outside the Archaean gneisses of the Lewisian, an interpre tation that agrees with the detrital zircon populations (from the Flowerdal e schists) that have a significant 2.2-2.0-Ga component. The Ard gneiss, fo rmerly regarded by some as a tectonic sliver of basement, is a strongly fol iated granodiorite that occurs in sheets intrusive into the LMG, and has gi ven a U-Pb crystallisation age of 1903 +/- 3 Ma, consistent with its syntec tonic relationship with the major D1/D2 phase of Proterozoic deformation. T he gneiss has a rather primitive geochemistry, which implies that it was no t generated by melting of the local metasediments but was derived by partia l melting of a more mafic source. The most likely model is that the LMG evo lved as an accretionary complex, modern parallels of which can be found in the Shimanto belt in Japan, Rhodope in north Greece and Colombia and the Ca ribbean. The various elements of the complex became tectonically intermixed and subject to extreme deformation during accretion to the overriding Lewi sian continent. Eventual relaxation and exhumation of the accretionary comp lex may have resulted in the generation of the Ard gneiss (possibly by melt ing of the underplated oceanic plateau) followed by collision with the cont inental crust of the lower plate. The younger D3 phase of the Palaeoprotero zoic deformation sequence was coincident with the emplacement of the Tollie pegmatites at 1.7 Ga, c 200 m. years after the main collisional event, and may be related to a younger accretionary event (Labradorian?). (C) 2001 El sevier Science B.V. All rights reserved.