The current nomenclature for the Lewisian Gneiss Complex has evolved from lithological and structural correlations made prior to any dating. Initial (flawed) geochronological studies gave some names an apparent chronological standing but, as work advanced, fitting events into a coherent regional framework became increasingly difficult. Modern dating studies have shown that the Lewisian Gneiss Complex was progressively assembled from disparate blocks of Archaean continental crust and juvenile Proterozoic arcs, which satisfy the definition of terranes. Each terrane had its own separate accretionary and metamorphic history followed by a common history once juxtaposed against other terranes. Based on a new compilation of modern geochronology allied to the many detailed structural and metamorphic studies, this paper proposes a new systematic terminology for the Lewisian Gneiss Complex that is more applicable to this new tectonic framework.
The so-called 'southern region' of the mainland Lewisian Gneiss Complex is shown to be a package of isotopically distinct tectonostratigraphic units, from south to north the Rona, Ialltaig and Gairloch terranes. These were deformed during and following their accretion to the previously assembled Gruinard, Assynt and Rhiconich terranes to the north. SHRIMP U/Pb zircon geochronology has identified the 2730 Ma granulite facies metamorphic signature of the Gruinard Terrane in a tonalitic gneiss from the Tollie antiform, which is in sheared contact with the Palaeoproterozoic Gairloch Terrane (Loch Maree Group and Ard Gneiss) along the Creag Bhan Belt. Current structural interpretations of the region suggest the tectonic contact may be folded over the Carnmore antiform, coinciding with the high-strain zone along the Gruinard Belt. South of the Gairloch Terrane the Archaean gneisses of the Rona Terrane show a quite different pattern to those of the Gruinard Terrane, yielding older zircon protolith ages (ca. 3135-2880 Ma), and no evidence for the 2730 Ma Gruinard granulite facies metamorphism. Instead they show evidence for metamorphism and melting at ca.
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