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The Lewisian Gneiss Complex of North-west Scotland represents a c. 250 km x 150 km fragment of the North Atlantic Craton, comprising tonalite-trondhjemite-granodiorite (TTG) gneisses that were formed in the Archaean and successively reworked during the Palaeoproterozoic. This talk will review new work on the event history of the Lewisian, and its relationship to the wider North Atlantic Craton. A significant programme of zircon dating over the last two decades showed that the protoliths of the Lewisian Gneiss Complex TTG gneisses were formed in the Archaean (c. 3100-2800 Ma) and that different protolith ages occur in different terranes within the Lewisian Gneiss Complex (Kinny et al., 2005). Similar relationships have been recognised in the Archaean gneisses of Greenland. However, most Lewisian gneisses are characterised by a ‘smear’ of zircon ages along concordia between c. 3000 and 2500 Ma, and it is difficult to disentangle protolith and metamorphic ages. Mafic to ultramafic rocks, and some supracrustals, are locally associated with the TTG gneisses; their ages are uncertain, but they may represent the remnants of Archaean greenstone belts.

The Lewisian Gneiss Complex of North-west Scotland represents a c. 250 km x 150 km fragment of the North Atlantic Craton, comprising tonalite-trondhjemite-granodiorite (TTG) gneisses that were formed in the Archaean and successively reworked during the Palaeoproterozoic. This talk will review new work on the event history of the Lewisian, and its relationship to the wider North Atlantic Craton. A significant programme of zircon dating over the last two decades showed that the protoliths of the Lewisian Gneiss Complex TTG gneisses were formed in the Archaean (c. 3100-2800 Ma) and that different protolith ages occur in different terranes within the Lewisian Gneiss Complex (Kinny et al., 2005). Similar relationships have been recognised in the Archaean gneisses of Greenland. However, most Lewisian gneisses are characterised by a ‘smear’ of zircon ages along concordia between c. 3000 and 2500 Ma, and it is difficult to disentangle protolith and metamorphic ages. Mafic to ultramafic rocks, and some supracrustals, are locally associated with the TTG gneisses; their ages are uncertain, but they may represent the remnants of Archaean greenstone belts.

Editors: Aug. 1859-July 1865, J. D. White, J. H. McQuillen, G. J. Ziegler.--Aug. 1865-Dec. 1871, J. H. McQuillen, G. J. Ziegler.--Jan. 1872-May 1891, J. W. White.--July 1891-Apr. 1930, E. C. Kirk (with L. P. Anthony, Dec. 1917-Apr. 1930).--May 1930-Dec. 1936, L. P. Anthony., Vols. 1-13 are called "new series.", Merged in Jan. 1937 with: Journal of the American Dental Association, ISSN 1048-6364, to form: Journal of the American Dental Association and dental cosmos, ISSN 0375-8451., The Dental cosmos; a monthly record of dental science. [Vol. 68] : Vol 68 : Issue 1, Page(s) 100, (dlps) volume: 0527912.0068.001, (dlps) article: 0527912.0068.001:32, http://quod.lib.umich.edu/t/text/accesspolicy.html

Although there is evidence for periodic geological perturbations driven by regular or semi-regular extra-terrestrial bombardment, the production of Earth’s continental crust is generally regarded as a function of planetary differentiation driven by internal processes. We report time series analysis of the Hf isotopic composition of zircon grains from the North Atlantic and Pilbara cratons, the archetypes of Archean plate tectonic and non-plate tectonic settings, respectively. An ~170–200 m.y. frequency is recognized in both cratons that matches the transit of the solar system through the galactic spiral arms, where the density of stars is high. An increase in stellar density is consistent with an enhanced rate of Earth bombardment by comets, the larger of which would have initiated crustal nuclei production via impact-driven decompression melting of the mantle. Hence, the production and preservation of continental crust on the early Earth may have been fundamentally influenced by exogenous processes. A test of this model using oxygen isotopes in zircon from the Pilbara craton reveals correlations between crust with anomalously light isotopic signatures and exit from the Perseus spiral arm and entry into the Norma spiral arm, the latter of which matches the known age of terrestrial spherule beds. Our data support bolide impact, which promoted the growth of crustal nuclei, on solar system transit into and out of the galactic spiral arms.