CONTRASTING MECHANISMS FOR GNEISS-DOME FORMATION IN PRECAMBRIAN DOME-AND-KEEL PROVINCES: A RECORD OF EVOLVING STYLES OF CRUSTAL FLOW

Archean and Paleoproterozoic crust includes dome-and-keel provinces (granite-greenstone terranes) in which felsic-intermediate granitoid, migmatite, and/or gneiss occurs in equant to elliptical domes. Domes are surrounded by deep keels (troughs) containing supracrustal assemblages of mafic-ultramafic volcanics, and iron-rich sediment. Although all dome-and-keel provinces share characteristics reflecting compositional and rheological contrasts between dome and keel rocks, comparisons among examples globally reveal that there are 3 distinct end-member types which differ based on mode of origin: Abitibi-type provinces: In these, domes are primarily elliptical and consist of juvenile TTG granitoids intruding similar-age greenstones. Domes lie within an anastamosing network of greenstone belts and shear zones. Numerous studies suggest that these provinces formed along oblique-convergent plate boundaries, where buoyant sea-floor from either subducting slabs or back-arc basins, as well as LIPs of mafic and ultramafic rocks, accreted in a transpressional strain field; Pilbara-type provinces: In these, domes consist dominantly of granitoids formed by anatexis of preexisting basement, and dome emplacement occurred 50-100 million years after emplacement of a greenstone blanket. Domes in these provinces are bordered by keel-side-down shear zones displaying lineations that trend radially outward relative to dome centers. Many authors argue that these provinces formed by diapiric remobilization of basement following incubation beneath the greenstone; Penokean-type provinces: In these, domes consist of basement that is more than a 0.5 billion years older than the youngest strata (including BIF) in keels, and keel borders are steep normal-sense shear zones. These provinces formed by chocolate-tablet extensional collapse of a fold-thrust belt. The tectonic setting in which Penokean-type provinces form are similar to that in which some Phanerozoic gneiss domes and core complexes form, but the style of deformation is different. Differences reflect the fact that Archean and Paleoproterozoic orogens differ from Phanerozoic orogens—in the former, temperature increased more rapidly with depth and the density of supracrustal sequences significantly exceeded that of basement than in the latter.