FORMATION OF GNEISS DOMES IN PRECAMBRIAN DOME-AND-KEEL PROVINCES: A CONSEQUENCE OF EXTENSIONAL TECTONICS

Precambrian crustal provinces include dome-and-keel architecture in which regional-scale domes of silicic/intermediate pluton and/or gneiss are separated from one another by deep troughs ("keels") of volcanic and sedimentary supracrustal rocks. We distinguish between two types of such architecture. Archean granite-greenstone belts contain Type-M (for magma) dome-and-keel. Here, domes consist of plutonic rocks that intrude keels of similar-age greenstone. Paleoproterozoic orogenic belts contain Type-B (for basement) dome-and-keel. Here, domes consist of basement gneiss and granitoid (i.e., the domes are "gneiss domes") half-a-billion older than keel rocks, for keels contain Paleoproterozoic platform (including BIF) and foreland-basin deposits. In some cases, Type-B architecture overprints Type-M architecture. Models in which domes are emplaced during extensional collapse of orogens explains characteristics of dome-and-keel architecture. For example, some Type-M domes form during collapse of accretionary orogens built from buoyant oceanic crust and oceanic plateaus, or during collapse of hot-spot mounds, for collapse triggers decompression melting. Type-B domes form during collapse of Paleoproterozic collisional orogens; keels in this setting are steep-sided grabens in which supracrustal rocks dropped down into spaces between stretched basement, a process that resembles boudinage formation. Buoyancy of dome rocks with respect to dense supracrustals may help drive relative displacement of domes with respect to keels. Mylonitic shear zones form along the borders of gneiss domes in dome-and-keel, but the foliation of gneisses in the dome interior is a relict of pre-doming events. Notably, Type-B domes form in the same tectonic setting as Phanerozoic Cordilleran metamorphic core complexes. Structural contrasts between Phanerozoic core complexes and Type-B domes, may reflect changes in lithosphere strength during the past two billion years.