PROTEROZOIC CRUSTAL EXTENSION AND THE GENERATION OF DOME-AND-KEEL STRUCTURE IN AN ARCHEAN GRANITE GREENSTONE TERRANE

ARCHAEAN granite-greenstone terranes 1-3, in which narrow belts of 'greenstone' (ultramafic and mafic volcanics) and overlying sedimentary rocks occur in association with broad provinces of 'granite' (granitic igneous rocks, gneiss and migmatite), have long puzzled geologists because of the lack of any clear modern analogues. Not only is there uncertainty about how the greenstone formed in the first place 4-6, but there is continuing debate about how and when the terranes developed their distinctive structural and metamorphic architecture. Many granite-greenstone terranes display a dome-and-keel geometry, in which belts of supracrustal (volcanic and sedimentary) rocks occur as structural troughs wedged between dome-shaped bodies of the granite-gneiss-migmatite complex. A metamorphic aureole typically occurs in the supracrustals adjacent to their contact with the domes, and the contact itself is a shear zone. Here we report the results of field studies of a granite-greenstone terrane in Brazil, showing that this architecture originated during the Proterozoic, more than 500 million years after extrusion of the greenstone. We suggest that the thermal regime and deformation kinematics necessary to create this architecture could be generated during an episode of crustal extension, when hot basement rocks were transported upwards along a transcrustal normal fault system to the base of the supracrustals.