On the allochthonous nature of auriferous greenstones, Guayana shield, Venezuela

The tectonic style of Paleoproterozoic orogeny in the Guayana shield has been considered by most workers to be similar to those commonly invoked for the development of Archean greenstone belts, in which diapirs of granitoid magma (Supamo complex) rose and deformed 2.1–2.0 Ga volcanic and sedimentary rocks of the Pastora supergroup to form keel-like greenstone belts. In this model, subsequent collision with the dominantly granulite-grade Imataca complex to the north deformed the greenstone belts during what is known as the Trans-Amazonian orogeny. However, more recent geological mapping demonstrated that the base of the greenstone belts is marked by a major mylonitic shear zone that placed rocks of the belts atop a regional unit of quartzite that sits unconformably upon granitic gneisses and plutons of both the Imataca and Supamo complexes. These relations were interpreted to indicate that the greenstone belts are allochthonous with respect to the Imataca and Supamo complexes, and as the quartzite oversteps the contact between the Imataca and Supamo complexes, juxtaposition of the two complexes predated deposition of the quartzite. New U–Pb zircon results show that volcanic rocks of the Pastora supergroup are older than the quartzite beneath them and confirm the hypothesis, based entirely upon field relations, that rocks of the greenstone belts were thrust upon the quartzite and its granitic and gneissic basement. At least two subsequent periods of deformation refolded rocks of the region. Similar structural relationships are common in Archean cratons of southern Africa, Australia, and the Canadian shield.