Autochthonous and allochthonous strata of the El Callao greenstone belt: Implications for the nature of the Paleoproterozoic Trans-Amazonian orogeny and the origin of gold-bearing shear zones in the El Callao mining district, Guayana shield, Venezuela

The tectonic style of the Paleoproterozoic Trans-Amazonian orogeny in the Guayana shield was considered by previous workers to represent non-actualistic tectonics in which rising masses of granitoid magma of the Supamo complex deformed 2.1–2.0 Ga supracrustal belts of volcanic and sedimentary rocks to form greenstone belts. Subsequent collision with the dominantly granulite-grade Imataca complex deformed the greenstone belts. However, the base of the greenstone belts is everywhere marked by a major mylonitic shear zone that placed rocks of the greenstone belts atop a regional unit of quartzite that sits unconformably upon both the Imataca and Supamo complexes. The simplest interpretation of these relationships is that the greenstone belts are allochthonous with respect to the Imataca and Supamo complexes. As the quartzite oversteps the contact between the Imataca and Supamo complexes, juxtaposition of the two complexes must predate deposition of the quartzite and deformation of the Trans-Amazonian orogeny. artzite and the thrust fault are used as paleohorizontal markers to unfold the greenstone belt in the El Callao area where auriferous shear zones, which clearly predate the thrusting, show reverse sense of movement. When restored to their pre-folding orientation they are normal faults. This, and the tholeiitic to calc-alkaline volcanic packages, are compatible with an extensional forearc to arc setting for both the mineralization and the volcano-sedimentary sequences. ortheasterly trending folds of both the allochthon and the autochthon, coupled with the northerly thickening of the quartzite, suggest that the allochthon was part of an unknown craton that formerly lay to the north. A second regional fold set, which trends northerly, refolds the earlier structures. It is possibly related to collision of the San Carlos terrane with the amalgamated Imataca–Supamo–Pastora terranes. The unconformably overlying 1.98 Ga Cuchivero arc terrane provides a minimum age for the two collisional events.