Precambrian geological evolution

Abstract

Geologists from Brazil, the three Guianas, Venezuela, the northern Andean countries, North America, and Europe are participating in a new project of the International Geological Correlation Program, concentrating on the Precambrian geological evolution of the Amazonian region. Most of the major Precambrian rock units of the region are currently being studied by geologists in the various countries that have exposures of the Guiana and Guaporé shields, which make up the Amazonian craton. The craton is notable because of the great expanses of crust that formed in the Early Proterozoic. There are granite‐greenstone terranes as extensive as those of the Archean provinces of the North American shield and high‐grade gneiss and granulite terranes of both Archean and Proterozoic age. These provide opportunities to compare continental formation and evolution of Proterozoic age with their more ancient counterparts in other shields. Most of the craton was established in the Early Proterozoic and was affected by the Trans‐Amazonian orogeny; about half of this was then covered by Middle Proterozoic intracratonic sedimentray basins, associated felsic volcanics, mafic intrusives, and epizonal granitoid rocks. The felsic magmatic rocks of the Middle Proterozoic are particularly extensive and well exposed: This must rank as one of the prime regions in the world to study such rocks. Alkaline complexes with associated carbonatites occur in Brazil, Guyana, and Venezuela. All of the rock types mentioned have important ore deposits associated with them, and one of the goals of the project is to develop further understanding of the mineral potential and metallogenesis of the region.