Markers of the last stages of the Palaeoproterozoic collision: evidence for a 2 Ga continent involving circum-South Atlantic provinces

Abstract

The reconstruction of a Palaeoproterozoic continental block requires correlation between the major structural and lithological features of the main continental provinces. The African and American circum-South Atlantic continents, for example, can be brought together in a pre-ocean-opening fit and have therefore been the subject of many attempts at such correlation. The tectonic evolution of the Palaeoproterozoic fluvio-deltaic deposits in the West African, Guiana, Congo and Sao Francisco provinces is related to the 2-Ga collision orogeny. These formations either rest directly upon Archaean blocks, as is the case for the Francevillian in Gabon and the Jacobina Unit in Brazil, or they rest on the upper part of the Palaeoproterozoic, as with the Tarkwaian in Ghana and in French Guiana. They were deposited, in Guiana and Gabon, in tectonic settings of extension, in foreland and pull-apart basins. They are composed mainly of conglomerate and sandstone, and range from fluviatile, locally with debris flows, to deltaic. These sedimentary rocks were deposited after the initial stages of a collision orogeny dated at more than 2.1 Ga. This event was recorded, in Guiana and West Africa, by the inheritance of detrital zircons and pebbles of foliated metamorphic rocks in the basal conglomerates. In Gabon and Brazil, these detrital formations were deposited on Archaean continental margins that became involved in the orogeny only at a late stage. They are all interpreted as products of the collapse and breakup of the early orogenic mountain belt formed during the Palaeoproterozoic collision. The structural and metamorphic evolution of these deposits show many similarities throughout the provinces. The margins of the basins are overthrust by older Palaeoproterozoic or by Archaean rocks. Structural geometry and kinematics of deformation in shear zones and recumbent folds are consistent with the overall tectonic evolution of the Palaeoproterozoic provinces during the latest stages of the collision orogeny. Metamorphism was generally in the greenschist facies, but muscovite-andalusite parageneses are common and may be replaced by kyanite-chloritoid assemblages in relation with the thickening and burial during overthrusting of the older rocks. The P-T conditions (450°C and 3.5 kbar to 600°C and 5 kbar) indicate that some of the fluvio-deltaic rocks were buried to depths of more than 10 km. This implies significant underthrusting and the formation of mountain belts. Comparison of the tectonic evolution on either side of the South Atlantic shows that major convergence of the Archaean and Palaeoproterozoic terranes took place at ∼ 2 Ga, contributing to the establishment of new continental blocks. The tectonic style of the fluvio-deltaic formations indicates a frontal collision between the Congo and Sao Francisco provinces, whereas the collision between West Africa and Guiana was oblique, at least during the latest stages of the orogeny.