MARKERS OF THE LAST STAGES OF THE PALAEOPROTEROZOIC COLLISION - EVIDENCE FOR A 2-GA CONTINENT INVOLVING CIRCUM SOUTH-ATLANTIC PROVINCES

The reconstruction of a Palaeoproterozoic continental block requires c orrelation between the major structural and lithological features of t he main continental provinces. The African and American circum-South A tlantic continents, for example, can be brought together in a pre-ocea n-opening fit and have therefore been the subject of many attempts at such correlation. The tectonic evolution of the Palaeoproterozoic fluv io-deltaic deposits in the West African, Guiana, Congo and Sao Francis co provinces is related to the 2-Ga collision orogeny. These formation s either rest directly upon Archaean blocks, as is the case for the Fr ancevillian in Gabon and the Jacobina Unit in Brazil, or they rest on the upper part of the Palaeoproterozoic, as with the Tarkwaian in Ghan a and in French Guiana. They were deposited, in Guiana and Gabon, in t ectonic settings of extension, in foreland and pull-apart basins. They are composed mainly of conglomerate and sandstone, and range from flu viatile, locally with debris flows, to deltaic. These sedimentary rock s 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 Afri ca, by the inheritance of detrital zircons and pebbles of foliated met amorphic rocks in the basal conglomerates. In Gabon and Brazil, these detrital formations were deposited on Archaean continental margins tha t became involved in the orogeny only at a late stage. They are all in terpreted as products of the collapse and breakup of the early orogeni c mountain belt formed during the Palaeoproterozoic collision. The str uctural and metamorphic evolution of these deposits show many similari ties throughout the provinces. The margins of the basins are overthrus t 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 Palaeoproterozoi c provinces during the latest stages of the collision orogeny. Metamor phism was generally in the greenschist facies, but muscovite-andalusit e parageneses are common and may be replaced by kyanite-chloritoid ass emblages in relation with the thickening and burial during overthrusti ng of the older rocks. The P-T conditions (450 degrees C and 3.5 kbar to 600 degrees C and 5 kbar) indicate that some of the fluvio-deltaic rocks were buried to depths of more than 10 km. This implies significa nt underthrusting and the formation of mountain belts. Comparison of t he tectonic evolution on either side of the South Atlantic shows that major convergence of the Archaean and Palaeoproterozoic terranes took place at similar to 2 Ga, contributing to the establishment of new con tinental blocks. The tectonic style of the fluvio-deltaic formations i ndicates a frontal collision between the Congo and Sao Francisco provi nces, whereas the collision between West Africa and Guiana was oblique , at least during the latest stages of the orogeny.