THE ARCHAEAN/PROTEROZOIC CONTACT ZONE IN WEST-AFRICA - A MOUNTAIN BELT OF DECOLLEMENT THRUSTING AND FOLDING ON A CONTINENTAL-MARGIN RELATED TO 2.1-GA CONVERGENCE OF ARCHEAN CRATONS

Lithotectonic and metamorphic studies in various parts of the Man Shield, and in particular at the periphery of the Archaean nucleus, have shown that the contact zone between the Archaean and the Palaeoproterozoic Birimian terranes is a major thrust zone. The thrusting took place during the earliest stage (D-1) of the Palaeoproterozoic Birimian tectonic and metamorphic evolution. Lithologic, sedimentologic, tectonic and geochronologic studies together provide a model of progressive D-1 deformation and thickening reflecting progressive tectonic accretion of Proterozoic cocks onto the Archaean nucleus. In particular, the sediments of part of the lower Birimian (B1) were being deposited while the earlier lithotectonic pile was being progressively built up through thrusting (early D-1). These Birimian B1 flysch-type sediments, derived from erosion of the domain undergoing thickening, were themselves later accreted through thrusting and folding (late D-1), which implies progressive burial of the earlier terrane by continued tectonic stacking. This is confirmed by prograde early metamorphic evolution (under increasing pressure) with the transition andalusitekyanite and the late-tectonic development of staurolite-kyanite assemblages. The tectonic and metamorphic evolution of the thrust belt is characteristic of a Precambrian mountain belt that underwent: (1) a collisional stage, with important crustal thickening and prograde metamorphic evolution, syntectonic granitization, and late-kinematic migmatization when sillimanite developed at the expense of kyanite; (2) a late-orogenic stage characterized by (a) strong granitization and locally intensified migmatization with doming, (b) retrograde metamorphism, with the development of andalusite or cordierite at the expense of sillimanite marking a lowering of pressure, related to isostatic and tectonic uplift, the latter in turn related to transcurrent tectonism, and (c) numerous post-D-1 intracontinental volcanic areas (tholeiitic, locally komatiitic, magmatism), developed in a distensive or transtensive tectonic setting (of Basin and Range style?) during the uplift and before the paroxysmal transcurrent activity, and associated molasse basins locally coeval with the latter calc-alkaline rocks. The interpretation proposed for this belt is ''a belt of decollement thrusting and folding on a continental margin'' that was built similar to 2.15-2.10 Ga during a period of convergence. This implies subduction for which geochemical evidence exists in Palaeoproterozoic mafic rocks of the northern Man Shield. This D-1 mountain belt appears to form part of broad orogenic domain characterized by thrusting related to a global eastward displacement of the Archaean Guapore, Man and Reguibat terranes. The cause of the displacement could have been subduction along a suture, a segment of which has been indicated in the northeast of the orogenic domain.