CHARACTERISATION OF THE VOLCANIC SERIEES IN THE DAMRANE (WESTERN OUGARTA, ALGERIA) AND IMPLICATION FOR THE GLOBAL MODEL OF THE WEST AFRICAN SHIELD NORTEASTERN MARGIN

In the variscan chain of Ougarta (South Algeria), the Proterozoic is only known from some anticline cores, southeastern equivalent of the Anti-Atlas inliers of Morocco; in general, it deals the uppermost ignimbritic levels, assigned to P.III structuro-lithostratgraphic unit. However, more ancient, levels have already been described in the southeastern Ougarta (Sebkha el Mellah and assigned to the P.II unit; by comparison, the lowermost levels underlying the ignimbrites in the Damrane (southwestern Ougarta), have been assigned to this structuro-lithostratigraphic unit (P.II). Moreover, on this basis of their mostly mafic volcanic composition and of the presence of a positive gravimetric anomaly in this area, it was concluded that the western Ougarta was a landmark, between Ahaggar and Anti-Atlas of the West- African Shield. It was thus inferred that is occurred there, first and oceanization and a further obduction and related continental collision.

The detailed studies of the Damrane we present here, lead to challenge most of these conclusions, and demonstrate that:

*) the geostructural origin of both and felsic volcanisms is of late-to post orogenic (compressive) intra- continental type, devoid of any obvious nearby subduction- zone relationships ;

**) it is thus inferred that, during the Middle- Upper Proterzoic, the western Ougarta belong to a thineed continental-crust domain, backwards a hypothecal defined (aborted subduction zone or active margin? rifting?) would at most have occurred in eastern Ougarta (Sebkha el Mellah) or even farther to the east (?). As a result, the Pan-African geosuture, as far as it really exists in Ougarta, is to be searched in this direction, not in the Damrane, which at these times was governed by intra-continental crust subaerial conditions.