Petrogenesis of the post-collisional Bled M'Dena volcanic ring complex in Reguibat Rise (western Eglab shield, Algeria)

The Palaeoproterozoic Bled M'Dena ring complex (BRC) dated at 2.01Ga (Rb-Sr isochron) situated in the eastern part of the Yetti-Eglab junction-zone, belongs to the Aftout orogenic province, intruding the Aftout granite (2.07Ga). It exhibits a centripetal evolution from gabbro to granodiorite. The western part of the Bled M'Dena ring complex is dominated by volcanic rocks varying in composition from basalt, trachyandesite, andesite, dacite to rhyolite. Andesite constitutes the major volcanic type in the Bled M'Dena ring complex. Chemistry of these rocks suggest two distinct magmatic trends: A transitional-tholeiitic suite localised at the outer part of the ring complex, and a high-K calc-alkaline suite situated within the inner ring structure. Basic volcanics exhibit flat primitive REE patterns with insignificant Eu anomalies, suggesting that they probably represent a parental basaltic melt from which the Bled M'Dena complex evolved, (Eu/Eu^* =2.06-4.07). The europium anomaly did not vary significantly, as it evolved progressively through the volcanic series by differentiation and feldspar accumulation.

It is proposed that a primitive basic magma from the mantle was contaminated by Eburnian crust (2.2-2.0. Ga). Thus the transitional-tholeiitic volcanic rocks, dominant in the region have developed a high-K calc-alkaline suite. Their source is melted shortly after its generation when the lithosphere was still hot, which restricts high-K calc-alkaline magmatism mainly to post-collisional settings. This has been linked to large movement along mega-shear zones (Yetti-Eglab junction) due to subduction of oceanic basins in a post collisional setting.