GEOCHEMISTRY AND PETROGENESIS OF ARCHEAN MAFIC VOLCANIC-ROCKS OF THE SOUTHERN ABITIBI BELT, QUEBEC

The Blake River Group of the late Archean Abitibi Greenstone Belt is characterized by the abundance of differentiated lava series as well as by the presence of coeval tholeiitic and calc-alkaline eruptives. Differentiated tholeiites from this group are interpreted as the product of magma mixing between asthenospheric tholeiites and subduction-related calc-alkaline basaltic andesites. From the lower to the upper southern Abitibi Belt, chemical variations recorded in tholeiites are indicative of a major change in the geodynamic regime. Tholeiites from the lower volcanic sequence were produced by a high degree of partial melting of adiabatically rising diapirs below a rifted lithosphere. Uncontaminated tholeiites from the upper part of the belt reflect a lower degree of partial melting probably due to decreasing lithospheric stretching. This may be related to the closure of the rift-related volcanic basins at the begining of the Kenorean deformational events in the Southern Abitibi Belt.