THOLEIITIC VOLCANIC-ROCKS OF THE LATE ARCHEAN BLAKE RIVER GROUP, SOUTHERN ABITIBI GREENSTONE-BELT - ORIGIN AND GEODYNAMIC IMPLICATIONS

The late Archean Blake River Group volcanic sequence forms the uppermost part of the southern Abitibi greenstone belt in Quebec. The group is mainly composed of mid-ocean-ridge basalt (MORB)-like tholeiites that show a progressive change of several incompatible trace element ratios (e.g., Nb/Th, Nb/Ta, La/Yb, and Zr/Y) during differentiation. The compositional variations are inferred to be the result of fractional crystallization coupled with mixing - contamination of tholeiites by calc-alkaline magma which produced the mafic - intermediate lavas intercalated with the tholeiites in the uppermost part of the sequence. The MORB-like tholeiites were probably emplaced in a back-arc setting.