EARLY HISTORY OF THE MIDCONTINENT RIFT AS RECORDED BY THE ALONA BAY LAVAS, ONTARIO

The Alona Bay lavas (ABL) of the Midcontinent rift system outcrop along the southeastern shore of Lake Superior in Ontario just north of the well-studied Mamainse Point Formation (MPF). The Alona Bay section is about 1200 m thick and contains approximately 107 picritic to basaltic lava flows. Although it has been suggested that the ABL lie stratigraphically below the MPF, detailed chemostratigraphy indicates that the ABL correlate with the basal portions of the MPF which outcrop in Mica Bay. The ABL can be subdivided into four distinct chemical groups which have identical counterparts in the basal MPF. Individual lavas and sequences of lavas in the uppermost and lowermost ABL show direct correlations with the Mica Bay section. The central portions of the two sections, although dominated by high-TiO₂ lavas, are different enough to indicate that they were, at times, fed by separate eruptive centers. Within the ABL, intra-group chemical variations confirm earlier suggestions that parental magmas early in the development of the Midcontinent rift experienced fractionation at moderate (lower crustal) pressures. Although the chemical characteristics of one chemostratigraphic group at Alona Bay is governed by extensive crustal contamination, the defining compositional distinctions of the three remaining groups require subcrustal explanations.