Two alkaline igneous complexes and three lines of diatreme breccias were emplaced in the East Arm of Great Slave Lake during the lower Proterozoic. Field relationships suggest that those rocks are broadly cogenetic and were emplaced about 2.1 Ga ago.One of the intrusions, the Easter Island dyke, was rotated subsequent to emplacement such that both top and bottom are now exposed. Field and petrographic data are indicative of progressive differentiation along (i.e., up) the dyke and are substantiated by chemical data. The differentiation history of the early gabbros of the Blachford Lake complex is similar. Late differentiates of both complexes closely resemble the igneous matrices of the breccias and petrographic and chemical data support the proposal of cogenesis and contemporaneity.The field data show that there was a period of significant faulting and concomitant alkaline igneous activity in the East Arm area in the lower Proterozoic.