Pseudomorphs after beryllian cordierite (possibly beryllian sekaninaite) occur along core margins of Archean peraluminous granitic pegmatites of the beryl-columbite subtype at Greer Lake, southeastern Manitoba, and in analogous position in pegmatitic pods of the parent leucogranite. The pseudomorphs are locally intergrown with primary columnar beryl I, and studded by spessartine that contains subparallel inclusions of beryl II. The muscovite>biotite>berthierine assemblage of the pseudomorphs after cordierite contains anhedral fine-grained beryl III, locally attaining approximately 20 vol.% of the breakdown products. This complex mineral association is interpreted as a product of two processes. (i) Magmatic crystallization from a Be-rich and Mn-enriched melt yielded cordierite and beryl I at approximately 550 degrees C and < or =2.8 kbar. Increased activity of Mn triggered intermittent stabilization of spessartine+beryl II during the crystallization of cordierite; this assemblage probably represents a Be-rich substitute for spessartine+aluminosilicate+quartz, the low-pressure equivalent of "manganocordierite", which is unstable above 1 kbar P(H 2 O). (ii) Subsolidus metasomatic alteration of cordierite by alkali- and F-bearing residual aqueous fluids at about 500-450 degrees C, approximately 2.7-2.6 kbar generated biotite>muscovite>>beryl III along the basal parting planes, followed by several textural varieties of muscovite>biotite>>beryl III, which replaced the bulk of the cordierite crystals. Low-temperature alteration of biotite yielded berthierine. The proportion of beryl III (up to approximately 20%) in the micaceous matrix of the pseudomorphs indicates up to approximately 2.6 wt.% BeO in the cordierite precursor, which closely corresponds to the experimentally established maximum value at the given P,T conditions of magmatic crystallization.