Mineral variation in Proterozoic diabase sills and dykes at Lake Nipigon, Ontario

Middle Proterozoic olivine tholeiite diabase sills and dykes of the Lake Nipigon area, Ontario, were emplaced into Archean basement and Proterozoic metasediments. The diabase sills are 150 to 200 m thick and were intruded near the Archean-Proterozoic unconformity under a probable lithostatic load of less than 0.4 kbars. The major silicate phases in the sills and dykes are plagioclase (An7S-7), vari-able proportions of augite (XMg 0.79-0.30, W046-2S) and pigeonite (XMg 0.77-0.52, W01S_S), olivine (Fon_9), and minor orthopyroxene and a micrographic quartz- alkali feldspar intergrowth. Augite compositions are widely scat-tered between W02S and W046 ' Variation of XMg at cons-tant Wo is attributed to stable equilibrium during fractional crystallization, whereas variations in Wo are probably due to metastable crystallization. Olivine grains are homogene-ous and vary systematically with stratigraphic height. Mg-rich olivine (Fon) in chilled diabase probably crystallized in equilibrium with basaltic liquid. Olivine in medium-grained diabase (F0S6-3V reacted with residual liquids. Fayalite (F09) occurs in pegmatitic diabase with a micro-graphic quartz-albite intergrowth. Plagioclase is the liqui-dus phase in all of the diabases. Three variants of crystal-lization sequence, however, are defined by the order of crystallization of mafic minerals. These are: 1) olivine, augite, late pigeonite, 2) olivine, augite + pigeonite and 3) olivine, pigeonite, augite. Olivine persists throughout the sill section only in the sequence with late pigeonite. These variations are interpreted to reflect increasing activity of silica and may relate to the degree of contamination with siliceous crust. Fractionation in the sills is attributed to flow-age differentiation and movement of residual liquids enriched in H20, Na, and Fe toward the top of the sec-tions.