GEOCHEMICAL AND PETROGRAPHIC CONSTRAINTS ON THE FORMATION OF A NIPIGON DIABASE SILL, ONTARIO, CANADA

The Nipigon complex, part of the ~1.1 Ga Midcontinent Rift system, consists of 150-250 m thick gabbroic sills located in the Nipigon Embayment north of Lake Superior. We have determined the geochemical, textural, and mineralogical variations in a 250 m thick Nipigon sill from Black Sturgeon Lake, southwest of Lake Nipigon, sampled from a continuous drill core transect. Through most of the sill, the composition varies between 47-52% SiO₂ and 6-11% MgO. These compositional variations are largely controlled by the mechanical concentration of olivine and plagioclase by means of crystal settling and flow differentiation. Near the upper contact, in a ~30 cm thick horizon interpreted as a segregation vein formed by solidification front instability, the composition reaches extreme values of 56% SiO₂ and 1.7% MgO, consistent with residual liquid composition after ~80% fractional crystallization. The mineralogy of the sill is dominated by plagioclase and clinopyroxene, with obvious variations in modal olivine content, ranging from 0-12%. Crystal size distributions of plagioclase highlight subtle reversals in the coarsening inwards trend, consistent with soft internal chills. Textural anisotropy (preferred orientation of plagioclase grains) suggests that crystal-mush compaction, which can redistribute interstitial liquid rich in incompatible elements, was an important process in the central portion of the sill.

Based on observed geochemical and petrographic variations, this sill appears to be a composite intrusion, emplaced as at least four separate injections of magma with entrained olivine + plagioclase phenocrysts. A better understanding of the injection history of composite intrusions such as the Nipigon sills may provide important insights into the relationships between volcanic and plutonic environments, particularly the frequency and consequences of magmatic recharge events.