Petrologic Evolution of a Nipigon Diabase Sill, Ontario, Canada: Insights From Compositional and Textural Profiles

摘要

Petrographic and geochemical profiles collected through a ~250-m-thick Nipigon diabase sill near Black Sturgeon Lake, Ontario, constrain the complex emplacement and differentiation processes that controlled the magmatic evolution of these rocks: several nested remjections of phenocryst-bearing magma, compaction-driven redistribution of mildly evolved interstitial liquids in the central portion of the sill, and segregation of highly-evolved interstitial liquids into micropegmatitic veins near the upper contact. These processes impact not only the silicate mineralogy of the rocks, but also distribution of sulfide phases. Both petrographic and geochemical data demonstrate that the sill was gradually inflated via several discrete injections of olivine-bearing magma. Plagioclase crystal size trends exhibit several reversals that are correlated with increases in olivine abundance and bulk-rock MgO content. These textural and compositional reversals are attributed to reinjections of olivine-bearing magma into partially solidified magma from earlier injections. Mixing between evolved resident magma and reinjections of less-evolved magma is commonly proposed as a mechanism for triggering sulfide mineralization. In this sill, which contains no significant mineralization, localized minor enrichments and depletions in copper content are associated with the margins of a well-constrained reinjection horizon.