THE BLACK STURGEON SILL: INSIGHTS INTO FORMATION OF MODAL AND COMPOSITIONAL LAYERING

The Black Sturgeon sill (BSS) is a 250 m thick Midcontinent Rift mafic intrusion from northwest of Nipigon, Ontario. A continuous drill core through the sill has enabled the construction of highly detailed mineralogical, textural, and compositional profiles. Sampling protocols have focused on three different scales: the entire sill is sampled at 2 m intervals; a central, olivine-rich zone is sampled at 0.5 m intervals; and an internal horizon within this zone is sampled at 0.1 m intervals. Principal component analysis (PCA) for each of these three sample sets demonstrates that compositional variations are largely controlled by the relative proportions of olivine, plagioclase, and evolved interstitial liquids.

On the largest scale, the BSS consists of four compositionally distinct zones (each on the order of 50 m thick). In these zones, PCA results can distinguish different amounts of crystal cargo and evolved interstitial liquid. On the smaller scales, PCA results distinguish different types of crystal cargo (olivine-dominated versus plagioclase-dominated), as well as tracking redistribution of interstitial liquid throughout the entire zone. Unexpectedly, accumulation of olivine and/or plagioclase operates independently of interstitial liquid abundance over different length scales, and produce distinct compositional signals in the PCA. Differential accumulation of crystal cargo produces variations in compatible elements with little effect on incompatible element abundances; redistribution of evolved liquids produces variations in incompatible elements with little effect on compatible element abundances.

When considered in a spatial context and in connection with petrographic observations, these results demonstrate that modal and compositional variations in the BSS were generated by two distinct processes. Smaller-scale modal layering, consisting of alternating olivine-dominated and plagioclase-dominated horizons over roughly 1 m length scales, was produced by mechanical sorting of crystal cargo that was originally entrained in the intruding magma. Larger-scale variations, consisting of compositionally distinct zones over 50 m length scales, was produced by a combination of emplacement of compositionally distinct magma batches and redistribution of evolved interstitial liquids.