Seamus Magnus Honours thesis abstract

Thesis Title: 
An Investigation of the Assimilation Hypothesis in the Navilus Sill, Thunder Bay, Ontario

Pink granophyric features in some of theLogan sills, particularly in the Navilus sill, just west ofThunder Bay, were attributed by Blackadar (1956) to in-situ assimilation of granitic material.  Petrographic and geochemical studies have been performed on both the Navilus and Terry Fox sills, with a more modern approach, in order to re-evaluate Blackadar's hypothesis.

Two unique zones consistently appear around the xenoliths found in the Navilus sill: a zone of quartzo-feldspathic intergrowths, or "granophyre", directly adjacent to the xenoliths, followed by a zone of discreet pyroxene grains on the interface between normal diabase magma and the granophyric zone.  Similar granophyric formations are found interstitial to plagioclase and pyroxene grains, along with closely associated iron-oxides, both of which appear to have formed during the latest stage of crystallization due to the immiscibility of a silica-rich and an iron-rich liquid which exsolved from the magma.  The similarity between the granophyre observed around the xenoliths, and the late-stage "groundmass" granophyre indicates that the former was also formed due to liquid immiscibility.  Introduction of silicate-rich xenoliths to the already silica-saturated, quartz-tholeiitic magma has been proposed as the cause for the premature exsolution of silica and iron-rich liquids from the magma.

Spherules have been observed in close proximity to the xenoliths and granophyric zones, with carbonate nuclei and chlorite rims.  Due to the high melting temperature of carbonate minerals, it is unlikely that these were derived from a melt, and were more likely formed as carbonate xenoliths from the overlying Gunflint Formation recrystallized and interacted with the magma to produce spherical chlorite rims.

REE diagrams for both the Navilus and Terry Fox sills indicate that geochemical variations between the diabase with inclusions (such as xenoliths and spherules) and the normal diabase have been caused by late stage fractionation during crystallization.

Variable enrichment of LILs as seen in the Spider diagrams for both sills may indicate that in-situ assimilation has taken place, though variable depletion of HFSEs may indicate that these patterns also represent the effects of fractionation.

The basal contact of the Terry Fox sill with Rove shale (SM011) displays REE and Spider diagram patterns that differ from the normal diabase in that sill, which may indicate that in-situ assimilation has taken place, though a more detailed analysis is required to fully understand the role of assimilation in these sills.

Plotting the samples of normal diabase from both sillson Gd/Yb vs. La/Sm and TiO2 vs. Mg# discrimination diagrams has grouped the Navilus and Terry Fox sills with the Nipigon sills, contrary to their previous designation as Logan sills.