CRYSTAL SETTLING AND IN-SITU MAGMA DIFFERENTIATION IN A NEOPROTEROZOIC SILL OF THE FRANKLIN IGNEOUS EVENT, VICTORIA ISLAND NWT, CANADA

This study examines an individual sill of the Franklin Igneous event for lateral changes in composition, thickness, sorting, and texture to better understand the crystallization history by observing its mineralogical and compositional variations. The sill is one of a series of thin, laterally continuous Proterozoic (~720Ma) sills exposed on the northern limb of the Holman Island Syncline, exposed at 72^o N latitude on Victoria Island, NWT. The sill in this study is < 25m meters thick, extends for tens of kilometers along strike, and has a 10 meter thick mafic base rich in olivine crystals, overlain by a thin augite-rich zone and a diabasic upper section. Vertical chemical profiles of the sill were taken, along with sections from drill core data, and examined for lateral changes in thickness and chemical composition of the various sill units. Although thin section examination indicates that the sill has been altered, geochemical modeling suggests that its major and trace element composition has not been significantly affected by the alteration process. Plots of Mg# vs. height have classic S-shaped profiles with a maximum Mg# of 78 and a minimum Mg# of 52.

Two favorable possibilities for the development of S-shaped profiles in thin (< 500m thick) sills are: (1) the emplacement of a phenocryst rich magma that undergoes crystal settling and rapid cooling, or (2) in-situ crystal fractionation with crystal settling and melt migration associated with thermal or compositional convection. It is also conceivable for both of these mechanisms to occur in the same sill to produce the chemical variations seen.

Chemical modeling using variation diagrams, Pearce-element-ratio-analysis, olivine mass-balance calculations, and trace element analysis, suggest that this thin sill was emplaced as a phenocryst-rich magma, followed by settling of the initial phenocrysts and rapid cooling, without undergoing significant post-emplacement in-situ differentiation. A thicker sill (~43m) of similar bulk composition, emplaced ~35m above the sill in this study, shows clear evidence for both crystal settling of initial phenocrysts and in-situ differentiation, suggesting that for this mafic magma composition a critical value for in-situ differentiation may be between 25 and 43 meters.