CRYSTAL SIZE DISTRIBUTION OF OLIVINE IN THE BLACK STURGEON SILL, ONTARIO

Igneous intrusions are often emplaced in multiple phases or pulses. To test whether textural data could be used to distinguish between these pulses, we examined a continuous drill core through the Black Sturgeon sill, a 250 m thick diabase intrusion located southwest of Lake Nipigon in Ontario. The most primitive and olivine-rich zone is from 120-200 m above base. In this study, we used crystal size distributions (CSDs) to characterize olivine populations in this zone. We collected images from 42 samples throughout the olivine zone and manually traced at least 200 olivine crystals from each sample, then calculated the CSD using the software CSDCorrections 1.5. The slope and intercept of best-fit lines to the CSDs were used to quantify the texture. Slope data ranged from 2.89 to 7.19, and intercept data from 1.18 to 5.20.

Cluster analysis of the slope-intercept data reveals four well-defined textural groups, which could correspond to distinct populations of olivine. These groups are typically found in separate parts of the olivine zone, although some overlap exists. Group 1 is characterized by low slope and intercept values, while Groups 2 and 3 have progressively higher values. Group 4 anomalously has similar intercepts to Group 2, but lower slopes. Although we have not found any consistent variation within groups, the breaks between the populations coincide with major whole-rock compositional changes. We interpret these populations as evidence of the episodic emplacement of the Black Sturgeon sill.

CSDs have proved an effective complement to other methods of textural and compositional analysis in identifying discrete magma pulses in this sill. Comparable methods could be useful in studies of other olivine-rich intrusions. The episodic nature of magma emplacement in the Black Sturgeon sill also raises the possibility that other layered mafic intrusions formed by a similar process.