Emplacement and differentiation of the Black Sturgeon Sill, Nipigon, Ontario: A principal component analysis

Abstract

Magmatic emplacement and differentiation processes typically produce compositional or petrographic signatures that are easily recognized and interpreted. However, these signatures can sometimes be indistinct or ambiguous, particularly when the available data sets are limited. Detailed sampling of a continuous drill core through the 250m thick Black Sturgeon Sill (Nipigon, Ontario) provides an ideal opportunity to rigorously characterize the compositional variance in a natural system using principal component analysis. We identify three significant principal components, or composite variables, that together account for ~90% of the total major-oxide variation in our data set and evaluate the petrogenetic significance of each. The first principal component is controlled by the contrast between residually-enriched liquids (which we interpret as evolved interstitial melt; PC1>0) and olivine + anorthitic plagioclase (which we interpret as entrained crystal cargo; PC1<0). The second principal component is controlled by the contrast between 'gabbroic' (rich in plagioclase and augite; PC2>0) and 'ferrous' (iron-rich; PC2<0) compositions. The third principal component is controlled by the contrast between compositions dominated by felsic (PC3>0) and mafic (PC3<0) minerals. Compositional groupings, identified using k-means cluster analysis, correspond closely to observed petrographic zonation in the sill and demonstrate that the sill is a composite intrusion constructed from at least three sequential phases of magma injections. The first phase, with PC1~PC2~PC3~0, is a typical olivine diabase. The second phase, with elevated PC2, is dominated by plagioclase and augite, with little or no olivine. The final phase, with PC1<0, is the most primitive and was intruded as a slurry of plagioclase and olivine crystals. After substantial (but not complete) crystallization, interstitial melt was redistributed, particularly within the uppermost 40m of the sill. This produced a heterogeneous upper zone, with PC1>0, in which highly evolved liquids collected in decimetre-scale lenses and patches. This study illustrates the value of principal component analysis for interpreting complex petrologic systems. Previous studies have demonstrated the value of principal component analysis for simplifying the representation of complex, multidimensional data sets. In this study, however, the primary benefit is the ease with which the petrogenetic processes responsible for compositional evolution in a particular system can be identified. By using the principal components as indices for the various processes and mapping their stratigraphic variations through the Black Sturgeon Sill, this study illustrates a broader application of principal component analysis that could be applied to many other systems of petrologic interest.