INTERPRETATION OF SYSTEM-WIDE INJECTION DYNAMICS IN MAFIC SILLS: THE NIPIGON DIABASE, ONTARIO,CANADA

The filling history of sills is an increasingly important area of research. Rather than a simplified model of magma chambers as instantaneously filled bodies of phenocryst-free liquid, it is becoming increasingly clear that even smaller bodies were generated by a process of pulsatile injections of magma batches containing variable phenocryst loads. In order to understand the chemical and physical evolution of these magma bodies, it is critical that the initial conditions of formation are well understood. In this study, texture variations are used to explore the filling history of a major sill system in northern Ontario.

The Nipigon diabase sills are olivine tholeiites associated with the 1100-Ma (Keweenawan) Midcontinent Rift system. Previous investigation of a section through a thick Nipigon sill at Kama Point has revealed a texture reversal located approximately 25 m above the basal contact. This reversal has been shown to be consistent with the effects of a reinjection event that took place approximately 10 years after the initial magma injection. A second section through the sill, approximately 30 km from the first section, was sampled in order to determine whether the reversal detected at Kama Point is a localized or system-wide phenomenon.

In the second section, samples from the lower contact to ~40 m above base were analyzed using Crystal Size Distributions (CSDs). The CSDs show a general coarsening inward trend from the base to a height of ~25 m, at which point the texture becomes significantly finer. Above this, the coarsening inward trend continues through the remainder of the measured section. This trend is nearly identical to that seen in the original profile, 30 km to the east. These results suggest that the two sections are indeed from the same sill, and that this sill experienced a large-scale reinjection event that impacted the textures of the rocks throughout the sill.