PETROLOGIC EVOLUTION OF A NIPIGON DIABASE SILL, ONTARIO, CANADA: INSIGHTS FROM COMPOSITIONAL AND TEXTURAL PROFILES
Through most of the sill, compositions vary from 47-52% SiO2 and 6-11% MgO. These compositional variations are largely controlled by mechanical concentration of olivine via crystal settling and/or flow differentiation. Approximately 32 m below the upper contact, there is a ~30 cm thick silicic segregation whose composition is consistent with residual liquid composition after ~70-80% fractional crystallization (based on MELTS). The rocks immediately beneath the segregation horizon have compositions consistent with a net addition of this late-stage interstitial liquid to a hypothetical phenocryst-free “carrier magma”. The mineralogy of the sill is dominated by plagioclase and clinopyroxene, with striking variations in modal olivine content, ranging from 0-12%. In the vicinity of the silicic segregation, olivine is absent, and granophyre is conspicuously abundant.
Crystal size distributions of plagioclase vary smoothly, but not monotonically, throughout the interior of the sill. Of particular interest, the textures gradually fine inwards in a zone between 40 and 140 m below the upper contact. This zone also corresponds with the strongest alignment of plagioclase crystals and the highest MgO and Ni abundances. Highly anisotropic fabrics are also correlated with low concentrations of incompatible trace elements such as Zr, which is consistent with compaction of a partially crystalline mush to form the foliations, and expulsion of incompatible-rich interstitial liquid. These variations suggest that mechanical processes such as olivine accumulation and compaction significantly impacted texture development in the sill, making it more difficult to infer cooling conditions directly from the textures. Further work will focus on documenting small-scale variations in texture, fabric, and composition to clarify these relationships.