The role of magma injection and crystal sorting in the formation of early gabbros at the Coldwell Complex, Ontario, Canada

Abstract

The Layered Series of the Midcontinent Rift related Coldwell Complex comprises thick sections of gabbro, without any known associated ultramafic rocks. It represents a major early intrusive unit of the Coldwell Complex and consists of thick accumulations of olivine gabbro and oxide augite melatroctolite. This study combines petrography, mineral chemistry, and lithogeochemistry to constrain the magma composition and petrogenesis of the Layered Series. The presence of cumulus orthoclase together with the observation that the Layered Series rocks plot in the alkaline field on a total alkali–silica diagram indicate that the Layered Series magma has an alkaline parentage. The stratigraphy of the Layered Series cannot be fully correlated between different areas using lithogeochemistry and mineral chemistry. This together with observed normal and reverse trends for mineral chemical compositions in different areas suggest that the processes related to magma emplacement and crystallization were different in different locations. The whole-rock concentrations of incompatible elements and the compositions of major minerals of the olivine gabbro and oxide augite melatroctolite units are chemically similar. However, major element lithogeochemistry is variable, dominantly due to differences in the abundances of olivine, clinopyroxene, plagioclase, and magnetite. An additional observation is that olivine and clinopyroxene are not in chemical equilibrium. Together, these observations are interpreted to reflect a combination of multiple injections of magma and crystal sorting in an open system.