GO WITH THE FLOW: A TEXTURAL RECORD OF DEPOSITION, MAGMATIC CURRENTS, AND REPLENISHMENTS IN THE RHUM COMPLEX, SCOTLAND

The Rhum Complex, Scotland is well known for igneous layering associated with troctolite and anorthosite. Spatially associated with troctolite-anorthosite contacts are chromite seams – a potential source of Platinum-Palladium Group Elements. Several processes, including multiple replenishments, gravitational settling, textural equilibration, compaction, slumping, and magma mixing have been proposed to explain the origin of these layers and the formation of chromite seams. We present results of a qualitative and quantitative petrographic analysis of changes in mineralogy, texture, and fabric across a troctolite, chromitite, and anorthosite layers from the Rhum Complex. Where the olivine orthocumulate troctolite, with intercumulus plagioclase, is in contact with chromitite - cumulus olivine crystals are extensively embayed. The orthocumulate chromitite layer, with intercumulus plagioclase, is sinusoidal and of variable thickness (1.25-1.68mm). The anorthosite layer consists of three subzones based on crystal orientation and compaction. At the contact with chromitite, adcumulate plagioclase crystals define an imbricate pattern, which transitions to a planar fabric, and then to a plagioclase mesocumulate with intercumulus pyroxene. Based on crystallization paths in the Ol-Di-An ternary phase diagram, we suggest replenishment of a high Al-tholeiite magma, containing plagioclase phenocrysts, resulted in dissolution of olivine and deposition and imbrication of plagioclase due to flow. Using this spatial arrangement as a topping indicator, chromite accumulation is thicker in the trough and thinner on the crest of this “unconformity”. Planar plagioclase may record flow or compaction. This suggests that locally igneous textures related to magmatic crystallization, replenishment, and flow sorting are preserved. Chromitite seams may form as the result of mixing of two distinct magmas followed by accumulation due to gravitational settling and magma flow.