Microstructural evidence for crystallization regimes in mafic intrusions: a case study from the Little Minch Sill Complex, Scotland.

The magma forming the 20 m thick crinanitic/picrodoleritic Dun Raisburgh sill, part of the Little Minch Sill Complex of NW Scotland, comprised a mafic carrier liquid with a crystal cargo of plagioclase and olivine (1 vol%). The olivine component of the cargo settled on the floor of the intrusion while the more buoyant plagioclase component remained suspended during solidification, resulting in a relatively high plagioclase content in the centre of the sill. The settled olivine grains form a lower fining-upwards sequence overlain by a poorly sorted accumulation formed of grains that grew within the convecting magma. The accumulation of olivine on the sill floor occurred over 5-10 weeks, synchronous with the upwards-propagation of a solidification front comprising a porous (~ 70 vol% interstitial liquid) plagioclase-rich crystal mush.