Magma rheology variations in sheet intrusions of the Ardnamurchan Central Complex (Scotland) inferred from gabbro inclusion characteristics

Abstract

Entrainment of xenoliths and their consequent assimilation are key processes in modifying the crystallization kinetics and magma dynamics of conduit systems. Here, an integrated textural and mineral chemical study of the evolution of a suite of gabbroic inclusions withina set of sheet intrusions from the Ardnamurchan Central Complex, NW Scotland, is presented. The key findings are as follows: (1) the host magma sheets and inclusions are not cognate; (2)there are microstructural and mineral chemical similarities between the gabbroic inclusionsand the textures and mineralogy of the major Hypersthene Gabbro on Ardnamurchan; (3) orientations of magnetic fabrics within the host sheet groundmass and within the gabbroic inclusionsare virtually identical. Field observations suggest that the inclusions were derived from the Hypersthene Gabbro andwere entrained in a few laterally restricted magma segments that subsequently coalesced with inclusion-free segments into continuous sheets. Using Stokes' Law and adaptations thereof, we calculate that the magma within the inclusion-free segments behavedas aNewtonian fluid, with a potential settling velocity of <0.028 m s-1. In contrast, the presenceof gabbro inclusions probably modified the magma dynamics to Bingham-like behaviour. We infer that this variation in the magma rheology of separate segments continued after coalescence and internally partitioned the magma sheet, preventing lateralmixing and inclusion transport. © The Author 2012. Published by Oxford University Press. All rights reserved.