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 within a 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 inclusions and 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 inclusions are virtually identical. Field observations suggest that the inclusions were derived from the Hypersthene Gabbro and were 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 behaved as a Newtonian fluid, with a potential settling velocity of <0·028 m s–1. In contrast, the presence of 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 lateral mixing and inclusion transport.
Highlights
The presence of 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 lateral mixing and inclusion transport
We report the results of integrated rock magnetic, quantitative textural and mineral chemical analyses of a suite of gabbro inclusions entrained in several closely spaced sheet intrusions from the southern part of the Ardnamurchan Central Complex in NW Scotland (Fig. 1) to assess the provenance of the inclusions, inclusion^host magma relationships and the transport mechanics of the system
From the observed groundmass grain size variations in the XBSa segments and the confined distribution of the gabbro inclusions, we suggest that high velocity gradients developed between the two segments owing to variations in magma rheology restricted lateral mixing and potentially provided zones suitable for inclusion accumulation (Fig. 13)
Summary
Mafic magmas that feed and drive many volcanic systems are often compositionally modified during transport from the mantle and lower crust to the Earth’s surface (Anderson, 1976; Huppert & Sparks, 1985; Sparks & Marshall, 1986; Blake & Fink, 2000; Polacci et al, 2001; Rutherford, 2008; Genareau & Clarke, 2010). We report the results of integrated rock magnetic, quantitative textural and mineral chemical analyses of a suite of gabbro inclusions entrained in several closely spaced sheet intrusions from the southern part of the Ardnamurchan Central Complex in NW Scotland (Fig. 1) to assess the provenance of the inclusions, inclusion^host magma relationships and the transport mechanics of the system. Richey & Thomas (1930) briefly described these inclusions and suggested that they represent parts of intrusive sheets that were subsequently fragmented and disrupted by the current sheet lithology that utilized the pre-existing pathway during magma ascent We disagree with this interpretation as the inclusions are gabbroic, with a well-developed cumulate texture, similar to that of the nearby (51km) Hypersthene Gabbro. The gabbro inclusions of interest here occur in a localized set of mafic sheet intrusions, with variable geometries, along a 500 m coastal section on the southwestern
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