Contact metamorphism and anatexis of Torridonian arkose by minor intrusions of the Rum Igneous Complex, Inner Hebrides, Scotland

Significant volumes of partial melt developed in the arkosic contact aureol es of two of the numerous mafic and ultramafic minor intrusions found in th e northern parts of the Isle of Rum, Scotland. Melting was essentially stat ic, with little movement of melt even on a thin-section scale, and no segre gation. The relative proportions of (now inverted) tridymite and high quart z inferred to have crystallized in the silica primary-phase field constrain the pressure of metamorphism to 150 +/- 50 bars. Melting attained 95 vol. %, and occurred up to 15 m from the contact with the 50 m diameter gabbro p lug. Melting around the adjacent 200 m diameter peridotite plug reached app roximate to 70 vol. %, and occurred up to 6 m from the contact. Simple ther mal models for the two aureoles, based on the isograds given by the onset o f melting, the breakdown of chlorite and the disordering of microcline, sup port the hypotheses that the peridotite plug was injected as a crystal-rich mush close to its solidus, whereas the gabbro plug was a relatively long-l ived feeder conduit. Time scales for the melting events are of the order of forty years for the aureole of the gabbro and ten years for that surroundi ng the peridotite body. The melt distribution resulting from the heating pa rt of the thermal history is controlled by reaction, and is far from textur al equilibrium. Crystallization was abrupt, being complete in ten years for the gabbro, and in only four for the peridotite, resulting in a fine-grain ed cotectic intergrowth and preservation of the melt distribution.