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

Abstract

Significant volumes of partial melt developed in the arkosic contact aureoles of two of the numerous mafic and ultramafic minor intrusions found in the northern parts of the Isle of Rum, Scotland. Melting was essentially static, with little movement of melt even on a thin-section scale, and no segregation. The relative proportions of (now inverted) tridymite and high quartz 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 plug. Melting around the adjacent 200 m diameter peridotite plug reached ≈ 70 vol. %, and occurred up to 6 m from the contact. Simple thermal models for the two aureoles, based on the isograds given by the onset of melting, the breakdown of chlorite and the disordering of microcline, support 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-lived 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 surrounding the peridotite body. The melt distribution resulting from the heating part of the thermal history is controlled by reaction, and is far from textural equilibrium. Crystallization was abrupt, being complete in ten years for the gabbro, and in only four for the peridotite, resulting in a fine-grained cotectic intergrowth and preservation of the melt distribution.