Abstract

The aureole of the Tertiary Rum Igneous Complex formed at shallow levels in amphibolite-facies migmatitic Lewisian gneisses and previously unmetamorphosed Precambrian arkose of the Torridonian Group. Contact metamorphism of arkose resulted in the breakdown of chlorite-rich cement and detrital muscovite, feldspar disordering, and melting. A simplified thermal model shows that magma chamber filling may have occurred over a few hundred years. Contact metamorphism of Lewisian gneiss, exposed only within the Main Ring Fault of the complex, resulted in the breakdown of hornblende to pyroxene, plagioclase, biotite and magnetite. Biotite in high-grade gneisses is replaced by feldspar and orthopyroxene. Extensive melting of the gneiss occurred only locally. Prominent migmatitic structures predate contact metamorphism. Significant mobility of Fe, Mg and Al occurred in the aureole, probably effected via circulation of aqueous fluids confined to the metamorphic peak within the Main Ring Fault but extending to lower temperatures outside. Significant differences between cooling rate of rocks within and outside the Main Ring Fault are inferred from textural observations. These probably relate to a combination of a relatively extended metamorphic event for the innermost rocks and a greater cooling influence of circulating aqueous fluids on the outermost rocks.

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