Magma mixing between rhyolite and pseudotachylite as the origin for the Glencoe ‘flinty crush rock’

Abstract

The Glencoe caldera is a well-studied example of a caldera system exposed to intermediate depths along the glacially excavated glen. We present a first quantitative assessment of clast-size population and matrix chemistry from the flinty crush rock that occurs on the main ring faults. Size–shape metrics of clasts differ from those of a ‘normal’ pseudotachylite from the Outer Hebrides. Both samples display good power-law clast-size populations, once allowance is made for dissolution of a portion of clasts into the melt that contained them, with fractal dimensions of 2.7 and 4.0 for the Outer Hebrides and Glencoe samples respectively. Mass-balance calculations of flinty crush rock matrix chemistry imply an origin by mixing between rhyolite and pseudotachylite that was derived from semipelitic host rock. This means that the flinty crush rock was transported some distance from the point of frictional heating, as previously proposed, as semipelitic rocks are not present at the surface at Stob Mhic Mhartuin but are likely to be present at depth. This transport, and mixing with rhyolite magma, would have provided the time and thermal energy for clast dissolution beyond that possible during normal pseudotachylite generation and quenching on fault wall rocks. Supplementary material: Results of simulations, major element compositions of the matrix in the flinty crush rock sample and a supplementary figure are available at https://doi.org/10.6084/m9.figshare.c.7084992