Rhythmic layering formed by deposition of plagioclase phenocrysts from influxes of porphyritic magma in the Cuillin Centre, Isle of Skye

Abstract

The Outer Bytownite Gabbros of the Cuillin Centre on the Isle of Skye include a 175 m rhythmically layered sequence consisting of alternating layers of coarse-grained massive gabbro and finer-grained laminated gabbro. We infer that the massive layers formed from repeated influxes of plagioclase-phyric magmas, which flowed into an existing magma chamber and deposited their entrained phenocrysts on the chamber floor. The phenocrysts accumulated rapidly, trapping large amounts of pore liquid in the cumulus pile. During the intervals between influxes, laminated gabbro layers formed by crystallization of the resident magma; these layers accumulated more slowly, by crystal settling or in situ growth, allowing sufficient time for their pore liquids to be expelled by compaction or adcumulus crystallization. The resulting fractions of trapped pore liquid were 510% in laminated gabbros and 20-40% in massive gabbros. High porosity in the massive layers is attributable to rapid accumulation of a lowdensity cumulus assemblage, with trapping of pore liquids facilitated by the confining effects of impermeable laminated cumulates above and below. Among layers, whole-rock Sr and Nd isotope ratios reveal variable degrees of contamination by the Archean crust beneath Skye. In the lower part of the section, influxes of plagioclase-phyric magma are associated with a shift to more primitive mineral compositions, lower 87Sr/86Sr, and higher 143Nd/144Nd, indicating that the replenishing magmas were initially more primitive and less contaminated than those already residing in the chamber. In accordance with this interpretation, massive layers have lower 87Sr/86Sr than laminated layers, consistent with derivation of the plagioclase phenocrysts from a less contaminated source. In the upper part of the section, the shift toward more primitive and less contaminated compositions is reversed, suggesting that over time the replenishing magmas underwent increasing degrees of fractionation and contamination within the magmatic plumbing system that fed the intrusion. These observations and inferences, coupled with published evidence for other isotopic reversals, support a model for growth of the Cuillin Centre by frequent replenishment of a small-volume magma chamber with pulses of magma that underwent complex histories of fractionation and contamination during their transit through the Archean crust; periods dominated by assimilation and fractional crystallization within the feeder system alternated with periods of vigorous recharge by relatively primitive and less contaminated magmas. These cycles were superimposed on an overall trend toward less contamination through time, consistent with increasing dominance of the system by uncontaminated tholeiitic magmas as the Hebridean magmatic province matured. © The Author 2014. Published by Oxford University Press. All rights reserved.