The cumulus and post-cumulus evolution of chrome-spinels in ultrabasic layered intrusions: evidence from the Cuillin Igneous Complex, Isle of Skye, Scotland

Abstract

Chrome-spinels from the layered Peridotilte Series of the unmetamorphosed, anorogenic 60 Ma Cuillin Igneous Complex, Isle of Skye, display a wide variety of compositions. Cumulus (within seams) chrome-spinels from the lowest exposed portion of the Peridotite Series exhibit features indicative of textural equilibrium, are rich in Al and Mg, and have low values of the ratio Cr/(Cr+Al). Cumulus chrome-spinels from higher up in the series are different from these: particularly, textural disequilibrium is evident, intercumulus plagioclase and olivine are present, and the chrome-spinels are rich in Cr, Fe and Ti, with high values of the ratio Cr/(Cr+Al). Intercumulus (dispersed) chrome-spinels tend towards anhedral forms and define enrichment trends towards Fe (both Fe2+ and Fe3+) with decreasing Mg, Cr and Al, and towards Al, with decreasing Fe2+ and Cr (and increasing Mg). Individual crystals are completely homogeneous and are devoid of reaction rims. The observed textural characteristics and compositional data of the chrome-spinels documented here suggest that the semi-quantitative peritectic reaction: aluminous chrome-spinel + melt→plagioclase + olivine + chromian chrome-spinel, is responsible for the observed parageneses, and that both the environment of crystallization (eumulus or intercumulus) and the role of plagioclase ±olivine crystallization are critical parameters for this geochemical trend in spinels within upper crustal magmatic systems. The effects of pyroxene crystallization on the development of this geochemical trend are also considered. This investigation highlights the need to consider the role of post-cumulus mineral-melt reactions and their influences upon the final compositions of major oxide and silicate phases within layered intrusions.