Adcumulate mafic dykes in layered intrusions: a case study of a late-stage dyke in the Bayantsagaan layered intrusion, Mongolia

Abstract

AbstractWe have discovered an adcumulate late-stage dyke in the Bayantsagaan layered intrusion. The 11-cm-thick dyke is composed of fresh troctolite with no signs of chilling against the host leucotroctolite. Texturally, both the dyke and its host are medium-grained plagioclase-olivine-magnetite cumulates. The dyke is however finer grained and contains less interstitial material. Two geochemical features characterize the dyke: it is compositionally more evolved than the host, as indicated by its lower real and normative An-content in plagioclase, whole-rock Cr and Mg-number; and it is highly depleted in all incompatible components (e.g. K2O, Y and rare earth elements or REEs) that have much lower concentrations than in the host. The depletion in REEs is extreme and has not been reported earlier for any troctolitic rocks. It has an internal reverse zonation with an inwards increase in compatible MgO and TiO2 and a decrease in all incompatible elements, suggesting an inwards decrease in the amount of trapped melt. We interpret this late-stage dyke as having formed from a residual melt that was channelled along a fissure in solidifying cumulates. The flow of the melt resulted in the very efficient removal of a boundary layer of evolved liquid from crystals growing on the walls of the dyke. This resulted in an almost perfect adcumulate that is extremely depleted in incompatible components.