Adcumulate mafic dykes in layered intrusions: a case study of a late-stage dyke in the Bayantsagaan layered intrusion, Mongolia
2015
We 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. K 2 O, 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 TiO 2 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.
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