Osmium isotopes and highly siderophile element fractionation in the massif-type anorthosites of the Mesoproterozoic Kunene Intrusive Complex, NW Namibia

Abstract

The behaviour of highly siderophile elements during differentiation of mafic melts and formation of cumulate rocks in the crust is little understood, mostly because abundances of some of these elements are very low and reliable data is scarce. Data on massif-type anorthosites and associated Fe–Ti ore from the Mesoproterozoic Kunene Intrusive Complex (NW Namibia), indicate the effects of fractional crystallisation, crustal contamination, cumulate formation and post-cumulus re-equilibration on the distribution of the highly siderophile elements and the osmium isotopic composition of the anorthositic cumulate rocks.Anorthosites and Fe–Ti ore of the Kunene Intrusive Complex are characterised by low concentrations and strongly fractionated chondrite-normalised patterns of the highly siderophile elements. The abundance of highly siderophile elements is dominated by Fe–Ti oxides and traces of sulphides. The osmium isotopic composition and highly siderophile element distribution of the anorthosites are consistent with their derivation from a mantle-derived parental magma that was subject to fractionation of mafic minerals and variable extent of crustal contamination during its igneous evolution. Under conditions of the anorthosite formation Re behaves as a compatible element in fractionating titanomagnetite. The other highly siderophile elements are incorporated to a lesser extent into Fe–Ti oxides. During post-cumulus re-equilibration of Fe–Ti oxides Re is preferentially incorporated in magnetite whereas Pt displays preference for ilmenite.