Experimental study of a Kiglapait marginal rock and implications for trace element partitioning in layered intrusions

Experiments on a chilled marginal rock from the Kiglapait Intrusion, Labrador, have been performed in order to constrain the partitioning behaviour of major and trace elements during formation of Kiglapait Lower Zone (LZ) cumulates. At 0.1 MPa and fo2 close to FMQ, the liquidus is bracketed between 1225 and 1212°C and the only liquidus phases are plagioclase (An73−74) and olivine,(Fo82–85) In contrast LZ troctolites contain Fo67±5 olivine and An63±4 plagioclase. Plagioclase-melt partition coefficients (Dʹs) for Li, Ti, Rb, Sr, Y, Zr, Ba, La and Eu, determined by ion-microprobe analysis of run products, are similar to those for other basaltic systems, but differ systematically from apparent Dʹs calculated from bulk analyses of LZ plagioclases and modelled liquid compositions obtained by summation. The discrepancies in Dʹs and phase compositions are inconsistent with diffusion-controlled boundary layer growth. Crystallisation of trapped intercumulus liquid is quantitatively plausible; however, (a) the volume of liquid required to reconcile experimental and model Dʹs (20±1%) is larger than that estimated from observed proportions of intercumulus phases (11 ± 2%), and (b) this mechanism alone cannot reduce XAn in LZ plagioclases due to very slow Al ⇔ Si interdiffusion. The discrepancy between experiment and model is best resolved if the parental melt to LZ cumulates was more evolved (higher Fe/Mg, Na/Ca, REE and alkalis, lower Sr) than the summed bulk composition, in accord with a recent revision of the Kiglapait Fe/Mg based on olivine compositions