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, L abrador, have been performed in order to constrain the partitioning be haviour of major and trace elements during formation of Kiglapait Lowe r Zone (LZ) cumulates. At 0.1 MPa and f(O2) close to FMQ, the liquidus is bracketed between 1225 and 1212 degrees C and the only liquidus ph ases are plagioclase (An(73-74)) and olivine (Fo(82-85)). In contrast LZ troctolites contain Fo(67 +/- 5) olivine and An(63 +/- 4) plagiocla se. Plagioclase-melt partition coefficients (D's) for Li, Ti, Rb, Sr, Y, Zr, Ba, La and Eu, determined by ion-microprobe analysis of run pro ducts, are similar to those for other basaltic systems, but differ sys tematically from apparent D's calculated from bulk analyses of LZ plag ioclases and modelled liquid compositions obtained by summation. The d iscrepancies in D's and phase compositions are inconsistent with diffu sion-controlled boundary layer growth. Crystallisation of trapped inte rcumulus liquid is quantitatively plausible; however, (a) the volume o f liquid required to reconcile experimental and model D's (20 +/- 1%) is larger than that estimated from observed proportions of intercumulu s phases (11 +/- 2%), and (b) this mechanism alone cannot reduce X-An in LZ plagioclases due to very slow Al reversible arrow Si interdiffus ion. The discrepancy between experiment and model is best resolved if the parental melt to LZ cumulates was more evolved (higher Fe/Mg, Na/C a, REE and alkalis, lower Sr) than the summed bulk composition, in acc ord with a recent revision of the Kiglapait Fe/Mg based on olivine com positions (Morse, S.A., 1996, Kiglapait mineralogy III: Olivine compos itions and Rayleigh fractionation models, J. Petrol. 37, 1037-1062). ( C) 1997 Elsevier Science B.V.