Original paper

Upper Zone of the Archean Windimurra layered mafic intrusion, Western Australia: insights into fractional crystallisation in a large magma chamber

Nebel, Oliver; Arculus, Richard J.; Ivanic, Timothy J.; Rapp, Robert; Wills, Kevin J. A.


Layered mafic intrusions (LMI) are large, stratified igneous bodies of great economic significance. The constitutive crustal magma chamber(s) supplied by mantle-derived magma pulses exhibit complex mineralogical and chemical layering. Current magma chamber models invoke mineral density separation, liquid density inversion, and small-scale convection, superimposed on possible magma recharge and crustal assimilation, but petrogenetic interpretation is difficult. Four new drill cores from the Upper Zone and uppermost Middle Zone of the Windimurra Igneous Complex, the largest layered mafic intrusion in Australia, sample a continuous section of ∼1300 m including several magnetitite horizons. The Upper Zone is free of primary hydrous minerals and exhibits ore-grade Fe-enrichment up to 62.7 wt% FeOT in some sections. Relatively high Cr in magnetite at the base and sustainedly lower Cr in the remainder of the Upper Zone is consistent with injection of a single magmatic pulse at the commencement of the zone. Major and trace element analyses of representative samples from macroscopically layered horizons from throughout the Upper Zone sequence define three magmatic trends that reflect physical separation of phases within a single magma chamber. Magnetite accumulated at the base of the Upper Zone and geochemical trends in major and trace elements indicate an early formation of magnetite with a subtle effect on oxygen fugacity in the magma chamber. Bulk Nb/Ta < 14 indicate that Fe enrichment was not the result of large-scale liquid immiscibility, but is interpreted here as the normal result of an advanced tholeiitic fractionation trend. Accumulation in magnetitite horizons close to the base of the Upper Zone is interpreted to result from density separation of magnetite crystals. Field evidence indicates a density inversion in the course of magma differentiation, probably caused by ongoing crystallisation that resulted in late-stage mass movement of magnetitites and anorthositic blocks. Strontium isotope data point to a mantle-derived origin with 87Sr/86Sr[2.8Ga]∼0.701, indicating little, if any crustal contamination. The Windimurra Upper Zone sequence is an ideal example to study closed system magma chamber processes in an evolving, dry mafic system in the absence of or with only very limited crustal interaction.


archeanigneous differentiationlarge igneous provincelayered intrusionsmagnetiteupper zonewindimurra