The late Archaean Munni Munni Complex is a layered mafic ultramafic intrusion emplaced into granitic rocks of the west Pilbara Block. It consists of a lower Ultramafic Zone with a maximum thickness of 1850 m and an overlying Gabbroic Zone at least 3600 m thick. There are strong geometrical and stratigraphic similarities to the Great Dyke of Zimbabwe. The Ultramafic Zone comprises multiple macrorhythmic cycles of olivine-clinopyroxene adcumulates and mesocumulates. Layering dips towards the centre of the intrusion and trends laterally into a narrow and variably contaminated chilled margin. Higher layers extend progressively further up the sloping floor of the intrusion. Cryptic layering is defined by rapid fluctuations in Cr content of cumulus clinopyroxene. accompanied by relatively small variation in Fe/Mg ratio. The base of the Gabbroic Zone is marked by the first appearance of cumulus plagioclase and the simultaneous appearance of pigeonite as a persistent cumulus phase. Magnetite appears as a cumulus phase 400-600 m above this. Gabbroic Zone cumulates show a gradual linear upward increase in Fe/Mg and an absence of cyclic layering. suggesting crystallization in a closed chamber. Chilled margin samples show evidence of in situ contamination, but indicate that the parent magma to the ultramafic portion of the intrusion was a high-Mg, low-Ti basalt with similarities to typical Archaean siliceous high-Mg basalts. Partial melting of granitic wall rocks occurred along steep side walls but was less extensive along the shallow-dipping floor. A pyroxenite dyke. the Cadgerina Dyke, intersects the floor of the intrusion at a level close to the top of the Ultramafic Zone, and appears to have acted as a feeder conduit to the Gabbroic Zone and the uppermost layers of the Ultramafic Zone. The contact zone between the Ultramafic Zone and the Gabbroic Zone is a distinctive 30-50 m thick pyroxenite layer. the Porphyritic Websterite Layer, which also extends laterally up the side walls of the intrusion to form a 200 m thick marginal border zone separating Gabbroic Zone cumulates from country rock granites. A distinctive suite of bronzite-rich xenoliths, some containing Al-rich, Cr-poor spinel seams, occurs within and just above the Porphyritic Websterite Layer in the central part of the intrusion. There is a steep gradient of decreasing Cr for and increasing Fe/Mg in cumulus clinopyroxenes across the upper 100 m of the Ultramafic Zone. A sharp downward step in Cr occurs a few metres below the base of the Gabbroic Zone, immediately beneath a strongly orthocumulate layer of augite cumulate containing disseminated platinum-group element (PGE)-rich sulphides. Lateral pyroxene composition trends within the Porphyritic Websterite Layer can be accounted for by an increase in cumulus porosity as this layer approaches the floor of the intrusion. Quantitative modelling of pyroxene composition trends indicates that Ultramafic Zone cumulates crystallized from relatively small volumes of magma, an order of magnitude less than the size of the magma body inferred from trends in the Gabbroic Zone. This conclusion. together with the geometry of the Porphyritic Websterite Layer, implies that the Porphyritic Websterite Layer marks a level at which the chamber expanded as a result of a major new influx of magma. Pyroxene composition trends indicate that this influx was of a distinctly different and more fractionated composition than that parental to the Ultramafic Zone. Injection of fractionated tholeiitic magma into more primitive high-Mg basalt resident magma formed a turbulent fountain, which entrained the resident magma and formed a cool, dense basal hybrid layer. Crystallization of the Porphyritic Websterite Layer occurred where the top of this hybrid layer impinged on the sloping floor. Continuing injection of tholeiitic magma expanded the thickness of the hybrid layer, causing the Porphyritic Websterite Layer to accrete progressively up the sloping floor and the walls. After the conclusion of the influx phase, the hybrid layer became homogenized to a final tholeiite-rich composition, which eventually crystallized to form the Gabbroic Zone. The xenolithic rocks within and above the Porphyritic Websterite Layer were probably derived initially by crystallization of a contaminated silica-enriched melt layer at the roof of the intrusion, followed by detachment and sinking or slumping to the floor. Orthopyroxene phenocrysts within the Porphyritic Websterite Layer may also have originated within this roof zone.
