THE MUNNI-MUNNI COMPLEX, WESTERN-AUSTRALIA - STRATIGRAPHY, STRUCTURE AND PETROGENESIS

The late Archaean Munni Munni Complex is a layered mafic ultramafic in trusion emplaced into granitic rocks of the west Pilbara Block. It con sists of a lower Ultramafic Zone with a maximum thickness of 1850 m an d an overlying Gabbroic Zone at least 3600 m thick. There are strong g eometrical and stratigraphic similarities to the Great Dyke of Zimbabw e. The Ultramafic Zone comprises multiple macrorhythmic cycles of oliv ine-clinopyroxene adcumulates and mesocumulates. Layering dips towards the centre of the intrusion and trends laterally into a narrow and va riably contaminated chilled margin. Higher layers extend progressively further up the sloping floor of the intrusion. Cryptic layering is de fined by rapid fluctuations in Cr content of cumulus clinopyroxene. ac companied by relatively small variation in Fe/Mg ratio. The base of th e Gabbroic Zone is marked by the first appearance of cumulus plagiocla se and the simultaneous appearance of pigeonite as a persistent cumulu s phase. Magnetite appears as a cumulus phase 400-600 m above this. Ga bbroic Zone cumulates show a gradual linear upward increase in Fe/Mg a nd an absence of cyclic layering. suggesting crystallization in a clos ed chamber. Chilled margin samples show evidence of in situ contaminat ion, but indicate that the parent magma to the ultramafic portion of t he 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, interse cts the floor of the intrusion at a level close to the top of the Ultr amafic Zone, and appears to have acted as a feeder conduit to the Gabb roic Zone and the uppermost layers of the Ultramafic Zone. The contact zone between the Ultramafic Zone and the Gabbroic Zone is a distincti ve 30-50 m thick pyroxenite layer. the Porphyritic Websterite Layer, w hich 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 xeno liths, some containing Al-rich, Cr-poor spinel seams, occurs within an d just above the Porphyritic Websterite Layer in the central part of t he intrusion. There is a steep gradient of decreasing Cr for and incre asing Fe/Mg in cumulus clinopyroxenes across the upper 100 m of the Ul tramafic Zone. A sharp downward step in Cr occurs a few metres below t he base of the Gabbroic Zone, immediately beneath a strongly orthocumu late layer of augite cumulate containing disseminated platinum-group e lement (PGE)-rich sulphides. Lateral pyroxene composition trends withi n the Porphyritic Websterite Layer can be accounted for by an increase in cumulus porosity as this layer approaches the floor of the intrusi on. Quantitative modelling of pyroxene composition trends indicates th at Ultramafic Zone cumulates crystallized from relatively small volume s 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 th e Porphyritic Websterite Layer marks a level at which the chamber expa nded as a result of a major new influx of magma. Pyroxene composition trends indicate that this influx was of a distinctly different and mor e 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 th e resident magma and formed a cool, dense basal hybrid layer. Crystall ization 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, causin g the Porphyritic Websterite Layer to accrete progressively up the slo ping floor and the walls. After the conclusion of the influx phase, th e hybrid layer became homogenized to a final tholeiite-rich compositio n, which eventually crystallized to form the Gabbroic Zone. The xenoli thic rocks within and above the Porphyritic Websterite Layer were prob ably derived initially by crystallization of a contaminated silica-enr iched 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.