Petrology of the Munni Munni Complex, Roebourne, Western Australia

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Abstract
The Munni Munni Complex is a layered sequence of clinopyroxenite, peridotite, and gabbro within the Archean Pilbara Block, Western Australia. The intrusion has been tilted, exposing some 2000 m of layered ultramafic rocks overlain by over 3500 m of gabbroic rocks. The top of the intrusion is hidden under younger sedimentary rocks. It displays many of the features of layered intrusions: rhythmic and cryptic layering, igneous lamination, pseudo‐gravity stratification, slump structures, and cumulus textures. Bulk‐rock analyses for major and trace elements show similar variation with structural height to that described from the Skaergaard, Bushveld, and Stillwater intrusions, although the mineralogical features of the Munni Munni Complex are slightly different. Olivine, clinopyroxene, and plagioclase are the only cumulus phases, and orthopyroxene, clinopyroxene, plagioclase, and Fe‐Ti oxides are the major intercumulus phases. The original magma was probably a tholeiitic basalt which differentiated in situ by fractional crystallization due to effective removal of early formed crystals by settling. Temperatures of crystallization were apparently in the range 1250° to 1200°C, and cooling rates were slow, as evidenced by exsolution features in the pyroxenes and oxides. Pressure was probably low (1–4 kb), indicating normal crystallization in the upper crust.