Geological framework and crustal evolution of the Giles mafic-ultramafic complex and environs, western Musgrave Block, central Australia

Layered mafic-ultramafic intrusions of the Giles Complex, western Musgrave Block are confined to a southern granulite facies block thrust northward over an amphibolite facies block and southward over sediments of the Officer Basin. The Tomkinson Ranges to Jameson Range region, at the westernmost end of the southern granulite facies terrane, consists of 17 medium to large-scale faulted segments or intact layered mafic-ultramafic sills and lopoliths emplaced into felsic to intermediate or mafic granulite facies orthogneiss. Protoliths of these gneisses, giving ages of -1.55 and 1.3 Ga, were metamorphosed at -1.2 Ga. Emplacement of the mafic-ultramafic bodies into multiply deformed felsic granulites (D1 and D2-pure shear; T>750°C, P=5±1 kb), previously believed to have occurred at about 1200-1188 Ma, is now thought more likely to be of 1.08--1.06 Ga--coeval with the Tollu Group volcanics. The western Musgrave Block displays crustal zonation of near-contemporaneous units, from deep crustal ultramafic-dominated intrusions in the north (south of the Woodroffe Thrust), to gabbro-pyroxenite intrusions in the Tomkinson Ranges, to troctolite intrusions in the southwest, to upper crustal volcanics of the Tollu Group-i.e. a southward rise in crustal level. The layered intrusions include: (1) large olivine-clinopyroxene-plagioclase troctolite to troctolite-anorthosite bodies, with Fe-rich olivine and plagioclase as liquidus phases, crystallised from highly evolved silica-undersaturated liquids and representing high-pressure orthopyroxene fractionation prior to intrusion. These bodies are commonly magnetite-rich, representing high oxygen fugacities, and include little or no ultramafic component, e.g. Jameson, Blackstone, Cavenagh, and Bell Rock intrusions; (2) large orthopyroxene-clinopyroxene-plagioclase gabbronorite to norite intrusions, including a significant (up to about 30%) ultramafic component, e.g. Michael Hills, Mount Davies, Kalka; (3) small to medium-sized layered pyroxenite-peridotite-gabbro intrusions, e.g. Murray Range, Claude Hills, The Wart, Gosse Pile, Ewarara, and (4) stratiform anorthosites forming lenses and recrystallised tongues interlayered with felsic granulites, mainly around Teizi bore. Ultramafic increments crystallised from little-fractionated primitive basaltic magmas saturated with olivinespinel form late magmatic pulses injected into above-solidus resident gabbroic bodies. Intrusion was followed by isobaric cooling (Wingellina Hills: P=6±1 kb; Blackstone: ~4 kb). Near-coeval relations between the Giles Complex and the Tollu Group volcanics imply rapid uplift and erosion of deep crustal zones followed by volcanic activity. Feeders for the volcanics are represented by type-A dykes correlated with the ~1.05-1.07 Ga Kulgera swarm of the eastern Musgrave Block, and by extensive granite veining and related granulite facies recrystallisation of large sectors of the Giles Complex (D3-simple shear; early stage T=650-700°C, P=11 kb; late stage P=4.5±1.1 kb). Northward thrusting of the granulite facies block over amphibolite facies gneisses along the western extension of the Woodroffe Thrust -550 Ma was associated with elevated pressures along the fault zone (P=14.0±1.1 kb; T=750°C), contemporaneous with the Petermann Ranges deformation.