Structural and pressure-temperature evolution of host rocks of the Giles Complex, western Musgrave Block, central Australia: evidence for multiple high-pressure events

The history of Mid-Proterozoic metasedimentary gneisses, felsic and mafic orthogneisses of the Tomkinson Ranges, western Musgrave Block, involved multiple low- to high-P metamorphic events and polyphase deformation. Isoclinal, gently inclined to recumbent D1/2 folding accompanied the intrusion of pre-S1 mafic and felsic orthogneisses and post-S1 , pre-S2 felsic orthogneisses under peak conditions of P=5±1 kbar and T-750 °C at ~1200 Ma. K-feldspar megacrystic granitoid stocks formed at ~1188 Ma. Post-S2 mafic to ultramafic magmatism resulted in the large layered intrusions of the Giles Complex at ~1080 Ma, whilst the terrane was at conditions of P=6±1 kb. At ~1080-1060 Ma, microgranitoid dykes and veins intruded Giles Complex sills in the Tomkinson Ranges, while comagmatic volcanics of the Tollu Group unconformably overlie other parts of the Musgrave Block further west. Type A mafic dykes, which may represent feeder veins to the Tollu Group, intruded rocks of the Tomkinson Ranges before a third deformation event, D3, resulted in a system of steep, southeast-trending mylonites with a granulite facies S3 foliation and a well-developed down-dip L3 stretching lineation. Type A mafic dykes and their host felsic granite gneiss show marginal to complete recrystallisation to granulite facies mylonitic S3 assemblages at conditions of P-11 kb and T-700°C. Near-isothermal decompression returned such parts of the Musgrave Block to conditions of P-4-5 kb late in D3. Parts of the Musgrave Block that are unconformably overlain by the Tollu Group probably remained closer to the earths surface during D3. Coarse-grained, ~800 Ma Type B mafic dykes and aphanitic, ~1000 Ma Type C mafic dykes cut S3, and are cut by a system of mylonite and retrograde shear zones, D4-7. East-trending ~550 Ma D6 ultramylonite pseudotachylite zones represent the effects of the Late Proterozoic to Cambrian Petermann Orogeny, which resulted in significant dislocation of parts of the Musgrave Block and the partial to complete recrystallisation of post-S3 dykes near the Woodroffe thrust zone at sub-eclogite facies conditions of P=14±1 kb and T=700-750°C. These late high-pressure assemblages suggest that the structural grain in rocks of south-central Australia resulted, to a major extent, from Late Proterozoic to Cambrian compression.