Musgrave Geology
Birksgate Complex of the Fregon Subdomain
The Birksgate Complex is composed of well-layered amphibolite to granulite facies felsic and mafic orthogneisses and quartzofeldspathic, pelitic and calc-silicate paragneisses. Granulite facies rocks are well exposed in the Tomkinson, Mann and Musgrave Ranges. The dominant lithology is felsic gneisses, comprising up to 90% of the rocks. Mafics account for up to 9%, with ultramafic, pelitic, quartzites, manganiferous and calc-silicate rocks making up the remainder. The protolith material is believed to be acid volcanics, associated sediment and minor mafic intrusions. Protolith age ranges from 1600 to 1300 Ma. Metamorphism during the ~1200 – 1170 Ma Musgrave Orogeny caused anhydrous melting, producing migmatites, granitic intrusions, and pegmatites, some of which contain abundant allanite.
Olia Gneiss of the Mulga Park Subdomain
The Olia Gneiss occurs north of the Woodroffe Thrust within the Mulga Park Subdomain and is predominantly composed of adamellite and granitic augend gneisses containing biotite, hornblende and allanite. There are some interlayered pelitic biotite-muscovite schists, and rare layers of amphibolite and quartzite. The augen gneisses are assumed to have originally been felsic volcanics and/or arkoses. The amphibolites were possibly basic igneous rocks and/or calc-silicate sediments. The age of the Olia Gneiss has been determined by extensive geochronology by the Northern Territory Geological Survey and ranges from 1600 - 1540 Ma.
Kulgera Suite Granites
The Musgravian Orogeny coincides with the intrusion of granites of the Kulgera Suite both north and south of the Woodroffe Thrust. Compositions range from alkali granite to quartz diorite and more mafic granites (charnockites) containing a variable mineralogy of feldspar, hypersthene, clinopyroxene, hornblende, biotite, garnet and ilmenite. Titanite (sphene) is a common accessory, with allanite also locally common.
Historically, most granites of the Musgrave Block were assigned to the Kulgera Suite, but recent work has shown that a second, equally voluminous suite which is thought to be synchronous with the intrusion of the Giles Complex (i.e. Angatja Granite ~1070 Ma).
The Giles Complex
The Giles Complex consists of generally layered basic and ultrabasic igneous rocks that form a discontinuous series of intrusions into granulite facies rocks of the Musgrave Block. Tholeiitic basic rocks predominate over ultrabasic and anorthositic rocks. The Complex forms an easterly trending belt about 85km wide, stretching from the Bedford Range in Western Australia to the eastern end of the Musgrave Province in South Australia. The level at which the Complex was emplaced ranges from the lower crust in South Australia to higher, near surface crust levels.
Igneous layering ranges from large scale cyclic units, through rhythmic layering, intercumulus layering and cryptic layering. Sedimentary-like structures are the result of crystallisation and fractionation.
Eleven main outcropping intrusions are recognised in South Australia. The Wingellina Hills, Hinckley Range and Michael Hills intrusions crop out mostly in Western Australia. The Claude Hills, Ewarara, Kalka, Mt Davies and Gosse Pile intrusions crop out in the north-west section of the MANN 1:250 000 map sheet, in the Tomkinson Ranges region.
The Mt Woodroffe-Trudinger Pass and Caroline intrusions crop out in the northern section of the WOODROFFE 1:250 000 map sheet, in the Musgrave Ranges region. Smaller outcrops of Giles Complex have been identified on the ALBERGA 1:250 000 map sheet.
Shear Zones
The Musgrave Province is traversed by a series of east-west trending major shear zones, which impart a prominent east-west ”grain” to the country. The main shear zones are the Hinckley-Scarface, Mann-Ferdinand, Davenport and Woodroffe systems. The shear zones may have originally developed during the Musgravian Orogeny between 1200 and 1170 Ma. Reactivation of these shear zones during the Late Neoproterozoic Petermann Orogeny (~550 Ma) resulted in the development of high strain mylonites and ultra-mylonites zones along these major structures.
Basic Dykes and Sills
Seven distinct intrusive episodes of basic dykes and sills, mainly gabbros and dolerites were emplaced post-Giles, these include the Alcurra and Amata Dyke swarms. Two of the earlier mafic dyke suites intruded whilst the Giles Complex was still cooling which resulted in these dykes experiencing contact metamorphism on intrusion.
Three following phases of mafic dykes that are the most obvious in the field and on geological maps, were not metamorphosed during their emplaced. These rocks are mostly gabbros, which predominate, and dolerites. They usually contain various amounts of olivine and orthopyroxene. Rare dykes of norite, diorite or troctolite also occur.
The majority of the mafic dykes pre-date movement on the Woodroffe Thrust, but small unmetamorphosed dolerite dykes and sills have intruded the Woodroffe Thrust mylonite zone, these dykes are potentially younger than mid-Cambrian.
Alcurra Dyke Swarm
The Alcurra Dyke swarm forms an east-west arcuate belt 90km long by 15km wide extending from South Australia to the Northern Territory. They are shallowly south dipping olivine dolerite dykes that truncate gneissic layering in granulite rocks from the eastern Mann Ranges. The ~1070 Ma Angatja Granite intrudes the Alcurra dykes providing a minimum age constraint for their emplacement.
Amata Dyke Swarm
Recent dating of the Gairdner Dyke Swarm has produced an age of 827 Ma. This correlates closer with a number of dykes in the Amata region which have been dated at ~800 Ma and have been tentatively classified as a separate swarm, the Amata Dyke Swarm.
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- Page Last Modified: November 8, 2011