Musgrave Block

Musgrave Block

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The Musgrave Block is an east-west trending belt of Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...

 granulite
Granulite
Granulites are medium to coarse–grained metamorphic rocks that have experienced high temperature metamorphism, composed mainly of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure...

-gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...

 basement rocks approximately 500km long. The Musgrave Block extends from western South Australia
South Australia
South Australia is a state of Australia in the southern central part of the country. It covers some of the most arid parts of the continent; with a total land area of , it is the fourth largest of Australia's six states and two territories.South Australia shares borders with all of the mainland...

 into Western Australia.
The Musgrave Block is primarily exposed through the actions of the Petermann Orogeny
Petermann Orogeny
The Petermann Orogeny was an intracontinental event that affected basement rocks of the northern Musgrave Province and Proterozoic sediments of the southern Amadeus Basin between ~550-535 Ma....

 at c. 535-550 Ma, which exhumed the orogenic belt
Orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...

 along the Woodroffe Thrust.

Geomorphology of Quaternary deposits


The Musgrave Block is currently passive geologically, with surficial processes described as residual erosion. The area currently experiences on average less than 150mm (6 inches) of rainfall per annum, which provides little surface runoff and hence virtually no erosion.

The landforms of the area are primarily composed of wide calcrete plains, often covered by Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....

 Age aeolian deposits of sand dunes, sometimes reworked into ephemeral sheetwash fans. Outcrop is rare, restricted primarily to the igneous rocks of the Giles Complex and several granite dome
Granite dome
A granite dome is a dome of granite, formed by exfoliation.-Formation:Granite forms plutons of igneous rock several kilometers below the surface as magma slowly cools and crystallizes. The granite is under great overhead pressure....

s, gneiss domes and isolated outcrops.

This area is also a distinct physiographic province of the larger West Australian Shield
Australian Shield
The Australian Shield, also called the Western Australian Shield or Western Plateau, occupies more than half of the continent of Australia. It occupies the portion of Australia west of a line running north-south roughly from the eastern shore of Arnhem Land on the Bay or Gulf of Carpentaria to the...

.

Palaeozoic rocks


Several Palaeozoic to neoproterozoic sedimentary basins onlap the Musgrave Block, and are exposed mostly around its edges. These were derived from erosion of the Musgrave Block basement during the neoproterozoic to Permian.

Proterozoic basement


The Proterozoic of the Musgrave Block is composed of seven main classes of rocks;
  • Surficial volcanics of c.1050 to 1080 Ma, both mafic (Mummawarrawarra Basalt) and andesitic to rhyolitic (Tollu, Smoke Hill Volcanics) and the Bentley Supergroup volcanics and sediments of c. 1080 Ma
  • Large intrusive caldera type granite intrusive complexes (Palgrave, Skirmish Hill, Smoke Hill), possibly of c. 1050 and likely no older than 1080 Ma
  • Partly metamorphosed, rarely dissected c. 1080 Ma granite
  • Usually unmetamorphosed, rarely dissected granites of a c. 1050 intrusive suite
  • Partly metamorphosed, dissected intrusive rocks of the mafic-ultramafic c. 1080 Ma Giles Complex
  • Highly metamorphosed metagranites of the c. 1200 Ma suite
  • High-grade crystalline metamorphic basement rocks of c. 1550-1300 Ma age


The Musgrave Block is flanked by several Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...

 to Palaeozoic sedimentary basins, whose sedimentary history can elucidate the timing of tectonic events in the Musgrave Block post-1080 Ma. These include the Proterozoic Amadeus
Amadeus Basin
The Amadeus Basin is a large intracratonic sedimentary basin in central Australia, lying mostly within the southern Northern Territory, but extending into the state of Western Australia. It is named after Lake Amadeus which lies within the basin...

 and Officer Basin
Officer Basin
The Officer Basin is a large intracratonic sedimentary basin in Australia, lying across the states of South Australia and Western Australia. It is named after Officer Creek which is a watercourse that drains a small part of the basin. Deposition of up to 10 km of marine and non-marine sedimentary...

s.

Events


The most illustrating way of considering the Musgrave Block is as part of a time-space plot in which geological events are arrayed in time against rock units, stratigraphic relationships and for correlative purposes.

The key events in the Musgrave Block are;
  • Protolith formation ~1550 Ma. Formation of crustal rocks of the Birksgate Complex, which are both mafic and felsic in composition.
  • Igneous event and orogeny ~1300 Ma inferred from geochronology and distribution of felsic “volcanic” supracrustal sequence of the high grade gneisses
  • Musgravian orogeny at ~1200 Ma including generation of voluminous Kulgera Suite of granite
    Granite
    Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

    s and metamorphosis of the Birksgate Complex and ~1300 supracrustal sequence to amphibolite
    Amphibolite
    Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...

    -granulite
    Granulite
    Granulites are medium to coarse–grained metamorphic rocks that have experienced high temperature metamorphism, composed mainly of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure...

     facies
  • Warakurna Large igneous province
    Large igneous province
    A Large Igneous Province is an extremely large accumulation of igneous rocks—intrusive, extrusive, or both—in the earth's crust...

     at 1076 ± 6 Ma including the intrusion of the Giles Complex and the Winburn Suite of granites, plus deposition of the Bentley Supergroup (including Tollu and Smoke Hill Volcanics)
  • Amata Dyke Suite at ~830-880 Ma
  • Petermann Orogeny
    Petermann Orogeny
    The Petermann Orogeny was an intracontinental event that affected basement rocks of the northern Musgrave Province and Proterozoic sediments of the southern Amadeus Basin between ~550-535 Ma....

    , 535-550 Ma; marginal foreland basin formation, trending to crustal consolidation and quiescence
  • Permian
    Permian
    The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

     glaciation and erosion of Petermann Orogeny mountains; deposition of Permian sequences in Officer and Amadeus basins
  • Intracontinental setting till present

Granites and Calderas


There are three main phases of granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

 intrusion into the Musgrave Block;
  • Kulgera Suite at 1200 Ma
  • At ~1080Ma synchronous with the Giles Complex
  • Winburn Suite at ~1050Ma postdating the Giles Complex.


The Kulgera Suite is a widespread, voluminous suite of fractionated amphibole-bearing plagioclase rich tonalite to granodiorites of an I-type affinity (Stewart, 2003). They are dated at ~1200 Ma and are considered to be related to melting of the lower crust during the ~1200 Ma Musgravian Orogeny.
The Windburn Suite is considered to be an anorogenic A-type granite suite produced by anatexis
Anatexis
Anatexis in geology, refers to the differential, or partial, melting of rocks, especially in the forming of metamorphic rocks such as migmatites.-Optimum Temperature Conditions for Crustal Melting:...

 of the lower crust at ~1080 to 1050 Ma by the injection of the mafic Giles Complex intrusions. They are in most cases poorly fractionated, biotite-muscovite granites, with little hornblende, a high radiometric count and the presence of fluorine. The 1080 Ma granites and 1050 Ma granites can be distinguished on geochemical grounds, the latter is perhaps created by th above process, and the earlier suite a hybrid between A-type and I-type granites formed by assimilation and mixing.

There are also significant granite caldera complexes, of many hundred square kilometers in area, which intrude the Musgrave Block. These are of the ~1050 Ma age, and are subvolcanic
Subvolcanic rock
A subvolcanic rock, also known as a hypabyssal rock, is an igneous rock that originates at medium to shallow depths within the crust and contain intermediate grain size and often porphyritic texture. They have textures between volcanic and plutonic rocks. Subvolcanic rocks include diabase and...

, possibly related to some of the ~1050 Ma volcanic rocks.

Palgrave Volcanic Association


The Palgrave Palgrave Volcanic Association is a large volcanic and granite caldera edifice of about 1500 square kilometers sandwiched between the Jameson Range Intrusion and the gneisses underlying the Bentley Supergroup volcanics.

The caldera is in two parts, an overlying volcanic edifice composed primarily of porphyritic rhyolite and dacite with occasional vent complex agglomerates, which shows prominent circular ring-complex faults, and the Winburn Granite which underlies the caldera and is primarily exposed in the east as a pink, potassium-feldspathic porphyry granite, the lower margins of which are weakly tectonised.

The whole caldera edifice is tilted to the southwest, which is why the Winburn Granite is exposed along the east and northeast.

Skirmish Hill Caldera


The Skirmish Hill Caldera is poorly exposed along the southern margin of the Musgrave Block and consists of granite and overlying? rhyolite. It has been traditionally seen as a potential source for the Tollu Volcanics.

The caldera is truncated on the north by a north-dipping thrust fault and is probably tilted to the south.

Other Calderas


Several other prominent gravity and magnetic highs are arranged along the Mugrave Block strike line, one of which was drilled by BHP
BHP Billiton
BHP Billiton is a global mining, oil and gas company headquartered in Melbourne, Australia and with a major management office in London, United Kingdom...

 in the 1990’s through 300m of Permian glacial sediments.

This caldera is composed of highly tectonised, stretched felsic volcanic rocks, interleaved with a significant thickness of equally sheared titaniferous differentiated mafic sills
Sill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...

. The best interpretation of this, and probably also of the Palgrave Caldera is that they represent hot spots along the Musgrave Block where significant magma flux penetrated, formed volcanic calderas with large subvolcanic granite intrusions, and associated mafic volcanism.

The relationship of the large granite calderas to the 1050-1080 Ma volcanics has been postulated as one in which the granite calderas were the source for the intermediate and felsic volcanic rocks.

Bentley Supergroup


The Bentley Supergroup Volcanics are a sequence of bimodal supracrustal volcanic rocks formed during the ~1080 Warakurna Large Igneous Province, and are widely considered comagmatic with the mafic to ultramafic
Ultramafic to mafic layered intrusions
A layered intrusion is a large sill-like body of igneous rock which exhibits vertical layering or differences in composition and texture. These intrusions typically are many kilometers in area covering from around 100 km2 to over 50,000 km2 and several hundred meters to over a kilometer...

 Giles Complex intrusions.

The Bentley Supergroup is composed primarily of bimodal volcanism
Bimodal volcanism
Bimodal volcanism refers to the eruption of both mafic and felsic lavas from a single volcanic centre with little or no lavas of intermediate composition...

, with several hundred-metres thicknesses each of alternating rhyolite
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...

 and basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

ic volcanism adding up to several kilometers true thickness in the area of the Warburton Range to the southwest of the Palgrave caldera. The Bentley Supergroup is divided into the Cassidy Group, Pussycat Group and Tollu Group.

The prevailing theory of the formation of the Bentley Supergroup is that the Warakurna Large Igneous Province, primarily represented by the Giles Complex intruding into the lower crust, breached the crust and erupted voluminous basaltic lava flows, and when enough heat had been added to the crust by the massive intrusions below, intracrustal felsic and intermediate melts were produced, forming A-type intracontinental granites of the Winburn Suite, and the felsic volcanic rocks.

This created the typical bimodal volcanic signature of the Cassidy Group and Pussycat Groups; the Tollu Group is a bit different, and it is considered the product of the large granite calderas which were formed immediately after the Giles Complex mamatism.
Giles (1980) and earlier mappers have assigned the MacDougall Formation, overlying Mummawarrawarra Basalt, intermediate Smoke Hill Volcanics and the Tollu Volcanics to the Bentley Supergroup.

There has been little real study done on the Bentley Supergroup Volcanics since the 1960’s. Geochemical and petrological observations are few and far between or lacking comprehensive rare earth and trace elements suites. The Bentley Supergoup is poorly exposed in South Australia (if at all).

See also

  • GLIKSON A.Y., STEWART A.J., BALLHAUS C.G., CLARKE G.L., FEEKEN E.H.J., LEVEN J.H., SHERATON J.W., AND SUN S.S. 1996. Geology of the western Musgrave Block, central Australia, with particular reference to the mafic-ultramafic Giles Complex. Australian Geological Survey Organisation Bulletin 239. 205 pp.