Мусграв бұлғасы: Австралия геологиялық құрылымы

Geologic formation in Australia

The Musgrave Block (also known as the Musgrave Province) is an east west trending belt of Proterozoic granulite gneiss basement rocks approximately long. The Musgrave Block extends from western South Australia into Western Australia. The Musgrave Block is primarily exposed through the actions of the Petermann Orogeny at c. 535 550 Ma, which exhumed the orogenic belt along the Woodroffe Thrust.

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 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 domes, gneiss domes and isolated outcrops. This area is also a distinct physiographic province of the larger West Australian Shield.

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.

The Palgrave Palgrave Volcanic Association is a large volcanic and granite caldera edifice of about 1500 square kilometres 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.

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.

Several other prominent gravity and magnetic highs are arranged along the Mugrave Block strike line, one of which was drilled by BHP in the 1990s 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. 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.

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 Giles Complex intrusions. The Bentley Supergroup is composed primarily of bimodal volcanism, with several hundred metres thicknesses each of alternating rhyolite and basaltic volcanism adding up to several kilometres 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 magmatism. 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 1960s. 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).