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Igneous rocks are formed when molten magma invades the Earth's upper layers and solidifies. Australia's geology includes almost all known rock types, spanning a geological time period of over 3.8 billion years. Igneous rocks can be found in the Pilbara region of Western Australia, providing a diversity of Archaean to Proterozoic igneous rocks in a relatively compact area. Granite, an igneous rock, can be found in New South Wales and Victoria, with the Bega batholith being a notable example. The Gawler Craton and Willyama Block are also sources of igneous rocks, flanked by the Musgrave Block of granulite gneiss and igneous rocks.
| Characteristics | Values |
|---|---|
| Location | Pilbara region, Western Australia |
| Area | 500 km x 500 km |
| Rock Diversity | Archaean to Proterozoic igneous rocks |
| Rock Types | Komatiite, basalt, granite, adamellite, monzogranite, syenogranite, granodiorite, tonalite, granophyre, etc. |
| Mineral Composition | Quartz, feldspar, mica, amphibole, hornblende, clinopyroxene, etc. |
| Geological History | Younging southward crustal history of igneous activity, sedimentation, tectonics, and metamorphism |
| Geological Time Period | Archaean (3.6–2.7 Ga) to Proterozoic (2.5–1.8 Ga) |
| Geological Terms | Mafic, ultramafic, granitoid, volcaniclastic, dolerite, gabbro, etc. |
| Geological Features | Granite intrusions, volcanic arcs, deep water sediments, volcanic lava rock, etc. |
| Mineral Wealth | Gold, bauxite, iron ore, uranium, opals, diamonds, sapphires, etc. |
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What You'll Learn
Granite in New South Wales and Victoria
Igneous rocks are formed when magma cools and solidifies. Granite is an igneous rock composed of between 20% and 60% quartz by volume and at least 35% alkali feldspar.
Granite in New South Wales
The Australian state of New South Wales is geologically diverse, consisting of seven main regions: the Lachlan Fold Belt, the Hunter-Bowen orogeny or New England Orogen (NEO), the Delamerian Orogeny, the Clarence Moreton Basin, the Great Artesian Basin, the Sydney Basin, and the Murray Basin. The Lachlan Fold Belt, formed in the Middle Paleozoic from 450 to 340 Mya, is particularly notable for its granite outcroppings. The Granite Belt, situated in the New England Orogen, is a popular tourist destination known for its wineries, national parks, and granite outcroppings such as Girraween National Park. The Granite Belt extends from Stanthorpe in Queensland to Liston in New South Wales. The Eulo Shelf, southeast of Hungerford, is also underlain by granite. The Yanco Glen deposit in New South Wales is another example of a granite-related mineral system.
Granite in Victoria
In northeastern Victoria, the Unicorn porphyry Cu-Mo-Ag deposit is a significant granite-related mineral system. This deposit is located south of the Holbrook disseminated Mo-Sn-Cu deposit and is interpreted as a Climax-type porphyry Mo deposit. The Ovens Graben (Ovens Valley Graben) extends from the Murrumbidgee River in New South Wales to Wangaratta in Victoria and is covered by the Oaklands Basin in New South Wales.
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Igneous rocks in the Pilbara region
The Pilbara region in Western Australia is known for its diversity of Archaean to Proterozoic igneous rocks, which record a younging southward crustal history of igneous activity, sedimentation, early life, tectonics, and metamorphism. The igneous rocks in the Pilbara region vary in age, type, and mode of occurrence. The region covers an area of approximately 500 km by 500 km and includes the Pilbara Craton, one of only two pristine Archaean 3.8–2.7 Ga (billion years ago) crusts identified on Earth. The youngest rocks in the Pilbara Craton are 1.7 Ga old.
The igneous rocks in the Pilbara region include komatiite, mafic volcanic/volcaniclastic rocks, basalt, tuff/volcanic breccia/accretionary lapilli, dolerite, gabbro, leucogabbro, pegmatitic gabbro, granite, and adamellite. There are also xenolithic dolerite/gabbro, andesite, dacite, rhyodacite, rhyolite, and granitoids such as adamellite, monzogranite, syenogranite, and granodiorite. The Pilbara Craton also contains well-preserved Archean granite-greenstone terranes, providing insights into the sources of felsic magmas and the processes of continental growth and evolution.
The geology of the Pilbara region was reassessed in 2007, leading to the separation of a thick succession of interbedded clastic or chemical sedimentary rocks and volcanic rocks forming the Fortescue, Hamersley, and Turee Creek basins. These rocks are typically aged from 2.78–2.42 billion years old, while the younger volcano-sedimentary Ashburton Basin is aged from 2.21–1.79 billion years ago. The Pilbara region is arid, allowing for excellent rock exposure and the observation of geological features.
The Pilbara region is internationally significant due to its unique geology and well-exposed geological features. The diversity of Archaean to Proterozoic igneous rocks in a relatively compact area makes it a globally unique potential Precambrian igneous-rock geopark. The region provides valuable insights into the early Earth's crust and the processes that shaped it. The igneous rocks in the Pilbara region offer a window into the Earth's geological history and the formation of the continent.
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Volcanic arcs in New England
Igneous rocks are formed when magma cools and solidifies. Volcanic arcs in New England, New South Wales, Australia, are evidenced by the presence of Late Carboniferous continental conglomerates interbedded with silicic ignimbrite sheets. These volcanic arcs are comparable to the Andean stratovolcanoes and ignimbrite centres, which provide insight into the reconstruction of the Late Carboniferous volcanic source terrain.
The geological record of these continental margin volcanic arcs is shaped by volcanism, specifically the eruption of voluminous ignimbrites, as well as uplift, deformation, and glaciation centred on the arc. The diversity in the flank sequences is influenced by variations in the importance of these factors over time and across different sections of the arc.
The Late Carboniferous volcanic source terrain, which is no longer exposed, was located to the west of the existing conglomerate-ignimbrite sequences. It was underlain by continental crust, with the northern Lachlan Fold Belt representing a portion of this crust. The regional Late Carboniferous palaeogeography shared similarities with the present-day western continental margin of South America.
The Andean stratovolcanoes and ignimbrite centres provide a valuable analogue for understanding the Late Carboniferous volcanic arc in western New England Orogen. The Hercynian uplift along the western margin of South America, caused by the collision of "Chilinia", initiated a long Late Palaeozoic glacial record that ended in western Australia. This glacial record provides insights into the preservation of glacially-influenced strata in newly-rifted intracratonic basins.
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Igneous rocks in Queensland
Igneous rocks are formed when magma cools and solidifies. They can be found in various locations across Queensland, a state in Australia.
The geology of Queensland includes a range of rock types, including igneous rocks. The Precambrian basement in Queensland, located west of the Tasman Line, includes elements such as the Mount Isa Orogen and the Georgetown Inlier. The Mount Isa Orogen started with sediments, volcanics, and intrusive rocks and was deformed and metamorphosed during the Barramundi Orogeny 1870 Mya. Over time, this region experienced the development of sedimentary layers, with felsic volcanic rocks forming from 1870 to 1850 Mya, followed by shallow water sediments from 1790 to 1705 Mya, and fine-grained sediments and carbonate rocks from 1675 to 1590 Mya.
The Murphy Province in Queensland, which extends into the Northern Territory, consists of the Murphy Metamorphics, the Nicholson Granite, and the Cliffdale Volcanics, dating from 1820 to 1730 Mya. North of the Murphy Province is the McArthur Basin, which includes the Tawallah Group of sediments and volcanics from 1725 Mya. The South Nicholson Basin straddles the border between Queensland and the Northern Territory, and the Savannah Province on Cape York was formed from sediments around 1585 to 1550 Mya and has since been converted into slate, phyllite, schist, gneiss, and quartzite.
Queensland has also witnessed volcanic activity, with the Glasshouse Mountains serving as an example of Tertiary intraplate volcanism. These mountains form a chain of small volcanic plugs that decrease in age towards the south, eventually leading to the younger maar volcanoes and basalts of the Newer Volcanics in South Australia and Victoria. Additionally, the Felsic and Intermediate Igneous Rocks of Australia Project, which concluded in 2007, focused on the Tasmanides of eastern Australia, including Queensland. This project aimed to assess the metallogenic potential of major Australian Phanerozoic granite suites and their associated host rocks, contributing to the understanding of mineralisation and the prediction of mineralised sites.
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Opals in the outback
Opals are considered precious gemstones and are Australia's national gemstone. They are found in the Australian outback and are highly sought after for their vibrant colours and patterns. Opals are composed of silica and water, and their appearance varies depending on the conditions under which they are formed.
There are several types of opals, including boulder opals, crystal opals, and black opals. Boulder opals are primarily mined in Queensland and are a combination of precious opal and the host rock in which it was formed. Crystal opals are translucent, semi-translucent, or transparent, and can display any colour of the spectrum. The most valuable type of opal is the black opal, which is only mined at Lightning Ridge in Queensland. Black opals have a dark grey to jet-black body, and the spectral colours appear extremely vibrant against this dark background.
The Outback Opal Hunters TV series has brought attention to the world of opal mining in Australia, and there is a thriving industry for opal jewellery and gemstones. Opal Auctions is a popular online marketplace for buying loose opals, rough opals, and opal jewellery, with sellers from all around the globe.
When it comes to opal care, it is important to note that opals are vulnerable to scratching due to their Mohs Hardness of 5 to 6.5. Additionally, opals should be protected from extreme temperature changes, bright light, and excessive dryness to prevent moisture loss and crazing.
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Frequently asked questions
Igneous rocks can be found all over Australia. The Pilbara region in Western Australia is known for its diversity of Archaean to Proterozoic igneous rocks. The Gawler Craton and the Willyama Block are also made up of igneous rocks.
Igneous rocks are formed when magma cools and solidifies. They can be further classified into volcanic and subvolcanic rocks.
Some examples of igneous rocks include granite, basalt, and porphyry.
Some unique igneous rocks found in Australia include komatiite, adamellite, and monzogranite. The Argyle diamond mine in Western Australia is also known for its lamproite, a type of volcanic rock that hosts diamonds.
