Yilgarn Craton
The Yilgarn Craton is a large craton that constitutes a major part of the Western Australian land mass. It is bounded by a mixture of sedimentary basins and Proterozoic fold and thrust belts. Zircon grains in the Jack Hills, Narryer terrane have been dated at ~4.27 Ga, with one detrital zircon dated as old as 4.4 Ga.
The Murchison Province of the craton contains the over 2 billion year old Yarrabubba impact structure, previously regarded as the oldest dated meteorite impact crater, until the discovery of evidence such as shatter cones confirming the existence of the 3.47 billion year old North Pole Crater near Marble Bar in the Pilbara.
Geology
The Yilgarn Craton appears to have been assembled between ~2.94 and 2.63 Ga by the accretion of a multitude of formerly present blocks or terranes of existing continental crust, most of which formed between 3.2 Ga and 2.8 Ga.This accretion event is recorded by widespread granite and granodiorite intrusions, which comprise over 70% of the Yilgarn craton; voluminous tholeiitic basalt and komatiite volcanism; regional metamorphism and deformation as well as the emplacement of the vast majority of the craton's endowment in gold mineralisation.
These accretion events occurred in several phases, probably by accretion of continental fragments separated by pauses in subduction, with renewed activity occurring episodically.
The craton is primarily composed of approximately 2.8 billion year old granite-gneiss metamorphic terrain, and three granite-greenstone terrains. Some greenstone belts and granites are as old as 3.1-2.9 Ga, and some are younger, at ~2.75-2.65 Ga.
The craton is one of the distinct physiographic provinces of the West Australian Shield physiographic division, which comprises the Stirling-Mount Barren Block, Darling Hills, and Recherche Shelf sections.
Western Gneiss terrane
The Western Gneiss terrane is a series of polydeformed high-grade early Archaean metamorphic belts, composed predominantly of feldspathic leucocratic granulite gneisses, which represent some of the oldest crustal fragments on Earth. The Western Gneiss terrane is distinct from the remainder of the Yilgarn Craton in that the latter has a predominance of metavolcanic rocks, both felsic and mafic, whereas the former consists of high-grade metasediments and gneisses of unknown protolith.The Western Gneiss terrane is exposed along the western half of the northern margin of the Yilgarn Craton as the Narryer Gneiss terrane, a composite of heavily polydeformed feldspathic metagranite and metasedimentary amphibolite-grade gneisses and migmatites, dated at greater than 3.3 Ga and up to 3.8 Ga in age, flanked by the Murgoo Gneiss terrane, as well as sheets of 2.75 Ga to 2.6 Ga granite, obducted ophiolite sheets and some 2.4 Ga to 2.0 Ga Proterozoic gneiss belts.
On the western edge of the Yilgarn Craton, partially covered by Phanerozoic sedimentary basins and in faulted contact with the 2.7 Ga to 2.55 Ga Yilgarn tectonic domains, lies the Jimperding Gneiss Complex of 2.75 to 2.65 Ga age, composed primarily of micaceous quartzite, quartz-feldspar-biotite-garnet gneiss, andalusite and sillimanite schists, banded iron formation and other exotics, intruded by minor masses of porphyritic granite.
Detrital zircons in the Jimperding Gneiss Complex range in age from 3267 ± 30 Ma to 3341 ± 100 Ma, with metamorphic overgrowth dated at 3180 Ma.
On the southwest of the Yilgarn Craton the Balingup Gneiss Complex is situated inboard from the Early Proterozoic Leeuwin Complex of metamorphic rocks. The Balingup Complex consists primarily of metasedimentary paragneiss, granite orthogneiss, with minor layers of calc-silicate, ultramafic and ortho-amphibolite gneiss. The metamorphic grade is considered to be peak granulite facies, but the majority has preserved peak amphibolite facies assemblages.
In total, the Western Gneiss terrane sub-blocks represent an earlier substrate upon which the majority of the Yilgarn Craton's about 2.70 to 2.55 Ga greenstone metavolcanic belts have been deposited and into which the voluminous Archaean trondhjemite-tonalite-granodiorite suite and trondhjemite-tonalite-diorite suite granites were emplaced.
Murchison Province
The Murchison Province is exposed in the western and northern third of the Yilgarn Craton. The Province is bounded by major transcrustal structures which separate it from the surrounding tectonic provinces of the craton and the Western Gneiss Belt.The Murchison Province Stratigraphy, after Watkins, is divided into six basic structural-stratigraphic components - two greenstone belt metavolcanic-metasedimentary sequences and four suites of granitoids.
- Luke Creek Group metavolcanics
- Mount Farmer Group
- Early granodiorite-monzogranite intrusive suite
- Monzogranite Suite
- Two post-tectonic differentiated suites of granitoid rocks
The Murchison Province contains the Yarrabubba crater, which is the oldest dated meteorite impact crater, at 2229 ± 5 Ma. The crater is heavily eroded and no surface expression remains of the original structure. The primary trace is an elliptical aero-magnetic anomaly, measuring approximately 20 km by 11 km, as well as the presence of shock-recrystallised minerals. This impact may have ended the Huronian glaciation by climate forcing with a significant pulse of greenhouse gases, as the age broadly overlaps with the youngest glacial deposits.
Southern Cross province
The Southern Cross province lies in the central area of the Yilgarn craton. The Marda–Diemals greenstone belt in the Southern Cross terrane can be divided into three layers: the lower greenstone belt characterized by mafic volcanic rock and banded iron formation, a felsic-intermediate volcanism layer, and an upper sedimentary layer of calc-alkaline volcanic and clastic sedimentary rocks.East–West orogeny occurred in two stages; an earlier folding phase and a late phase that resulted in deposition and deformation of the Diemals Formation. Subsequent orogeny resulted in shear zones and arcuate structures.
The lithostratigraphy of the Marda–Diemals greenstone belt are similar to the northern Murchison terrane, but has older greenstones and deformation events than the southern Eastern Goldfields terrane. This indicates that the Eastern Goldfields terrane may have accreted to an older Murchison–Southern Cross granite–greenstone nucleus.
Eastern Goldfield province
The Archaean Norseman-Wiluna Greenstone Belt in the Eastern Goldfield province contains most of Australia's lode gold deposits, including the famous Kalgoorlie Golden Mile containing the Super Pit.These gold deposits are generally of large tonnage and are confined to the volcanic-intrusive-sedimentary sequences of the greenstone belts and not the granites. There is a pattern of gold distribution along the Archean Boulder-Lefroy shear zone.
Extrusive komatiites occur along the Norseman-Wiluna Greenstone Belt. A change from volcanic-dominated to plutonic-dominated magmatism occurred in the Norseman-Wiluna Greenstone Belt approximately 2685–2675 Ma. Voluminous high-Ca granite intrusions occurred 2670–2655 Ma. Much of the gold was deposited between 2650–2630 Ma, with much of this associated with strike-slip reactivation of earlier faults.
An earlier gold event 2660-2655 Ma was associated with major extension resulting in the formation of late basins and the intrusion of mantle-derived magmas and tight anticlockwise PTt paths.
Bounding terranes
The Yilgarn Craton is bound on all sides by younger terranes of various ages, but predominantly of Proterozoic age. The boundaries between the various flanking terranes provide considerable evidence of the post-Archaean events which have involved the Yilgarn Craton.Perth Basin
The Yilgarn Craton is bound on the western side by the Perth Basin, of Jurassic age, and is separated from this basin by the Darling Fault. The Perth Basin is considered to be a rift fill basin formed on a passive margin.Gascoyne Complex
The Perth Basin is bound on the north by the Gascoyne Complex, Glengarry Basin and Yerrida Basin, which are all part of a middle Proterozoic mobile belt which leads east to the Musgrave Block. The Gascoyne complex and other metamorphic belts of this age including reactivation of the Yarlarweelor Gneiss and Narryer Gneiss terrane, indicate prolonged multi-phased strike-slip movement from the late Archaean through to neoproterozoic and even into the Palaeozoic.Albany-Fraser Orogen
The Yilgarn Craton is bounded on the east-southeast by the ~1,300 Ma Albany-Fraser Orogen, composed primarily of amphibolite to greenschist facies sedimentary protolith gneisses, migmatites and granites. The Albany-Fraser Orogen displays both subduction-related and prolonged strike-slip tectonic structures and is intimately interconnected with the other Proterozoic basins and mobile belts of Australia.Sedimentary basin cover
The Yilgarn Craton is partially covered by onlapping sedimentary basins of Palaeozoic and Phanerozoic age in the east and north-east, including the Canning Basin. It is bounded on the western edge by the Darling Scarp and Darling Fault which separate the Yilgarn Craton from the Perth Basin to the west, and is covered by several remnant sedimentary basins of Jurassic age such as the Collie Sub-Basin.The Yilgarn Craton also has a considerable Tertiary and younger sedimentary veneer of palaeochannel deposits derived from prolonged erosion, sedimentation and redeposition of older cover sequences and regolith as well as the Archaean basement itself.
Recognised Tertiary cover sequences include the Bremer Basin, Officer Basin and others.