The Gascoyne Complex is a

terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its own ...

Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...

granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...

and

metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...

in the central-western part of

Western Australia Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to th ...

. The complex outcrops at the exposed western end of the Capricorn Orogen, a 1,000 km-long arcuate belt of folded, faulted and metamorphosed rocks between two

Archean The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic. The Earth Earth ...

craton A craton (, , or ; from grc-gre, κράτος "strength") is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and ...

s; the

Pilbara craton The Pilbara Craton is an old and stable part of the continental lithosphere located in the Pilbara region of Western Australia. The Pilbara Craton is one of only two pristine Archaean 3.6–2.7 Ga (billion years ago) crusts identified on the ...

to the north and the

Yilgarn craton The Yilgarn Craton is a large craton that constitutes the bulk 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 be ...

to the south. The Gascoyne Complex is thought to record the collision of these two different Archean continental fragments during the Capricorn Orogeny at 1830–1780 Ma. The Gascoyne Complex is separated from the Yilgarn Craton to the south by a major fault, the Errabiddy Shear Zone. To the east and northeast rocks of the complex are overlain unconformably by fine-grained

Mesoproterozoic The Mesoproterozoic Era is a geologic era that occurred from . The Mesoproterozoic was the first era of Earth's history for which a fairly definitive geological record survives. Continents existed during the preceding era (the Paleoproterozoic), ...

sedimentary Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followed by cementation (geology), cementation. Sedimentati ...

rocks of the Edmund Basin and Collier Basin (formerly known as the Bangemall Basin). Several inliers of granite within these sedimentary basins also belong to the Gascoyne Complex. To the west, the Gascoyne Complex is overlain unconformably by sedimentary rocks of the

Phanerozoic The Phanerozoic Eon is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 538.8 million years to the present, and it began with the Cambrian Period, when anima ...

Carnarvon Basin The Carnarvon Basin is a geological basin located in the north west of Western Australia which extends from the Dampier Archipelago to the Murchison bioregion, and is the main geological feature that makes up the North West Shelf. The onshore ...

. To the north,

schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes o ...

of the Gascoyne Complex probably pass with decreasing intensity of metamorphism into metamorphosed sedimentary rocks of the upper Wyloo Group. The Gascoyne Complex is divided into two parts, the 1840–1620 Ma northern and central Gascoyne Complex, and the 2005–1970 Ma Glenburgh Terrane in the southern Gascoyne Complex. The two are separated by a major east-southeast trending fault, the Chalba Shear Zone. Rocks of the Glenburgh Terrane do not outcrop at surface north of the Chalba Shear Zone, but it is unclear as to whether or not rocks of this terrane floor all or part of the central and northern Gascoyne Complex. The Gascoyne Complex has been shaped by four

orogenies Orogeny is a mountain building process. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An ''orogenic belt'' or ''orogen'' develops as the compressed plate crumples and is uplifted t ...

, the most important and widespread of which were the 1830–1780 Ma Capricorn Orogeny and the 1680–1620 Ma Mangaroon Orogeny. Both of these orogenies were marked by extensive folding, faulting and metamorphism, and were accompanied by the intrusion of large volumes of granite referred to as supersuites. The effects of the oldest orogeny, the 2005–1960 Ma Glenburgh Orogeny, although known only from the southern end of the complex, reflect a period of substantial granite magmatism and intense deformation and metamorphism. The

Neoproterozoic The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago. It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is ...

Edmundian Orogeny mainly consists of the reactivation of earlier formed faults in the Gascoyne Complex, along with folding and faulting of the overlying Edmund and Collier basins.

Glenburgh Terrane

In this section the component rock units of the Gascoyne Complex, and the orogenies that shaped the complex, are discussed in order of decreasing age.

Halfway Gneiss

This rock unit is exposed only in the Glenburgh Terrane at the southern end of the Gascoyne Complex. The gneiss is a composite of c. 2540 Ma granites and c. 2000 Ma granites that were together deformed and metamorphosed during the Glenburgh Orogeny. Although the c. 2540 Ma granites are Archean in age, they are younger than any known granites from the Yilgarn Craton to the south. The c. 2000 Ma component of the Halfway Gneiss overlaps with the age of the Dalgaringa Supersuite (2005–1960 Ma), and probably represents an intensely deformed portion of the supersuite.

Moogie Metamorphics

The Moogie Metamorphics consist of

and

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...

, which represent deformed and metamorphosed

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

, and carbonate rocks. This rock unit is known only from the Glenburgh Terrane. These rocks were first deformed during the 2005–1960 Ma Glenburgh Orogeny, when they were also intruded by

s of the Dalgaringa Supersuite.

Dalgaringa Supersuite

The Dalgaringa Supersuite comprises sheets, dykes and veins of 2005–1985 Ma foliated and gneissic

tonalite Tonalite is an igneous, plutonic ( intrusive) rock, of felsic composition, with phaneritic (coarse-grained) texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with alkali feldspar making up less than 10% of the total ...

granodiorite Granodiorite () is a coarse-grained (phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar. The term banatite is sometimes used informally for various rocks ranging from gra ...

, quartz

diorite Diorite ( ) is an intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is intermediate in composition between low-silic ...

and

monzogranite Monzogranites are biotite granite rocks that are considered to be the final fractionation product of magma. Monzogranites are characteristically felsic (SiO2 > 73%, and FeO + MgO + TiO2 < 2.4), weakly

pluton In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

of c. 1975 Ma mesocratic and leucocratic tonalite. The oldest, and possibly most abundant, rock type is a mesocratic, foliated to gneissic diorite to tonalite, which is typically pegmatite banded. At any given locality this rock type is intruded by several granite phases, typically in the following order: foliated biotite monzogranite and leucocratic tonalite, then biotite granodiorite and monzogranite and, finally, biotite monzogranite, syenogranite and

pegmatite A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic com ...

Central and northern Gascoyne Complex

Morrissey Metamorphics

The Morrissey Metamorphics are a group of metamorphic rocks dominated by

pelitic A pelite (Greek: ''pelos'', "clay") or metapelite is a metamorphosed fine-grained sedimentary rock, i.e. mudstone or siltstone. The term was earlier used by geologists to describe a clay-rich, fine-grained clastic sediment or sedimentary rock, ...

and psammitic schists derived from the metamorphism of shales and sandstones. This unit also includes some metamorphosed mafic igneous rocks and carbonate rocks. The sedimentary precursors to the Morrissey Metamorphics were deposited after about 1840 Ma, and were deformed and metamorphosed at amphibolite facies during the 1830–1780 Ma Capricorn Orogeny, before being intruded by granites of the Moorarie Supersuite at 1810–1780 Ma.

Pooranoo Metamorphics

The Pooranoo Metamorphics are a sequence of

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...

muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula K Al2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavage ...

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...

+/-

sillimanite Sillimanite is an aluminosilicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman (1779–1864). It was first described in 1824 for an occurrence in Chester, Connecticut. Occurrence S ...

schists and gneisses with an interpreted psammitic and pelitic (sedimentary) protolith. They have attained a peak of

amphibolite Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flaky ...

facies, within the sillimanite grade. The Pooranoo Metamorphics were deposited at the end of the Capricorn Orogeny. The maximum depositional age of the Pooranoo Metamorphics is constrained by detrital zircon dates of 1680 +/- 14 Ma derived from arenaceous metasediments. The Pooranoo Metamorphics are intruded by granites of the Durlacher supersuite, which has been dated at 1680 Ma. The metamorphic grade of the Pooranoo Metamorphics varies, with two belts of different metamorphic character and timing during the Mangaroon orogeny recognised. These are divided by the Ti Tree Creek Lineament, a multiply reactivated fault which bisects the Gascoyne Complex. : North of the Ti Tree Creek Lineament the Pooranoo Metamorphics and Durlacher granites endured Abukuma Facies Series low-pressure, high-temperature Sillimanite grade metamorphism, with a date of 1680-1675 Ma returned from zircons : South of the Ti Tree Lineament, the Pooranoo Metamorphics and Durlacher granites endured a high-pressure, high-temperature Barrovian Facies Series metamorphic event, typically reaching garnet grade. This occurred at 1665 - 1650 Ma.

Granitic intrusive suites

Moorarie Supersuite

The Moorarie Supersuite consists of voluminous granites intruded across the Gascoyne Complex at c. 1830–1780 Ma and are syntectonic with the Capricorn Orogeny. The granites are predominantly biotite granites. The Moorarie Supersuite includes granites of the Minne Creek batholith which include a

molybdenum Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lea ...

tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isolat ...

mineralised leucocratic

porphyry.

Durlacher Supersuite

The Durlacher Supersuite is a granite suite which was intruded into the Gascoyne Complex during the Mangaroon Orogeny. It is composed of two suites of granites * A northern belt of S-type (two-mica) granites (the Minnie Creek Suite) * A southern belt of S-type and I-type granites Most intrusions of this age are heavily sheared orthoclase

porphyroclast 350px, A mylonite showing a number of (rotated) porphyroclasts: a clear red garnet left in the picture while smaller white feldspar porphyroclasts can be found all over. ''Location'': the tectonics, tectonic contact between the wiktionary:autochth ...

ic granites. Geochronology on the granites is sparse, but has constrained the intrusions to 1680 to 1620 Ma, which is syntectonic to post-tectonic with the Mangaroon Orogeny.

Peripheral units

Several fold and thrust belts and sedimentary basins are peripheral, and potentially related, to the Gascoyne Complex. Between approximately 2000-1800 Ma, on the northern margin of the

Yilgarn Craton The Yilgarn Craton is a large craton that constitutes the bulk 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 be ...

, the c. 1890 Ma Narracoota Volcanics of the Bryah Basin formed in a transverse back-arc rift sag basin during collision. Culmination of the cratonic collision resulted in the foreland sedimentary Padbury Basin. To the east the Yerrida and Earaheedy Basins were

passive margin A passive margin is the transition between oceanic and continental lithosphere that is not an active plate margin. A passive margin forms by sedimentation above an ancient rift, now marked by transitional lithosphere. Continental rifting creat ...

s along the Yilgarn's northern margin. The c. 1830 Ma phase of the Capricorn Orogeny resulted in northeast-southwest deformation of the Bryah-Padbury Basin with

flood basalt A flood basalt (or plateau basalt) is the result of a giant volcanic eruption or series of eruptions that covers large stretches of land or the ocean floor with basalt lava. Many flood basalts have been attributed to the onset of a hotspot reach ...

s in the Yerrida Basin. The Gascoyne Complex rocks, namely the Yarlarweelor Gneiss are thrusted eastward onto the upper rock units of the Bryah Basin and the whole succession of the Padbury Basin. The Yerrida Basin was affected by this wast-west compression adjacent to the Goodin Fault. This event is incongruous with the c. 1830 Ma Capricorn Orogeny, is strike-slip to oblique-slip in nature and is most likely the Mangaroon Orogeny.

References

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