Gascoyne Complex
Encyclopedia
The Gascoyne Complex is a terrane
of Proterozoic
granite
and metamorphic rock
in the central-western part of Western Australia
. 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
craton
s; the Pilbara craton
to the north and the Yilgarn craton
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
sedimentary 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
Carnarvon Basin
. To the north, schist
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
, 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
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.
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.
-muscovite
-quartz
-plagioclase
+/-sillimanite
schists and gneisses with an interpreted psammitic and pelitic (sedimentary) protolith. They have attained a peak of Amphibolite
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 contrained 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.
-tungsten
mineralised leucocratic granite
porphyry
.
Most intrusions of this age are heavily sheared orthoclase porphyroclast
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.
Between approximately 2000-1800 Ma, on the northern margin of the Yilgarn Craton, 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 margins 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 basalts in the Yerrida Basin.
The Gascoyne Complex rocks, namely the Yarlarweelor Gneiss are thrust 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.
Terrane
A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...
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"...
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...
and metamorphic rock
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
in the central-western part of Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
. 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
Archean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...
craton
Craton
A craton is an old and stable part of the continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by...
s; the Pilbara craton
Pilbara craton
The Pilbara craton , along with the Kaapvaal craton are the only remaining areas of pristine Archaean 3.6-2.7 Ga crust on Earth...
to the north and the Yilgarn craton
Yilgarn craton
The Yilgarn Craton is a large craton which constitutes the bulk of the Western Australian land mass. It is bounded by a mixture of sedimentary basins and Proterozoic fold and thrust belts...
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
Mesoproterozoic
The Mesoproterozoic Era is a geologic era that occurred between 1600 Ma and 1000 Ma . The Mesoproterozoic was the first period of Earth's history with a respectable geological record. Continents existed in the Paleoproterozoic, but we know little about them...
sedimentary rocks of the Edmund Basin and Collier Basin (formerly known as the Bangemall Basin). Several inliers 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...
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
Phanerozoic
The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared...
Carnarvon Basin
Carnarvon Basin
The Carnarvon Basin is a geological basin located in the north west of Western Australia. This is the main geological feature that makes up the North West Shelf...
. To the north, schist
Schist
The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...
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
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...
, 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
Neoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1,000 to 542.0 ± 1.0 million years ago. The terminal Era of the formal Proterozoic Eon , it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods...
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 consists of schist and gneiss, which represent deformed and metamorphosed sandstone, siltstone and shale, 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 granites of the Dalgaringa Supersuite.Dalgaringa Supersuite
The Dalgaringa Supersuite comprises sheets, dykes and veins of 2005–1985 Ma foliated and gneissic tonalite, granodiorite, quartz diorite and monzogranite, intruded by a large pluton 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.Morrissey Metamorphics
The Morrissey Metamorphics are a group of metamorphic rocks dominated by peliticPelite
Pelite is old and currently not widely used field terminology for a clayey fine-grained clastic sediment or sedimentary rock, i.e. mud or mudstone. It is equivalent to the Latin-derived term lutite. More commonly, metamorphic geologists currently use pelite for a metamorphosed fine-grained...
and psammitic schists
Psammite
Psammite is a general term for sandstone. It is equivalent to the Latin-derived term arenite. Also, it is commonly used in various publications to describe a metamorphosed sedimentary rock with a dominantly sandstone protolith. In Europe, this term was formerly used for a fine-grained, fissile,...
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 biotiteBiotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
-muscovite
Muscovite
Muscovite is a phyllosilicate mineral of aluminium and potassium with formula KAl22, or 236. It has a highly-perfect basal cleavage yielding remarkably-thin laminæ which are often highly elastic...
-quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
-plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
+/-sillimanite
Sillimanite
Sillimanite is an alumino-silicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman . It was first described in 1824 for an occurrence in Chester, Middlesex County, Connecticut, USA....
schists and gneisses with an interpreted psammitic and pelitic (sedimentary) protolith. They have attained a peak of 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...
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 contrained 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.
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 molybdenumMolybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
-tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
mineralised leucocratic 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...
porphyry
Porphyry (geology)
Porphyry is a variety of igneous rock consisting of large-grained crystals, such as feldspar or quartz, dispersed in a fine-grained feldspathic matrix or groundmass. The larger crystals are called phenocrysts...
.
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
Porphyroclast
thumb|350px|right|A [[mylonite]] showing a number of porphyroclasts: a clear red [[garnet]] left in the picture while smaller white [[feldspar]] porphyroclasts can be found all over...
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 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 margins 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 basalts in the Yerrida Basin.
The Gascoyne Complex rocks, namely the Yarlarweelor Gneiss are thrust 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.