The Narryer Gneiss Terrane is a geological complex in

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 ...

that is composed of a tectonically interleaved and polydeformed mixture of

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 under ...

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks incl ...

intrusions and

metasediment In geology, metasedimentary rock is a type of metamorphic rock. Such a rock was first formed through the deposition and solidification of sediment Sediment is a naturally occurring material that is broken down by processes of weathering and e ...

ary rocks in excess of 3.3 billion years old, with the majority of the Narryer Gneiss Terrane in excess of 3.6 billion years old. The rocks have experienced multiple metamorphic events at amphibolite or

granulite Granulites are a class of high-grade metamorphic rocks of the granulite facies that have experienced high-temperature and moderate-pressure metamorphism. They are medium to coarse–grained and mainly composed of feldspars sometimes associated ...

conditions, resulting in often complete destruction of original

igneous Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...

sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

(

protolith A protolith () is the original, unmetamorphosed rock from which a given metamorphic rock is formed. For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other kind of non-metamorphic rock and ...

) textures. Importantly, it contains the oldest known samples of the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

's crust: samples of

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of t ...

from the

Jack Hills The Jack Hills are a range of hills in Mid West Western Australia. They are best known as the source of the oldest material of terrestrial origin found to date: Hadean zircons that formed around 4.39 billion years ago. These zircons have enabl ...

portion of the Narryer Gneiss have been radiometrically dated at 4.4 billion years old, although the majority of zircon

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...

s are about 3.6-3.8 billion years old. The Narryer Gneiss Terrane is adjacent to the northernmost 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 b ...

and is abutted on the north by the Gascoyne Complex metasedimentary and metagranite

orogen An orogenic belt, or orogen, is a zone of Earth's crust affected by orogeny. An orogenic belt develops when a continental plate crumples and is uplifted to form one or more mountain ranges; this involves a series of geological processes collecti ...

. The Narryer Gneiss Terrane also includes parts of the Yarlarweelor Gneiss which abuts to Nabberu Basin metamorphic sequences of the Bryah-Padbury Basins, where it is present as discontinuous slivers of

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, caus ...

pelite 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, ...

metaconglomerate Metaconglomerate is a rock type which originated from conglomerate after undergoing metamorphism. Conglomerate is easily identifiable by the pebbles or larger clasts in a matrix of sand, silt, or clay. Metaconglomerate looks similar to conglome ...

s 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 a ...

es caught up within regional strike-slip oblique thrust faults. The Narryer Gneiss in this far-eastern region may form the basement to the 2.0-1.8 billion year old Proterozoic rocks, and the

unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

surface may be preserved within the thrust sheets. The Narryer Gneiss Terrane is divided into four major rock sequences (Myers 1990); the Dugel Gneiss, Meeberrie Gneiss, Manfred Complex, and unassigned polydeformed leucocratic gneisses and metasediments.

Dugel Gneiss

The Dugel Gneiss is syenogranitic or monzogranitic in composition and this is interpreted as the protolith. The gneiss is leucocratic, with only minor amounts of biotite and

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 cleavag ...

. The rock exhibits variable intensity of metamorphic banding which may include grain size variations, and has a high degree of deformation around the margins, and preserves an amphibolite facies metamorphic assemblage. The gneiss is pervasively shot through with pegmatite veins. Within the low-strain zones, the Dugel Gneiss exists as a medium-grained leucocratic metagranite with

phenocryst 300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...

s of K-feldspar which has recrystallized into granulite facies. The rock is greasy-looking with annealed

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 ...

and

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...

, and syn-granulite facies leucosomes cutting across metamorphic banding, and subsequently deformed by later

metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of ch ...

. The Dugel Gneiss is considered to be intruded into the older Meeberrie Gneiss, possibly as sheets or sills, but most contacts are overprinted by ductile metamorphic banding or

mylonite Mylonite is a fine-grained, compact metamorphic rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. Mylonites can have many different mineralogical compositions; it is a ...

zones.

Meeberrie Gneiss

The Meeberrie Gneiss is a generally ductilely deformed banded gneiss of monzogranitic composition, and its protolith is interpreted as a set of monzogranite sills or lopolithic intrusions. The gneiss is heavily banded by layers of amphibolite-K-feldspar-quartz of varying grainsize, plus networks of pegmatite veins. Most is strongly deformed, but least-deformed areas show a relict porphyritic to equigranular texture.

Manfred Complex

The Manfred Complex is a heavily attenuated and discontinuous series of ultramafic to mafic cumulates contained within a matrix or wall-rocks of mixed Dugel and Meeberrie Gneisses. The rock types are primarily pyroxene

gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...

to amphibolite, with rare serpentinised peridotite and dunite, occasionally containing relict igneous or metamorphic

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...

. These boudins of material range from centimetre-scale to ~100m thick and one kilometre long and, based on their position within

anticline In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is the ...

s and synclines in the Mount Narryer area, are interpreted to have intruded subparallel to bedding and are now strung out by shearing. The Manfred Complex is interpreted to represent an early Archaean mafic to ultramafic layered intrusion which has been disaggregated. This disaggregation is partly tectonic, but in some areas evidence suggests that this was mostly achieved by the intrusion of the Dugel and Meeberrie gneisses as sills or sheets.

Geochronology Geochronology is the science of determining the age of rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, whereas relative geochronology is ...

on the Manfred Complex places its age at around 3.73 billion via Pb-Pb on zircon. This makes it the oldest recognised intrusive rock on the Earth, containing the oldest known igneous textures and mineral assemblages.

Metasedimentary rocks

Metasedimentary rocks from within the Narryer Gneiss terrane account for about 10% of the outcrop and are variably deformed but uniformly of at least amphibolite grade metamorphism. The most abundant rock types are

quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tec ...

and

banded iron formation Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert. They can be up to several hundred meters in thickness ...

, with subordinate metamorphic gneisses, metaconglomerates and pelitic to semi-pelitic quartz-muscovite

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 ...

s. The conglomerates are primarily orthoquartzite monomict vein- pebble conglomerates, or polymict pebble conglomerates. In low-strain zones they preserve graded bedding,

cross-bedding In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The origina ...

and heavy mineral-rich horizons. These rocks provide the bulk of age determinations from the Narryer Gneiss Terrane from detrital zircons, with the bulk of readings bracketing 3.5 to 3.75 billion years old, 3.35 to 3.45 billion years old, and a population of 4.1 to 4.2 billion years, with outliers of up to 4.4 billion years old.

Structure

The Narryer Gneiss Terrane has undergone many high-grade polyphase deformation events, with the most notable being at 2600 to 2700 Ma associated with granite- greenstone

magmatism Magmatism is the emplacement of magma within and at the surface of the outer layers of a terrestrial planet, which solidifies as igneous rocks. It does so through magmatic activity or igneous activity, the production, intrusion and extrusion of ...

in the

, following an event at ~3350 Ma of amphibolite facies, resulting in widespread re-equilibration of geochronometers, preceded by a deformation loosely constrained to ~3680 Ma and another predating the Meeberrie Gneiss and postdating the Manfred Complex, between 3680 and 3730 Ma. Older structural grain is preserved in areas of low stress in subsequent deformations; the 2700 to 2600 Ma Yilgarn Craton associated deformation has overprinted most other deformations, resulting in rotation of previous structures into parallelism with NE-trending upright folds and metamorphic banding. Proximal to the Proterozoic orogens and thrust belts, the gneiss belt has become variably overprinted by later deformations. In the Bryah-Padbury Basin fold-thrust belt, Yarlarweelor Gneiss rafts exist as undeformed thrust plates of pelitic schist bounded by discrete mylonite zones, as well as deformed heavily overprinted gneiss blocks caught up within shear zones, some of which appear to preserve the unconformity surface.

Sequence of events

A simplified sequence of events known from the Narryer Gneiss Terrane of the Western Gneiss Belt of the Yilgarn craton is reproduced after A. F. Trendall (1991); * 2000-1600 Ma;
Basic dykes, related to the Gascoyne Complex and Capricorn Orogeny * 2700-2600 Ma;
Granite sheets and juxtaposition with the Yilgarn craton. * 3350-3300 Ma; Amphibolite to granulite facies metamorphism * ~3350-3400 Ma; deposition of metasedimentary rocks * 3400 Ma; Intrusion of syenogranites to form Dugel Gneiss, basic and ultramafic dykes * >3400-3680 Ma; Deformation * ~3680 Ma; Meeberrie Gneiss * >3680-3730 Ma; Deformation * ~3780 Ma; Manfred Complex ultramafic-mafic layered intrusion * >4100 Ma; Metasedimentary precursor gneiss to host the Manfred Complex

References

Fletcher, I.R., D.K. Paul, and A.F. Trendall. (1990) ''Sm‐Nd and geochemical characteristics of metasedimentary rocks at Mt Narryer, Western Australia''. Australian Journal of Earth Sciences 39:67-78.