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The Pilbara craton (the Pilbara province in northwest Western Australia), along with the Kaapvaal craton (the Kaapvaal province of South Africa) are the only remaining areas of pristine Archaean 3. For the Fly Genus, see Pilbara (fly. The Pilbara is a Western Australia is a state occupying the entire western third of the Australian continent. The Kaapvaal craton ( Limpopo province of South Africa) along with the Pilbara craton of Western Australia, are the only remaining areas of pristine The Republic of South Africa (also known by other official names) is a country located at the southern tip of the continent of Africa 6-2. 7 Ga crust on Earth. Annum is one form of the Latin noun meaning Year, not a form normally used for derivatives in modern languages the accusative singular EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 Similarities of their rock records, especially the similarities in the overlying Late Archean sequences of both these cratons, suggest that they were once part of the Vaalbara supercontinent, and then believed to have belonged to Ur continent. Vaalbara is the Earth's theorized first Supercontinent. According to radiometric data of the encompassing Cratons that constituted Vaalbara it is believed In Geology, a supercontinent is a Landmass comprising more than one Continental core or Craton. Ur is the first known continent that probably formed 3 billion years ago in the early Archean Eon.

The Pilbara Craton comprises a mid-Archaean granite-greenstone terrane and an overlying late-Archaean volcano-sedimentary sequence called the Hamersley Basin. Granite (ˈɡrænɪt is a common and widely occurring type of intrusive, Felsic, igneous rock. Greenschist - also known as greenstone - is a general field petrologic term applied to metamorphic and/or altered Mafic Volcanic rock A terrane in Geology is a fragment of crustal material formed on or broken off from one Tectonic plate and accreted — " sutured " Plate tectonics and hotspots Divergent plate boundaries At the Sedimentary rock is one of the three main rock types (the others being igneous and Metamorphic rock) A sequence in Geology refers to a sequence of geological events processes or rocks arranged in chronological order Depression in Geology is a Landform sunken or depressed below the surrounding area The Tabba Tabba Shear Zone is the major division between the East and West Pilbara craton. Study of geological shear is related to the study of Structural geology, Rock microstructure or rock texture and fault mechanics. A craton ( Greek kratos / κρἀτος ( neut. "strength" is an old and stable part of the Continental crust that has survived The Tabba Tabba Shear Zone is a granodioritic suite that forms the eastern boundary fault of the Mallina Basin. Granodiorite (ˌgrænəˈdaɪəraɪt/ /ˌgreɪn- is an intrusive Igneous rock similar to Granite, but contains more Plagioclase than Potassium feldspar In Geology, petrology (from Greek πέτρα petra, rock and λόγος logos, knowledge is the study of rocks and the conditions on which In Geology a fault, or fault line, is a planar rock fracture which shows evidence of relative movement Depression in Geology is a Landform sunken or depressed below the surrounding area

Contents

Chronology

Ca. 3. 6 First Major Tectonic Cycle

The Pilbara craton evolved over two ~360 myr tectonic cycles. This article discusses the geologic usage for the philosophical or architectural usage see Architectonics ' Or see Plate tectonics. Zircon geochronology indicates that the bulk of the intermediate to silicic igneous rocks in the Pilbara formed during seven periods of paired volcanic and plutonic activity. Zircon is a Mineral belonging to the group of nesosilicates. Its chemical name is Zirconium silicate and its corresponding chemical formula is In the Natural sciences under the umbrella of Natural history, Geochronology is the Science of determining the absolute age of rocks, Fossils Silicic is a term used to describe Magma or Igneous rock rich in Silica. Igneous rocks (etymology from Latin ignis, fire are rocks formed by solidification of cooled Magma (molten rock In Geology, an intrusion is a body of Igneous rock that has Crystallized from molten Magma below the surface of the Earth. The extent of pre-3. 5 Ga rocks is uncertain, but appears limited to the greenstone belts and batholiths in the eastern Pilbara. Greenstone belts are zones of variably metamorphosed Mafic to Ultramafic Volcanic sequences with associated Sedimentary rocks that A batholith (from Greek bathos, depth + lithos, rock is a large emplacement of Igneous intrusive (also called plutonic rock that forms This period was the major episode of crustal growth in the eastern Pilbara domains with calc-alkaline basalts, andesites and dacites with intrusive anorthosites in most greenstone belts, and tonalite-trondhjemite-granodiorite (TTG) suite granitoids in most batholiths. Calc-alkaline and calc-alkalic are equivalent terms applied to groups of Igneous rocks that commonly occur together and that have compositions related by the characteristic Dacite ( deɪsaɪt) is an igneous, volcanic rock with a high iron content In Geology, an intrusion is a body of Igneous rock that has Crystallized from molten Magma below the surface of the Earth. Anorthosite (æˈnɔrθəsaɪt/ /ə- is a Phaneritic, Intrusive Igneous rock characterized by a predominance of Plagioclase Feldspar Tonalite – Trondhjemite – Granodiorite (TTG series are an aggregation of rocks that are formed by melting of Hydrous Mafic crust at A batholith (from Greek bathos, depth + lithos, rock is a large emplacement of Igneous intrusive (also called plutonic rock that forms The compositions of calc-alkaline volcanic rocks resemble those from modern supra-subduction environments with TTG magmas derived via melting of underplated or subducted mafic crust. In Geology, a subduction zone is an area on Earth where two tectonic plates meet and move towards one another with one sliding underneath the other

According to Bagas (2002), other major magmatic events occurred at 3. Magma (Plurals magmas and magmata) is molten rock that sometimes forms beneath the surface of the Earth (or any other Terrestrial planet 47–3. 41, 3. 33–3. 10, 3. 00–2. 93 and 2. 85–2. 83 Ga. with calc-alkaline basalts, andesites and dacites that formed in most greenstone belts, and TTG suite granitoids in most batholiths. Basalt (bəˈsɔːlt ˈbeisɔːlt ˈbæsɔːlt is a common Extrusive Volcanic rock. For the extinct cephalopod genus see Andesites. Andesite (ˈændəsaɪt is an igneous, Volcanic rock, of intermediate Dacite ( deɪsaɪt) is an igneous, volcanic rock with a high iron content [1] Some of the granitoids are as old as 3. 4 Ga. The compositions of the calc-alkaline volcanic rocks resemble those of modern supra-subduction environments with TTG magmas derived via melting of underplated or subducted mafic crust.

3. 49-3. 41 Ga Eastern Pilbara Domain

The period 3. 49 to 3. 41 Ga was a major episode of crustal growth in the Eastern Pilbara Domain. The 3. 47–3. 41 Ga period included significant Tonalite-Trondhjemite-Granodiorite (TTG) magmatism representing high-pressure melting of a mafic source. Most magmatism after ca. 3. 4 Ga represents remelting of older crust, including the TTG older than 3. 4 Ga, to produce moderate- to high-potassium monzogranite. Potassium (pəˈtæsiəm is a Chemical element. It has the symbol K (kalium from qalīy Atomic number 19 and Atomic mass 39 Monzogranites (MGr are Biotite Granite rocks that are considered to be the final fractionation product of Magma. The Archaean granite–greenstones are dominated by TTG formed by melting of hydrous mafic crust at high pressure, but a much greater degree of crustal reworking has occurred in the Pilbara Craton than is required by TTG-dominated crust.

Ca. 3. 3 Second Major Tectonic Cycle

A second major magmatic episode at ~3. 33 Ga in the eastern Pilbara involved rhyolites and I-type granitoids derived via extensive melting of older silicic crust. This page is about a volcanic rock For the ghost town see Rhyolite Nevada, and for the satellite system see Rhyolite/Aquacade. Silicic is a term used to describe Magma or Igneous rock rich in Silica. After this time the magmatism shifted to domains in the western and central Pilbara with tonalite-trondhjemite-granodiorite (TTG) magmatism in the western Pilbara and calc-alkaline magmatism in the central Pilbara between 3. 27 and 3. 23 Ga. The bulk of west Pilbara greenstone belts and granite batholiths were generated in magmatic episodes at ~3. 11 and 3. 00 to 2. 98 Ga with both episodes including calc-alkaline and TTG magmas. Late magmatism in the western Pilbara resulted from crustal melting by plume-derived mafic magmas at ~2. 93 Ga. Western Pilbara domains were probably accreted to eastern Pilbara domains by 2. 88 Ga with localized crustal melting in the eastern Pilbara producing fractionated Sn- and Ta-bearing granites and pegmatites. Pegmatite is a very coarse-grained Igneous rock that has a grain size of 20 mm or more such rocks are referred to as pegmatitic. [2]

3. 315 Ga Corunna Downs Granitoid Complex

The Archaean Corunna Downs Granitoid Complex (CDGC) in the southeastern part of the East Pilbara Granite–Greenstone Terrain (EPGGT) consists of 80% ca. 3. 315 Ga highly fractionated monzogranites, with trace elements consistent with remelting of an older TTG crust at a mid-crustal level. The remaining 20% is TTG formed through high-pressure melting of hydrated mafic crust. It is thought that as the mid-crustal melting of TTG occurred to form the monzogranites, melting of an associated mafic intraplate formed the TTG.

3. 25 Ga Tabba Tabba Shear Zone

The Tabba Tabba Shear Zone intruded the area at ~3. In Geology, an intrusion is a body of Igneous rock that has Crystallized from molten Magma below the surface of the Earth. 25 Ga, followed by gabbroic suite at 3. Gabbro (ˈɡæbrəʊ is a dark coarse-grained intrusive Igneous rock chemically equivalent to Basalt. 235 Ga. The area was then affected by an early dextral compressive event that incorporated granodiorites and gabbros that formed the Tabba Tabba Shear Zone. This article is about the geologic usage for human physiology usage see the articles Laterality. In Geology the term compression refers to the system of forces that tend to decrease the volume of or shorten rocks Gabbro (ˈɡæbrəʊ is a dark coarse-grained intrusive Igneous rock chemically equivalent to Basalt. A granitoid suite intruded the shear zone at 2. 94 Ga. with xenocrystic populations of 3. 115-3. 015 Ga and 3015 Ma. [3]

Geography

Northern Pilbara, Hamersley Basin and Hamersley Range

The Hamersley basin covers the Pilbara archean craton in the north. Granite is exposed in the Hamersley basin as batholiths up to a 100 km (62 miles) in length; these light rocks are diapiric intrusions into the dark greenstones (metamorphosed basalt). There are also banded iron formations. To the south is the Hamersley Range and the smaller Opthalmia Range, bordered on the south by the Ashburton Trough and the Bangemall basin. The Hamersley Range is a mountainous region of the Pilbara, Western Australia. Much of the region is marked by hills of low relief; the highest area is Mt Meharry(1235 m; 4013 ft) which is also located in the Hamersley Ranges.

Eastern Pilbara, Warrawoona Group

The greenstones in Eastern Pilbara comprise dominantly greenschist-facies volcanic rocks of the Warrawoona Group, which is dated between 3. Greenschist - also known as greenstone - is a general field petrologic term applied to metamorphic and/or altered Mafic Volcanic rock The term " Facies " can also refer to distinctive Facial expressions associated with conditions such as Williams syndrome. 517 and 3. 325 Ga, and lesser amounts of metamorphic sedimentary rocks, and ultramafic, mafic, felsic, and intrusive rocks. Ultramafic (also referred to as ultrabasic) rocks are igneous and meta -igneous rocks with very low Silica content (less than 45% generally This succession is unconformably overlain by the ca. An unconformity is a buried Erosion surface separating two rock masses or strata of different ages indicating that Sediment deposition was not 3. 31 Ga Budjan Creek Formation, which in turn is unconformably overlain by the dominantly clastic rocks of the Gorge Creek Group dated at younger than 3. Clastic rocks are composed of fragments or clasts, of pre-existing rock. 235 Ga. The entire volcano-sedimentary succession dips and youngs away from the CDGC, and all granite–greenstone contacts are intrusive. Several generations of granitic magmatism have been documented from granitoid complexes of the EPGGT. None of the rocks of the CDGC conform to a classic Archaean TTG suite. This suggests that the majority of true TTGs in the Pilbara Craton are restricted to the older (>3. 44 Ga) rocks of the granitic complexes of the East Pilbara Granite–Greenstone Terrane, and that extensive recycling of old TTG to produce voluminous high-K magmatism was not restricted to the late Archaean.

From the above analysis, a two-step process for the formation of the CDGC can be inferred. First, high-pressure melting of young mafic lower crust produced TTG magmas, such as those presently exposed in the Shaw Granitoid Complex. The thermal anomaly was also associated with basaltic magmatism that formed a mid-crustal intraplate. A second thermal event at c. 3. 3 Ga then caused widespread crustal melting at a depth of 35–40 km. This event involved the re-melting of the older TTG to produce the monzogranites of the CDGC, whereas re-melting of the mafic intraplate produced the tonalitic to granodioritic rocks of the complex. Monzogranites (MGr are Biotite Granite rocks that are considered to be the final fractionation product of Magma. Tonalite is an igneous, Plutonic ( Intrusive) rock, of Felsic composition with Phaneritic texture [1]

Volcanic rocks in the lower Warrawoona Group vary in preservation from virtually undeformed lower greenschist to severely altered meta-amphibolites. Greenschist - also known as greenstone - is a general field petrologic term applied to metamorphic and/or altered Mafic Volcanic rock Amphibolite (æmˈfɪbəlaɪt is the name given to a rock consisting mainly of hornblende Amphibole, the use of the term being restricted however to Metamorphic U/Pb zircon dating of felsic formations indicates that emplacement of the lower Warrawoona group volcanics occurred before ca. Uranium-lead is one of the oldest and most refined Radiometric dating schemes with a routine age range of about 1 million years to over 4 Zircon is a Mineral belonging to the group of nesosilicates. Its chemical name is Zirconium silicate and its corresponding chemical formula is 3. 47 Ga. The mafic rocks of the Warrawoona Group have overlying komatiitic basalts with thin sections of bedded chert. Komatiites are Ultramafic mantle-derived Volcanic rocks They have low SiO2 low K2O low Al2O3 and high to extremely high Chert (ˈtʃɝt is a fine-grained Silica -rich Microcrystalline, Cryptocrystalline or Microfibrous Sedimentary rock that may contain Geochemical signatures in these thin sections of bedded chert (3-6 meters thick) suggest that they were most likely formed by weak hydrothermal activity associated with hot-spot volcanism. In Geology, a hotspot is a location on the Earth's surface that has experienced active volcanism for a long period of time

The Apex cherts are a series of silicic deposits within pillow lavas of the Apex Basalt, dated at 3. 465-3. 458 Ga, and preserve eleven taxa of prokaryotes. The prokaryotes (proʊˈkærioʊts singular prokaryote /proʊˈkæriət/ are a group of Organisms that lack a Cell nucleus (= karyon or any other Siliceous mudstones and sandstones of the uppermost clastic rocks have geochemical signatures analogous to those of felsic plutonic/volcanic rocks. The Chemical compound silicon dioxide, also known as silica or silox (from the Latin " Silex " is an Oxide Mudstone (also called mudrock) is a fine grained Sedimentary rock whose original constituents were Clays or Muds Grain size is up Sandstone is a Sedimentary rock composed mainly of Sand -size Mineral or rock grains. Some of the siliceous mudstones have differentiated granitoids that were exposed in the Early Archean. The Chemical compound silicon dioxide, also known as silica or silox (from the Latin " Silex " is an Oxide Studies show that the Warrawoona Group cherts were deposited in a variety of environments ranging from mid-oceanic spreading to converging tectonic plate boundaries via a hotspot. A mid-ocean ridge or mid-oceanic ridge is an underwater Mountain range typically having a valley known as a Rift running along its axis formed by It is thought that the depositional variations here were caused by horizontal plate motions in the Early Archean.

"Geological and geochemical evidence shows that the Warrawoona Group was erupted onto a continental basement, and that these basalts assimilated small amounts of Carlindi granitoid. As the Coonterunah basalts have similar compositions, they probably formed likewise, although they were deposited >60 myr before. . . . An older continental basement was probably critical for the early Pilbara craton evolution. The geochemical geological and geophysical characteristics of the Pilbara greenstone successions can be best explained as flood basalt successions deposited onto thin, submerged continental basement. A flood basalt or trap basalt is the result of a giant Volcanic eruption or series of Eruptions that coats large stretches of land or the Ocean floor This magmatism was induced by thermal upwelling in the mantle, although the basalts themselves do not have compositions which reflect derivation from an anomalously hot mantle. The Carlindi granitoids probably formed by fusion of young garnet-hornblende-rich sialic crust induced by basaltic volcanism. The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives Hornblende is a complex inosilicate series of Minerals Hornblende is not a recognized mineral in its own right but the name is used as a general or field term to Sial or SiAl is also the name for the upper layer of the Earth's crust, which is also known as the Continental crust because Early Archaean rocks have Nd-Hf isotope compositions which indicate that the young mantle had differentiated into distinct isotopic domains before 4. Isotopes (Greek isos = "equal" tópos = "site place" are any of the different types of atoms ( Nuclides Isotopes (Greek isos = "equal" tópos = "site place" are any of the different types of atoms ( Nuclides 0 Ga. Such ancient depletion was associated with an increase of mantle Nb/U ratios to modern values, and hence this event probably reflects the extraction of an amount of continental crust equivalent to its modern mass from the primitive mantle before 3. 5 Ga. Thus, a steady-state model of crustal growth is favoured whereby post ~4. 0 Ga continental additions have been balanced by recycling back into the mantle, with no net global flux of continental crust at modern subduction zones. It is also proposed that the decoupling of initial e(Nd) and e(Hf) from its typical covariant behaviour was related to the formation of continental crust, perhaps by widespread formation of TTG magmas. "[4]

Pilganoora Belt

In the Pilgangoora Belt the 3. 517 Ga Coonterunah Group and 3. 484-3. 468 Ga Carlindi granitoids underlie the 3. 458 Ga Warrawoona Group beneath an erosional unconformity, thus providing evidence for ancient emergent continental crust. [4]

A new informative study by Green (2006): The uppermost units of the regionally extensive In the Pilgangoora Belt the 3. 517 Ga Coonterunah Group was intruded by 3. 484-3. 468 Ga Carlindi granitoids that underlie the 3. Granite (ˈɡrænɪt is a common and widely occurring type of intrusive, Felsic, igneous rock. 458 Ga Warrawoona Group. [5] The combined terrain was uplifted and eroded to form an erosional unconformity. The uppermost units of the regionally extensive 3. 458 Ga Warrawoona Group were deposited onto the unconformity. This is the oldest-known evidence for emergent continental crust. The basalts on either side of the unconformity are remarkably similar, with N-MORB-normalised enrichment factors for LILE, Th, U and LREE (low rare earth elements) greater than those for Ta, Nb, P, Zr, Ti, Y and M-HREE (high rare earth elements), and initial e(Nd, Hf) compositions which systematically vary with Sm/Nd, Nb/U and Nb/La ratios. Thorium (ˈθɔːriəm is a Chemical element with the symbol Th and Atomic number 90 Uranium (jʊˈreɪniəm is a silvery-gray Metallic Chemical element in the Rare earth elements and rare earth metals are according to IUPAC, the collection of seventeen Chemical elements in the Periodic table, namely Tantalum (ˈtæntələm (formerly tantalium /tænˈtæliəm/ is a Chemical element with the symbol Ta and Atomic number 73 Niobium (naɪˈoʊbiəm or columbium (/kəˈlʌmbiəm/ is a Chemical element that has the symbol Nb and Atomic number 41 Phosphorus, (ˈfɒsfərəs is the Chemical element that has the symbol P and Atomic number 15 Zirconium (zɚˈkoʊniəm /ˌzɝˈkoʊniəm/ is a Chemical element with the symbol Zr and Atomic number 40 Titanium (taɪˈteɪniəm is a Chemical element with the symbol Ti and Atomic number 22 Yttrium (ˈɪtriəm is a Chemical element with symbol Y and Atomic number 39 Rare earth elements and rare earth metals are according to IUPAC, the collection of seventeen Chemical elements in the Periodic table, namely Neodymium (ˌniːoʊˈdɪmiəm is a Chemical element with the symbol Nd and Atomic number 60 Hafnium (ˈhæfniəm is a Chemical element that has the symbol Hf and Atomic number 72 Samarium (səˈmɛəriəm is a Chemical element with the symbol Sm and Atomic number 62 Lanthanum (ˈlænθənəm is a Chemical element with the symbol La and Atomic number 57 Geological and geochemical evidence shows that the Warrawoona Group was erupted onto continental basement, and that these basalts assimilated small amounts of Carlindi granitoid. As the Coonterunah basalts have similar compositions, they probably formed likewise, although they were deposited 60 myr before. Such a model is applicable to the other early Pilbara greenstone successions, and so an older continental basement was probably critical for early Pilbara evolution. The geochemical, geological and geophysical characteristics of the Pilbara greenstone successions can be best explained as flood basalt successions deposited onto thin, submerged continental basement. A flood basalt or trap basalt is the result of a giant Volcanic eruption or series of Eruptions that coats large stretches of land or the Ocean floor This magmatism was induced by thermal upwelling in the mantle, although the basalts themselves do not have compositions which reflect derivation from an anomalously hot mantle. The Carlindi granitoids probably formed by fusion of young garnet-hornblende-rich sialic crust induced by basaltic volcanism. The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives Hornblende is a complex inosilicate series of Minerals Hornblende is not a recognized mineral in its own right but the name is used as a general or field term to Early Archaean rocks have Nd-Hf isotope compositions which indicate that the young mantle had differentiated into distinct isotopic domains before 4. 0 Ga. Such ancient depletion was associated with an increase of mantle Nb/U ratios to modern values, and hence this event probably reflects the extraction of an amount of continental crust equivalent to its modern mass from the primitive mantle before 3. 5 Ga. Thus, a steady-state model of crustal growth is favoured whereby post ~4. 0 Ga continental additions have been balanced by recycling back into the mantle, with no net global flux of continental crust at modern subduction zones. It is also proposed that the decoupling of initial e(Nd) and e(Hf) from its typical covariant behaviour was related to the formation of continental crust, perhaps by widespread formation of TTG magmas.

The lower part of the North Pole succession (see below) must have been deposited while the Coonterunah-Carlindi terrain Pilgangoora Belt was emergent. "These two successions provide critical constraints for determining the tectonic setting of the Pilbara greenstone belts. Evidence from both greenstone belts can be used to define some criteria which must be satisfied by proposed tectonic setting models. These include:

  1. Eruption onto continental base-ment.
  2. Derivation from a mantle with a generally uniform (depleted) composition.
  3. Eruption of thick basaltic successionswith only minor komatiitic and felsic volcanism.
  4. No strati-graphic trends of basalt composition.
  5. Coeval granitoid emplacement.
  6. Emergence of the Coonterunah-Carlindi terrain.
  7. Persistent shallow subaqueous to subaerial eruption of the Warrawoona Group
  8. Extensional setting for the Warrawoona Group
  9. Very low-grade metamorphism throughout the Warrawoona volcanic pile. Metamorphism can be defined as the solid state recrystallisation of pre-existing rocks due to changes in heat and/or pressure and/or introduction of fluids i
  10. Minor regional deformation.

Some derived constraints are that the potential mantle temperature was 1400 °C, partial melting was shallow and did not involve garnet, and that the pre-Warrawoona basement must have been significantly extended and thinned during deposition of the Warrawoona succession to maintain shallow subaqueous to subaerial conditions. These criteria preclude many of thepossible tectonic settings for greenstone development. The favoured model for the Pilbara is a setting similar to Phanerozoic continental flood basalt provinces, but differing from recent analogues in that it was deposited onto submerged basement. The base-level of deposition was most likely controlled by the thickness of the continental basement and the rates of extension and eruption. "[6]

North Pole Dome

The North Pole Dome (NPD), 10 km of the Warrawoona Group are exposed. The upper 3 km correlates lithologically and geochemically with the Warrawoona Group in the Pilgangoora Belt. In Geology, petrology (from Greek πέτρα petra, rock and λόγος logos, knowledge is the study of rocks and the conditions on which The field of geochemistry involves study of the chemical composition of the Earth and other Planets chemical processes and reactions that govern the composition Therefore, the lower part of the North Pole succession must have been deposited while the Coonterunah-Carlindi terrain was emergent. These two successions provide critical constraints for determining the tectonic setting of the Pilbara greenstone belts.

The NPD is a relatively high-level dome that has a flanking syncline preserving some of the youngest rocks of the Fortescue Group of the craton. In Structural geology, a syncline is a downward-curving fold, with layers that dip toward the center of the structure A craton ( Greek kratos / κρἀτος ( neut. "strength" is an old and stable part of the Continental crust that has survived Basaltic greenstones range in age from 3. 5 to 2. 7 Ga. The greenstone belts in the North Pole Dome (NPD) have undergone metamorphism from prehnite-pumpellyite facies to greenschist-amphibolite facies. The prehnite-pumpellyite facies is a metamorphic facies typical of subseafloor alteration of the Oceanic crust around Mid-ocean ridge spreading centres The southern North Pole area is outside the metamorphic aureole. An aureola or aureole (diminutive of Latin aurea, "golden" is the radiance of luminous cloud which in Paintings of sacred personages Metamorphism of the North Pole greenstone belts are comparable to ocean-floor metamorphism.

The approximately 3. 46 Ga North Pole Monzogranite, a volumetrically insignificant intrusive granite body, intrudes the greenstones in the apex of the dome. At the apex of the NPD is a small intrusive granite that is thought to have been the top of a large underlying domal granite batholith, but no marginal shear zones occur around the intrusion. A new study/model done by Bell et al. (2004) suggests that the granite intrusion is plug-like, up to 1. 5 km thick and does not represent the exposed top of a larger underlying domal batholith. [7] "Results from potential field modelling show that the dome is relatively flat bottomed, with a base around 5. 5–6. 5 km deep. The NPD has no significant granitic material within the dome, but like all greenstones, is underlain by felsic crust (granite) below its base. The development of the NPD (and flanking syncline) was a multistage process. The first stage of doming involved relatively minor doming/tilting, possibly associated with the emplacement of the monzogranite, because palaeocurrents of synchronous volcanic rocks flowed radially outward from the dome. It is likely that this doming was minor as there are no recorded unconformities in the Warrawoona Group (in the NPD) above these volcanic rocks. A major dome-forming event (tilting >20°) occurred in the period between 3. 24 and 2. 772 Ga, and was unrelated to the emplacement of the small granite plug (diapirism). A diapir (ˈdaɪəpɪər) ( French, from Greek diapeirein, to pierce through is an Intrusion caused by Buoyancy and Pressure Regional folding and refolding from horizontal compression deformed the area into a domal shape. Uplift and erosion of the dome was superseded by extension and deposition of flood basalts in the Fortescue Group that flowed towards the dome. Three further stages of shortening folded the regional unconformity and the underlying and overlying units, further amplified the underlying dome, developed the flanking Marble Bar Syncline, as well as fold interference patterns in the Fortescue Group. In Structural geology, a syncline is a downward-curving fold, with layers that dip toward the center of the structure The NPD was developed over a 800 Ma time frame, ostensibly by a process of fold interference due to multiple stages of horizontal compression. This work shows that diapirism was not the cause of the development of the domal geometry of the NPD, and its flanking syncline, rather folding and refolding due to horizontal compression was the principal controlling factor. "

Talga Talga Section, Marble Bar Belt

Cataclastic breccias and hdrothermal faults are well exposed in the Marble Bar cherts. Cataclastic rocks contain angular fragments formed by cataclasis Breccia (ˈbrɛtʃiə ˈbrɛʃiə breach is a rock composed of angular fragments of several Minerals or rocks in a matrix, that is a cementing material Studies of the downward facing pillow basalts, the geometry of the breccias, and oxygen isotope data for rocks and the breccia matrix, suggest the rocks were steeply overturned on the flank of the Mt Edgar Dome prior to brecciation. The breccias are thought to represent steep conjugate fault zones developed by local trans-tension. Studies show that the overturning and brecciation occurred before the formation of dome foliation and metamorphism. The deposition of the underlying Duffer Formation occurred at 3. 46 Ga and the intrusion of the Mt. Edgar Batholith occurred at 3. 32 Ga. The overturning of the Marble Bar sequence prior to brecciation suggests that the main phase of the dome formation was very protracted.

According to a study by Nelson et al. (1999): "The Mt. Edgar Batholith near Marble Bar is a NE-SW granitoid complex of magma genesis with a sequence of pre-, syn- and post-tectonic intrusive phases, containing an originally subhorizontal mid-crustal detachment zone. Along the southwestern margin, the structure indicates this zone was tilted partly actively and partly passively during deformation to form the 70 km long, now steeply dipping, 2-3 km wide, Southern Edgar Marginal Shear Zone (SEMSZ). Early movement on this zone juxtaposed magmatitic gneisses adjacent to greenschist and lower greenschist facies supracrustals. Gneiss (ˈnaɪs is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from preexisting formations that were originally Greenschist - also known as greenstone - is a general field petrologic term applied to metamorphic and/or altered Mafic Volcanic rock The term " Facies " can also refer to distinctive Facial expressions associated with conditions such as Williams syndrome. Supracrustal rocks are rocks that were Deposited on the existing Basement rocks of the crust, hence the name Kinematic analyses consistently give a greenstone belt up movement. Zircon SHRIMP U-Pb crystallization ages for granitoid sheets range between 3. True shrimp are swimming decapod Crustaceans classified in the Infraorder Caridea, found widely around the world in both fresh 312 and 3. 465 Ga. . . . Evidence for an early deformation phase in the SEMSZ comes from a gabbro/diorite complex (U/Pb age 3. Gabbro (ˈɡæbrəʊ is a dark coarse-grained intrusive Igneous rock chemically equivalent to Basalt. Diorite (ˈdaɪəraɪt is a grey to dark grey intermediate intrusive Igneous rock composed principally of Plagioclase Feldspar (typically 465 Ga) with syn-tectonic dolerite sills. A related swarm of dolerite dykes (Ar age >3. 4 Ga) exploited a conjugate set of NE-SW extensional faults in a felsic extrusive unit. An Extensional Fault is a fault that extends the pre-existing geological layering The dykes are feeders for the overlying basaltic units, which are now, as are the felsics, part of a thrust sheet. Part of the SEMSZ footwall is formed by ~3. 315 Ga TTG sheets and plutons. Less deformed plutons of similar age have intruded into the hanging wall of the SEMSZ. . . . This study indicates that a mid-crustal detachment played a major role in the emplacement of the circa 3. 315 Ga Mt Edgar granitoid suites and that this occurred during a uni-directional tectonic transport to the NE. Structures within the magmatitic gneisses and the thermal gradients across the detachment at this time are consistent with an extensional tectonic regime, the same regime proposed for the earlier phase of granitoid emplacement at circa 3. 46 Ga in the Eastern Pilbara. "[8]

North Star Basalt

The North Star Basalt in the Marble Bar Belt is the lowermost formation of the Warrawoona Group and one of the oldest greenstones sequences in the Archaean Pilbara Cratot. In a thesis written by Beintema (2003): "It consists mainly of pillowed and massive basalts, minor gabbro, and comprises a large number of mafic and ultramafic dykes. Geochemical studies have shown that the upper part of the North Star Basalt comprises enriched tholeiitic basalts, probably due to contamination of the magmas by assimilation of crustal material. They do not resemble modern mid-oceanic ridge basalts (MORB). The lower, ultramafic part of the stratigraphy may not be part of the North Star Basalt, as indicated by its different trace element geochemistry. A 40Ar/39Ar cooling age of about 3. 47 Ga indicates that these rocks may be the same age as the Talga Talga Subgroup of the Warrawoona Group, to which the North Star Basalt belongs. Only a small fraction of the dykes that occur in the area, is genetically related to the extrusive pile; the majority has been emplaced later, probably during regional extension at ca 3. 3 Ga. Granite intrusions at ca 3. 3 Ga post-date emplacement of all of the dyke suites, and have destroyed the lower section of the greenstone sequence. There is no firm evidence for large displacements on any of the structures within the unit. Therefore the Talga Talga anticline may still be a suitable type area for the North Star Basalt, but the presence of low angle unconformities should not be disregarded. "[9]

According to a study by Kloppenburg et al. (1999): The excellent preservation of the 3. 49 Ga greenschist amphiboles from the North Star basalt in the Talga Talga section suggests that metamorphism occurred soon after extrusion. [10] "Similar lithologies have been recognised throughout the area in the Marble Bar Belt, the Kelly Belt, the Gorge Ranges and are remnants of a formerly wide spread upper plate. Granodiorites of 3. 46 Ga (U/Pb zircon ages) have intruded into this upper plate sequence in the north Shaw and in the Mt. Edgar Batholith near Marble Bar. The upper plate sequence consists of an imbricated stack of thrust sheets with contrasting degrees of metamorphic overprinting, and is separated from lower plate gneisses by prominent mid crustal detachments. This configuration has been recognised in the northern and eastern Shaw Batholith, the southern Mt. Edgar Batholith, and in the northern margin of the Kurrana Batholith. The lower plate typically consists of banded grey gneisses that show evidence for a complex thermal history. The detachments have typically been the focus of late intrusion ranging in composition from gabbroic to muscovite bearing granitic sheets. Muscovite (also known as Common Mica, Isinglass, or Potash mica) is a phyllosilicate Mineral of Aluminium Although similar in setting, a combination of kinematic and geochronological arguments suggests that the three identified detachments are not connected: the Split Rock Shear Zone in the Shaw Batholith is 3. Kinematics ( Greek κινειν, kinein, to move is a branch of Classical mechanics which describes the motion of objects without 46 Ga old although reactivation as young as 3. 2 Ga cannot be ruled out. The South Edgar Marginal Shear is 3. 31 Ga old although 3. 47 Ga old gabbro sheets may point to an earlier component in this shear zone. The Kurrana Shear Zone predates 3. 2 Ga as measured from the cooling age of metamorphic hornblende from the Middle Creek basement complex. The deposition age of the Mosquito Creek metasediments, which tectonically overly the Kurrana Shear Zone, is bracketed between 3. 2 Ga, the age of high grade metamorphism in the Kurrana basement, and 2. 9 Ga, the age of mafic sills in the eastern sector of the Mosquito Creek domain. The mid crustal detachments consistently yield kinematic data indicative of large scale horizontal motions at different periods in the Mid Archean tectonic evolution of the eastern Pilbara Craton. These we relate to cycles of extensional and compressional tectonics, which pre-date the final amalgamation of the East and West Pilbara Terranes at ca 2. 9 Ga.

West and Central Domains

Younger TTG-type rocks are present in the West Pilbara Granite–Greenstone Terrane and Central Pilbara Tectonic Zone.

Mount Bruce Supergroup, Wyloo Group, Fortescue Group, Hamersley Group

The Paleoproterozoic Mount Bruce Supergroup of the Pilbara Craton is overlain by the Wyloo Group with a maximum thickness of 10 km. The Paleoproterozoic (ˌpeɪlɪoʊˌproʊtərəˈzoʊɪk also spelled Palaeoproterozoic) is the first of the three sub-divisions ( eras) of the and maximum age of Archaean. Neoarchaean comprises the Fortescue Group, Carawine Dolomite (Hammersley Group/Hamersley Basin). A layer of probable impact melt spherules occurred in the Late Archaean Jeerinah Formation, Fortescue Group. Spherulites, in Petrology, are small rounded bodies that commonly occur in Vitreous Igneous rocks They are often visible in specimens of Obsidian Magmatism caused doming of the Archaean Shaw Granitoid Complex, Pilbara Craton.

Physiography

The Pilbara craton (or Pilbara block), is distinct physiographic section of the larger Nullagine Platform province, which in turn is part of the larger West Australian Shield division. Australian Shield, also called the Western Australian Shield or Western Plateau, occupies more than half of the continent of Australia.

Australia map. Pilbara Region is coloured in red.
Australia map. Pilbara Region is coloured in red.

See also

References

  1. ^ a b Bagas, L. Australia is a Continent situated on the Indo-Australian Plate. Australian Shield, also called the Western Australian Shield or Western Plateau, occupies more than half of the continent of Australia. The Western Plateau is Australia 's largest drainage division and is composed predominantly of the remains of the ancient rock shield of Gondwanaland. The Kaapvaal craton ( Limpopo province of South Africa) along with the Pilbara craton of Western Australia, are the only remaining areas of pristine The Yilgarn Craton is a large Craton which constitutes the bulk of the Western Australian land mass The Gawler Craton covers approximately 440000 square kilometres of central South Australia Vaalbara is the Earth's theorized first Supercontinent. According to radiometric data of the encompassing Cratons that constituted Vaalbara it is believed , D. C. Champion, and R. H. Smithies. (2002) "Geochemistry of the Corunna Downs Granitoid Complex,East Pilbara Granite–Greenstone Terrane, Western Australia. " Geological Survey of Western Australia, 2001-02 Annual Review. [1]
  2. ^ Barley, Mark. (1999) "Growth and Recycling of Archaean Continental Crust in the Pilbara Craton. " Journal of Conference Abstracts, Vol. 4, No. 1. Symposium A08, Early Evolution of the Continental Crust. [2]
  3. ^ Beintema, Kike Anneke, P. R. D. Mason, D. R. Nelson, S. H. White, and J. R. Wijbrans. (2003a) "New constraints on the timing of tectonic events in the Archaean Central Pilbara Craton, Western Australia. " Journal of the Virtual Explorer 13. [3]
  4. ^ a b Green, Michael Godfrey. (2001) "Early Archaean crustal evolution: evidence from ~3. 5million year old greenstone successions in the Pilgangoora Belt, Pilbara Craton, Australia. " thesis, Geosciences, University of Sydney. Online Abstract:[4]
  5. ^ Green, Michael Godfrey. (2006) "Early Archaean crustal evolution: evidence from ~3. 5 million year old greenstone successions in the Pilgangoora Belt, Pilbara Craton, Australia. " Geosciences, University of Sydney. Online Abstract:[5]
  6. ^ Green, Michael Godfrey. (2000) "Early Archaean crustal evolution: evidence from ~3. 5 million year old greenstone successions in the Pilgangoora Belt, Pilbara Craton, Australia. " Journal of Conference Abstracts Volume 5(2), 455. Online Abstract: [6]
  7. ^ Bell, B, R. S. Blewett and S. Shevchenko. (2004) "The North Pole Dome: a non-diapiric dome in the Archaean Pilbara Craton, Western Australia. " Precambrian Research, Vol. 133, Issues 1-2, 5 August, pp. 105-120. Online Abstract:[7]
  8. ^ Nelson, David R. , Armelle Kloppenburg, Jan R. Wijbrans and Stan H. White. (1999) "Tectonic Evolution of a Mid-Crustal Shear Zone in the Eastern Pilbara. " Journal of Conference Abstracts, Vol. 4, No. 1. Symposium A08, Early Evolution of the Continental Crust. [8]
  9. ^ Beintema, Kike Anneke. (2003) "Lithology, structure and geochemistry of the ca. 3. 5 Ga North Star Basalt in the Marble Bar Greenstone Belt, Archaean Pilbara Craton, Western Australia. " thesis, School of Earth Sciences, Utrecht University. [9]
  10. ^ Kloppenburg, Armelle, David R. Nelson, Stan H. White, Jan R. Wijbrans, and Tanja E. Zegers. (1999) "Timing Tectonothermal Processes in the Early Earth Crust: A Combined 40Ar/39Ar and U/Pb Study of Basement Gneisses and Associated Supracrustal Rocks from the Eastern Pilbara Craton, W. Australia. " Journal of Conference Abstracts, Vol. 4, No. 1. Symposium A08, Early Evolution of the Continental Crust. [10]

Bibliography

External links

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