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Pilbara craton
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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 3.6-2.7 Ga crust on Earth. 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.
The Pilbara Craton comprises a mid-Archaean granite-greenstone terrane and an overlying late-Archaean volcano-sedimentary sequence called the Hamersley Basin.
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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 3.6-2.7 Ga crust on Earth. 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.
The Pilbara Craton comprises a mid-Archaean granite-greenstone terrane and an overlying late-Archaean volcano-sedimentary sequence called the Hamersley Basin. The Tabba Tabba Shear Zone is the major division between the East and West Pilbara craton. The Tabba Tabba Shear Zone is a granodioritic suite that forms the eastern boundary fault of the Mallina Basin.
Chronology
Ca. 3.6 Ga First Major Tectonic Cycle
The Pilbara craton evolved over two ~360 myr tectonic cycles. 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. The extent of pre-3.5 Ga rocks is uncertain, but appears limited to the greenstone belts and batholiths in the eastern Pilbara. 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. 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.
According to Bagas (2002), other major magmatic events occurred at 3.47–3.41, 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. 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. 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 Ga 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. 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.
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 ~3.25 Ga, followed by gabbroic suite at 3.235 Ga. The area was then affected by an early dextral compressive event that incorporated granodiorites and gabbros that formed the Tabba Tabba Shear Zone. A granitoid suite intruded the shear zone at 2.94 Ga. with xenocrystic populations of 3.115-3.015 Ga and 3015 Ma.
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. 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 3.517 and 3.325 Ga, and lesser amounts of metamorphic sedimentary rocks, and ultramafic, mafic, felsic, and intrusive rocks. This succession is unconformably overlain by the ca. 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 3.235 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.
Volcanic rocks in the lower Warrawoona Group vary in preservation from virtually undeformed lower greenschist to severely altered meta-amphibolites. U/Pb zircon dating of felsic formations indicates that emplacement of the lower Warrawoona group volcanics occurred before ca. 3.47 Ga. The mafic rocks of the Warrawoona Group have overlying komatiitic basalts with thin sections of bedded chert. 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.
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. Siliceous mudstones and sandstones of the uppermost clastic rocks have geochemical signatures analogous to those of felsic plutonic/volcanic rocks. Some of the siliceous mudstones have differentiated granitoids that were exposed in the Early Archean. 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. 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. 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. 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."
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.
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 3.458 Ga Warrawoona Group. 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. 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. 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. 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:
- Eruption onto continental base-ment.
- Derivation from a mantle with a generally uniform (depleted) composition.
- Eruption of thick basaltic successionswith only minor komatiitic and felsic volcanism.
- No strati-graphic trends of basalt composition.
- Coeval granitoid emplacement.
- Emergence of the Coonterunah-Carlindi terrain.
- Persistent shallow subaqueous to subaerial eruption of the Warrawoona Group
- Extensional setting for the Warrawoona Group
- Very low-grade metamorphism throughout the Warrawoona volcanic pile.
- 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."
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. 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. 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 southern North Pole area is outside the metamorphic aureole. 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. "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). 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. 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. 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. Kinematic analyses consistently give a greenstone belt up movement. Zircon SHRIMP U-Pb crystallization ages for granitoid sheets range between 3.312 and 3.465 Ga....Evidence for an early deformation phase in the SEMSZ comes from a gabbro/diorite complex (U/Pb age 3.465 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. 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."
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."
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. "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. 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 3.46 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. 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. 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.
See also
Bibliography
- Cawood, P.A, and N.H.S. Oliver. (2001) "Early tectonic dewatering and brecciation on the overturned sequence at Marble Bar, Pilbara Craton, Western Australia: dome-related or not?" Precambrian Research, fascicolo: 1, Vol. 105, pp. 1-15. Online Abstract:
- Dann, J., M. J. de Wit, S. H. White, and E. Zegers. (1998) Vaalbara, Earth's oldest assembled continent? A combined. structural, geochronological, and palaeomagnetic test."
- Kato, Yasuhiro and Kentaro Nakamura. (2003) "Origin and global tectonic significance of Early Archean cherts from the Marble Bar greenstone belt, Pilbara Craton, Western Australia." Precambrian Research, Vol. 125, Issues 3-4, 25 August, pp. 191-243.
- Masadab, Yuki, Hiroaki Ozawa, and Masaru Terabayashi. (2003) "Archean ocean-floor metamorphism in the North Pole area, Pilbara Craton, Western Australia." Precambrian Research, Vol. 127, Issues 1-3, 10 November, pp. 167-180.
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