Applying the Th/Yb–Nb/Yb plot of Pearce (2008) to the well-studied Archean greenstone sequences of Western Australia shows that individual volcanic sequences evolved through one of two distinct ...processes reflecting different modes of crust–mantle interaction. In the Yilgarn Craton, the volcanic stratigraphy of the 2.99–2.71 Ga Youanmi Terrane mainly evolved through processes leading to Th/Yb–Nb/Yb trends with a narrow range of Th/Nb (‘constant-Th/Nb’ greenstones). In contrast, the 2.71–2.66 Ga volcanic stratigraphy of the Eastern Goldfields Superterrane evolved through processes leading to Th/Yb–Nb/Yb trends showing a continuous range in Th/Nb (‘variable-Th/Nb’ greenstones). Greenstone sequences of the Pilbara Craton show a similar evolution, with constant-Th/Nb greenstone evolution between 3.13 and 2.95 Ga and variable-Th/Nb greenstone evolution between 3.49 and 3.23 Ga and between 2.77 and 2.68 Ga. The variable-Th/Nb trends dominate greenstone sequences in Australia and worldwide, and are temporally associated with peaks in granite magmatism, which promoted crustal preservation. The increasing Th/Nb in basalts correlates with decreasing εNd, reflecting variable amounts of crustal assimilation during emplacement of mantle-derived magmas. These greenstones are typically accompanied in the early stages by komatiite, and can probably be linked to mantle plume activity. Thus, regions such as the Eastern Goldfields Superterrane simply developed as plume-related rifts over existing granite–greenstone crust – in this case the Youanmi Terrane. Their Th/Nb trends are difficult to reconcile with modern-style subduction processes. The constant-Th/Nb trends may reflect derivation from a mantle source already with a high and constant Th/Nb ratio. This, and a lithological association including boninite-like lavas, basalts, and calc-alkaline andesites, all within a narrow Th/Nb range, resembles compositions typical of modern-style subduction settings. These greenstones are very rare, and were probably only preserved when fortuitously stabilised by granitic magmatism related to the evolution of later variable-Th/Nb greenstones. The rarity of constant-Th/Nb trends suggests that either processes forming them never dominated Archean greenstone evolution, or that such greenstones simply were rarely preserved. Metamorphic mobility of Th renders the Th/Yb–Nb/Yb plot inappropriate for interpreting Eoarchean greenstone units worldwide. Nevertheless, such sequences appear dominated by volcanic rocks that, in modern settings, reflect only the embryonic or initiation stages of subduction. They probably record subduction failure rather than anything resembling modern-style subduction.
•Archean greenstones show either constant or highly variable Th/Nb trends.•Both trends dominate different times in different regions in the Australian Cratons.•Variable trends dominate and are incompatible with subduction (Pearce, 2008).•Constant Th/Nb greenstones only remain when followed by variable Th/Nb greenstones.•Subduction never dominated, and the early Archean record is of subduction failure.
► Presents extensive geological data for the Archean Murchison Domain, Yilgarn Craton. ► Outcrop and zircon data indicate autochthonous crustal development from 2.95 to 2.6Ga. ► Geochemistry of ...(ultra)mafic rocks indicate a long-lived, evolving plume source. ► Common history with adjacent crust is at odds with arc-accretion tectonic models.
Map, geochemical, and geochronological data are used to develop a new stratigraphic scheme and unravel the Archean tectonic evolution of the Murchison Domain of the Yilgarn Craton. Greenstones are divided into four groups: (1) c. 2960–2935Ma Mount Gibson Group of mafic and felsic volcanic and volcaniclastic rocks, in the southern part of the domain; (2) the widespread 2825–2805Ma Norie Group of mafic volcanic rocks, felsic volcaniclastic sandstones and banded iron-formation; (3) 2800–2735Ma Polelle Group of mafic-ultramafic volcanic rocks, intermediate to felsic volcanic and volcaniclastic sedimentary rocks, and banded iron-formation; (4) the 2735–2700Ma Glen Group of coarse clastic sedimentary rocks, komatiitic basalt, and minor rhyolite. Younger groups each have an unconformable relationship with older, underlying, greenstones, whereas the base of the Mount Gibson Group is intruded by younger granites. Very large layered mafic–ultramafic complexes of the Meeline and Boodanoo suites (e.g. Windimurra Igneous Complex) accompanied eruption of the Norie Group during crustal extension at 2825–2805Ma. Less voluminous mafic-ultramafic intrusive suites accompanied eruption of the Polelle and Glen groups. Common c. 2950Ma xenocrystic zircons in these rocks, combined with similar-age detrital zircons in 2820–2720Ma greenstones, implies autochthonous development of post 2820Ma greenstones on older crust.
Greenstone belt volcanism was accompanied by widespread intrusion of syn-volcanic plutons and outlasted by 110Ma of widespread and voluminous granitic magmatism, from 2720 to 2600Ma, including 2640–2600Ma post-tectonic granites. All granites are crustal melts, indicating an extremely long period of crustal melting and thus an external thermal input, with or without the effects of thermal blanketing from newly erupted greenstones.
Deformation consists of four events, including two early periods of greenstone tilting (D1=2930–2825Ma; D2=2735Ma) – possibly associated with crustal extension – and two later (c. 2680–2640Ma) periods of deformation resulting in tight to isoclinal folding of greenstones. D3 structures include steeply-plunging, east–west trending folds of greenstones and open domes of granitic rocks, which formed during a period of inferred partial convective overturn of dense greenstone upper crust and partially molten granitic middle crust at c. 2675Ma. Overprinting D4 structures developed in response to strong east–west compression, resulting in broad, splayed, north-northeast striking dextral shear zones, upright, north- to north-northeast trending folds, and minor north-northwest striking sinistral shear zones. Gold mineralization tends to be focussed in regions of D4 dextral shear and/or low-pressure domains in fold interference structures.
Much of the late history of the domain, from 2720 to 2630Ma, is similar and contemporaneous with events that also affected the Eastern Goldfield Superterrane (EGS) of the craton. Shared events include komatiitic-basaltic volcanism at c. 2720Ma, followed by widespread felsic magmatism (2690–2660Ma), early deformation at 2675Ma, shear-hosted gold mineralisation at 2660–2630Ma, and post-tectonic granites at c. 2630Ma. In addition, the whole craton experienced a period of mafic-ultramafic magmatism (komatiitic–basaltic volcanic rocks, layered mafic-ultramafic complexes, and gabbros) at c. 2810Ma, indicating a shared early history. These findings, together with the low overall metamorphic grade (prehnite-pumpellyite to upper greenschist facies), lack of evidence for significant thrusting, and lack of passive margin/foreland basin/accretionary prism successions suggest that a re-evaluation of subduction-accretion tectonic models for craton development is warranted.
Lithostratigraphy of the Late Archaean Marda–Diemals greenstone belt in the Southern Cross Terrane, central Yilgarn Craton defines a temporal change from mafic volcanism to felsic-intermediate ...volcanism to clastic sedimentation. A ca. 3.0
Ga lower greenstone succession is characterised by mafic volcanic rocks and banded iron-formation (BIF). It is subdivided into three lithostratigraphic associations and unconformably overlain by the ca. 2.73
Ga upper greenstone succession of calc-alkaline volcanic (Marda Complex) and clastic sedimentary rocks (Diemals Formation). D
1 north–south, low-angle thrusting was restricted to the lower greenstone succession and preceded deposition of the upper greenstone succession. D
2 east–west, orogenic compression ca. 2730–2680
Ma occurred in two stages; an earlier folding phase and a late phase that resulted in deposition and deformation of the Diemals Formation. Progressive and inhomogeneous east–west shortening ca. 2680–2655
Ma (D
3) produced regional-scale shear zones and arcuate structures. The lithostratigraphy and tectonic history of the Marda–Diemals greenstone belt are broadly similar to the northern Murchison Terrane in the western Yilgarn Craton, but has older greenstones and deformation events than the southern Eastern Goldfields Terrane of the eastern Yilgarn Craton. This indicates that the Eastern Goldfields Terrane may have accreted to an older Murchison–Southern Cross granite–greenstone nucleus.
Zircon grains in rocks from the Yilgarn Craton record crust formation dating back to shortly after the formation of the Earth. However, much of the evidence is cryptic and not apparent in the mapped ...geology. New Lu-Hf isotopic results, combined with existing Lu-Hf and Sm-Nd isotopic data, indicate five model-age probability peaks in the central and eastern Yilgarn Craton: at ca 4200, ca 3500, ca 3100, ca 2800 Ma and ca 2700 Ma. The ca 3100 Ma, ca 2800 Ma and ca 2700 Ma model-age peaks likely correspond to crust formation events. Evidence of the earlier peaks is not seen directly in the rock record, although zircon crystals in rocks of the Southern Cross Domain of the Youanmi Terrane show a long history of reworking pointing back to mantle extraction more than 4200 million years ago. The earliest peak is not recorded in the Eastern Goldfields Superterrane, indicating that crust formation in this region post-dated the earliest development of the Yilgarn Craton. Subsequent, broadly contemporaneous, episodes of mantle extraction and crustal reworking are indicated by the datasets for both the Eastern Goldfields Superterrane and the Southern Cross Domain. Magmas in the Eastern Goldfields Superterrane had a substantial juvenile input whereas those in the Southern Cross Domain recorded major reworking of older crust. The rock records for both the Eastern Goldfields Superterrane and the Southern Cross Domain share common elements of history after ca 2960 Ma. Both regions appear to have been subjected to major heating at ca 3100 Ma and ca 2800 Ma that resulted in the generation of juvenile crust in the east and reworking of older crust in the west. The ca 3500 Ma event is not readily evident in the rock record and may reflect a mixed age. However, the ca 3100 Ma and ca 2800 Ma events are recorded by both granite suites and greenstone successions across the craton. The ca 2700 Ma event is most evident in rocks from the Eastern Goldfields Superterrane.
New in situ Lu–Hf data on zircons from GSWA geochronology samples has provided a unique isotopic dataset with a high temporal resolution for the Murchison Domain of the Yilgarn Craton in Western ...Australia. These data identify extended periods of juvenile mantle input (positive εHf values) into the crust firstly at c. 2980Ma and then from c. 2820Ma to c. 2640Ma with significant pulses of crustal recycling at c. 2750Ma and c. 2620Ma (highly negative εHf values). Geochemical data from well-characterised granitic suites of the Murchison Domain provide additional constraints on the crustal evolution of the area and indicate a prolonged period of crustal melting and remelting at progressively shallower depths from c. 2750 to c. 2600Ma.
At c. 2760–2753Ma, widespread calc-alkaline, intermediate to silicic volcanic rocks of the Polelle Group were erupted, accompanied by intrusion of felsic to intermediate melts derived from a variety of crustal sources that likely formed by partial mixing with basaltic melts. The intrusive rocks include a wide geochemical array of rocks in the Cullculli and Eelya suites that were sourced over a wide range of crustal depths. At this time a major departure to negative εHf values (<−5) occurred, indicating sampling of c. 3.80Ga model aged source rocks as well as continued juvenile input. Post-volcanic granitic rocks emplaced between c. 2710 and c. 2600Ma show geochemical evidence for progressive fractionation through time and derivation from an evolving crustal source.
We interpret the driving force for this protracted history of mantle and crustal melting to be two mantle plumes at 2.81 and 2.72Ga. These data document the process of cratonization through progressive melt depletion of the lower crust, progressively fractionating and shallower melts, culminating with a final phase of crustal recycling (εHf<−5) and the cessation of juvenile input at c. 2630–2600Ma during intrusion of the Bald Rock Supersuite, resulting in cratonization of this part of the Yilgarn Craton.
► Murchison Hf data are consistent with plume activity at 2.81 and 2.72Ga. ► A significant juvenile event is identified at 3.04Ga. ► Polelle Group is unique in that indicates significant reworking of older crust. ► Reworking is accompanied by diverse melt sources. ► Granites become shallower sourced and more fractionated, leading to cratonization.
New geological mapping and geochronology in the northeast Yilgarn Craton has changed our geological understanding of this region. The Yilgarn Craton had previously been divided into a series of ...terranes, with the easternmost Eastern Goldfields Superterrane separated from the Youanmi Terrane, which forms the core of the protocraton, by the Ida Fault zone. The Eastern Goldfields Superterrane was subdivided into the western Kalgoorlie, central Kurnalpi, and eastern Burtville terranes, with the latter, easternmost terrane the focus of the new field mapping and geochronology. Four main episodes of greenstone crustal growth have been recognised in the northeast Yilgarn Craton: ca 2970-2910 Ma, ca 2815-2800 Ma, 2775-2735 Ma, and ca 2715-2630 Ma. Rather than a single Burtville Terrane, as previously proposed, the distribution of greenstone magmatism reveals a previously unrecognised young (<2720 Ma) Yamarna Terrane in the northeast corner of the craton. The Yamarna Terrane is separated from the older (>2735 Ma) redefined Burtville Terrane by the Yamarna Shear Zone, which is now regarded as a terrane boundary. The correlation of lithologies and ages of magmatism in the northeast Yilgarn Craton with the rest of the craton indicates that the Burtville Terrane has affinities with the Youanmi Terrane that forms the nucleus of the craton, whereas the Yamarna Terrane has affinities with the Kalgoorlie Terrane in the west of the Eastern Goldfields Superterrane. The Burtville and Youanmi terranes shared a common history from ca 2970 Ma until ca 2720 Ma, when regional extension accommodated deposition of the Kambalda Sequence in the Kalgoorlie Terrane. It appears that extension also occurred along the Yamarna Shear Zone after ca 2720 Ma, accommodating the deposition of greenstones in the Yamarna Terrane. Like the Kalgoorlie and Kurnalpi Terranes, the Yamarna Terrane contains inherited zircon and local older rocks. This suggests that the ca 2720 Ma extension did not result in widespread rifting and the formation of extensive oceanic crust. Rather, there was thinning of older crust that extended right across the current Yilgarn Craton.
SHRIMP U‐Pb zircon analysis indicates that detrital zircons from extensive quartzite units in the Southern Cross Granite–Greenstone Terrane of the central Yilgarn Craton have ages ranging from ...ca 4350 Ma to ca 3130 Ma. Regional mapping studies indicate that the quartzites lie at the stratigraphic base of the exposed succession. The detrital zircon age profiles of the Southern Cross Granite–Greenstone Terrane quartzites are remarkably similar to those of quartzites in the Narryer and South West Terranes, in the northwest and southwest of the Yilgarn Craton respectively, and are significantly older than any igneous rocks that have been dated anywhere in the Yilgarn Craton other than the Narryer Terrane. Similar detrital‐zircon‐bearing quartzites have not been identified in the Murchison Granite–Greenstone Terrane. These age profiles suggest that the quartzites have a common depositional history. Granites in the central Yilgarn Craton are mainly younger than ca 2750 Ma and contain rare xenocrystic zircons older than 3100 Ma. If the central and western Yilgarn quartzites were all deposited at approximately the same time, the lack of preserved continental crust in the Southern Cross and Murchison Granite–Greenstone Terranes, and the South West Terrane, that is older than 3100 Ma, suggests that pre‐3100 Ma Narryer‐like continental crust may have been rifted or extensively reworked during deposition of greenstone successions between ca 3000 and ca 2700 Ma. If not, then a ca 4350 Ma detrital zircon in the Southern Cross Granite–Greenstone Terrane indicates more widespread, very old, continental crust than has previously been identified.
The Pilbara and Yilgarn Cratons in Western Australia record the Earth's early history from ca. 3.7 Ga to ca. 2.5 Ga. This paper highlights recent scientific work, and advances in understanding the ...similarity and differences in granitic rocks, greenstone stratigraphy, structural styles, and tectonic settings of the major granite-greenstone terranes within the cratons, and then discusses the crustal evolution trends from the Paleo- to Neoarchaean. We conclude that in the Pilbara and Yilgarn Cratons, the Paleoarchaean areas are characterized by vertical tectonics, whereas the Meso- to Neoarchaean areas are dominated by horizontal tectonics - plate tectonic processes may have started in the Mesoarchaean.
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