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D R Nelson - One of the best experts on this subject based on the ideXlab platform.
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Lithostratigraphy and tectonic evolution of contrasting Greenstone successions in the central Yilgarn Craton, Western Australia
Precambrian Research, 2003Co-Authors: She Fa Chen, Stephen Wyche, Angela Riganti, John E. Greenfield, D R NelsonAbstract:Abstract 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.
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evolution of the archaean granite Greenstone terranes of the eastern goldfields western australia shrimp upb zircon constraints
Precambrian Research, 1997Co-Authors: D R NelsonAbstract:Abstract Thirty-five high-precision SHRIMP UPb zircon dates (with an average 95% confidence error of ±6 Ma) have been obtained from the granite-Greenstone terranes in the southern part of the Eastern Goldfields of Western Australia. These dates establish the timing of major deformation, metamorphism and granite intrusion events, and test inferred stratigraphic relationships within the Greenstones. These new data reveal that felsic volcanic rocks were erupted throughout the southern part of the Eastern Goldfields between c. 2713 and c. 2672 Ma. Analytically indistinguishable dates of c. 2705 Ma were obtained for felsic volcanic rocks interbedded with komatiitic lavas at three sites. Dates on felsic volcanic rocks within tectonostratigraphic packages that include komatiites are also consistent with an inferred eruption age for these komatiites of c. 2705 Ma. Komatiitic lavas in the southern part of the Eastern Goldfields were probably emplaced during a single voluminous eruption that resulted in the flooding of a vast area of the late Archaean landscape, filling the topographic lows around older felsic volcanic vents. The stratigraphic complexity within tectonostratigraphic domains is attributed to the existence of an undulating topography during the eruption of the mafic and ultramafic lavas, and to the rapid and localized deposition of felsic volcanic rocks close to a number of isolated volcanic centres prior to, and after, the komatiite lava eruption event. Between c. 2685 and c. 2675 Ma, early (pre-D2) intrusive equivalents of the felsic volcanic rocks were emplaced as thick granite sheets into the base of the Greenstone sequences. Regional east-west compression between c. 2675 and c. 2657 Ma reactivated early extensional structures and resulted in the stacking of the Greenstones. This was accompanied by further intrusion of granite within developing antiforms. Burial of Greenstone slices by stacking, and the effect of hydrothermal fluids emanating from both the granites and buried Greenstones, resulted in regional metamorphism to greenschist facies prior to c. 2640 Ma. Monzogranite and granodiorite emplaced near the western, southern and eastern margins of the Greenstones were deformed and metamorphosed to upper amphibolite facies, and converted into gneisses during D2 and D3 compression events. Movement along large-scale transcurrent D3 shear zones had ceased by c. 2635 Ma. Remnants of a c. 2958 to c. 2930 Ma Greenstone sequence preserved near Norseman and Ravensthorpe in the southeastern Yilgarn Craton, and xenocryst zircons ranging in age from c. 2910-2730 Ma in the c. 2713-2672 Ma felsic volcanic rocks, indicate that the exposed Greenstone sequences were deposited on continental basement. The Eastern Goldfields granite-Greenstone terranes were formed by the closure of a series of narrow back-arc rift basins that developed along a continental margin and above an active subduction zone located to the east of the rift basins.
Carl R. Anhaeusser - One of the best experts on this subject based on the ideXlab platform.
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Archaean Granitoid–Greenstone Geology of the Southeastern Part of the Kaapvaal Craton
Regional Geology Reviews, 2019Co-Authors: Axel Hofmann, Carl R. Anhaeusser, John Dixon, Alfred Kröner, Lopamudra Saha, Allan H. Wilson, Hangqiang XieAbstract:The southeastern Kaapvaal Craton is a Palaeoarchaean granitoid–Greenstone terrain. Supracrustal rocks are dominated by metamorphosed mafic–ultramafic volcanic rocks intercalated with minor felsic volcanic and chemical sedimentary rocks, including carbonaceous chert and minor iron formation. Siliciclastic sedimentary rocks are rare. The Greenstones occur in the Schapenburg and Dwalile fragments close to the Barberton Greenstone belt, the Assegaai, De Kraalen, Witrivier and Commondale fragments in the vicinity of Piet Retief, and the Nondweni and Ilangwe Greenstone belts together with several smaller fragments in the southern part of the craton. The Greenstones are locally in tectonic contact with compositionally layered grey gneisses as old as ~3.5 Ga and, in the southern part, are intruded by the ~3.25 Ga Anhalt granitoid suite, providing a minimum age for the Greenstones. The oldest felsic volcanic rocks so far identified are 3.53 Ga in age. The belts show evidence of polyphase deformation, with metamorphism at greenschist- to amphibolite-facies grade, the peak of which was at ~3.2 Ga. Ore deposits are restricted to sub-economic gold and rare massive sulphide Cu–Zn deposits.
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Metamorphism of the granite-Greenstone terrane south of the Barberton Greenstone belt, South Africa: an insight into the tectono-thermal evolution of the "lower" portions of the Onverwacht Group
Precambrian Research, 2001Co-Authors: Annika Dziggel, Carl R. Anhaeusser, Gary Stevens, Marc Poujol, Richard ArmstrongAbstract:Abstract This paper reports new geochronological and metamorphic data from Onverwacht Group Greenstone remnants exposed in the granitoid-dominated terrain to the south of the Barberton Greenstone belt. The Greenstone remnants consist of metamorphosed mafic and ultramafic metavolcanic sequences, with minor sedimentary units that comprise thin chert and banded iron formation layers, interbanded with the (ultra) mafic volcanic units, as well as an up to 8 m-thick clastic sedimentary unit that contains well-preserved primary sedimentary features such as trough cross-bedding. Coarse-grained portions of the clastic sediments contain up to 4.5 wt.% K 2 O and represent metamorphosed impure arkoses. SHRIMP and conventional U–Pb dating of detrital zircons reveal dates ranging between ca . 3521 and 3540 Ma, indicating that at least two protoliths for these sediments predate the formation of the bulk of the Barberton Greenstone belt. A minimum age of 3431±11 Ma for the formation of the metasediments is given by a trondhjemite gneiss that intrudes the Greenstone remnant. Thus, these metasediments were deposited between ca. 3521 and 3431 Ma, contemporaneously with the erosion of spacially associated older, and presumably potassium-rich, granitoid rocks. Other portions of the clastic metasediments have a mafic affinity and are characterised by the peak-metamorphic mineral assemblage diopside+andesine+garnet+quartz. This assemblage, and garnet in particular, is extensively replaced by epidote. Peak-metamorphic mineral assemblages of magnesio−hornblende+andesine+quartz, and quartz+ferrosilite+magnetite+grunerite have been recorded from adjacent amphibolites and interlayered iron formation units, respectively. In these rocks, retrogression is marked by actinolitic rims around peak metamorphic magnesio–hornblende cores in the metamafic rocks, and by a second generation of grunerite that occurs as fibrous aggregates rimming orthopyroxene in the iron formation. PT calculations, using a variety of geothermometers and barometers, for the peak-metamorphic mineral assemblages in all these rock types vary between 650 and 700 °C and 8 and 11 kbar. This implies a tectonic setting comparable to some modern orogenic belts and that the granite–Greenstone terrane investigated in this study possibly represents an exhumed mid- to lower-crustal terrane that formed a ‘basement’ to the Barberton Greenstone belt at the time of the peak metamorphic event at ca. 3230 Ma.
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Emplacement features of Archaean TTG plutons along the southern margin of the Barberton Greenstone belt, South Africa
Precambrian Research, 1995Co-Authors: Alexander F. M. Kisters, Carl R. AnhaeusserAbstract:Abstract The Barberton Greenstone belt in South Africa and its surrounding granitoid terrane represents one of the type localities for Archaean granite-Greenstone tectonics. Contact relationships between early Archaean granitoids and Greenstone sequences along the southern margin of the Greenstone belt illustrate multiple, simultaneously operating emplacement mechanisms for the plutons. These emplacement mechanism, which can be identified in individual plutons comprise ductile wall-rock deformation, stoping, assimilation, and the intrusion of ring dykes. The interaction between these various mechanisms is interpreted to be responsible for the complex and highly variable discordant and concordant contact relationships observed in the granite-Greenstone terrane as well as the deformation observed within wall rocks and country-rock xenoliths. The absence of regionally consistent fabrics within the plutons suggests considerable competence contrasts between the tonalitic-trondhjemitic plutons and, in many cases, the hydrated Greenstone cover-sequence during later subhorizontal crustal shortening which occurred parallel to the Greenstone-granitoid interface. This competence contrast is interpreted to have resulted in the accentuation of cuspate-lobate folds along the granitoid-Greenstone interface, being partly responsible for the typical arcuate, dome-and-keel geometry of the Barberton Greenstone belt.
Van Kranendonk - One of the best experts on this subject based on the ideXlab platform.
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model for the development of kyanite during partial convective overturn of archean granite Greenstone terranes the pilbara craton australia
Journal of Metamorphic Geology, 1999Co-Authors: Van KranendonkAbstract:Restricted occurrences of early, syn- and late-kinematic kyanite adjacent to large domal batholiths in the Archean granite–Greenstone terrane of the east Pilbara craton, Australia, are considered to result from partial convective overturn of the crust. The analogue models of Dixon & Summers (1983) and thermo-mechanical models of Mareschal & West (1980), involving gravitionational overturn of dense Greenstone crust that initially overlay sialic basement, successfully explain the geometry, dimension, kinematics and strain patterns of the batholiths and Greenstone rims. Application of these models suggests that andalusite and sillimanite are the stable aluminosilicate polymorphs in domal crests and rims, where prograde clockwise P–T–t paths, with small pressure changes, should be recorded. Both aluminosilicates are predicted to overprint kyanite, which is observed locally around the east Pilbara domes. Kyanite is the predicted aluminosilicate polymorph in the deeper parts of domal rims and within sinking Greenstone keels, reflecting rapid, near-isothermal burial. The narrow zones of kyanite-bearing schists adjacent to some batholiths in the Pilbara craton are metamorphosed, highly strained equivalents of altered felsic volcanic rocks in the low-grade Greenstone succession, dragged to mid-crustal depths (6 kbar) during Greenstone sinking. The schists rebounded as an arcuate tectonic wedge along the southern Mount Edgar batholith rim, during the later stages of doming, and were juxtaposed against regional, greenschist facies, low-strain Greenstones. Thus, kyanite was preserved: if the walls had remained at depth, it would have been overprinted by the higher-temperature aluminosilicate polymorphs during thermal recovery. Kyanite growth in the Pilbara craton is unlikely to have resulted from ballooning of plutons, mantled gneiss doming, metamorphic core complex formation, or early crustal overthickening. The typical subvertical foliations and lineations of the tectonic wedge suggest that subvertical fabrics extended to mid-crustal depths (c. 20 km) before rebound, providing a three-dimensional glimpse of Archean dome-and-keel structures. The general occurrence of large granitoid domes in Archean granite–Greenstone terranes, restriction of rare kyanite to the adjacent, high-strain batholith margins, and its absence from the batholiths, suggest that partial convective overturn of the crust may have been a common process at this early stage of Earth history.
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Model for the development of kyanite during partial convective overturn of Archean granite–Greenstone terranes: the Pilbara Craton, Australia
Journal of Metamorphic Geology, 1999Co-Authors: Collins, Van KranendonkAbstract:Restricted occurrences of early, syn- and late-kinematic kyanite adjacent to large domal batholiths in the Archean granite–Greenstone terrane of the east Pilbara craton, Australia, are considered to result from partial convective overturn of the crust. The analogue models of Dixon & Summers (1983) and thermo-mechanical models of Mareschal & West (1980), involving gravitionational overturn of dense Greenstone crust that initially overlay sialic basement, successfully explain the geometry, dimension, kinematics and strain patterns of the batholiths and Greenstone rims. Application of these models suggests that andalusite and sillimanite are the stable aluminosilicate polymorphs in domal crests and rims, where prograde clockwise P–T–t paths, with small pressure changes, should be recorded. Both aluminosilicates are predicted to overprint kyanite, which is observed locally around the east Pilbara domes. Kyanite is the predicted aluminosilicate polymorph in the deeper parts of domal rims and within sinking Greenstone keels, reflecting rapid, near-isothermal burial. The narrow zones of kyanite-bearing schists adjacent to some batholiths in the Pilbara craton are metamorphosed, highly strained equivalents of altered felsic volcanic rocks in the low-grade Greenstone succession, dragged to mid-crustal depths (6 kbar) during Greenstone sinking. The schists rebounded as an arcuate tectonic wedge along the southern Mount Edgar batholith rim, during the later stages of doming, and were juxtaposed against regional, greenschist facies, low-strain Greenstones. Thus, kyanite was preserved: if the walls had remained at depth, it would have been overprinted by the higher-temperature aluminosilicate polymorphs during thermal recovery. Kyanite growth in the Pilbara craton is unlikely to have resulted from ballooning of plutons, mantled gneiss doming, metamorphic core complex formation, or early crustal overthickening. The typical subvertical foliations and lineations of the tectonic wedge suggest that subvertical fabrics extended to mid-crustal depths (c. 20 km) before rebound, providing a three-dimensional glimpse of Archean dome-and-keel structures. The general occurrence of large granitoid domes in Archean granite–Greenstone terranes, restriction of rare kyanite to the adjacent, high-strain batholith margins, and its absence from the batholiths, suggest that partial convective overturn of the crust may have been a common process at this early stage of Earth history.
Hangqiang Xie - One of the best experts on this subject based on the ideXlab platform.
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Archaean Granitoid–Greenstone Geology of the Southeastern Part of the Kaapvaal Craton
Regional Geology Reviews, 2019Co-Authors: Axel Hofmann, Carl R. Anhaeusser, John Dixon, Alfred Kröner, Lopamudra Saha, Allan H. Wilson, Hangqiang XieAbstract:The southeastern Kaapvaal Craton is a Palaeoarchaean granitoid–Greenstone terrain. Supracrustal rocks are dominated by metamorphosed mafic–ultramafic volcanic rocks intercalated with minor felsic volcanic and chemical sedimentary rocks, including carbonaceous chert and minor iron formation. Siliciclastic sedimentary rocks are rare. The Greenstones occur in the Schapenburg and Dwalile fragments close to the Barberton Greenstone belt, the Assegaai, De Kraalen, Witrivier and Commondale fragments in the vicinity of Piet Retief, and the Nondweni and Ilangwe Greenstone belts together with several smaller fragments in the southern part of the craton. The Greenstones are locally in tectonic contact with compositionally layered grey gneisses as old as ~3.5 Ga and, in the southern part, are intruded by the ~3.25 Ga Anhalt granitoid suite, providing a minimum age for the Greenstones. The oldest felsic volcanic rocks so far identified are 3.53 Ga in age. The belts show evidence of polyphase deformation, with metamorphism at greenschist- to amphibolite-facies grade, the peak of which was at ~3.2 Ga. Ore deposits are restricted to sub-economic gold and rare massive sulphide Cu–Zn deposits.
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generation of early archaean felsic Greenstone volcanic rocks through crustal melting in the kaapvaal craton southern africa
Earth and Planetary Science Letters, 2013Co-Authors: Alfred Kröner, Hangqiang Xie, Elis J Hoffmann, Carsten Munker, E Hegner, J Wong, Yusheng Wan, Dunyi LiuAbstract:Abstract High-potassium felsic volcanic rocks interlayered with basalt and komatiite in early Archaean Greenstone sequences in the Barberton Greenstone Belt of South Africa and Swaziland, previously considered to be derived from melting of mafic precursors, are shown to be the result of melting of significantly older felsic crust. This is documented by a combination of SHRIMP zircon dating with Hf-in-zircon and whole-rock Lu–Hf and Sm–Nd isotopic data. Zircons from felsic rocks of the oldest Barberton unit, the 3.53 Ga Theespruit Formation, yielded predominantly negative e Hf ( t ) -values, whereas whole-rock e Hf ( t ) - and e Nd ( t ) -data are slightly negative to slightly positive. Similar results were obtained for ca. 3.45 Ga felsic rocks in the Hoeggenoog and Noisy Formations higher up in the Greenstone stratigraphy. These data rule out derivation of the felsic units from melting of basaltic precursors and favor a crustal source, most likely of TTG composition. The isotopic data are not compatible with an entirely oceanic origin of the Barberton Greenstone sequences and favor a pre-Greenstone basement beneath the volcanic rocks. Crustal melting of Eo- to Paleoarchaean lower crust probably generated the felsic volcanic rocks and is likely to have been responsible for gradual stabilization of the Kaapvaal craton.
Donald R Lowe - One of the best experts on this subject based on the ideXlab platform.
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Chapter 4 Archean Greenstone-Related Sedimentary Rocks
Archean Crustal Evolution, 1994Co-Authors: Donald R LoweAbstract:Publisher Summary Archean sedimentary rocks provide the primary record of surface conditions and crustal evolution on the early earth. Although there is a sparse development of Proterozoic- and Phanerozoic-style craton cover and craton margin sedimentary rocks older than 2.5 Ga, the principal present-day repositories of the Archean sedimentary record are Greenstone belts. This record begins with the 3.9 Gy-old banded iron-formation, felsic metatuffs and metavolcanic breccias, cherts, and calc-silicate rocks of the Isua supracrustal suite in West Greenland includes the well-preserved Greenstone sequences of the 3.55-3.2 Gy-old Barberton Greenstone Belt, South Africa and Swaziland, and of the 3.5-3.2-Gy-old eastern Pilbara Block, Western Australia; and culminates with a host of 3.0-2.5 Gy-old, Late Archean belts, present on virtually all continents but best studied in the Superior and Slave Provinces of Canada (Card, 1990) and the Yilgarn Block, Western Australia. While the Archean sedimentary record is dominated by largely simatic Greenstone-belt supracrustal sequences, the latest Archean saw the formation of enormous, buoyant blocks of continental crust that dominated both tectonic and sedimentary systems throughout post-Archean time.
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chronology of early archaean granite Greenstone evolution in the barberton mountain land south africa based on precise dating by single zircon evaporation
Earth and Planetary Science Letters, 1991Co-Authors: Alfred Kruner, Gary R Byerly, Donald R LoweAbstract:Abstract We report precise 207 Pb 206 Pb single zircon evaporation ages for low-grade felsic metavolcanic rocks within the Onverwacht and Fig Tree Groups of the Barberton Greenstone Belt (BGB), South Africa, and from granitoid plutons bordering the belt. Dacitic tuffs of the Hooggenoeg Formation in the upper part of the Onverwacht Group yield ages between 3445 ± 3 and 3416 ± 5 Ma and contain older crustal components represented by a 3504 ± 4 Ma old zircon xenocryst. Fig Tree dacitic tuffs and agglomerates have euhedral zircons between 3259 ± 5 and 3225 ± 3 Ma in age which we interpret to reflect the time of crystallization. A surprisingly complex xenocryst population in one sample documents ages from 3323 ± 4 to 3522 ± 4 Ma. We suspect that these xenocrysts were inherited, during the passage of the felsic melts to the surface, from various sources such as Greenstones and granitoid rocks now exposed in the form of tonalite-trondhjemite plutons along the southern and western margins of the BGB, and units predating any of the exposed Greenstone or intrusive rocks. Several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 Ma, coeval with or slightly older than Onverwacht felsic volcanism, while the Kaap Valley pluton along the northwestern margin of the belt is coeval with Fig Tree dacitic volcanism. These results emphasize the comagmatic relationships between Greenstone felsic volcanic units and the surrounding plutonic suites. Some of the volcanic and plutonic units contain zircon xenocrysts older than any exposed rocks. These indicate the existence of still older units, possibly stratigraphically lower and older portions of the Greenstone sequence itself, older granitoid intrusive rocks, or bodies of older, unrelated crustal material. Our data show that the Onverwacht and Fig Tree felsic units have distinctly different ages and therefore do not represent a single, tectonically repeated unit as proposed by others. Unlike the late Archaean Abitibi Greenstone belt in Canada, which formed over about 30 Ma, exposed rocks in the BGB formed over a period of at least 220 Ma. The complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient Greenstones. A surprising similarity in rock types, tectonic evolution, and ages of the BGB in the Kaapvaal craton of southern Africa and Greenstones in the Pilbara Block of Western Australia suggests that these two terrains may have been part of a larger crustal unit in early Archaean times.
