Granite-greenstone Terrain Research Articles

Geologic observations across a tectono-metamorphic boundary in the Babangu area, Giyani (Sutherland) Greenstone Belt, South Africa

A narrow corridor across the tectono-metamorphic transition between “low-grade” and “high-grade” granite-greenstone terrains, respectively from the Giyani (also known as the Sutherland) Greenstone Belt into the hydrated granulites of the Southern Marginal Zone of the Limpopo Belt, has been mapped in detail. The two terrains are separated by a sharp tectonic (mylonitic) break. The “low-grade” greenstone terrain to the south of this break is part of a southward-dipping flattened antiformal thrust-stack, and is thus probably allochtonous. Regional variation of the dominant tectonic fabric is from shallow to steep; this is interpreted to be due to progressive deformation during northward-directed tectonic transport. The greenstone belt may lie on a flattish southward-dipping sole- thrust, “lubricated” by synkinematic granitoids in the foot and hanging walls. The northern margin of the greenstones is abruptly terminated by a late, steeply oriented shear zone, defined by mylonites. North of the shear zone, “high-grade” granite-greenstone terrain rocks (hydrated granulites) have been juxtaposed against and across the “low-grade” greenstone terrain. The tectonic break is either part of a transcurrent fault or a lateral ramp as part of a late southward-directed overthrust system. Although the rocks of the low-grade terrain have retained their early tectonic fabrics, their metamorphic character more reflects that of the “overriding” hydrated granulites.

The geology and tectonic evolution of the Pietersburg Greenstone Belt, South Africa

The Pietersburg Greenstone Belt occurs near the northern margin of the “low-grade” granite-greenstone terrain of the Kaapvaal Craton. To the north lies the “high-grade” granite-greenstone terrain which constitutes the Southern Marginal Zone of the Limpopo Belt. The original relationship between the two terrains has been overprinted by the effects of the Limpopo Orogeny and are therefore not known. A better understanding of the tectonic evolution of the Pietersburg Greenstone Belt may help to clarify this relationship. The rocks of the southwestern part of the Pietersburg Greenstone Belt have been mapped in considerable detail. The rocks can be subdivided into two sequences. The first comprises a sequence of massive to pillowed metabasalts, metagabbros and metaperidotites, as well as BIF, fine-grained mafic tuffs and sediments. The sequence was subjected to extensive hydrothermal metamorphism at low pressures and variable temperatures in the greenschist to amphibolite facies. This suggests an oceanic-like environment. The second sequence, known as the Uitkyk Formation, comprises a sequence of terrestrial clastics, which overlie the first sequence with a profound unconformity. Sedimentology indicates that the Uitkyk Formation formed in a prograding alluvial fan or braided alluvial plain. The Uitkyk sediments were deformed and infiltrated by fluids that precipitated quartz-tourmaline (±gold) and siderite soon after deposition. Age relationships, bedding/cleavage relationships and younging directions indicate that both the sediments and simatic rocks were involved in a phase of thrust-related deformation (D 2) that resulted in south over north stratigraphic repetition under generally low-grade conditions of metamorphism. Post-D 2 granites date this thrusting at pre 2.69 Ga. D 2 was probably diachronous, and at surface, the deformation was accompanied by syntectonic terrestrial sedimentation and placer gold deposition. The tectonic environment was probably one of an intermontane accretionary or terrestrial foreland basin. Within this tectonic environment, the basement and cover became tectonically stacked across evolving shear zones. Combined field and microstructural relationships within a major fault zone have revealed a complex history of progressive D 2 deformation, metamorphism, mineralisation and associated sedimentary processes. The zone represents a deformed syn-sedimentary duplex that was carried piggy-back above an allochthonous thrust sheet of simatic rocks. At the deepest levels this fault, and other shear zones, were intruded by syntectonic granitoids. This late-Archaean, northward-verging D 2 thrust-stack model for the greenstone belt, with the syntectonic granites intruding the thrusts from beneath the greenstone belt, is in good accord with the recent geophysical observations. The age of cover sedimentation is older than ∼2.69 Ga but younger than 2.98 Ga in age; this supports suggestions that these deposits are age equivalent of the upper, gold-bearing sediments of the Witwatersrand basin; and that the northwards-thrusting in both basins may be of the same age and related to the same regional tectonic forces. The simatic rocks of the Pietersburg Greenstone Belt may have been part of a more extensive allochthonous simatic terrain, which includes the “similar” greenstone remnants of the Southern Marginal Zone of the Limpopo belt. Such a terrain must have been disrupted and metamorphosed during the northward-directed imbrication and tectonic burial recorded in the Pietersburg Belt, prior to the climax of the Limpopo Orogeny. The latter subsequently severely disrupted the Pietersburg Greenstone Belt during Ventersdorp times (2.6–2.7 Ga).

P-wave velocity in granulites from South India: implications for the continental crust

P-wave velocities ( V p ) were measured in 160 high-grade metamorphic rocks from the South Indian granulite terrain (SGT). The wide variations observed in the V p of charnockites and gneisses could be due to the complex prograde and retrograde metamorphic histories of the two major rock types of the SGT. The velocity-density relation showed distinct trends for charnockites and gneisses. Initial stages of retrograde metamorphism in charnockites significantly affected their magnetic properties, however, its effect on velocity and density is not diagnostic. Contrasting physical properties on either side of the Palghat-Cauvery (P-C) shear zone lends support for the contention that the P-C shear zone is a major paleosuture. The laboratory mean V p of the rocks from the northern SGT are comparable with the mid-crustal DSS velocity in the adjacent granite greenstone terrain (GGT), suggesting that the GGT is possibly underlain by a felsic granulite basement. The physical properties of the high-grade metamorphic rocks from SGT are significantly lower than that of the lower crust. The physical properties and tectonic considerations show that the granulites of South India may not be of lower crustal origin and hence not representative of the lower crust, as generally thought. A simplified two-layer crustal model with a predominantly felsic granulite upper crust and a mafic granulite lower crust, is suggested for the SGT.

Isotope and rare earth element evidence for a late archaean terrane boundary in the southeastern pilbara craton, western australia

RbSr, SmNd and REE data have been obtained from granitic rocks exposed in Archaean granite-greenstone terrain in the southeastern part of the Pilbara Craton. Samples were collected from the Kurrana Batholith; the Cooninia, Billinooka, and Sylvania Inliers; and from the bottom of a borehole drilled through the Hamersley Basin succession in the Fortescue Valley. Almost all the samples have 87 Rb/ 86 Sr < 4.0, 144 Nd < 0.12 and SmNd model ages ( T DM) in the range 3.1 to 3.3 Ga. These characteristics, together with REE patterns, differ from published data for granitic rocks of the northern Pilbara granite-greenstone terrain that have similar geological histories. RbSr isotopic systematics have been variably disturbed, with data arrays on isochron plots all indicating ages younger than the accepted age of the base of the Hamersley Basin succession (2.76 Ga). This is attributed to pervasive burial metamorphism beneath the Hamersley Basin, and to later hydrothermal events. Some rock units appear to be younger than their northern Pilbara equivalents. A shear zone separating the Mosquito Creek Synclinorium from the Kurrana Batholith has the characteristics of a suture zone, and is postulated to be the boundary between two distinct terranes that joined to form the Pilbara Craton between 3.0 and 2.76 Ga.

Heat flow and heat generation in the Archaean Dharwar cratons and implications for the Southern Indian Shield geotherm and lithospheric thickness

We report terrestrial heat flow density measurements from 11 new sites in the Archaean Eastern and Western Dharwar cratons of the Southern Indian Shield. The new values, together with already published data, have yielded a mean heat flow of 40 ± 3.4 (s.d.) mW m −2 ( n = 11) for the Eastern Dharwar craton, which is a predominantly gneiss-granite terrain. The mean heat flow in the Archaean supracrustal belts of Western Dharwar craton, which is a granite-greenstone terrain, is 31 ± 4.1 (s.d.) mW m −2 ( n = 4). The observed difference in surface heat flow between the two cratons is ascribed to differences in radiogenic heat production in upper crustal layers, inferred to be related to different crust-forming processes. It has been argued that low heat production appears to be an inherent characteristic of the crust similar to that of the Western Dharwar craton, which evolved during the early Archaean and suffered depletion of the lithophile elements of its lower crust prior to the late Archaean. Radiogenic heat production measurements were carried out on borehole core samples for one heat flow site (Sortur), and data have been compiled from the literature on three other sites and on a variety of rocks of the Southern Indian Shield. Heat flow and radiogenic heat production for Sortur, when combined with the published data, define a linear relation ( r = 0.97) with a slope of 11.5 km and an intercept of 23 mw m −2. The thickness of the thermal lithosphere beneath the Dharwar cratons is over 200 km, thereby indicating, beneath the cratons, the existence of deep continental roots-a concept which is also supported by teleseismic P-wave travel time data. The geothermal data has clearly demonstrated that the Archaean Dharwar cratons of the Indian landmass are associated with low mean surface heat flow (37.7 mW m −2), reduced heat flow, Moho heat flux (12–17 mW m −2), and a cool and thick lithosphere—all characteristics of many Archaean-early Proterozoic terrains of other shields. It is inferred that the thermal structure of the South Indian lithosphere is identical with that of other cratons.

Partial melting of amphibolite/eclogite and the origin of Archean trondhjemites and tonalites

The generation of trondhjemites and tonalites on a massive scale during the Archean (3.8-2.5 Ga ago) marked the transition from a simatic to a sialic crust, and represents the magmatic contribution to cratonization. Petrogenetic models for the origin of these rocks based on their highly fractionated, HREE-depleted rare earth patterns suggest a mafic crustal source, either through a process of partial melting of amphibolite, garnet-amphibolite, or eclogite, in which hornblende and/or garnet are essential residual phases, or by hornblende-controlled fractionation of hydrous basaltic magma. A series of vapor-absent (i.e., Pfluid< Ptotal) melting experiments on four natural basaltic compositions were conducted at 8, 16, 22 and 32 kbar in order to assess the validity of models for the origin of Archean granitoids which assume a mafic crustal source. Melt compositions produced by 10–40% melting are tonalitic-trondhjemitic at all pressures investigated; residual assemblages are amphibole+plag±opx±FeTi at 8 kbar, garnet+cpx±amphibole±plag±opx±FeTi oxide at 16 kbar, and garnet+cpx±rutile at 22 and 32 kbar. REE patterns for most of the trondhjemitic-tonalitic partial melts, calculated on the basis of estimated modal proportions of melt and residual phases, are highly fractionated (LaYb is 30–50), heavy rare earth-depleted (YbN is 1–10) when garnet is present to some extent in the residue; these REE patterns are similar to those of trondhjemitic and tonalitic gneisses from several Archean “grey gneiss” and granite-greenstone terrains. A consideration of estimated Archean geotherms with respect to the experimental P-T conditions indicates that a temporally diminishing Archean geotherm might have progressively swept through a P-T regime in which trondhjemitic-tonalitic melts could have been generated initially from a water-saturated (Pf=Pt) to undersaturated (Pf<Pt) amphibolite source by partial melting at 5–8 kbar. Subsequent relaxation of the geotherm through the mid- to late-Archean would have produced similar melts by vapor-absent melting of garnet-amphibolite at 16 kbar and eclogite at 22–32 kbar. However, the degree of melting required to produce melts of trondhjemitic-tonalitic composition increases with pressure, 10–15% melting being appropriate at 8 kbar in a amphibole-dominated residue, but 25–35% melting being required at 22–32 kbar, where garnet dominates the residue. Supposing the tendency for melt segregation and/or magma mobilization mechanisms to be more effective at higher degrees of melting, an origin by partial melting of eclogite seems to be the most likely source for massive plutonic trondhjemite-tonalite contributions to the juvenile continents. Such a source is consistent with the generation of trondhjemite-tonalite protocontinental cores in any number of plausible Archean tectono-thermal scenarios, though not necessarily in a conventional subduction-zone setting.

The Late Archaean to Early Proterozoic lead isotopic evolution of the Northern Baltic Shield of Norway, Sweden and Finland

The Late Archaean rocks of the Northern Baltic Shield comprise high-grade gneiss terrains and granite-greenstone terrains. The high-grade gneiss terrains consist mainly of older upper-crustal material which became metamorphosed during the Lopian orogeny (ca. 2.9–2.6 Ga). In contrast, the ∼2.7 Ga-old granite-greenstone terrains comprise mainly mantle-derived materials, At the southern and western edge, the Archaean craton became partially remobilized during the Svecofennian orogeny (2.0–1.75 Ga). This orogeny represents a major crust-forming event in the Baltic Shield, during which mantle material was added to the crust and older material was remobilized. Whole-rock, galena and sulfide trace lead are used to outline lead provinces on the Northern Baltic Shield. The Archaean lead from the Baltic Shield defines three secondary lead isochrons at ∼ 2.65 Ga with different initial lead compositions which reflect average pre-2.65 Ga μ 1 ratios of 8.0, 8.1 and 8.3, respectively. The variation of the initial lead composition of the high-grade gneisses and tonalites (Lofoten and Koitelainen area) probably indicates (1) variable crustal residence time and/or (2) different degrees of crustal contamination/assimilation before the 2.65 Ga metamorphic event, and the lead from the Eastern Finland greenstone belts indicates a heterogeneous mantle with respect to U/Pb before 2.65 Ga. Lead in Proterozoic rocks of the different lead provinces defines sets of subparallel mixing fields which can be modelled by a ≈2.65 Ga lead component and a ∼ 1.8 Ga lead component. The parallel offset of these fields reflects the pre-2.65 Ga μ 1 ratio of these terrains, i.e. it reveals the pre-2.65 Ga μ 1 of the involved Archaean rocks. Proterozoic terrains with involvement of Archaean lower-crustal lead show a poor correlation in the 206Pb/ 204Pb- 208Pb/ 204Pb diagram, whereas the other Proterozoic terrains show a good correlation in the 206Pb/ 204Pb- 208Pb/ 204Pb diagram.

Archaean sedimentation on the Kaapvaal craton in relation to tectonism in the granite-greenstone terrains: geophysical and geochronological constraints

The Kaapvaal craton served as depository for Archaean sedimentary sequences such as the Fig Tree, Moodies, Dominion, Witwatersrand, Pongola and Ventersdorp. Geophysical models for the greenstone belts and the Limpopo belt in combination with age constraints on these tectonic entities and on the sedimentary sequences, are analysed. It is concluded that although none of the present age data in the Limpopo belt provide direct information on the joining of the Zimbabwe and Kaapvaal cratons, it is clear that the final overthrusting event coincided with Ventersdorp volcanism. The final Limpopo orogenesis occurring at about 2700 M.a. is too young to be considered as a source region for the Witwatersrand sediments. The Witwatersrand and Pongola successions are coeval and coincide with the formation of granites in the ca.3000 to 2800 M.a. age bracket. The compiled geochronologic data set shows that the typical duration of a sedimentary cycle in the Archaean is less than 100 M.a. (e.g. Fig Tree, Dominion, lower Witwatersrand and Pongola sequences) which is comparable to that of Phanerozoic sequences. Where the successions studied appear to have much longer histories, this seems to relate to inaduquate geochronological control.

Regional metamorphic controls on alteration associated with gold mineralization in the Eastern Goldfields province, Western Australia: Implications for the timing and origin of Archean lode-gold deposits

Alteration assemblages associated with Archean epigenetic gold mineralization in the Menzies-Kambalda area of the Yilgarn craton, Western Australia, vary systematically with regional metamorphic grade, up to and including upper amphibolite facies rocks. K-metasomatism is recorded by the presence of muscovite, biotite, and K-feldspar at progressively higher metamorphic grades. High metamorphic grades are commonly developed in broad thermal aureoles around fate syntectonic granitic intrusions. Metamorphic recrystallization of alteration assemblages is common in the thermal aureoles. These relations suggest broad contemporaneity among granitoid intrusion, regional metamorphism, and mineralization during the final stages of the tectonic evolution of the granite-greenstone terrain. Late syntectonic granitoids acted as centers of heat and fluid flux in large-scale, synmetamorphic hydrothermal systems that deposited gold, possibly from modified mantle-derived fluids.

An early thrust duplex in the Kalgoorlie-Kambalda greenstone belt, Eastern Goldfields Province, Western Australia

Extensive regional mapping of the stratigraphic sequence throughout the Kalgoorlie - Kambalda greenstone belt in Western Australia has outlined the geometry of early subhorizontal thrust faults. These thrusts stacked and repeated the stratigraphy to form a major thrust duplex south of Kambalda. The roof thrust to the duplex becomes a major thrust flat to the north at Kalgoorlie. Later transpressional deformation (F2 upright folds, D3 sinistral shear zones and folding) has rotated and displaced the D1 thrusts. Both sub-horizontal deformation and subsequent transpression indicate that compressional tectonics, rather than vertical tectonics involving solid-state granitoid diapirs, were crucial in the development of this granite-greenstone terrain.

The northern margin of the Capricorn Orogen, Western Australia—an example of an Early Proterozoic collision zone

The Early Proterozoic (2200-1600 Ma) Capricorn Orogen is a major zone of deformation, metamorphism and granite emplacement between the Archaean Yilgarn and Pilbara Cratons of northwest Australia. Capricorn Orogen structures are recognized along the southern margin of the Pilbara Craton deforming the Archaean granite-greenstone terrain of the Sylvania Inlier together with rocks of the Hamersley, Ashburton and Blair Basins. In the Sylvania Inlier deformation is characterized by large-scale shear zones interpreted as NNE-directed thrusts. In the Hamersley Basin, orogenic deformation forms the Ophthalmia Fold Belt and can be divided into two groups of structures: in the southwest large-scale dome-and-basin folds (pre-Ashburton Basin) are present; in the southeast folds and faults are part of a younger (syn-Ashburton Basin) N-directed foreland fold and thrust belt directly linked to deformation in the Sylvania Inlier. The deformed Ashburton Basin and adjacent parts of the Hamersley Basin form the Ashburton Fold Belt. Two periods of deformation are recognized: early recumbent folding ( D 1a, post-Ashburton Basin-pre-Blair Basin), and later dextral wrench faulting and associated folding ( D 2a, syn- or post-Blair Basin). Capricorn Orogen structures are interpreted as the result of an oblique continent-continent collision between the Pilbara and Yilgarn Cratons. Along the southern Pilbara margin, collision first occurred in the southeast and migrated northwest, with the Ashburton Basin evolving as a foreland basin. Late-stage dextral wrench faulting was the result of westward extrusion of material caught between the opposing craton margins.

A two-stage model for the formation of the granite-greenstone terrains of the Kalgoorlie-Norseman area, Western Australia

Accurate dating of granites and greenstones from the Kalgoorlie-Norseman area shows that there is considerable overlap in time between the formation of granites and greenstones. However, within any given area, the oldest granites are always younger than the oldest greenstones. It is suggested that the long-lived (100 Ma) thermal anomaly in the uppermost asthenosphere that gave rise to the greenstones is also responsible for regional metamorphism and widespread crustal melting, leading to the production of granites. The time lag between greenstone and granite formation reflects the time taken to conduct heat from the mantle to the lower crust. Anatexis of the lower crust produces a layer of light granitic magma that may temporarily stop the ascent of basaltic magma. Where this happens the basaltic magma ponds at the base of the crust, supplying additional heat and accelerating the melting process. Eventually the granitic melt layer becomes unstable and rises within the crust. Areas intruded by granite become topographic highs, with topographic lows forming above areas where light melt has been withdrawn from the lower crust. When the topographic highs protrude above sea-level they are eroded to shed sediments into the topographic lows which contain the basaltic greenstones. Thus the change from a basalt-dominated lower greenstone succession to a sediment and felsic volcanic-dominated upper whitestone sequence, seen in many granite-greenstone terrains, results from the intrusion of granitic magma into the upper crust. The crust of the Kalgoorlie-Norseman area formed in two distinct stages, separated by a time break of up to 700 Ma. The first stage involved the formation of primitive crust, probably above subduction zones. This primitive crust was characterized by low Rb/Sr and high Sm/Nd ratios. The second stage resulted from a major change in mantle convection that produced thermal upwellings in the mantle below the continents. This led to the eruption of basaltic magmas and to metamorphism and anatexis of the lower crust to produce granitic magmas. Melting reset the Rb/Sr, Sm/Nd and U/Pb systems, giving the impression that the second event, which occurred at ∼ 2.7 Ga, was a major crust-forming event. It was, in fact, a crustal reworking process although some basalt was added to the crust at this time. Although the two-stage history of the Kalgoorlie-Norseman area is clearly seen in the lead isotopes, and in zircons, it is not readily discernible from the Rb Sr and Sm Nd systematics, due to mantle-like ratios of those elements in the primitive crust.

Structural deformation and mineralization in the early Proterozoic Liaojitite suite, Eastern Liaoning Province, China

The Liaojitite Suite, located in eastern Liaoning Province, China, formed in the eugeosyncline of an Early Proterozoic mobile zone. Structural deformation occurred during several orogenic episodes, especially during the second deformation of the third episode, the Luliang Orogeny, 1900–2000 Ma ago. Superposed dome-shaped structures and their unevenly distributed groups formed during the Luliang Orogeny constitute the principal tectonic style in the mobile zone. The tectonic style may be considered transitional in the evolution from an oval style in Archean granite-greenstone terrains to a linear style in Phanerozoic terrains. The superposed dome-shaped structures contain the Liaoji Granite in the cores, ringed successively by the Boron-bearing Series and the outer Turbidite Series. The simple appearances of the dome-shaped structures mask their complicated, multiply deformed nature, which is revealed by tracing their formation and evolution from primary volcanic domes to granitic diapirs and finally to superposed dome-shaped structures. The evolution of the superposed dome-shaped structures represents a change in crustal activity from simple shear to horizontal compression. Mineralization in the Liaojitite Suite was closely related to the three deformations of the Luliang Orogeny. Metallogenic elements were concentrated during the first deformation; physical re-emplacement of orebodies occurred during the second and third deformations.

Early archaean narryer gneiss complex, Yilgarn Craton, Western Australia

An outline is given of the Narryer Gneiss Complex that forms the northwest part of the Yilgarn Craton. It consists of two major groups of quartzo-feldspathic gneiss: Meeberrie gneiss mainly derived from ∼ 3680 Ma monzogranite, and Dugel gneiss derived from ∼ 3400 Ma syenogranite and monzogranite. The gneisses contain inclusions of an ∼ 3730 Ma layered gabbro-anorthosite intrusion called the Manfred Complex and are tectonically interleaved with metasedimentary rocks, mainly quartzite and banded iron formation. The complex was deformed and metamorphosed in granulite facies about 3350-3300 Ma ago. The gneiss complex is in tectonic contact with a granite-greenstone terrain that forms most of the Yilgarn Craton. These terrains were juxtaposed, intruded by sheets of granite, intensely deformed and metamorphosed about 27002600 Ma ago. The Narryer Gneiss Complex is a substantial addition to known areas of early Archaean crust. In contrast to most previously described terrains of this antiquity, it is largely derived from granite, it is devoid of metavolcanic rocks, and metasedimentary rocks are predominantly siliceous. The latter contain detrital zircons with U-Pb ages of 4275-4000 Ma, far older than any known terrestrial rocks. These features all suggest the former presence of a substantial amount of very old sialic crust (> 3700 Ma) and that remnants of it may still survive in this vast complex.

K-feldspar PbPb isotope systematics of Archaean post-kinematic granitoid intrusions of the diemals area, central Yilgarn block, Western Australia

The initial Pb isotopic composition of voluminous syn- to post-kinematic granitoids in the Archaean granite-greenstone terrain of the central Yilgarn Block should provide important constraints of crustal evolution within the previously established geological and temporal framework of the region. The initial Pb isotopic composition was sought from K-feldspars in samples of the intrusions at Kellys Rock and Milky Soak in the Diemals area of the Southern Cross Province. Most of the whole-rock samples studied show substantial recent U loss, and K-feldspars appear to have been similarly affected showing decoupled removal of U and Pb during acid leaching of mineral separates. These processes preclude the determination of primary U/Pb in the K-feldspars and do not allow a meaningful correction for in situ U decay since formation of the mineral. In addition, post-emplacement remobilisation of Pb is suspected at Kellys Rock. Whole-rock PbPb geochronology of the Kellys Rock intrusive yields an age of 2687 ± 76 Ma, which is synchronous with the combined whole-rock, mineral and leach PbPb isochron age of 2693 ± 7 Ma for Milky Soak. The precision of the latter result was due to the radiogenic nature of the Pb leached from K-feldspar separates and whole-rock powders and good linear coherence of all data. The syn- to post-kinematic intrusives of the Diemals area appear to have formed at 2723 ± 25 Ma, with 2693 ± 7 Ma representing the minimum age of the tectono-thermal event deforming the greenstones and granitoids in the area.

Upb zircon ages bearing on the nature of early archaean greenstone belt evolution, barberton mountainland, southern africa

Abstract UPb zircon systematics for granitic and gneissic clasts from the Moodies conglomerate of the Barberton Greenstone Belt, South Africa, exhibit discordancy patterns that suggest variable and multi-episodic lead loss and indicate crystallisation ages for the parent granitoids between 3.3 and 3.47 Ga. Metamict zircons in some of these clasts indicate complete resetting of the older material or new growth at about 2.8–2.9 Ga ago that is ascribed to a regional thermal event as also shown by published RbSr data for a nearby granitoid pluton. Since the Moodies clasts consist largely of potassic granites our ages suggest the presence of highly evolved continental crust in the source region of the greenstone belt sediments, a feature that contradicts the commonly accepted model for the primitive nature of the granitoid crust in early Archaean granite-greenstone terrains. The maximum age of 3.3 Ga for the Moodies Group also shows that greenstone belt evolution in the Barberton region took place over a period of at least 150 Ma from about 3450 to 3300 Ma ago. The Kaap Valley and Stentor plutons intrude the lower part of the greenstone succession, and their diapiric emplacement caused large-scale deformation of earlier structures in the entire greenstone belt sequence at about 3.23–3.25 Ga ago. Detrital zircons in the sediments of the Fig Tree Group date the mean age of their source terrain at c. 3520 Ma which is also the maximum age of deposition for these strata. This age is indistinguishable from the oldest age reported from the Barberton greenstones and suggests the possibility that pre-greenstone granitoid crust may exist in the eastern Kaapvaal craton.

High-grade terrains in and around the Yilgarn Block of Western Australia

Summary Two kinds of high-grade terrain occur in the western part of the Archaean Yilgarn Block. One consists of repeatedly deformed and metamorphosed granites (3650 and 3500 Ma), layered basic rocks (3750 Ma), and siliceous metasedimentary rocks, which were metamorphosed to granulite facies at 3300 Ma. The other consists of much less deformed granites and some basic rocks which form deeper, granulite-facies equivalents of the adjacent high-level 3000–2600 Ma granite-greenstone terrain. The Yilgarn Block is bounded by Proterozoic belts of intense deformation and high-grade metamorphism. These belts were active between 2000 and 1600 Ma and comprise both reworked Archaean and early Proterozoic rocks. Two belts were mobile again and metamorphosed to granulite or amphibolite facies between 1300 and 1000 Ma, and one of these belts was also intruded by plutonic rocks, deformed and metamorphosed to granulite facies at about 650 Ma.

Igneous and Tectonic Evolution of the Batchawana Greenstone Belt, Superior Province: A U-Pb Zircon and Titanite Study

U-Pb isotopic dating of zircon and titanite from all the major litho-tectonic units of the Batchawana belt, an Archean greenstone belt of the Abitibi Subprovince of the Superior Province in Canada, shows that the belt evolved during a period of about 60 Ma between about 2730 and 2670 Ma ago. The earliest date of 2729 + 3/-2 Ma was found in a predominantly tholeiitic metavolcanic sequence the base of which is intruded by a tonalite dated at 2716 ± 2 Ma. Younger ages of 2720 ± 10 Ma, 2709 ± 2Ma,2711 ± 2Ma,2701 ± 2Ma, and 2698 ± 2 Ma date rocks at increasingly higher stratigraphic levels of a mixed tholeiitic-calc-alkalic metavolcanic sequence which is itself overlain by a thick succession of clastic metasedimentary rocks. Subsequent deformation of the supracrustal sequences produced isoclinal folding and culminated in metamorphism ranging from lower greenschist to amphibolite facies and anatexis related to the intrusion of syn- to late-tectonic plutons, four phases of which have ages of 2678 +4/-2Ma, 2677 ± 2 Ma, 2677 ± 3 Ma, and 2676 ± 2 Ma. Two post-tectonic granitoid plutons in the center of the belt were intruded 2674 ± 3 Ma and 2673 ± 5 Ma ago and were followed by the emplacement of a composite mafic to felsic intrusion; a monzonite and a hornblendite from this intrusion yield identical ages of 2668 ± 2 Ma. Titanite ages are identical or younger than the ages of coexisting zircons and reflect regional metamorphism and post-tectonic plutonism, but in a few cases they are younger and may record increased fluid activity along faults and the intrusion of mafic dikes. U-Pb zircon systematics, together with age and lithological relationships, suggests that the greenstone belt formed in an oceanic environment from material derived initially mainly from the mantle. Subsequent melting at the base of the thickening volcanic succession produced intermediate to felsic volcanic rocks, tonalites, and later granodioritic to granitic plutons leading to the final consolidation of the granite-greenstone terrain.

Relation between archaean high-grade gneiss and granite-greenstone terrain in western Australia

Two major regions of Archaean crust occur in Western Australia called the Pilbara and Yilgarn Blocks. All the Pilbara and most of the Yilgarn Block consist of granite-greenstone terrain with metamorphic grades in greenschist or low amphibolite facies, but these terrains developed at different times in the two regions: 3500–3000 Ma ago in the Pilbara and 3000–2600 Ma ago in the Yilgarn. High-grade gneisses are only known to occur in a belt along the western margin of the Yilgarn Block in a region called the Western Gneiss Terrain. Direct observations of the relation between this high-grade gneiss terrain and typical granite-greenstone terrain can therefore only be made in this region. Two kinds of relation are seen. In the north, the high-grade gneiss terrain developed between 3800–3300 Ma ago. It is distinctly older than, and contains a different assemblage supracrustal rocks from, the granite-greenstone terrain. The gneiss terrain could have formed a sialic basement to the greenstones but direct evidence of this relation is not preserved. The gneiss terrain is intruded by granites similar to those of the granite-greenstone terrain and the two terrains are deformed and metamorphosed together. Prograde metamorphism of the granite-greenstone terrain increases from mainly greenschist facies in the east, to high amphibolite facies in the west, where it is associated with retrograde metamorphism of the gneiss terrain from granulite to high amphibolite facies. In the south, a portion of the Western Gneiss Terrain also consists of gneisses which pre-date the granite-greenstone terrain. However, most of the gneiss terrain consists of younger granitoid intrusions, similar to those that form a major component of the granite-greenstone terrain, but metamorphosed in granulite facies. As in the north, a regional eastward decrease in metamorphic grade suggests that the exposed crustal section is tilted eastwards. The tectonic significance of the high-grade gneiss and granite-greenstone terrains remains uncertain. Regional relationships on the scale of the Yilgarn Block suggest three possible models: (1) cratonic basinal model, whereby large greenstone basins formed on gneissic basement; (2) continental marginal model, whereby greenstone terrains developed adjacent to pre-existing protocontinent; or (3) collision involving two unrelated segments of crust.

Geological evolution of the karelian granite-greenstone terrain

Abstract The Karelian craton ( > 2.3 Ga ) has been recognized within the Baltic Shield. The basement of the craton is a late Archaean granite—greenstone terrain. The greenstone belts from different regions of Karelia show variations in age and composition of volcanic rocks. Two older belts ( > 2.9 Ga ) - one with multimodal volcanism and the other with bimodal volcanism - constitute the eastern part of the terrain (central and eastern Karelia). Two younger belts (2.8–2.6 Ga) with the same modal distributions of volcanic rocks make up the western part of the terrain (western Karelia and eastern Finland). According to magnetic anomaly patterns there are four types of granite—gneiss areas that separate the greenstone belts. Three types represent the greenstone basement reworked at different degrees. The fourth type corresponds to tonalites and granites surrounding the greenstone belts. The emplacement and evolution of the tonalites and granites are related to the evolution of the greenstone belts. Geochronological data on the granites of the basement demonstrate a decrease in age from E to W. Late Archaean low-pressure granulite metamorphism took place almost synchronously (2.65 Ga ago) in both the western and eastern domains.