Archean Granite-greenstone Terranes Research Articles

Formation of early Archean Granite-Greenstone Terranes from a globally chondritic mantle: Insights from igneous rocks of the Pilbara Craton, Western Australia

The continental crust grows via juvenile additions from the mantle. However, the timing of initial continent stabilisation and the rate of subsequent continental growth during the first billion years of Earth history is widely debated, in part due to uncertainty over the composition of the mantle source of new crust. Well-preserved Archean granite-greenstone terranes, as present within the Pilbara Craton (Western Australia), provide insights into the sources of felsic magmas and the processes of continental growth and evolution in the distant geological past at the regional scale. Here, we present zircon U-Pb, O and Hf isotope data from ancient gneissic and granitic rocks of the Pilbara Craton, to decipher magma sources and the timing and processes of craton growth. There is no evidence for depleted mantle compositions, and the simplest interpretation is that the crust of the Pilbara Craton was generated from mantle with a chondritic Hf isotope composition. Our results indicate crustal addition at ~3.59 Ga, represented by emplacement of gabbroic to anorthositic rocks. We suggest that the formation of these igneous rocks, and the foundering of the complementary residues, triggered extensive melting of hot, upwelling mantle, leading to the subsequent accumulation of the >12 km thick greenstone belt eruptive sequences from 3.53 Ga, with emplacement of coeval felsic magmas at depth. This process shaped the initial crustal configuration of the proto-craton, which subsequently underwent gravitationally driven overturn and reworking to generate stable, cratonic continental crust with the distinctive dome and keel architecture. The zircon Hf and O isotope signatures of the Pilbara igneous rocks from ~3.59–3.4 Ga do not support remelting of an ancient (> 3.8 Ga) basement, and reinforce the overwhelmingly chondritic to near-chondritic zircon Hf isotope composition of Eoarchean meta-igneous rocks from a number of different Archean cratons. A corollary of this remarkable global consistency is that a significant volume of the mantle maintained a chondritic composition for the Lu-Hf system from the formation of the Earth into the Paleoarchean (up to 3.6–3.5 Ga), as would be the case if stabilised volumes of felsic continental crust prior to 3.5 Ga were relatively small. One implication is that the common assumption of a linear evolution of depleted mantle from 4.5 Ga to the present day is inappropriate for determining the timing and volume of continental crust extraction in the Archean. The nearly identical early evolution of the Pilbara and Kaapvaal cratons suggests a common process to generate Archean granite-greenstone terranes that does not require extensive reworking of ancient crust, but rather involves juvenile crustal addition above persistent zones of upwelling, chondritic mantle.

Geochronological constraints on the formation and evolution of the Huangling basement in the Yangtze craton, South China

The Huangling basement has the largest area and highest metamorphic grade of exposed Archean and Paleoproterozoic rocks in the Yangtze craton basement. Hence, it provides an excellent opportunity for examining the formation and evolution of the Yangtze craton. The Huangling basement consists mainly of the Kongling complex, with a chronotectonic framework established through the geochronological study of amphibolites, khondalites, ultramafic rocks, and diabase dikes. Our results indicate that magmatic events in the Huangling basement are divided into four phases; the first phase is represented by the emplacement of the intermediate–basic volcanic rocks (3.2–3.0 Ga) that formed a typical Archean greenstone belt. The greenstone belt was subsequently intruded by tonalite–trondhjemite–granodiorite rocks (3.0–2.9 Ga) and A-type granite (2.7–2.6 Ga) constituting a typical Archean granite–greenstone terrane. The granite–greenstone terrane was transformed into an Archean continental nucleus during the first metamorphism (2.6–2.5 Ga), resulting in an unconformity between Archean and Proterozoic strata. Stable cratonization and sedimentation are considered to have lasted for 500 Ma (2.5–2.05 Ga), during which sedimentary rocks with a thickness of 2500 m were deposited. Later, the sedimentary rocks were metamorphosed to form the khondalite series during the second metamorphic event at 2.05–1.9 Ga, which caused an angular unconformity between Paleoproterozoic and Mesoproterozoic strata. The Paleoproterozoic high-grade terrane and Archean continental nucleus simultaneously merged to form the Huangling basement. The Kongling complex was intruded by A-type K-feldspar granite and diabase dikes, which were emplaced at 1.87–1.85 Ga and had not undergone deformation and metamorphism, indicating that the formation and evolution of the Kongling complex and the Huangling basement ended before 1.87–1.85 Ga.

A Yilgarn seed to the Pilbara Craton (Australia)? Evidence from inherited zircons

Abstract Knowledge of the age and compositional architecture of Archean cratonic lithosphere is critical for models of geodynamics and continental growth on early Earth, but can be difficult to unravel from the exposed geology. We report the occurrence of numerous >3.7 Ga zircon crystals in 3.45 Ga rhyolites of the eastern Pilbara Craton (Western Australia), which preserve evidence for an Eoarchean meta-igneous component in the deep Pilbara crust. This inherited zircon population shares similar and distinctive age and Hf-O isotope characteristics with the oldest gneissic components of the Yilgarn Craton ∼500 km farther south, suggesting a common ca. 3.75 Ga felsic crustal nucleus to these two Archean granite-greenstone terranes. We infer a pivotal role for such ‘seeds’ in facilitating the growth and persistence of Archean continental lithosphere.

Early earth geodynamics: cross examining the geological testimony.

Many studies link the presence of continents on Earth to the operation of plate tectonics. Radiogenic isotope data have, however, long consigned the bulk of crust generation and preservation to the murky realm of the Precambrian Earth, where the prevailing geodynamic systems are highly uncertain due to the sparse and complex nature of the geological record of these early eons. The purpose of this paper is to examine the nature of this geological record, considering the biases and artefacts that may undermine its fidelity, and to assess what are the most robust lines of evidence from which meaningful geodynamic inferences can be drawn. This is pursued with reference to Hadean detrital zircons, Archean gneiss complexes and Archean granite-greenstone terranes, and by considering isotopic proxies of crust-mantle interaction. The evidence reinforces long held views that the formation of some of the oldest continental nuclei involved a distinctive mode of planetary geodynamics that rests uneasily within definitions of modern style plate tectonics. A detailed interrogation of the oldest rocks, integrating multi-scale information from the best preserved whole-rock and mineral archives, and emphasizing careful selection at the sampling and analytical stages, will lead to the most robust input data for petrological and thermodynamic models of early Earth processes.This article is part of a discussion meeting issue 'Earth dynamics and the development of plate tectonics'.

Application of Lidar to resolving tectonic and glacial fabrics in glaciated terrain: An example from an Archean granite-greenstone belt, northeastern Minnesota

In this study we examine an Archean granite-greenstone terrane in northeastern Minnesota (USA) to illustrate the application of high-resolution Lidar (light detection and ranging) altimetry to mapping regional tectonic fabrics in forested glaciated areas. We describe the recognition of lineaments and distinguish between tectonic and glacial lineament fabrics. We use a 1-m Lidar derived bare earth digital elevation model to construct multiple shaded-relief images for lineament mapping with sun elevation of 45° and varying sun azimuth in 45° intervals. Two suites of lineaments are apparent. Suite A has a unimodal orientation, mean trend of 035, and consists of short (

Neoarchean metagabbro and charnockite in the Yinshan block, western North China Craton: Petrogenesis and tectonic implications

Properly calibrating the magmatic record of mantle and crustal origin in Archean granite-greenstone terranes is crucial for understanding the petrogenetic and geodynamic processes that generated early continental landmasses. This geochronological and geochemical study documents a Neoarchean bimodal metaplutonic suite of metagabbro and charnockite from the Yinshan block, North China Craton. The meta-gabbroic rocks show an SiO2 range from 45.9 to 51.7% and high MgO content from 6.7 to 16.4%, with enrichment in Ba and light rare earth elements (LaN/YbN=3.11–5.99) and depletion in high field strength elements. Together with their enriched whole-rock Nd (ɛNd(t)=−0.23 to 1.49) and zircon Hf (ɛHf(t)=0.1–6.9) isotopic signatures relative to the Archean depleted mantle in the craton, these rocks are supposed to originate from the second-stage high-temperature partial melting of refractory depleted mantle that experienced prior basaltic magma extraction and subsequent metasomatism by subduction-related fluids. The associated charnockites range in SiO2 from 61.3 to 69.6% and exhibit a magnesian, calc-alkalic and metaluminous character, with variable Sr/Y and LaN/YbN ratios. These elemental features, plus their evolved isotopic compositions (ɛNd(t)=0.93–1.85, zircon ɛHf(t)=−0.3 to 2.5), are consistent with partial melting of newly underplated mafic lower crustal protolith. In combination with widespread occurrence of metasomatized lithospheric mantle-derived magmas (e.g., high-Mg basalts, sanukitoid suites) and juvenile crust-extracted potassic granites in the NCC during Late Neoarchean time, such a mafic and felsic magma association not only attests to the establishment of a craton-scale subduction-related metasomatized sub-continental lithospheric mantle, but also encapsulates a scenario of coupled lithospheric mantle-crust formation at ∼2.7Ga and juvenile crustal reworking at ∼2.5Ga within a modern-style convergent continental margin possibly featuring episodic slab break-off events.

Paleoproterozoic high δ13Ccarb marbles from the Ruwenzori Mountains, Uganda: Implications for the age of the Buganda Group

The Paleoproterozoic Buganda Group forms part of the arcuate Ruwenzori Fold Belt, in southern Uganda, separating the Archean granite–greenstone terranes of the Tanzanian craton from the Archean high-grade gneisses of the North Uganda terrane. It consists of a metasedimentary succession of mica schists, phyllites, quartzites, and rare marbles and calc-silicates, overlain by a thick succession of amphibolites and mafic metavolcanic rocks, including tholeiitic pillow lavas, that have a MORB-like composition. The volcano-sedimentary succession was once part of the ocean floor, and was later obducted onto the Tanzanian craton during the Ruwenzori orogeny at around 1.9Ga. The Ruwenzori Fold Belt and its rock protoliths are poorly dated, and the age of the volcano-sedimentary succession is only constrained between c. 2.13 and 1.99Ga. We report here on the stable isotope (C and O) geochemistry of marbles from the Ruwenzori Mountains in western Uganda, which has a bearing on the age of the Buganda Group.A suite of samples of impure dolomitic and calcitic marbles from the High Peaks Schist and the Kilembe Schist subgroups, Buganda Group, was analyzed. The marbles, which are metamorphosed to upper amphibolite grade, show a wide spread of δ13Ccarb values, ranging from +11.3 to −9.3‰ VPDB, and a range of δ18O values from −9.9 to −24.0‰ VPDB. There is a generally positive correlation between δ13C and δ18O values in the marbles as well as between δ13C and Mn/Sr ratios. Six of the 12 measured samples from the High Peaks Schist Subgroup have anomalously high δ13Ccarb values ranging between +11.3 and +10.0‰ VPDB.The Ruwenzori carbonate rocks are interpreted to have been deposited during the c. 2.22–2.06Ga Lomagundi positive carbon isotope excursion in seawater composition, and these rocks were subsequently affected by regional metamorphism during the c. 1.9Ga Ruwenzori orogeny, contributing to the wide range of δ13C and δ18O values found in this study. The Ruwenzori marbles, being interbedded in the lower part of the Stanley Volcanics Formation, also indicate an age for the volcanics between c. 2.22 and 2.06Ga, during the peak of the Lomagundi carbon isotope excursion. These age constraints are broadly consistent with a recently obtained age of c. 2.13Ga for a granulite in the basement for the Buganda Group.

Komatiite Magmas and Sulfide Nickel Deposits: A Comparison of Variably Endowed Archean Terranes

Komatiites are found in most Archean granite-greenstone terranes, but sulfide Ni mineralization associated with these rocks has a very biased distribution. The global endowment of sulfides Ni in Archean komatiites is overwhelmingly dominated by the Kalgoorlie terrane of the Eastern Goldfields Superterrane in the Yilgarn Craton. The question of whether the Kalgoorlie terrane komatiites possess any exceptional attributes which could explain this bias is addressed through an exhaustive compilation of geochemical data from this terrane and a number of others: the southeastern Youanmi terrane of the Yilgarn Craton; the eastern terranes of the Eastern Goldfields superterrane (Kurnalpi, Burtville and others); and the Abitibi greenstone belt of the Superior province. High MgO komatiite magmas, with MgO in the 25 to 30% range, are found in all the terranes sampled, but the proportion of these compositions appears to be higher in the Kalgoorlie terrane, and the abundance of strongly adcumulate olivine cumulates is much higher. Crustal contamination is apparently more extensive and advanced in the Kalgoorlie terrane than in all the others, but there is no systematic difference in the lithophile trace element (LILE) characteristics and degree of source depletion evident in the most primitive magmas from all the terranes. Consideration of variable-valency transition metals V and Cr indicates there are no systematic variations in oxidation state between the terranes and komatiites uniformly evolved close to the quartz-fayalite-magnetite buffer. Platinum-group element (PGE) variations imply that komatiites in all the terranes were emplaced sulfide undersaturated and were derived from sources with remarkably similar PGE contents. There is no evidence that the Kalgoorlie terrane magmas were in any way exceptional. The Forrestania and Lake Johnston Belts of the Youanmi terrane include the only known examples of Al-depleted komatiites hosting significant Ni sulfide resources, and they are also the only komatiites of this type that form adcumulate dunite bodies. The presence of adcumulate dunites, formed by high magma fluxes in central conduits, is the common feature between these belts and the Kalgoorlie terrane. Coupled with evidence for higher degrees of contamination in the Kalgoorlie terrane, and the availability of accessible crustal S sources, it appears that magma flux, rather than primitive magma composition, is the critical factor, and that craton-scale deep lithospheric structure is the ultimate control on the rate of magma supply between mantle source and crustal emplacement site. The Kalgoorlie terrane komatiites were emplaced at exceptionally high rates, giving rise to well-developed long-lived magma conduits, either lava tubes or subvolcanic channelized sills, which are interpreted to be the essential condition for forming large deposits.

Geochemical features of the Riphean fine-grained terrigenous rocks of the Southern Urals, Uchur-Maya region, and Yenisei Range: Estimation of the maturity of the pre-Riphean continental crust and its evolution within 1.65–0.6 Ga

The geochemical features of basal fine-grained terrigenous rocks from the Riphean sedimentary megasequences of the Southern Urals, Uchur-Maya region, and Yenisei Range were compared in order to estimate the maturity of the continental crust that was formed by the beginning of the Riphean. It was shown that initial shales from the base of the Riphean sequence of the Yenisei Range and fine-grained aluminosiliciclastic rocks from the base of the Riphean sections of the Southern Urals were formed by the erosion of a rather mature continental crust. In contrast, fine-grained terrigenous rocks from the base of the Riphean of the Uchur-Maya region were derived from immature Late Archean protoliths or their Early Proterozoic analogs. The fine-grained terrigenous rocks of the three sedimentary megasequences show different variations in the (La/Yb)N ratio. In the Southern Urals, this ratio is high (12–15) in the Burzyan Group and decreases upsection to 6–10. In the shales of the Uchur-Maya region, the (La/Yb)N ratio decreases upsection, and the La/Sc ratio shows a sympathetic behavior. This is due to a decrease in the proportion of “primitive” tonalite-trondhjemite associations of the Archean granite-greenstone terranes in the provenance area with time and the appearance of intra-plate (riftogenic?) granitoids and significant amounts of basic and ultrabasic rocks. The latter marks the onset of large rift-forming events in the Uchur-Maya region at the beginning of the Late Riphean. The (La/Yb)N of the studied rocks from the Yenisei Range are mostly similar to the PAAS ratio, but higher values were found in the Upper Vorogovka and Chingasan groups, which was related to the contribution of strongly LREE-enriched granitoids and rift felsic and alkali basaltic volcanic associations to the formation of the terrigenous material. A comparison of Rb, Sr, Y, Zr, Ba, Hf, Th, U, Cr, and Ni contents and Zr/Y, (La/Yb)N, Ni/Co, Cr/Th, Cr/Sc, and La/Th ratios in the fine-grained terrigenous rocks of the Riphean megasequences of the Southern Urals, Uchur-Maya region, and the Yenisei Range with those in the model geochemical objects (PAAS, UCPR1, UCAR2, and others) showed that, in terms of most of the parameters, the Riphean fine-grained terrigenous rocks from the three regions are similar to each other, PAAS, and Proterozoic cratonic shales. This indicates a fairly high general maturity of the protoliths that were eroded during the Riphean in the eastern East European craton and in the southeastern and southwestern parts of the Siberian craton.

Diamond deposits of the Siberian craton: Products of post-1200 Ma plume events affecting the lithospheric keel

The Siberian craton was affected by more voluminous plume events during last 1200 Ma than any other craton on the Earth. These events produced many economically important deposits, of which the tectonic setting of diamond deposits and related alkaline magmatism is analysed in this paper. In space and time, they can be grouped into several subprovinces: Meso- to Neoproterozoic Yenisei–Sayan; Late Devonian to Early Carboniferous Vilyui; Permo-Triassic Tunguska; Late Jurassic Olenek; and Late Jurassic to Early Cretaceous Aldan. Regardless of their age and subprovince affinity, the alkaline intrusions, including kimberlites, preferentially occur within the Archean granulite-gneiss terranes, forming a north–south-trending ‘Central Horde’, framed by Archean granite–greenstone terranes. These terranes in the basement of the Siberian craton constitute Tungus–Anabar and Aldan domains of similar composition, sinistrally offset for about 870 km. Despite such similarity, diamond deposits are discovered to date only in the Tungus–Anabar domain. The seismic data show that the Central Horde in the Tungus–Anabar domain has a lithospheric keel, extending to a depth of greater than 250 km, whereas it is absent in the Aldan domain. At the surface, the Central Horde forms an uplift that controls lithofacies of Riphean to Cenozoic sedimentary basins in the Siberian craton, thus representing a long-lived and relatively stable feature, with storage of the diamonds in the subcontinental lithosphere at depth. Previous direct dating of diamonds from different Siberian kimberlites indicated that they were formed in Archean to Paleoproterozoic times. The reconstructions suggest that, in Meso- to Neoproterozoic times, the Siberian craton might have formed part of the supercontinents Columbia and Rodinia. Within them, Siberian craton was attached by its northern edge to the present western margin of the North American craton, whereas the southern margin of Siberia might have been facing a present northern margin of Australia. Together, they were part of a very long supercontinent, and plotting of all presently globally known diamond deposits shows that they would all occur along its axis, mapping its then possibly single lithospheric keel, or a Super-Horde. After breakup of Rodinia, due to penetration of the spreading ridges between the Australian and Siberian cratons, Siberia was translated towards Eastern Europe for about 5000 km during 500 Ma. It was during this translation that it was periodically affected by Neoproterozoic to Mesozoic plumes, which delivered the diamonds to the surface into all subprovinces except Aldan.

Central Antarctic provenance of Permian sandstones in Dronning Maud Land and the Karoo Basin: Integration of U–Pb and TDM ages and host-rock affinity from detrital zircons

In conjugate SE Africa and Antarctica, Early Permian sandstones of the Swartrant Formation of the Ellisras Basin, Vryheid Formation of the Karoo Basin, and Amelang Plateau Formation of Dronning Maud Land (DML) were deposited after Gondwanan glaciation on a westward paleoslope. We analysed detrital zircons for U–Pb ages by a laser ablation microprobe-inductively coupled plasma mass spectrometer (LAM-ICPMS) and attached age significance only to clusters of three or more overlapping analyses. We analysed Hf-isotope compositions by a multi-collector spectrometer (LAM-MC-ICPMS) and trace elements by electron microprobe (EMP) and ICPMS. These analyses indicate the rock type and source (whether crustal or juvenile mantle) of the host magma, and a “crustal” model age ( T DM C). The integrated analysis gives a more distinctive, and more easily interpreted, picture of crustal evolution in the provenance area than age data alone. Zircons from the Ellisras Basin are aged 2700–2540 Ma with minor populations about 2815 Ma and 2040 Ma, which correspond with the ages of the upslope parts of the proximal Kaapvaal Craton and Limpopo Belt. Mafic rock is the dominant host rock, and it reflects the Archean granite–greenstone terrane of the Kaapvaal Craton. The three Karoo Basin samples and the two DML samples have zircons with these common properties: (1) 1160–880 Ma, host magma mafic granitoid (< 65% SiO 2) derived from juvenile depleted mantle sources ( ε Hf positive) at 1.65 Ga and 1.35 Ga, with T DM C of 2.0–0.9 Ga; (2) 760 to 480 Ma, host magma granitoid and low-heavy rare earth element rock (?alkaline rock–carbonatite), derived from mixed crustal and juvenile depleted mantle sources ( ε Hf positive and negative) at 1.50 Ga and 1.35 Ga, with T DM C of 2.0–0.9 Ga. Together with similar detrital zircons in Triassic sandstone of SE Australia, these properties reflect those in upslope central Antarctica, indicating a provenance of ∼ 1000 Ma (Grenville) cratons embedded in 700–500 Ma (Pan-Gondwanaland) fold belts. Detrital zircons in Cambrian sediments of the Ellsworth–Whitmore Mountains block and Cambrian metasediments of the Welch Mountains with comparable properties suggest that the central Antarctic provenance operated also in the ∼ 500 Ma Cambrian.

The Archean BIF-hosted Cuiabá Gold deposit, Quadrilátero Ferrífero, Minas Gerais, Brazil

The Cuiabá Gold Deposit is located in the northern part of the Quadrilátero Ferrífero, Minas Gerais State, Brazil. The region constitutes an Archean granite–greenstone terrane composed of a basement complex ( ca. 3.2 Ga), the Rio das Velhas Supergroup greenstone sequence, and related granitoids (3.0–2.7 Ga), which are overlain by the Proterozoic supracrustal sequences of the Minas (< 2.6–2.1 Ga) and Espinhaço (1.7 Ga) supergroups. The stratigraphy of the Cuiabá area is part of the Nova Lima Group, which forms the lower part of the Rio das Velhas Supergroup. The lithological succession of the mine area comprises, from bottom to top, lower mafic metavolcanics intercalated with carbonaceous metasedimentary rocks, the gold-bearing Cuiabá-Banded Iron Formation (BIF), upper mafic metavolcanics and volcanoclastics and metasedimentary rocks. The metamorphism reached the greenschist facies. Tectonic structures of the deposit area are genetically related to deformation phases D 1, D 2, D 3, which took place under crustal compression representing one progressive deformational event (E n). The bulk of the economic-grade gold mineralization is related to six main ore shoots, contained within the Cuiabá BIF horizon, which range in thickness between 1 and 6 m. The BIF-hosted gold orebodies (> 4 ppm Au) represent sulfide-rich segments of the Cuiabá BIF, which grade laterally into non-economic mineralized or barren iron formation. Transitions from sulfide-rich to sulfide-poor BIF are indicated by decreasing gold grades from over 60 ppm to values below the fire assay detection limit in sulfide-poor portions. The deposit is “gold-only”, and shows a characteristic association of Au with Ag, As, Sb and low base-metal contents. The gold is fine grained (up to 60 μm), and is generally associated with sulfide layers, occurring as inclusions, in fractures or along grain boundaries of pyrite, the predominant sulfide mineral (> 90 vol.%). Gold is characterized by an average fineness of 0.840 and a large range of fineness (0.759 to 0.941). The country rocks to the mineralized BIF show strong sericite, carbonate and chlorite alteration, typical of greenschist facies metamorphic conditions. Textures observed on microscopic to mine scales indicate that the mineralized Cuiabá BIF is the result of sulfidation involving pervasive replacement of Fe-carbonates (siderite–ankerite) by Fe-sulfides. Gold mineralization at Cuiabá shows various features reported for Archean gold–lode deposits including the: (1) association of gold mineralization with Fe-rich host rocks; (2) strong structural control of the gold orebodies, showing remarkable down-plunge continuity (> 3 km) relative to strike length and width (up to 20 m); (3) epigenetic nature of the mineralization, with sulfidation as the major wall–rock alteration and directly associated with gold deposition; (4) geochemical signature, with mineralization showing consistent metal associations (Au–Ag–As–Sb and low base metal), which is compatible with metamorphic fluids.

Petrology and geochemistry of rocks from the basement of the Pechenga paleorift

The basement of the Early Proterozoic Pechenga paleorift was penetrated by the Kola Superdeep Borehole over a depth interval of 6842-12,261 m. The principal types of basement rocks are gneisses of trondhjemite-tonalite composition in association with amphibolites, gneisses containing aluminous minerals, and banded iron formation (BIF). It was demonstrated that the paleorift basement and rock associations exposed at the surface in the northwestern part of the Kola-Norwegian block are of similar age, composition of their protoliths, and correspond to the main types of sedimentary- volcanic and plutonic rocks of the Archean granite-greenstone terrane, which was broken up into separate segments in the Early Proterozoic. Rocks of the Pechenga Archean basement were aected by Proterozoic magmatism and metasomatism related to the rift development. The most intense Proterozoic processes in the Pechenga basement and its nearest surroundings were intrusion of numerous mafic-ultramafic bodies, retrograde metamorphism to the medium-low-temperature amphibolite and epidote amphibolite facies, synmetamorphic migmatization, and emplacement of postkinematic granite dikes. Most Proterozoic processes were determined to have been related to mantle sources. The overall amounts of Proterozoic material introduced into the Archean rocks penetrated by the lowermost part of the borehole plus the remobilized Archean crustal material were estimated at 30% ( 12-15% amphibolites, 3% granite veins, and 15% migmatized rocks).

Gold deposits and occurrences of the Crixás Goldfield, central Brazil

The Crixas Goldfield is a part of the Tocantins province and consists of a ~2.9 Ga Archean granite–greenstone terrane. The gold field contains one large deposit (Mina III, 65 tons Au) and several smaller occurrences, which are structurally controlled by regional shear zones that are either thrust or strike-slip faults. The deposits can be classified as auriferous massive sulfide (pyrrhotite + arsenopyrite) hosted ores within sulfidized banded iron formations, auriferous quartz veins, and disseminated gold ores in carbonaceous schists. On a more local scale, the ore bodies coincide with dilation zones surrounded by hydrothermal alteration haloes of various widths within carbonaceous schists, iron formations, and metabasalts. Hydrothermal alteration related to deposits within thrust faults consists of sulfidization and carbonatization, whereas, within strike-slip faults, propylitic, potassic, and sericitic alteration zones occur in concentric haloes. In both cases, quartz veins and veinlets may be abundant. Absolute ages of mineralization are uncertain, but structural data suggest multiple events probably occurred during Archean, Paleoproterozoic and Neoproterozoic tectonic events.

Geometry and kinematics of large arcuate structures formed by impingement of rigid granitoids into greenstone belts during progressive shortening

Regional-scale arcuate structures in the central part of the Archean Yilgarn craton, Western Australia, were generated by impingement of competent granitoid blocks into less competent greenstone belts during progressive east-west shortening. Sinistral shear zones developed along northwest-trending margins of the granitoid blocks, whereas dextral shear zones developed along northeast-trending margins. In apex regions, these northeast- and northwest-trending shear zones are linked by north-trending contractional zones along which shortening was accommodated by the formation of folds and reverse faults in the greenstone belts and a coaxial flattening fabric in granitoid rocks. Lateral escape of the greenstone belts is indicated by the progressive rotation of early macroscopic folds into parallelism with the strike-slip shear zones during granitoid impingement. This recently recognized deformation style may have important tectonic implications for other Archean granite-greenstone terranes and where rock heterogeneity and competency differences are significant.

U–Pb geochronological constraints for Paleoproterozoic evolution of the Core Zone, southeastern Churchill Province, northeastern Laurentia

The Core Zone of the Southeastern Churchill Province, northeastern Laurentia, is a composite of Paleoproterozoic lithotectonic domains assembled in the vise between obliquely colliding Archean Superior and North Atlantic cratons between ca. 1860 and 1810 Ma. Detailed geological and U–Pb geochronological studies of domains in the southern Core Zone highlight significant differences in Archean and Paleoproterozoic geology between domains, and constrain timing and models for Paleoproterozoic assembly. In the south-central part of the Core Zone, Crossroads and Orma domains consist of Late Archean granite–greenstone terrane crust. Crossroads domain also contains a significant number of granite–charnockite intrusions belonging to the 1840–1810 Ma De Pas batholith; magmatism pre-dates and overlaps with ca. 1820–1775 Ma high-grade metamorphism and attendant deformation. In marked contrast, Orma domain, which occurs to the east of Crossroads domain, contains only local evidence of Paleoproterozoic (De Pas related?) intrusions and appears to have mainly escaped Paleoproterozoic metamorphism and penetrative deformation. A model compatible with available data suggests the De Pas batholith is a continental magmatic arc formed above an east-dipping subduction zone. Paleoproterozoic metamorphism and deformation is concentrated in regions contiguous with arc magmatism. West of Crossroads domain, and separated from it by the Lac Tudor shear zone, McKenzie River domain is dominated by Archean orthogneiss (>80 m.y. older than gneisses in Crossroads and Orma domains), minor amounts of Paleoproterozoic supracrustal rocks (absent in domains to the east) and 1815 Ma tonalite. It does not contain De Pas intrusions. U–Pb geochronology suggests juxtaposition of the McKenzie River domain with domains to the east may have occurred around 1810 Ma. One possible model envisages the McKenzie River domain as a piece of reassembled Superior craton crust, incompletely rifted from the Superior margin during ca 2.17 Ga rifting. Domains occurring east of the Lac Tudor shear zone are interpreted to be exotic with respect to the Superior craton.

Laser 40Ar/39Ar thermochronology of Archean rocks in Yellowknife Domain, southwestern Slave Province: insights into the cooling history of an Archean granite-greenstone terrane

Laser 40Ar/39Ar ages for hornblende, muscovite, and biotite from Archean plutonic rocks of Yellowknife Domain, southwestern Slave Province, complement U-Pb ages from minerals with higher closure temperatures (zircon, monazite, titanite). Together, the data indicate that the plutonic suites of different ages (Defeat, Prosperous, Morose) coring three major thermal culminations in the domain have distinct Archean cooling histories until below approximately 250°C (biotite closure). The cooling trajectories for successive suites appear to document a decrease in cooling rate over the course of orogeny, possibly reflecting the build-up of heat within the orogen, coupled with a transition to lower rates of exhumation and (or) erosion. Greater structural relief over the Sleepy Dragon basement culmination may be responsible for slower cooling of the Morose Granite situated in its core. At or below the closure temperature of biotite the cooling curves converge with one another, documenting a transition from "localized" to "regional" cooling. The fact that cooling was locality specific until at least the biotite closure temperature suggests that, just as intrusion and related peak metamorphism were diachronous across the region, so was cooling to approximately 250°C. Uniform regional isotherms thus were not established until &gt;200 Ma after the youngest Archean plutonism. The variable disturbance of biotite Ar spectra, but generally not muscovite or hornblende, indicates that the Paleoproterozoic thermal disturbance of Archean rocks in Yellowknife Domain did not exceed 200-300°C.

Growth of granite–greenstone terranes at convergent margins, and stabilization of Archean cratons

Archean granite–greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events. Most Archean granite greenstone terranes appear to have acquired their first-order structural and metamorphic characteristics at convergent plate margins, where large accretionary wedges similar in aspect to the Chugach, Makran, and Altaids grew through offscraping and accretion of oceanic plateaux, oceanic crustal fragments, juvenile island arcs, rifted continental margins, and pelagic and terrigenous sediments. Buoyant slabs of parts of Archean oceanic lithosphere may have been underplated beneath these orogens, forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere. Back-stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges. Dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas (sanukitoid suite) in the mantle wedge, whereas other magmas (tonalite, trondhjemite, granodiorite or TTG suite) appear to have been generated by melting of hot young subducted slabs. Eventual collision of these juvenile orogens with other continental blocks formed anatectic granites, then thickened the crust beyond its ability to support its own mass, which initiated gravitational collapse and decompressional release of syn- to late-tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs, and aided in the cratonization of the granite–greenstone terranes. Deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid- and upper-crust, forming ore deposits and plutons. Most preserved granite–greenstone terranes have been tectonically stable since the Archean, and form the cratonic interiors of many continents.

Model for the development of kyanite during partial convective overturn of Archean granite–greenstone terranes: the Pilbara Craton, Australia

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 &amp; Summers (1983) and thermo‐mechanical models of Mareschal &amp; 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.

Laser &lt;sup&gt;40&lt;/sup&gt;Ar/&lt;sup&gt;39&lt;/sup&gt;Ar thermochronology of Archean rocks in Yellowknife Domain, southwestern Slave Province: insights into the cooling history of an Archean granite-greenstone terrane

Laser 40Ar/39Ar ages for hornblende, muscovite, and biotite from Archean plutonic rocks of Yellowknife Domain, southwestern Slave Province, complement U-Pb ages from minerals with higher closure temperatures (zircon, monazite, titanite). Together, the data indicate that the plutonic suites of different ages (Defeat, Prosperous, Morose) coring three major thermal culminations in the domain have distinct Archean cooling histories until below approximately 250°C (biotite closure). The cooling trajectories for successive suites appear to document a decrease in cooling rate over the course of orogeny, possibly reflecting the build-up of heat within the orogen, coupled with a transition to lower rates of exhumation and (or) erosion. Greater structural relief over the Sleepy Dragon basement culmination may be responsible for slower cooling of the Morose Granite situated in its core. At or below the closure temperature of biotite the cooling curves converge with one another, documenting a transition from "localized" to "regional" cooling. The fact that cooling was locality specific until at least the biotite closure temperature suggests that, just as intrusion and related peak metamorphism were diachronous across the region, so was cooling to approximately 250°C. Uniform regional isotherms thus were not established until >200 Ma after the youngest Archean plutonism. The variable disturbance of biotite Ar spectra, but generally not muscovite or hornblende, indicates that the Paleoproterozoic thermal disturbance of Archean rocks in Yellowknife Domain did not exceed 200-300°C.