Cratonic Domains Research Articles

Crustal structure of the northern Yukon and Mackenzie Delta, northwestern Canada

Travel time inversion and amplitude forward modeling have been applied to two seismic refraction profiles from the northern Yukon‐Mackenzie Delta region of northwestern Canada. The two‐dimensional crustal P wave velocity models feature a near‐surface layer which is 1–7 km thick and has an average velocity of 4 km/s; this overlies three crustal units, each having an average thickness of 11–15 km and with average velocities of 5.9, 6.1, and 7.1 km/s. The Moho is at ∼37 km with little relief and overlies an upper mantle with a poorly constrained velocity. Tectonically, the study area lies between cratonic and Cordilleran North America and adjacent Mesozoic polar continental margin. The velocity models clearly illustrate a domainal crustal structure in the study area. A cratonic domain is characterized by a middle and lower crust with homogeneous velocities of 6.6–6.8 km/s. The other domain (“Yukon domain”) is characterized by midcrustal velocities near 6 km/s and a lower crustal layer with velocities near 7.1 km/s. The transition zone between these domains is well‐defined and is interpreted as a Proterozoic paleocontinental margin, supporting previous interpretations based on geological trends and potential field data. Lateral homogeneity of the crustal velocity structure within Yukon domain supports interpretations that Arctic Alaska was not emplaced into its present position on strike‐slip faults. Local variations in lower crustal thickness, together with clear wide‐angle Moho reflections, suggest a lower crustal and Moho signature possibly related to rifting, crustal extension, and magmatic intrusion and underplating during the Jura‐Cretaceous development of the Arctic Ocean and polar continental margin.

Stratigraphy of the Southern part of the Sergipano Belt, NE Brazil : tectonic implications

The Sergipano Belt (NE Brazil) is a ESE-WNW trending volcano-sedimentary wedge polydeformed and metamorphosed (700-600Ma) due to the collision of the Pernambuco-Alagoas Massif, to the north, with the Sao Francisco Craton, to the south, during the Pan-African/Brasiliano orogeny. According to previous, regional-scale studies, the belt comprises three longitudinal lithotectonic domains with cratonic, miogeoclinal and eugeoclinal affinities, respectively from S to N, separated by regional, WNW-ESE trending, generally high-angle thrust-strike slip faults. Divergent tectonic evolution models were produced based on interpretations for and against the lateral continuity between these domains. Detailed stratigraphic-structural analysis from a 1:50,000 scale mapping carried out (1987-1995) in a 4000km2 area encompassing the interface between the three domains and surrounding two basement gneiss domes in the southern part of the belt, revealed: (1) the lithostratigraphy of the rocks deposited in the cratonic and miogeoclinal segments; (2) innequivocal evidence for the sedimentological, structural and metamorphic continuity across the domains boundary faults; (3) the upper section of the sediments deposited in the cratonic domain records the deposition of coarsening-upwards mudstones, siltstones, arkosic sandstones and lithic wackes that spreaded from the craton, to the south, across the craton-basin interface and graded into metasiltites and phyllites towards the miogeoclinal basin, where they occur in the core of a major basement-cored antiform and are overlain by a distinct diamictite formation, thus building up a siliciclastic mcgascquence. The sedimentological characteristics and innequivocal stratigraphic position of these arkosic sandstones and lithic wackes do not fit in previously suggested thrust-fold belt/foreland basin models; (4) very strong evidence for the miogeoclinal and eugeoclinal domains being also continuous across their boundary faults, allowing to erect a new stratigraphic template for the Sergipano Belt. The data allow to interpret that sedimentation was tectonicaly controlled by the basement domes and normal faults likely to have been inverted during the closure of the basin. The origin of the upper section of the sediments deposited in the cratonic domain and the tectonic controls of the sedimentation have relevant implications for the evolution of both the Sergipano Belt and the Sao Francisco Craton.

The importance of late- and post-orogenic crustal growth in the early Proterozoic: evidence from SmNd isotopic studies of igneous rocks in the Makkovik Province, Canada

SmNd isotopic studies suggest that large tracts of 1900–1700 Ma old crust in Laurentia-Baltica is of ‘juvenile’ origin. This crust has generally been ascribed to arc magmatism, sustained over long periods, and most conceptual models for crustal growth emphasize this process. The late- to post-orogenic granitoid rocks that areally dominate many ancient orogenic belts are commonly viewed as anatectic derivatives of the earlier arc-type crust. However, in regions of short crustal residence, the time resolution of isotopic tracers, such as the SmNd system, does not permit discrimination between this model and continued growth of the crust during post-orogenic magmatism. The relative contributions of recycled crust and new, mantle-derived material in late- to post-orogenic magmas can only be assessed where they also transect much older crustal blocks. The Nd isotopic signatures of 1800–1720 Ma igneous suites in the Makkovik Province define such a boundary between an Archean craton and a juvenile Proterozoic domain. In the juvenile domain, the Nd signatures of most igneous suites are equivocal (initial ϵ Nd = 0 to +2), and they could be anatectic derivatives of slightly older orthogneisses; however, addition of new, mantle-derived material is documented by ‘A-type’ granites with initial ϵ Nd up to +4. In the cratonic domain, temporally and compositionally equivalent igneous suites mostly have initial ϵ Nd of −7 to −3, significantly above local Archean basement, which has ϵ Nd of −15 at 1800 Ma. Conservative calculations suggest that most of these suites contain more than 50% new, mantle-derived material. These results suggest significant crustal growth via late-stage magmatism, and direct interaction of mantle-derived magmas and lower crustal rocks. In contrast, 1650 Ma igneous suites lack clear systematic variation in ϵ Nd, and are interpreted as representing crustal growth via later, distal, arc-type magmatism, with crust-mantle mixing mainly facilitated by sediment subduction. Crustal growth in this area thus involved significant addition of new material through late- to post-orogenic magmatism. This process may partially explain the unusually high growth rates calculated for some segments of Proterozoic crust, which approach or exceed modern global arc addition rates. This ‘stage 3’ crustal growth may have been proportionally more important in early Earth history due to higher ambient mantle temperatures.

Self-indentation of a heterogeneous continental lithosphere

The Neoproterozoic Ribeira-Aracuai belt stretches along the southeastern edge of the Sao Francisco craton of southeastern Brazil and extends south of the cratonic domain for >1000 km. The termination of the craton is spatially correlated with a significant modification of the deformation pattern in the belt: (1) the structural trend bends from due north to northeast, (2) the dominant tectonic flow shifts from orogen-transverse to orogen-normal, and (3) the metamorphic conditions of deformation decrease southwestward from high to medium grade. Finite-element modeling suggests that the presence of a craton within a continent favors strain localization, initiation of continental-scale shear zones, and differential vertical deformations. The southward termination of the Sao Francisco craton may have triggered the development of the complex deformation pattern that characterizes the Ribeira-Aracuai belt.

SÍNTESE DOS RECURSOS MINERAIS DO CRÁTON DO SÃO FRANCISCO E FAIXAS MARGINAIS EM MINAS GERAIS

The organization of the main mineral resources of Minas Gerais State, Brazil, in the São Francisco Craton and its marginal belts permits the establishment of mineral provinces, districts and fields. This has been performed, taking into account the types of resources, their distribution in time and space, and the similar geological contexts of their host rocks. The representation of these units at a scale of 1:2 500 000 is overlain on a base map of regional geological environments, and is the result of an extensive bibliographic survey focusing on this past decade.The Archaean granite-greenstone terrains in the cratonic domain are host to auriferous mineralizations, the subject of controversy regarding their syn- or epigenetic origins in the rocks of the Nova Lima Group, Quadrilátero Ferrífero. Gold- and sulphide-bearing quartz veins occur in rocks correlative to this Group, in the Pitangui-Pará de Minas, Mateus Leme and Conselheiro Lafaiete regions, respectively at the northwest and south of the Quadrilátero Ferrífero. The two former regions comprise agalmatolitic (massive pyrophyllitic) districts, and the latter is manganiferous. Mylonitic metaconglomerates of the Maquiné Group are mineralized in gold near Capanema. Chromitie boudinage lenses are associated with peridotites at Piumhi.The main mineralizations hosted in metasedimentary cover sequences of the Transamazonian cratonic basement are those of the Quadrilátero Ferrífero Itabirite District. It comprises extensive deposits of high-grade hematite, friable itabirites and weathered hematite, in the Minas Supergroup. Gold-bearing quartz and hematite boundins occur in shear zones affecting the itabirites. At Mariana, gold is associated with quartz veins along the contact between the Cauê Itabirite and underlying schists.Gold and uranium are present in paleoplacer metaconglomerates in the Gandarela and Ouro Fino Synclines. Quartz veins contain topaz near Ouro Preto. Graphitic Iodes occur at Itapecirica. Transamazonian tin-tantalum-lithium-niobium-bearing pegmatites crop out near São João del Rei.The Neoproterozoic cratonic cover hosts lead-zinc mineralizations in the Bambuí Group rocks in Januária region. Gas wells are present in places in the sediments of the Bambuí Group. In the Alto Paranaíba region, alluvial diamonds are, in part, considered to be derived from Precambriam metasediments.In the Araçuai Belt, chrome occurrences of the Serro meta-ultramafic bodies, with subordinate fold, are significant. Manganese is present in gondites of the Dom Silvério Group. Gold-related hydrothermal mineralization in shear zones is hosted in metavolcanosediments in the granite-gneissic Porteirinha Complex. Oxide-facies banded iron formations constitute iron ore at Morro do Pilar. Graphitic Iodes occur in metamorphic rocks at Pedra Azul. Nickel is associated with meta-ultramafic rocks at Ipanema, in the Atlantic Belt.Diamond is associated with fluviatile metaconglomerate units of the Sopa-Brumadinho Formation of the Espinhaço Supergroup, and is recovered from their alluvial deposits. Gold and quartz occur in hydrothermal veins near Diamantina.Iron is present in glacial-marine hematitic diamictites of the Macaúbas Group in the Rio Vacaria Valley. In southern exposures of this Group, surficial chemical weathering produces manganiferous laterites. Tungsten occurs in contact aureoles around intrusive granites between Coronel Murta and Itinga.Pegmatites related to the Brasiliano-age magmatism are divided into six districts, in the Araçuaí, Governador Valadares, Santa Maria de Itabira, Caparaó and Juiz de Fora regions. They are exploited for alexandrite, aquamarine, beryl, emerald, tourmaline, lithium, niobium, tantalum, tin and kaolin.In the Alto Rio Grande Belt, gold and limestone are present in rocks of the São João del Rei Group. Lode gold is related to shear zones truncanting metamorphic rocks of the Andrelândia Group at São Gonçalo do Sapucaí. Nickel, south of Liberdade, and kyanite also occur in the Andrelândia Group.In the Brasília Belt, the Morro do Ferro Greenstone Belt hosts nickel-cooper-cobalt and platinum group elements in gossans associated with komatiitic peridotites near Fortaleza de Minas. At Nova Rezende, it hosts nickel-cooper-zinc sulphide mineralizations which occur in fault zones in komatiitic ultrabasics and metasediments, and along the contacts between them. Lead-zinc and phosphate respectively comprise deposits in the Vazante and Lagamar regions, in the Vazante Formation. Gold-bearing boudins constitute ore in the Paracatu Formation.During the last decade, the volume of scientific contributions to the field of study is significant. Discussions on hydrothermal, structurally-controlled shear zone-hosted, and granitoid-related mineralizations are highlighted as the most notable. In particular, tha syn-and/or epigenetic relations remain the subject of academic debate.

An introduction to the crystalline basement of Africa

Abstract The crystalline basement of Africa comprises three major suites of rocks: the granite-gneiss-greenstone association of Archaean cratonic nuclei; strongly deformed metamorphic suites in mobile belts, mainly of Proterozoic age; and anorogenic intrusions which include Phanerozoic intrusive magmatic rocks related to rifting. This heterogeneous basement is extensively concealed beneath a variable thickness of diverse, unmetamorphosed sedimentary and extrusive volcanic rocks and weathering products. The Archaean cratons have similar geological histories, which generally culminate with anorogenic potassic granites emplaced at about 2500 Ma. The history of the Kaapval Craton is prematurely curtailed at about 3050 Ma. The Limpopo Mobile Belt is uniquely Archaean: its development is linked to differential movement between the oldest cratonic elements of southern Africa. Elsewhere in Africa, the oldest Archaean record suggests an absence of thick continental crust: early greenstone belts formed above mantle plumes on mobile, thin crust. The development of the Proterozoic mobile belts is related both to collision of older cratons (Wilson cycle orogenesis) and to ensialic disruption of single cratons. Early extensional orogenic phases produced both active (new oceanic crust development) and failed rifts. The earliest Proterozoic mobile belts cut across Archaean cratonic domains and later belts may be superposed. Older fractures are commonly reactivated by new stress systems. Much of the Phanerozoic anorogenic magmatism is related to Mesozoic and Cenozoic continental fragmentation and rifting, possibly associated with hot spots (White & McKenzie 1989). Notwithstanding the relative antiquity of most of the basement, it is the effects of high-level brittle fracturing and weathering which largely control groundwater storage. The fracturing varies in age from Archaean, within the greenstone belts, to Phanerozoic, for faults related to movement of the African Plate, with ongoing Quaternary faulting in the major tensional rifts such as the East African Rift System. The thickest weathering profiles occur above the oldest erosion surfaces; regional thickness variations are controlled by past and present climatic differences.

Lead-isotopic signatures of porphyry copper deposits in oceanic and continental settings, Colombian Andes

Three discrete sub-belts of porphyry copper-type mineralization are recognized in the Colombian Andes: a western Eocene sub-belt, an eastern Jurassic to early Cretaceous sub-belt and, between them, a central Miocene sub-belt. The western sub-belt is part of an oceanic domain constituted by oceanic crust and overlying immature island-arc rocks, the eastern sub-belt is within a continental domain underlain by the leading edge of the Guayana shield, and the central sub-belt spans the faulted boundary between them. The thicker continental crust includes important granulitic rocks which crop out locally, as in the vicinity of the Mocoa porphyry copper deposit. Pb-isotopic ratios were determined for pyrite samples collected from 6 porphyry copper centers, 3 in the western sub-belt, 2 in the eastern sub-belt, and one in the central sub-belt. Ratios fall into 3 discrete populations: the most radiogenic values represent the western sub-belt, the least radiogenic represent the eastern sub-belt, and an intermediate value corresponds to the central sub-belt. Ratios therefore become progressively less radiogenic from the western oceanic domain to the eastern cratonic domain. Comparison of the Pb-isotopic ratios with those given in the literature for possible source materials for Colombian porphyry copper leads enables the subcontinental mantle wedge, subducted oceanic crust and subducted metalliferous sediments to be discounted as principal sources. The relatively radiogenic signatures of 5 of the porphyry copper centers appear to be broadly compatible with either a subducted pelagic sediment source or an upper continental crust source, whereas the sixth center, Mocoa, is characterized by a distinctly less radiogenic 206Pb 204Pb ratio. An admixture of a relatively small percentage of non-radiogenic Pb from granulitic material in the upper crust with the more radiogenic Pb typical of the western sub-belt centers could account for the Mocoa data. Because much of the upper crustal section in the region of Mocoa is thought to be dominated by granulitic material, the radiogenic Pb component could be best derived from subducted pelagic sediments. If this is the case, then a subducted pelagic sediment source could dominate the Pb-isotopic signatures of all Colombian porphyry copper mineralization, with some of the Pb-isotopic differences being produced by selective contamination with upper crustal leads of variable character. If this conclusion is correct, then the fundamental metal budgets of Colombian porphyry copper systems are probably largely independent of upper crustal influences.

Single-pyroxene thermobarometry

Enstatites in peridotites from the Vourinos ophiolites, Greece, have exceptionally high Cr/Al ratios, though the pyroxenes are in equilibrium with picotite. Phase-chemistry data yield new empirical relations for the critical effect of Cr on Al solubility, in full agreement with local Cr/Al ratio variations observed in some massifs. These new relations allow application of the single-pyroxene thermobarometer to Cr-rich parageneses and revision of the general method of calculation of pressure and temperature estimates previously suggested for both spinel and garnet facies. The revised method is still based on thermodynamics, but emphasis is given to the fundamental distinction between experimental pure systems and natural complex ones as a result of the observed non-ideality of many solutions dealt with and of the interaction between Al and Cr. The impossibility of extrapolating experimental spinel-facies Al-solubility data for Cr-free systems to natural peridotites, leads to construction of an empirical Al(px) model based on geophysical, petrological and geochemical constraints, such a model yielding realistic estimates. Geotherms constructed fall into three categories, depending on the regional tectonic environment: oceans and continental extension zones, advanced rifting/ridges, and cratonic domains. Typical geotherms so obtained are similar to those proposed earlier (Mercier and Carter, 1975) but with the addition of the new high-temperature rift/ridge type.

The tectonic evolution of Australia

Fifteen non-palinspastic palaeotectonic maps, and accompanying explanatory text, are presented to illustrate the progressive development of the Australian continental block from the Archaean to the present. They summarise the structural and chronological framework of tectonic events in Australia as a data base for further research. They are a development from the Tectonic Map of Australia and New Guinea (GSA, 1971). Areas on the maps are classified into Precratonic (Orogenic), Transitional, and Cratonic Domains, and these are further subdivided into various subunits. Areas of known outcrop are distinguished from concealed or inferred rocks. Australia and New Guinea may be divided into major crustal blocks, each of which has its own history and tectonic style, and each of which represents an important stage in the evolution of the Australian continent. Although significant differences are shown between the tectonic patterns developed during the Proterozoic and the Phanerozoic, even more significant parallels exist: the same scheme of tectonic analysis and classification may be applied to both. The fundamental tectonic cycle of geosynclinal deposition and orogenesis, through transitional tectonism, to cratonisation and platform cover deposition, is evident throughout.