Archaean Cratons Research Articles

Vertical zonation and petrogenesis of the early Precambrian crust in Western Australia

Variations in metamorphic grade, structural style, isotopic ages and granite geochemistry observed within the Yilgarn craton, and between the Yilgarn and Pilbara cratons, Western Australia, are interpreted in terms of vertical zonation of the Archaean crust. We correlate the gneiss-granulite suite of the Wheat Belt (southwestern Yilgarn) with concealed coeval infracrustal roots of the low-grade granite—greenstone Kalgoorlie terrain (eastern Yilgarn). Differences between the Pilbara, Southern Cross and Laverton granite—greenstone blocks and the downfaulted linear greenstone belts of the Kalgoorlie block are interpreted in terms of deeper-level exposure in the first three blocks. Ultramafic—mafic volcanic sequences in the Yilgarn craton can be divided into at least two major groups — the lower greenstones, regarded as relicts of a once extensive simatic crust, and the significantly younger upper greenstones, believed to have formed within linear troughs following the intrusion of Na-rich granites. At least three major Archaean granite phases occur in Western Australia: (1) 3.1-2.9 b.y. old (recognized to date only in the western Yilgarn and in the Pilbara craton); (2) 2.8-2.7 b.y. old, and (3) 2.6 b.y. old (the two latter phases can only be separated from each other in the eastern Yilgarn, and phase (3) is also identified in the Pilbara). In the main, granites of phases (1) and (2) are Na-rich and those of phase (3) are K-rich. There is evidence for a secular increase in Rb levels and initial 87Sr/ 86Sr ratios. It is suggested that the K-rich granites grade down into Na-rich granites, and the former were generated by ensialic anatexis resulting in upward migration of K, Rb, U, and Th-enriched magmas. A review of data from several Archaean cratons in other continents suggests that evidence from these regions can be interpreted in terms of the general model of crustal evolution proposed for Western Australia. Implications of this model concerning petrogenesis of Archaean plutonic and volcanic suites, geothermal gradients and tectonic evolution of greenstone belts are discussed. Partial melting associated with mantle diapirism is thought to have given rise to the ultramafic—mafic volcanic cycles. Widespread subsidence and partial melting of this crust yielded Na-rich acid magmas. The development of the upper greenstones was confined to linear belts in a partly cratonized crustal environment. About 2.6 b.y. ago a rise in the geothermal gradient resulted in regional metamorphism and crusctal anatexis which gave rise to the K-rich granites.

Evolution of Proterozoic basement patterns in the Lewisian Complex

Two contrasting structural elements, both of which have affinities with the two Proterozoic mobile belts of Greenland, have contributed to the pattern of structures in the Lewisian complex of the northern British Isles. The evolution of these structural patterns can be interpreted in terms of an integral system of movements which developed under the influence of an evolving linear dislocation that flanked the major Archaean craton of Greenland.

Age relationships between greenstone belts and “granites” in the Rhodesian Archaean craton

Whole-rock Rb-Sr age measurements on rocks from the Rhodesian Archaean craton of Southern Africa demonstrate that (a) the Mashaba area gneisses, which are typical of the Rhodesian Basement Complex, are approximately 3600 m.y. old, (b) volcanic rocks from the main greenstone belts, assigned to the Bulawayan Group, were extruded approximately 2600–2700 m.y. ago, (c) a cross-cutting pluton, the Sesombi tonalite, was emplaced 2690 ± 70 m.y. ago, (d) the Gwenoro Dam migmatites, which field evidence suggests could be older than the main greenstone belts, were emplaced 2780 ± 30 m.y. ago. The initial 87Sr/ 86Sr ratios of most of the rock units of groups b, c and d are in close agreement at about 0.701, suggesting that the later granitic (sensu lato) and andesitic rocks so far analysed were not produced by remelting of, or contamination with, ancient gneissic basement.

Age differences between Archaean cratons of eastern and southern Africa

ARCHAEAN greenstone belts have commonly been correlated on the basis of similarities in their stratigraphic successions1. Conclusive radiometric tests of these correlations have rarely been made, because of the difficulty of dating the rocks of the greenstone belts themselves, and because of uncertainties in their relationships to the more easily dated granitoid rocks with which they are normally associated. We report here radiometric evidence from eastern Africa, which clearly contradicts previous lithostratigraphic correlations between the Tanzanian and Kaapvaal Cratons by establishing a minimum age difference between the uppermost parts of their respective greenstone belt successions.

A zircon age from gabbro-anorthosite inclusions in the gneisses of the Angmagssalik area, South-East Greenland

Age determinations were made on 3 zircon fractions extracted from gabbroanorthosite inclusions which occur as tectonically dislocated pods in quartzofeldspathic gneisses on the eastern end of Kitak island, South-Bast Greeniand (fig. 5). The anorthosites were selected for study because they are derived from a lithologically distinctive suite of rocks found over a large part of the Archaean craton of the North AtIantic area (Bridgwater et al., 1973b). While there is no evidence that all the anorthosites of this type are contemporaneous their formation appears to require special conditions in the ernst and they are thus likely to be a useful stratigraphic tool. Furthermore the coarse-grained refractory nature of these rocks means that they retain their lithological identity during later tectonic, anatactic and metasomatic events to a far greater degree than the quartzofeldspathic, semipelitic and amphibolitic units making up the major part of the gneiss complex in which the anorthosites occur.

Alpine-type structures in the Rhodesian basement complex at Selukwe

The Rhodesian Archaean Craton consists of greenstone schist belts tightly folded between gneiss domes. The Selukwe Schist Belt, situated near the centre of the craton, consists of mainly Sebakwian Group greenstones, ultra-mafics and sediments, older than intrusive 2900 m.y. tonalities. It appears to be part of an imbricated and overturned lower limb of a large recumbent fold, resting allochthonously on a gneissic basement. Four main tectonic slices, separated by mylonite shear zones were recognized. The inverted succession together with the shear zones were refolded during two subsequent tectonic episodes, before being arched and tilted by intrusion of the tonalite diapir in the southeast.

A Discussion on the evolution of the Precambrian crust - The Archaean craton of the North Atlantic region

The North Atlantic Archaean craton, exposed in parts of Greenland, Labrador and northwest Scotland, is a high-grade gneiss terrain which contrasts with Archaean granite-greenstone-belt terrains such as those of southern Africa. The tonalitic to granitic banded or agmatitic gneisses which occupy most of the craton are considered to be derived largely from granitic bodies emplaced within the crust. Early granitic gneisses of this type in Godthabsfiord are at least 3800 Ma in age and it is suggested that a granitic basement of similar age extended over much of the craton. Most of this early basement was reworked and interleaved with metamorphosed supracrustal rocks, with layered anorthositic complexes and with abundant tonalitic gneisses derived from younger intrusions. Identifiable metavolcanics and metasediments, forming narrow belts in the gneisses, occupy less than 20% of the craton; they include highly-metamorphosed basic, ultrabasic and intermediate-acid volcanic rocks with associated intrusions and predominantly chemical sedimentary rocks. Clastic sediments are preserved in the lower part of the Isua supracrustal belt where they are overlain by banded ironstones and metavolcanics. All these rocks suffered profound deformation and metamorphism which destroyed their primary relationships and culminated in the development of fold interference patterns without linear grain and in granulite or amphibolite-facies metamorphism ending at about 2800 Ma. Tectonic and metamorphic episodes over the next thousand million years were more localized and served to differentiate the Archaean craton from border-zones of early Proterozoic mobility.

A Discussion on the evolution of the Precambrian crust - Crustal development in the Precambrian

The oldest Archaean rocks in most shield regions are largely granulites and gneisses, and in west Greenland there is evidence of 1000 Ma of crustal history before the final high-grade metamorphism. Archaean greenstone belts are mostly younger than the high-grade terrains although in some areas, such as southern Africa, this has not yet been proved reliably. The greenstone belts may have developed as oceanic crust in connexion with plate movements, the earlier continents being represented by the more deeply eroded high-grade regions. Stabilization of the Archaean cratons is signalled by continental-scale intrusion of dolerite dyke swarms. Proterozoic mobile belts are exposed at two structural levels. Some early linear basins have mio- and eu-geosynclinal parts and may have been located along Proterozoic suture lines. More deeply eroded mobile belts are often floored by extensive, partly reworked, crystalline basement and probably developed along linear rifted zones which acted as loci for high heat flow and igneous activity; they lack ophiolites and are difficult to interpret as collision-type mountain belts. Most probably there were intra-continental plate movements in the Proterozoic.

A Discussion on the evolution of the Precambrian crust - The Rhodesian Archaean craton — an essay in cratonic evolution

The Rhodesian Archaean craton consisting largely of granite-greenstone terrain is briefly described, and is discussed mainly in the light of points arising from recent work in the south and central parts of the country. The Sebakwian-Bulawayan-Shamvaian terminology and the evidence for more than one age of greenstone belt development are reviewed. An attempt is made to trace the evolution of the craton from the earliest recognized rocks up to the emplacement of the Great Dyke (±2500 Ma). Evidence is presented for the existence of an earlier sialic basement to the main greenstone belt cover, and to show that the main greenstone belts developed during the approximate time-span 3300 to 2900 Ma. Between approximately 2900 and 2500 Ma the main events are traced both in the craton and the Limpopo mobile belt. Proterozoic events at 2000 to 1800 Ma affecting the craton and mobile belt are briefly described and discussed.