Archaean Nuclei Research Articles

Where are the Eburnian–Transamazonian collisional belts?

The reconstruction of Early Proterozoic crustal evolution and geodynamic environments, in Africa and South America, is incomplete if cratonic areas alone are studied. If the presence of high-grade gneisses is considered as a first clue to past collisional behaviour, 2 Ga high-grade gneisses are more abundant within the Pan-African–Brasiliano mobile belts than in the intervening pre-Late Proterozoic cratons. The West African craton and the Guiana–Amazonia craton consist of relatively small Archaean nuclei and widespread low- to medium-grade volcanic and volcanoclastic formations intruded by Early Proterozoic granites. By contrast, 2 Ga granulitic assemblages and (or) nappes and syntectonic granites are known in several areas within the Pan-African–Brasiliano belts of Hoggar–Iforas–Air, Nigeria, Cameroon, and northeast Brazil. Nappe tectonics have been also described in the Congo–Chaillu craton, and Early Proterozoic reworking of older granulites may have occurred in the São Francisco craton. The location of the Pan-African–Brasiliano orogenic belts is probably controlled by preexisting major structures inherited from the Early Proterozoic. High-grade, lower crustal assemblages 2 Ga old have been uplifted or overthrust and now form polycyclic domains in these younger orogenic belts, though rarely in the cratons themselves. The Congo–Chaillu and perhaps the São Francisco craton are exceptional in showing controversial evidence of collisional Eburnian–Transamazonian assemblages undisturbed during Late Proterozoic time.

Compositional distinction between oceanic and cratonic lithosphere

Modal and chemical data for variably-depleted peridotites of oceanic origin together with estimates of the composition of fertile parental peridotite are used to define an oceanic trend. This trend is most clearly illustrated in a plot of modal olivine against mg number. The oceanic residues represented by abyssal peridotites, ophiolite tectonites and some alpine peridotites are characteristically olivine-rich with mg numbers of 90.5–91.5. In contrast, peridotite xenoliths with equilibration temperatures below 1100–1200°C from the Archaean Kaapvaal craton in southern Africa are more enstatite-rich with mg numbers of 91.5–93.5. The structures and compositional relations of other Archaean cratons are believed to be similar to the Kaapvaal on the basis of tectonic, geophysical and sparse petrological data. Oceanic residues may have formed in a variety of tectonic environments, but are believed to be of relatively shallow origin. Mg-rich peridotite xenoliths from mafic volcanics erupted in continental areas marginal to Archaean cratons have compositions that fit the oceanic trend. Some of these xenoliths may be derived from oceanic lithosphere that has been accreted to Archaean nuclei. Others may have originated in sub-continental magmatic events at relatively shallow depths, comparable to those at which lithosphere has formed beneath the oceans. The Archaean cratonic peridotites have had a history that differs from those forming oceanic lithosphere and it is conjectured that they are high-pressure residues of komatiite formation. The cratonic peridotites have a lesser density than other peridotites and they are believed to have floated and coalesced to form continental nuclei.

Petrology, geochemistry and origin of a major Australian 1880-1840 Ma felsic volcano-plutonic suite: a model for intracontinental felsic magma generation

In Australia, a major period of felsic volcanism and granite emplacement occurred between 1880 and 1840 Ma, towards the end of the early Proterozoic. These volcanics and granites, covering at least 37 000 km 2, are compositionally uniform over large areas. The felsic magmas are predominantly I-type, being derived from infracrustal sources which have never been exposed to surficial weathering processes. When compared with early Archaean and Phanerozoic I-type granites, the 1880-1840 Ma suite has distinctively high levels of K 2O, Rb, Zr, La, Ce, Th, U, and low MgO and CaO contents even at relatively low SiO 2 levels of 60–65%. The 1880-1840 Ma rocks are characterized by low initial 87Sr/ 86Sr ratios, and negative ϵ Nd values. Chemical and isotopic modelling on these volcanics and granites suggest that they formed from mantle-derived sources that are around 2300 2000 Ma old. There is no evidence of a major Archaean component in these granites. The sources are interpreted to have formed as a result of polygonal small-scale mantle convection, which led to extensive underplating of Archaean crust. During this underplating event, large volumes of incompatible element-enriched mantle-derived material accreted to the base of the lower crust, and fractionated significantly. The more fractionated part of this source was remelted between 1880-1840 Ma to produce the voluminous felsic volcanics and comagmatic granites present in nearly every Proterozoic domain of Australia. The actual underplating event between 2300-2000 Ma, is believed to have triggered the formation of early Proterozoic sedimentary basins, which occur in a polygonal pattern, and appear to wrap around Archaean nuclei. The composition of these felsic melts and the tectonic setting operating during their emplacement precludes genesis by contemporaneous subduction of oceanic crust: all are generated by processes of vertical accretion, in an intracontinental environment.

Diversity in the lower continental crust

Summary Arguments based upon tectonic style and process suggest that the lower continental crust is laterally exceedingly inhomogeneous on the scale of tens and hundreds of kilometres. At least twenty basic variations exist apart from unmodified Archaean nuclei. Some 80% of the continental crust was generated by 2.5 × 10 9 years and most of this has been modified and/or redistributed by tectonic and sedimentary processes. Two general secular trends have occurred to increase homogeneity in the continental crust; addition of mafic igneous rocks to the lower crust, and differentiation, in both extensional and shortening environments, into a more mafic, refractory lower crust and an upper, wetter, more silicic, crust enriched in lithophile and radiogenic elements.

A greenstone belt—basement relationship in the Tanganyika shield

SummaryAn Archaean greenstone belt, occupied by rocks of the Nyanzian System, rests apparently unconformably on a basement composed of the metamorphites of the Dodoman System in the central part of the Tanganyika shield. The latter were originally metamorphosed in granulite facies. Two episodes of migmatization were imposed on this earlier crust producing two complexes of anatectic granitoids (Puma and Ikungi) which locally have a charnockitic character. Differences in structural setting suggest that they were developed in connection with two major tectonic events. A basin-like structure was built up during the second – Ikungi – in which the Nyanzian System occurs. The Nyanzian rocks are not migmatized and their juxtaposition with the migmatites suggests that they are of younger age. Another tectonism took place after deposition of the greenstones as a result of which they were intensely deformed. Two types of intrusive granites were emplaced in the greenschists upgrading them to hornfelses during this tectonism. An interesting feature is the coincidence of the general tectonic axes of these three events. For the successively established metamorphic processes in this Archaean nucleus a gradual downgrading of metamorphic facies is characteristic, showing its irreversible development.

Some Observations on the Structure, Metamorphism, and Geological Evolution of Peninsular India

Abstract Peninsular India, like other shield areas of the world, displays an ingrained pattern of successive orogenic belts. The description of these belts is accompanied by a tectonic map in which are noted the main structural trends as well as many of the reliable ages so far determined. The variations in the grades of metamorphism which can be recognised in South India are delineated in a sketch map. There is a general progressive increase in the intensity of metamorphism from north-west to south-east. The highest grades are met within the southern and south-eastern parts of the Peninsula where charnockites and khondalites are prevalent. The successive stages in the geological evolution of Peninsular India are next described. The structural pattern as well as relative ages indicate that the oldest rocks are found in the type area for Dharwar Schists in Mysore State. This region is a continental nucleus, and the Eastern Ghats and Satpura provinces are later Precambrian accretions which have been folded by pressures directed towards this Archaean nucleus.