Showing papers on "Archean published in 1976"

Precambrian atmospheric oxygen: evidence in the sedimentary distributions of carbon, sulfur, uranium, and iron

Abstract: The sedimentary distributions of carbon, sulfur, uranium, and ferric and ferrous iron depend greatly upon ambient oxygen pressure and should reflect any major change in proportion of oxygen in the atmosphere or hydrosphere. The similar distributions of these elements in sedimentary rocks of all ages are here interpreted to indicate the existence of a Precambrian atmosphere containing much oxygen.Organic carbon contents and distributions are similar in Precambrian and Quaternary sedimentary rocks and sediments, although distributions in both would have been sensitive to variations in rates of organic productivity and atmospheric oxygen pressure. Sedimentary pyrite is almost invariably closely associated with organic carbon, suggestive of formation by sulfate reduction, in sedimentary rocks of any age. Archean and Middle Precambrian cherty iron formations and uranium ores resemble Phanerozoic ores and probably formed similarly by diagenetic concentration. In general, we find no evidence in the sedimentary d...

Carbon and sulfur isotope abundances in Archean iron-formations and early Precambrian life

Abstract: The volcanically enclosed Michipicoten and Woman River iron-formations, both approximately 2.75 b.y. old, in Superior Province of the Canadian Shield lie at lithologically similar stratigraphic contacts between underlying felsic pyroclastics and overlying mafic lava flows. Both iron-formations display similar lateral transitions of, from west to east, oxide, carbonate, and sulfide facies, a common facies pattern attributed to chemical deposition of volcanically derived components upon eastward-inclined slopes forming parts of local volcano-tectonic basins of deposition. Both iron-formations contain numerous, thin graphitic units.Six samples of Michipicoten graphitic carbon give delta 13 C values ranging from --20.4ppm to --27.7ppm with a mean of -- 24.7ppm. A sample of Michipicoten inorganic carbon in the form of massive siderite gave a delta 13 C value of +2.4ppm, which is similar to Phanerozoic marine carbonates. These data indicate extensive biological activities in the Archean basin during deposition of iron-formation.Sulfur isotope abundances in 62 samples of Michipicoten iron-formation taken across live main stratigraphic sections have a mean delta 34 S value of +0.02ppm with a range of 20.6ppm (--10.5 to +10.1ppm) and in 33 samples of Woman River iron-formation taken across three sections have a mean delta 34 S value of --1.3ppm with a range of 15.0ppm (--6.8 to +8.2ppm). The delta 34 S distribution patterns, which show large fluctuations over short distances, in the sediments are essentially the same for the different iron ranges and for the different members, chert, siderite, and sulfide (pyrite), within the Michipicoten iron-formations. The results strongly indicate isotopic fractionation in the biological reduction of sulfate under anaerobic conditions from a restricted reservoir at low sulfate concentrations.The sulfur and carbon isotope data provide strong evidence for the existence of autotrophic organisms and reducing bacteria in Archean times, but taken together with geological considerations as to the physical and chemical environment of deposition, the evidence appears overwhelming.Further detailed sampling and isotope analysis of Archean iron-formation is under way in order to advance the understanding of biogenic activities and basin deposition during accumulation of these ancient Precambrian chemical sediments.

Early and Middle Proterozoic history of the Great Lakes area, North America

Abstract: Two major supracrustal sequences, the Huronian Supergroup in Ontario and the Marquette Range Supergroup and Animikie Group of Michigan and Minnesota, overlie an Archean basement. These sequences are about 2200—2300 Ma and 1900-2000 Ma old respectively. The major Early Proterozoic tectonic event is the ‘Penokean Orogeny’, which occurred about 1850-1900 Ma ago and included deformation, high-grade regional metamorphism, and extrusive and intrusive igneous activity. This was followed by formation of rhyolitic, ignimbritic volcanic rocks and emplacement of associated granites about 1790 Ma ago. The entire region was subsequently subjected to low-grade regional metamorphism 1650-1700 Ma ago, followed by emplacement of anorogenic quartz-monzonite, in part rapakivi, plutons 1500 Ma ago. Late Proterozoic Grenville and Keweenawan events represent the youngest major Precambrian activity in the region. The rocks involved in the Penokean Orogeny lie along the southern margin of the Archean craton of the Superior Province and are interpreted as representing Early Proterozoic cratonic-margin orogenic activity. The distribution of rocks types and structures associated with the Penokean Orogeny and with similar orogenic belts along the margin of the Archean craton of North America suggest that these orogenic belts may have formed as a result of processes similar to modern plate tectonics, although the data are far from conclusive at present.

Stratabound gold deposits in Archean banded iron-formation, Rhodesia

Abstract: Stratabound mineral deposits, particularly of gold and associated sulfides, are widely distributed within beds of banded iron-formation in the Archean greenstone belts of Rhodesia. The occurrence of gold at the Vubachikwe gold mine has been studied together with samples of mineralized iron-formation from eight other deposits. These are the Beehive, Camperdown, Connemara, Empress, Marvel, Nelly 404, Pickstone, and Sherwood Starr mines.At the Vubachikwe mine the orebodies occur in several thin beds of banded iron-formation which are interlayered with mafic and felsic aquagene tuffs. These rocks form part of a mafic-ultramafic volcanic assemblage characteristic of the Sebakwian Group and are overlain by basaltic and andesitic lavas and tuffs characteristic of the Bulawayan Group. The orebodies are stratiform and are confined to beds of sulfide and mixed sulfide-carbonate facies banded iron-formation, which consist of layers alternately rich in quartz (cherty), arsenopyrite, pyrrhotite, and ankerite. The gold is occluded within arsenopyrite grains and the average ore grade is 11 ppm. The petrographic and structural evidence indicates that the gold and sulfides are premetamorphic and predeformation in age.It is proposed that, for all the deposits studied, the gold, the sulfides, and the iron-formation are consanguineous. Experimental evidence on gold solubility and data from active fumarolic-stage geothermal areas support the view that thermal brines are capable of transporting significant quantities of gold and other metals to the surface as soluble complex ions. A volcanogenic model for the origin of banded iron-formation, the sulfides, and the gold is favored. Such deposits are considered to represent submarine chemical precipitates deposited from solutions extruded from subaqueous (and possibly subaerial) active fumaroles.There is a regional stratigraphic control to the distribution of these Stratabound mineral deposits, most of which occur in iron-formation associated with the lower mafic-ultramafic volcanic assemblage (Sebakwian Group).

Archaean high grade complexes and modern continental margins

Abstract: Two kinds of geological terrain formed before 2,500 Myr ago. First, the greenstone belts, which are linear basins full of sediments and volcanics, possibly comparable with the marginal basins that occur behind modern island arcs. Second, and the subject of this paper, are high grade complexes containing rocks that have been deformed and recrystallised at high temperatures and pressures—granitic gneisses derived from granitic rocks are particularly common. Examples occur in north-western Scotland, Greenland, the Labrador coast, the Limpopo Belt in southern Africa, southern India, and probably the Aldan and Kola shields in the USSR. These complexes are so similar lithologically that broad generalisations about their tectonic evolution are justifiable. No major attempt has been made to see if they share any similarities with modern geological environments whose origins are better understood. We suggest that they are ancient analogues of the modern deep seated granitic batholiths located along the main arc axis of active plate margins of North and South America, for example in California and the Andes; accordingly, they are complementary to the greenstone belts in their back-arc setting.

Late Archean metamorphism in the Buksefjorden region, Southwest Greenland

Abstract: Standard state thermodynamic data extracted from experimental studies and applied to mineral assemblages in orthogneisses, metasedimentary gneisses and metabasites show that conditions of late Archean (2,850 m.y.) upper amphibolite facies were P solid≈7.0 kb, T≈630° C, and rose to P solid≈10.5 kb, T≈810° C in adjacent granulite facies. The estimates of solid pressure for the granulite facies suggest a late Archean crustal thickness of ca. 35 km, comparable to present day continental crust. Upper amphibolite facies assemblages were in equilibrium with $$P_{H_2 O}$$ about one half P solid, while granulite assemblages equilibrated at much lower $$P_{H_2 O}$$ , varying from about one tenth P solid in quartzofeldspathic gneisses to one third P solid in more basic layers.

Archean metallogeny in southern Africa

Abstract: The Archean granite-greenstone terrain of southern Africa possesses a number of unique characteristics. Not only is the shield area one of the smallest in the world but it is also one of the oldest crustal fragments still preserved. Estimates of lithic proportions show that, in the southern African Archean, the granitic rocks outnumber those of the volcanic belts (greenstone belts) by approximately 6:1--in marked contrast to the situation on the Canadian Shield where granite: greenstone ratios have been estimated to be approximately 1:2.In order to establish further details of the lithological proportions, the southern African Archean greenstone belts are described in terms of a three-stage stratigraphic model involving a Lower Ultramafic Unit at the base, followed by a Mafic-to-Felsic Unit and a terminal Sedimentary Unit. Planimeter studies demonstrate that on average the southern African greenstone belts possess a relatively high proportion of ultramafic and mafic volcanic and plutonic rock types and a correspondingly lower proportion of rocks of intermediate to felsic composition. The relative abundances of the various volcanic components of the lower two stratigraphic units are tabulated, and the lithic proportions are compared with similar data available from greenstone belts of the Superior Province in Canada. Geochemical data illustrating the nature of Archean volcanism in southern Africa and elsewhere are made available for purposes of comparison.The distribution patterns of a wide variety of Archean mineralization types, including the deposits associated with the ancient granites and pegmatites, are illustrated by means of mineral distribution maps. A close genetic relationship exists between the various lithological subdivisions of the Archean greenstone belts and the mineralization. Important deposits of chrome, nickel-copper, gold, and chrysotile asbestos are associated with the ultramafic-mafic component of the volcanic pile, whereas antimony, gold, and lesser amounts of mercury, barite, corundum, copper-lead-zinc, and massive sulfide iron-formations occur throughout the greenstone piles and commonly occur as chemical sediments terminating cycles of volcanism. These well-banded rocks, which probably formed by fumarolic-exhalative processes, also contain significant stratiform gold mineralization.The Archean mineralization of southern Africa is briefly compared and contrasted with that of the Canadian Shield and elsewhere. It is concluded that the disparity of lithological types and proportions between the Canadian and southern African shields is, together with differences in age, the underlying cause of the variability in the mineralization thus far encountered in the two regions.

Basement-Cover Relationships of Peninsular Gneiss With High Grade Schists and Greenstone Belts of Southern Karnataka

Abstract: The low grade Archaean terrain of southern Karnataka is composed of greenstone belts and high grade schists set in a sea of gneiss complex (Peninsular gneiss). Peninsular gneiss contains three major components which are inter-gradational, namely: (a) a trimodal macrolayered unit of amphibolites, ultramafites and granite gneiss (b) normal migmatites with characteristic megascopic structures and (c) nebulitic, schlieric and homophanous granitoids. On the whole it is a polymigmatite encompassing several episodes. The ubiquitous enclaves of mafic-ultramafic rocks within Peninsular gneiss probably represent residues of repeated reworking of the crustal rocks. The composition of the gneiss complex varies in the indestructible range of tonalite through granodiorite to granite. The high grade schists or supracrustals consist of shelf sediments, volcanics and ironstones with profuse emplacements of ultramafites. Their contacts with the gneiss complex are concordant due to intense deformation, high metamorphic grade and extensive migrnatisation. This high grade thermal event (Pantectogenesis) dated at 2900-3000 m.y. has acted as an effective smokescreen blurring earlier episodes. Subsequent to the pantectogenesis, greenstone belts were evolved on a basement of gneiss complex. Two types of greenstone belts of mutually exclusive geographic distribution are seen in the craton: (i) dominantly volcanic belts, lacking shelf facies rocks and ultramafics at the base, called the Keewatin type, which are broadly comparable to the Archaean greenstone belts and (ii) Platformal volcanic and protogeosynclinal belts of Dharwar type, which are similar to the early Proterozoic basins and geosynclines. The contacts of the Dharwar type greenstone belts with the gneiss complex are almost always unconformable, whereas in the case of Keewatin type of belts, the contacts are obscured either by soil cover or by massive invasion of later granites. This major granitic event possibly corresponds to the 2500-2600m.y. isochron in the gneiss complex. This evolutionary picture reconstructed mainly from newer field data is consistent with the available limited information on geochronology, geochemistry and mineralisation trends.

Granitoid sheets, thrusting, and Archean crustal thickening in West Greenland

Abstract: Archean gneisses in part of West Greenland mainly formed from sheets of tonalite and granodiorite emplaced into more mafic volcanic and plutonic rocks. Sheet emplacement was syntectonic and associated with dilation and thrusting, and it led to considerable and irreversible thickening of the continental crust. Large recumbent nappelike folds of these rocks were compressed and refolded on steep axes while still ductile. They were then deformed by steep shear zones as they dried out, stiffened, and became part of a stable craton about 2,850 m.y. ago. Although similar in composition to many Phanerozoic intrusions, these Archean granitoid sheets appear to be unusually extensive. An analog for their generation is the present tectonic setting of the Himalayas and Tibetan plateau.

Archean rocks from the eastern Lac Seul region of the English River Gneiss Belt, northwestern Ontario, part 2. Geochronology

Abstract: The region is divided into two domains, which are defined by their contrasting petrology, structure, and aeromagnetic anomaly pattern. The northern domain is underlain by east-trending, steeply south-dipping, migmatized garnet–biotite paragneiss. The southern domain is underlain by gneissic tonalite and trondhjemite, intruded by granite plutons, and with abundant amphibolite inclusions.Twelve U–Pb isotopic age determinations of zircons from two gneisses and four granite bodies from the eastern Lac Seul region of the English River Gneiss Belt indicate the following sequence of events. The tonalite gneiss of the southern domain was emplaced definitely more than 3008 m.y. ago and probably more than 3040 ± 40 m.y. ago. Local anatexis or injection associated with regional metamorphism produced a local pegmatite leucosome in the paragneiss 2681 ± 20 m.y. ago. Late- or post-orogenic granites, which invade both the paragneiss and the tonalite, were emplaced 2660 ± 20 m.y. ago, after the major metamorphism in the ...

Lead isotopes and trace elements from sulfides of Archean greenstone belts in South Africa; a contribution to the knowledge of the oldest known mineralizations

Abstract: Lead isotopic investigations of sulfides from gold-sulfide deposits in the Swaziland Sequence of the Barberton Mountain Land indicate that the primary lead of the sulfides is a two-stage lead forming a secondary isochron. From the slope of the secondary isochron a maximum age of 3,800 m.y. and a minimum age of 3,450 m.y. are indicated for the Swaziland Sequence rocks. The two-stage leads show that differentiation of the uranium-lead system had already occurred within this time span of the formation of the early crust. New least radiogenic ore lead was found in the vein-type deposit of the Old Star Gold Mine in the Murchison Range. The variable isotopic ratios of different galena grains also indicate a multistage history for this least radiogenic lead. The trace element patterns observed in pyrites from gold-sulfide deposits in the Barberton Mountain Land support the metallogenetic concepts of their having been formed by lateral secretion of elements from the ultramafic to felsic units of the Onverwacht Group of the Swaziland Sequence during regional metamorphism at the time of the emplacement of the surrounding granites.

On the age of Rhodesian greenstone belts

Abstract: Isochron ages and initial Sr-87/Sr-86 ratios are derived from Rb-Sr isotopic analyses on metavolcanic rocks from some greenstone belts of the Rhodesian Archean craton. Three mineral isochron ages (about 2.4 billion years) are tentatively suggested to represent the time of an important low-grade metamorphic event.

Proterozoic crustal evolution in parts of southern Africa and evidence for extensive sialic crust since the end of the Archaean

Abstract: The Proterozoic mobile belts of southern Africa were subjected to multiphase deformation since the Late Archaean and formed part of an extensive Early Proterozoic crustal segment termed here the Kalahari-Malagasy Protoshield. This shield underwent tectogenesis and differential vertical movement at various times with considerable uplift in the mobile zones and relative stability in the Archaean granite-greenstone terrains. The evolution of the Namaqua, Limpopo and Malagasy mobile segments suggests no large-scale dispersive movements of major continental fragments during the Early and Middle Precambrian. The basement complex of Angola and northern South West Africa constitutes a mobile zone which includes reworked Archaean sialic crust and a tectonized Early Proterozoic supracrustal cover. This complex may have formed part of another Early Proterozoic segment termed here the Kasai-Angola Protoshield, which was probably connected with the Kalahari-Malagasy Protoshield during the Proterozoic prior to reworking during the Kibaran-Burundian, Rehoboth-Irumide and Pan African tectogenetic cycles. It is proposed that most, if not all, Proterozoic mobile zones in southern Africa constitute ensialic belts and the widespread occurrence of sialic rock units older than $\pm $ 2.5 Ga support the conclusion that a large crustal plate of predominantly granitoid composition and of continental proportions was already in existence since the end of the Archaean. The Proterozoic evolution of southern Africa is therefore characterized by plate destruction rather than by plate accretion and progressive cratonization, and only some of the granite-greenstone 'nuclei' have escaped this process.

Earliest Precambrian ultramafic-mafic volcanic rocks: Ancient oceanic crust or relic terrestrial maria?

Abstract: Data on aspects of Archean shield geology are consistent with, although not necessarily indicative of, impact of extraterrestrial matter, possibly contemporaneous with impacts that affected the Moon about 4.1 to 3.8 × 109 yr ago. The isotopically oldest gneisses of several shield areas abound in xenoliths of ultramafic-mafic volcanic rocks, minor silicic volcanic rocks, and derived and (or) chemical sedimentary rocks. No confident oldest age limits were set on these xenoliths, which antedate the development of the true greenstone belts in these areas and are thought to represent relics of a once-widespread volcanic crust. This crust differed from modern oceanic crust in the following ways: (1) peridotitic, high-Mg, and silicic volcanic rocks were more abundant in the Archean crust than in the modern crust; (2) sea-floor spreading and crustal overturn in early Archean time should have resulted in the rapid development of large continents at that stage, contrary to observations; and (3) the apparent retention of a coherent parallel Archean tectonic pattern on Gondwanaland reconstructions renders large-scale plate movements, and thus sea-floor spreading, unlikely. Had the early Archean ultramafic-mafic crust been genetically related to impact, the recurrence of chemically similar volcanic rocks within greenstone belts up to about 2.6 × 109 yr ago may signify repetition and gradual waning of mantle activity initially triggered by such events. Evidence bearing on the impact theory may be gained by a search for shock metamorphic effects in the oldest agmatites and in fragments and detrital grains incorporated in the earliest sedimentary intercalations of volcanic sequences.