Showing papers on "Archean published in 1974"

Crustal Evolution and Global Tectonics: A Petrogenic View

Abstract: Studies of the relative abundances and characteristics of rock types, series, and complexes through time reflect both general and subtle features of crustal and upper-mantle environments. These studies also suggest salient aspects of the dynamothermal and global tectonic history of the last 3,500 m.y. Among the many petrochemical indices, the ratio K 2 O/Na 2 O reflects both the degree of differentiation (fractionation) of igneous rocks and the more or less mature (residuate) nature of clastic sediments. Accordingly, this ratio is a guide to the thickness, composition, and stability of source and crusial site of rock emplacement. Average K 2 O/Na 2 O of rocks, rock complexes, and terranes decreasing from 1 are typical of rock series formed in primitive borderland and arc-to–oceanic-crustal sites. Average values of K 2 O/Na 2 O increasing from 1 characterize rock series evolving in and on more mature arc-to-continental sites. Relative abundances of the most common and characteristic rock assemblages of various ages and their weighted average K 2 O/Na 2 O suggest profound episodicity in crustal evolution and global tectonics. At least three macro-episodes of major significance are defined: the Archean, >2,500 m.y. B.P.; the Proterozoic-Paleozoic, 250 m.y. B.P.; and the Mesozoic-Cenozoic, ∼250 m.y. B.P. to present. Within these macro-episodes, there are innumerable subordinate episodes and variously developed rock cycles. Most major Archean rocks, rock complexes, and terranes have a K 2 O/Na 2 O of Archean thermal gradients varied abruptly both vertically and laterally from very steep to moderate and created highly unstable, thin to thick, labile lithosphere and protocrusts commonly inhospitable to the evolution of widespread, highly fractionated calc-alkaline series or mature sediments. Heat transfer was largely via convection and advection. The advective loss was undoubtedly large, associated especially with the extrusion of floods of ultramafic to felsic magmas and related upward streaming of volatiles. Culmination of major Archean orogenies and subsequent thermal decay about 2,500 m.y. B.P. induced relative crustal quiet and resetting of most Archean Rb/Sr and K/Ar radiomentric clocks between 2,600 and 2,400 m.y. B.P. In the early Proterozoic (∼1,700 to 2,300 m.y. B.P.), many segments of the megacontinent(s) were sufficiently cool, thickened, and fractionated to remain quasi-coherent, deforming and cracking internally as well as marginally above convecting mantle forces. Widespread, relatively ensialic orogenic regions evolved above many thinner sialic zones. These were repeatedly refractionated and redated as they became populated by igneous series with K 2 O/Na 2 O commonly 2 O/Na 2 O as high as 2 or more. Sedimentary K 2 O/Na 2 O increased much faster than did the igneous ratio, as blankets, basins, and prisms of more mature, frequently recycled sediments formed on all the continents. Two of the several major Proterozoic orogenic episodes culminated at about 1,700 m.y. B.P. and 1,000 m.y. B.P. The latter, often called the “Greenville Event,” between 1,000 and 1,200 m.y. B.P., is characterized by the evolution of both K- and Na-rich terranes and distinctive granulite and anorthositic-charnokitic complexes. Most of these Grenville rock complexes evolved between and on relatively thicker, more fractionated continental crust than did the Archean granulites, as indicated by the relative abundance of less telescoped prograde terranes. The Grenville thermal pulses and decay involving much advective and convective heat loss again reset numerous Rb/Sr and K/Ar ages of rocks emplaced 100 to 2,000 m.y. earlier. A post-Grenville relatively amagmatic period of 400 to 500 m.y. after 1,000 m.y. B.P., preceded the incipient rifting of large continental segments as the great Pan–African-Appalachian-Her-cyian-Caledonian orogenies waxed. But large-scale drift of most of these continental segments seems precluded by the continuing alignment of the older, including Archean, fold belts and by the predominantly continental rock component emplaced in most Proterozoic and Paleozoic orogens. Weighted average K 2 O/Na 2 O of Proterozoic igneous complexes are commonly > 1.2 and, in sedimentary sequences, often > 2. Nevertheless, the occurrences of lower Paleozoic ophiolite, blueschist, and less fractionated rock complexes in many Phanerozoic orogens suggest the onset of the large-scale continental rifting and global drift characteristic of post-Permian time. These events are reflected in variously depressed lower Paleozoic K 2 O/Na 2 O and, subsequently, in the plunge of average K 2 O/Na 2 O of major Mesozoic rock complexes to near Archean lows of ∼0.5 to 0.9, especially in the circum-Pacific. The petrochemical and structural data indicate the significant differences as well as similarities between the Archean and post-Permian. The Archean was dominated by the evolution and aggregation of protocontinents, arcs, and intervening oceanic crusts. The post-Permian was dominated by the unique fragmentation and widespread drift (>1,500 km) of large, thick, cool, fractionated, continental fragments and by the birth of new large ocean basins and island-arc chains.

Rare-earth element distributions in volcanic rocks from Archean greenstone belts

Abstract: Rare-earth element distributions in Archean volcanic rocks from the South Pass (Wyoming), Yellowknife (NW Canada) and Abitibi (Quebec) greenstone belts and from the Upper Fig Tree Group of the Barberton (S. Africa) greenstone belt reveal two distinct types of Archean volcanism. One type, herein referred to as the arc-type, is characterized by flat (or slightly enriched) REE distributions in tho leiites and enrichment in total and light REE and a variable negative Eu anomaly in more siliceous volcanic members. The second type, herein referred to as the Abitibi-type, is characterized by rather flat REE patterns and negative Eu anomalies in all volcanic rock types. REE distributions in the arc-type volcanic successions can be produced by either progressive shallow fractional crystallization of tholeiitic magma or by decreasing amounts of equilibrium melting of a plagioclase-bearing mantle source. REE distributions in the Abitibi volcanic rocks are most readily explained in terms of progressively decreasing amounts of fractional melting of a source area in which REE are contained chiefly in minor minerals (with low melting temperatures) that are depleted in Eu. The melting models seem to necessitate the existence of one or more pre-greenstone magmatic episodes as well as a continuously replenished mantle source. Replenishment of source material could be accomplished in either of the melting models in subduction zones but the analogy to Phanerozoic plate tectonics should be used with caution. Melting models also imply either (or both) a decreasing geothermal gradient with time or systematic changes in mantle source-area composition.

Archean Quench-Texture Tholeiites

Abstract: Tholeiitic metabasalts and metaandesites from sub-greenschist metavolcanics in the Abitibi volcanic belt northeast of Noranda–Rouyn display delicate and ornamental crystals indicative of rapid or s...

An Outline of the Geology of Labrador

Abstract: Labrador forms the eastern portion of the Precambrian Canadian Shield. It is divisible into four geologic provinces, each characterized by different orogenic history. Superior Province, in Western Labrador, and Nain Province, along the eastern and northern coast, represent parts of Archean orogenic belts. Both consist primarily of high grade metamorphic rocks, which in Nain Province are overlain locally by less deformed Proterozoic sedimentary and volcanic assemblages. Churchill Province trends northward across central Labrador, between the two older orogens. It is composed of a western belt of relatively little deformed sedimentary and volcanic rocks, and an eastern zone of high grade metamorphic rocks, both of which were last deformed in the Early Proterozoic Hudsonian orogeny. Grenville Province trends east-northeast across southern Labrador. It is composed largely of quartzo-feldspathic gneisses, last deformed in the Grenvillian orogeny of the Middle Proterozoic. The metamorphic rocks of Grenville, Nain and Churchill Provinces are intruded by large anorthosite-adamellite plutons, emplaced during the Middle Proterozoic. Exploration in Labrador has been concentrated in two areas: the Labrador Trough, in the western part of Churchill Province, and the "Central Mineral Belt" of Labrador, which extends eastward across the southern parts of Churchill and Nain Provinces. Mining in the Labrador portion of the Trough accounts for about half of Canada's iron ore production. Uranium, copper, beryllium and molybdenum occurrences are being investigated in the central mineral belt.

Paired Metamorphic Belts in Precambrian Granulite Rocks in Gondwanaland

Abstract: Examination of all available data on Precambrian granulite rocks centered around the Indian portion of Gondwanaland reveals the possible existence of a paired metamorphic belt that can be traced from the Ceylon–south India area west to Madagascar and possibly to Uganda, and east to southwest Australia and east Antarctica. This belt appears to be formed of low-pressure–high-temperature (Abukuma-type) rocks with an adjacent complementary belt with rocks formed by intermediate pressure and intermediate temperature (Barrovian-type). The belt of low pressure and high temperature is characterized by cordierite in pelitic, intermediate, and even mafic rocks, some of which are associated with sapphirine. Andalusite is present locally, coexisting with sillimanite, and wollastonite is found in some calc-silicate assemblages. The belt of intermediate-pressure and intermediate-temperature rocks contains sillimanite-garnet in metapelite, and scapolite-diopside with calcite-quartz in the calc-silicate rocks. This relation seems common to the granulite rocks of Gondwanaland and serves to illustrate geologic continuity of these segments when brought together in their suggested pre-drift positions. From the scant information available concerning these rocks, a gross metamorphic-structural-chronologic unity may be recognized. Highly deformed granulite rocks of at least Archean age (about 3,000 m.y.) are characteristic; these rocks were metamorphosed and (or) remobilized at about 2,500, 1,300, and 1,000 m.y., ending with a widespread metamorphic event at about 600 m.y. These paired belts form a pattern on models of reconstructed Gondwanaland, which suggests that plate tectonics processes between Indian, Antarctic-Australian, and African protoplates were in operation in Precambrian time.

Archean Rocks of Shoshonitic Affinities at Bijou Point, Northwestern Ontario

Abstract: The Archean volcanic and intrusive rocks of Bijou Point, northwest Ontario show several chemical characteristics which suggest affinities to the alkali or shoshonitic series characteristic of late stages of evolution in modern island arcs. The rocks have unusually high total alkalis (7% at 58–65% SiO2), high Al2O3 and high trace element contents (Ba 800–2500 ppm, Rb 50–350 ppm, Sr 600–2000 ppm). The presence of such rocks in the greenstone belts of the Superior Province serves to further emphasize the similarities between these ancient volcanics and their modern counterparts.

Possible thermal explanation of contrasting Archean and Proterozoic geological regimes

Abstract: THE Earth underwent a remarkable change of behaviour at the Archean–Proterozoic boundary. Greenstone belts intruded by granites dominate the geological record before about 2.5 × 109 yr ago. After that time, essentially no such greenstone belts were formed, and long linear orogens characterised by high–grade metamorphic rocks made their first appearance.

Lead Isotope Evidence on the Genesis of the Silver-Arsenide Vein Deposits of the Cobalt and Great Bear Lake Areas, Canada

Abstract: Silver veins of Proterozoic age in the Cobalt area, Ontario, are associated with the thick Nipissing diabase sheet, and those at Great Bear Lake, NWT, are also, in part, associated with a diabase sheet Host rocks in the Cobalt area are flat-lying Aphebian sedimentary rocks, steeply dipping Archean volcanic and associated sedimentary rocks, and diabase, while host rocks in the Great Bear Lake area are intermediate and felsic volcanic rocks and tuffs of the Echo Bay groupModel-lead ages for ordinary galenas are about 1,630 and 2,280 my for the Great Bear Lake and Cobalt veins, respectively, whereas the pitchblende U-Pb age for the Great Bear Lake veins is about 1,445 my, and the apparent Rb-Sr age for the Cobalt veins is about 2,160 my An Rb-Sr isochron age of 1,425 + or - 48 my has been obtained for the diabase sill in the Port Radium area, Great Bear LakeIn the Cobalt area essentially identical lead isotope compositions were obtained for interflow base metal mineralization in Archean volcanic rocks, galena along bedding in sedimentary rocks of the Cobalt group, and some late-stage sulfide veins One interflow sample with a more primitive composition has a model age of at least 2,805 my Mineralization, including deposition of most of the lead in Archean interflow beds, is interpreted to have taken place at about 2,160 my and to be genetically related to the Nipissing diabase In addition, two anomalous lead lines are defined, one of which suggests an age of 3,260 + or - 100 my for Archean volcanic rocks A shallow line, represented in only late-stage sulfide veins, is difficult to interpret but could be due to the late addition of a radiogenic component during Paleozoic or younger events not normally considered as causing mineralization, or even from ground waterLeads from the Great Bear Lake area could represent a single anomalous lead line with a slope of 01085 + or - 00070, and thus have a maximum possible age of 1,130 my Geological events younger than this are known in the Bear Province, but not in the vicinity of the veins, and the same problems of interpretation exist as for the Cobalt areaThe source of lead is not defined in either area However, a source with homogeneous lead is required and the lack of extensive wall-rock alteration suggests that the lead was not leached from the country rocks The diabase is tentatively favored as the source, but a connate brine or other source is possible

The Superior Geotraverse Project

Abstract: The Superior Geotraverse Project is a cooperative geological and geophysical research program being carried out by the Precambrian Research Group of the University of Toronto, assisted by scientists from other universities The purpose of the project is to study the origin of the Archean crust north of Lake Superior in a 300 mile long, N-S corridor which crosses three volcanic-plutonic belts (Uchi, Wabigoon and Shebandowan) and two metasedimentary-gneiss belts (English River and Quetico) Six different models for the origin of the Archean crust are under consideration: no decision between competing models will be possible until further work is done One possible model, however, is that the crust developed in three stages: 1 ) development of Archean "oceanic" crust, 2) development of Archean "island arc" crust as a result of subduction, 3) accumulation of greywackes in foreland basins

Quench Plagioclase from some Archean Basalts

Abstract: Quench plagioclase has been recognized in well preserved Archean submarine basalts from four localities in the superior province of the Canadian Shield. The crystals are skeletal in form with hollow cross sections and swallow tail terminations. In some cases the microphenocrysts are zoned. The widespread occurrence of quench plagioclase suggests that this feature was rather common in Archean basalts, but it is only locally preserved.

Geochemistry of early precambrian carbonate rocks from the Brazilian Shield: Implications for archean carbonate sedimentation

Abstract: Petrological and geochemical studies were carried out on early Precambrian carbonate rocks metamorphosed under granulite facies conditions from three areas of Bahia State (Brazil). Older rocks attributable to the Archean or lower Proterozoic consist of carbonates (mainly dolomite) and abundant, partially-serpentinized forsterite. Chemical data confirm their strong magnesian character and the almost complete absence of elements like Al, Ti, Na, K, Zr etc., which are normally found in the non-carbonate detrital part of impure carbonate rocks. Comparison with other carbonate rocks of similar silica content further emphasizes the scarcity of these elements. Samples of probably more recent origin belonging to migmatized complex of Central Bahia are more calcic in character and have a higher Sr and Ba content. The MgCO3 solvus thermometer in calcites coexisting with dolomite was determined both by chemical and X-ray procedures. Temperature values for carbonates under granulite facies range between 472 and 640° C. These temperatures seem to represent quench or recrystallization temperatures. The general geochemistry of carbonate rocks suggests two main hypotheses for their formation: a) chemical precipitation of pure carbonates during the Archean and later silica enrichment by metasomatic reactions; b) chemical co-precipitation in the Archean or early Proterozoic of carbonates and silica from silica-rich sea water. In this case precipitation would have occurred locally owing to increasing CO2 pressure (due, for example, to exhalative volcanism) or in limited evaporitic basins in areas of temporary stability bordering the continents.

Geochemistry of Granite-Greenstone Terrain of South India

Abstract: A critical evaluation of the available major and trace element data on rocks of the granite-greenstone belts or South India leads to the following inferences: Barrovian and Abukuma facies series of metamorphism are recognizable in the terrain. Rocks of peridotitic and basaltic komatiite affinities occur in the pre-Dharwar and early Dharwar ultramafic-mafic sequence. The bulk of the greenstones are island arc tholeiites with a strong calc-alkaline affinity. The presence of detrital pyrites in the oldest conglomerates, and the occurrence of red beds in the topmost Dharwar sequence, indicate that an oxygenic atmosphere evolved through the Dharwar times. The pre-Dharwar granitoids were probably tonalitic, and through time, alkali fractionation gave rise to the Champion gneisses, Peninsular gneisses and Closepet granites. The available data are inadequate to elucidate the fractionation pattern of the granitic crust of Peninsular India. Pyrolite and eclogite mantle are invoked for tectonomagmatic events during the pre-Dharwar and Dharwar times, respectively. Convection, and eclogite sinkers, are proposed as alternative geodynamic models. From the geochemical evolution, it is contended that the granite-greenstone belts of Peninsular India represent a more evolved crustal condition as compared to the Archaean granite greenstone belts of South Africa and Western Australia. Obviously, the granite-greenstone belts of all the shield areas, did not evolve simultaneously.

Structural Analysis of the Mount Wright Map-Area, Southernmost Labrador Trough, Quebec, Canada

Abstract: The Mount Wright map-area, covering approximately 7,200 km 2 of the Canadian Precambrian Shield, is situated at the southern end of the Labrador trough and lies across the Grenville metamorphic front. Subhorizontally interbanded charnockitic, granulite-facies Archean rocks, which are now restricted to the smaller northern part of the region and form a part of the Superior province, were deformed at least twice during the Kenoran orogeny (2,390 to 2,590 m.y. B.P.). The Archean rocks subsequently formed the basement on which lower Proterozoic (Aphebian) rocks of the Kaniapiskau Supergroup of the Labrador trough were deposited. South of the Grenville front, the supracrustal sequence and the Archean basement were later polyphase deformed and metamorphosed, mainly at the amphibolite-facies level, during the Hudsonian orogeny (1,640 to 1,830 m.y. B.P.) and the Grenville orogeny (1,150 to 850 m.y. B.P.). A study of the several tectonic episodes that affected this part of the Grenville province has been greatly enhanced by the occurrence of a thin but widespread group of ironstone, quartzite, and marble formations (the Gagnon Group) that provides excellent lithostratigraphic marker bands near the base of the lower Proterozoic succession. Analysis of the interference patterns within the Gagnon Group reveals the effects of at least three major phases of folding, the earliest of which postdated an initial period of metamorphism and migmatization. Isolated areas of granulite-facies rocks, occurring south of the front, probably represent zones of partially recrystallized Archean basement. Although it has been shown that, north of the Grenville front, the eastern part of the Labrador trough was strongly involved in the Hudsonian orogeny, it is difficult to assess the effects of this event in the Mount Wright map-area. We present a structural sequence in which the earliest folding of the lower Proterozoic supracrustal rocks is tentatively assigned to the Hudsonian orogeny.

Proterozoic Quench-Texture Basalts from the Labrador Geosyncline

Abstract: The occurrence of quench-crystal basalts from the Proterozoic Knob Lake and Doublet Groups of the Labrador Geosyncline is reported. The microscopic textures are similar to those in both modern abyssal basalts and Archean basalts from the Superior Province of the Canadian Shield. Textures of quench plagioclase and pyroxene have been well preserved but the plagioclase has been altered.

The Most Ancient Continental Margins

Abstract: Each continent of the world contains a Precambrian core or continental platform that includes one or more early Precambrian or Archean cratons. A pre-drift reconstruction of continents indicates that (1) the Precambrian continental crust formed a single asymmetric (one-sided) mass upon the earth’s sphere, and (2) Archean crust was confined to a narrow, irregular cresent, concave to the east, within it

Iron Ore Deposits of Western Australia--Geology and Development: ABSTRACT

Abstract: Although abundant iron ore was reported in Western Australia in the 1890s, only a few easily accessible deposits attracted interest before 1960. Intensive exploration since 1960 has increased the indicated and inferred reserves from a few hundred million to 24 109 MT. Most of the ore is high grade, and 95% of it is in the northwest of the state. Export began in 1965, and in 1974 six mines were operating solely for export, one for domestic use, and two for both purposes. Total production in 1973 was 74 million MT, and the total export tonnage (1965-73), including pellets, was 281.5 million MT. Geologically, there are three distinct ore types: hematite enrichment ore, pisolitic limonite ore, and sedimentary ore. Hematite enrichment ore developed during Proterozoic time within both Archean and Proterozoic banded iron formations (BIFs). Structurally controlled deep-water circulation removed Si, Ca, Mg, Na, K, and CO2, and introduced Fe, to effect metasomatic conversion of the BIFs to hematite and develop stratiform orebodies, a few of which exceed 1 109 MT. Secondary development of goethite has lowered the grade to about 63.5% iron. Pisolitic limonite formed along Tertiary drainages off the Hamersley basin. This material now caps sinuous mesas; it contains 40-60% iron. Sedimentary ore is qu ntitatively minor, and is restricted to the small high-grade Proterozoic coastal deposits of Yampi Sound. A total capital investment of A$1,570 million since 1965, the extent and quality of Western Australian ore reserves, and proximity to the Japanese market--all justify confidence that the present export status will be maintained; however, several problems must be solved before export and domestic-processing growth rates can increase.

Tests of Plate Tectonics: PART 1

Abstract: Geological and geophysical data from the ocean basins and the continents are now sufficiently abundant to demonstrate that all proposed models for the new global tectonics contain serious errors. For example, several sets of paleoclimatic data--specifically, the distribution on the continents and shelves of ancient evaporites, carbonate rocks, coals, and tillites--appear to be explicable only if the present positions of the rotational axis, continents, and ocean basins have been constant for at least 1,600 m.y. The distributions of fossil invertebrate and tetrapod faunas and floras likewise indicate the constancy of position of the rotational axis, continents, and ocean basins for at least 570 m.y. Faunal-realm studies are proving to be extremely useful, because many of t em demonstrate that the continents have been in about their present relative positions since Proterozoic time. A possibly fatal flaw in the plate-tectonics hypothesis is that the topologic requirements for moving the Americas away from Eurafrica eliminate any possibility of such movement unless the earth has expanded greatly during the last 150-200 m.y. Studies of climatic and biozoologic zones of the Mesozoic and Cenozoic show that expansion has not taken place. Other space requirements for the continents do not permit east-west movements since Archean time in the Northern Hemisphere, a conclusion now confirmed by the presence of continental crust across the North Atlantic beneath the Faeroe-Iceland-Greenland ridge. North-south movements of individual continents are limited to a few hundred kilometers--on the basis of paleoclimatic and paleontologic data. Movements involving the "opening" and "closing" of the Tethys--from present-day Spain to New Guinea--are restricted to distances of less than 300 km by detailed field geological and geophysical studies. If seafloor spreading is taking place in the Tethyan belt, lateral movements have been--and are--restricted to mantle movements and the overlying lithosphere is detached from the mantle. Ocean-basin studies show that island-arc trench fills, where subduction supposedly takes place, are undeformed. The volumes of undeformed sedimentary rocks in Layer 1 indicate that either (1) seafloor spreading has not taken place since late Mesozoic or earlier time, (2) subduction must take place seaward from the island arc trenches, or (3) there is no such process as subduction. Detailed studies of the Lesser Antilles and Tonga-New Zealand arcs prove that aseismic island chains seaward from both arcs have been in their same relative positions since mid-Mesozoic and late Paleozoic times, respectively. Preliminary studies of several other island-arc systems lead to similar conclusions. Sediment fills in fracture zones crossing midocean ridges also are undeformed--a remarkable fact if seafloor spreading is taking place. Many of these fractures continue onshore into the continents, where the proved senses of movement are the opposite of those predicted by "transform-fault" solutions. "Transform-fault" solutions, moreover, are accounted for more logically by known gravity models of midocean ridges than by hypothetical "plate" motions. JOIDES drilling results have been hailed as a "remarkable confirmation" of plate-tectonics predictions. Numerous alleged basalt basements have baked upper contacts, so that it is doubtful that such basalts really are basement. The first dating of such basalts from JOIDES End_Page 43------------------------- coreholes beneath Mesozoic rocks showed the basalt to be late Tertiary or younger. Thus another prop of the new global tectonics begins to crumble, and the age of the ocean basins remains unknown. A possible clue to the age of the ocean basins is the increasing number of discoveries of ancient rocks from the ocean basins and midocean ridges. Rocks ranging in age from 797 m.y. to 1,590 m.y. have been found from the Atlantic, the Pacific, and possibly the Indian Oceans. None of the ancient samples fit modern notions of plate tectonics.