U–Pb geochronology and Hf isotope ratios of magmatic zircons from the Iberian Pyrite Belt
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The Brazilian Northern region mostly corresponds to the Amazonian region of Brazil. The granitogenesis of this region covers the span of time from the Archaean to the beginning of the Late Proterozoic. Five groups of granitoids have been recognized: 1. Archaean (> 2.5 Ga) granitoids; 2. Transamazonian (2.0 ± 0.1 Ga) granitoids; 3. Middle Proterozoic (1.8 to 1.4 Ga) anorogenic granites of the Central Amazonian Province; 4. Middle Proterozoic (1.7 to 1.4 or 1.2 Ga) granitogenesis of the Rio Negro Province; and 5. Middle to Late Proterozoic (1.4 to 0.9 Ga) anorogenic granites of the Juruena and Rondonia Provinces. The Archaean granitoids are better preserved in the Eastern Amazonian region. The older ones are sodic granitoids, with dominant tonalitic composition, following the calc-a1kaline trondhjemitic trend. Archaean granodiorites are widespread in the Rio Maria region. They follow the calc-alkaline granodioritic trend. Potassic granites are associated with the sodic granitoids in the Amapa and Altamira regions. The Transamazonian granitoids are very abundant in the northern part of the Guyana Shield. The Agua Branca and similar granitoids (1.95 Ga) are the best example of Transamazonian granitoids in northern Brazil. They appear to be related to a high-alumina calc-alkaline magmatism and could represent a magmatic reflex in the Archaean Craton of the activity of the Transarnazonian Orogenic Cycle, The Middle Proterozoic anorogenic granites of the Central Amazonian Province form one of the large stanorogenic provinces of the world. Two-types of granites have been distinguished: a. alkaline granites similar to the A-type granites, probably generated by crustal anatexis of granulitic rocks; and b. magnetite-series granites, possibly derived by anatexis of meta-igneous rocks of granite-greenston e terranes. The ano rogenic granites of the Juruen a and Rondonia Provinces are similar, but younger than those found in the Central Amazonian Province. Three subgroups of granites were distinguished according to their ages. In the Rio Negro Province two peculiar granitoids occur: a. the sphene-amphibole bearing biotite granitoids that follow the suba1kaline monzonitic trend and which genesis probably involved mantIe derived magmas, associated with crustal ones; and b. the crust al two-mica granites derived by anatexis of metasedimentary rocks. In order to compare the characteristics of the five granitoid groups some petrological diagrams are presented and discussed. A brief comparison between the northern Brazilian region granitogenesis and those described in neighbouring regions is done.
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As now known, the Archean and Proterozoic appear to have been different worlds: the geology (tectonic style, basinal distribution, dominant rock types), atmospheric composition (O2, CO21, CH4), and surface environment (day-length, solar luminosity, ambient temperature) all appear to have changed over time. And virtually all paleobiologic indicators can be interpreted as suggesting there were significant biotic differences as well: (1) Stromatolites older than 2.5 Ga are rare relative to those of the Proterozoic; their biotic components are largely unknown; and the biogenicity of those older than approx. 3.2 Ga has been questioned. (2) Bona fide microfossils older than approx. 2.4 Ga are rare, poorly preserved, and of uncertain biological relations. Gaps of hundreds of millions of years in the known record make it impossible to show that Archean microorganisms are definitely part of the 2.4 Ga-to-present evolutionary continuum. and (3) In rocks older than approx. 2.2 Ga, the sulfur isotopic record is subject to controversy; phylogenetically distinctive bio-markers are unknown; and nearly a score of geologic units contain organic carbon anomalously light isotopically (relative to that of the Proterozoic and Phanerozoic) that may reflect the presence of Archaeans (Archaebacteria of earlier classifications) but may not (since cellularly preserved Archean-age Archaeans have never been identified).
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The geochronological understanding of the Precambrian crystalline shield of North-West Greenland (75–79°N) is at a rudimentary stage. Isotopic data from three major rock complexes—the Etah meta-igneous complex, the Kap York meta-igneous complex, and the Kivioq Havn gneiss and supracrustal complex—all show scatter indicating disturbed Rb–Sr isotope systems. This may reflect widespread reactivation of the crust in Proterozoic (Hudsonian) time. However, the majority of the samples define errorchrons that are regarded as geologically significant, and although the ages are poorly constrained, the data demonstrate the presence of both Archean and Proterozoic terranes.The Etah complex is Proterozoic in age (errorchron age ca. 1850 Ma), whereas the Kap York and Kivioq Havn complexes represent late Archean material (errorchron age ca. 2700 Ma). These ages compare favourably with isotopic age information from adjacent Canada in southeast Ellesmere Island and Devon Island, where correlatable Archean and Proterozoic rock complexes occur.
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A 1.5-2 m wide fine-grained undeformed acid dyke, cutting the Early Archaean Isua supracrustal succession, was found in 1978 (D.B., J.B.). Preliminary Rb-Sr isotope measurements (F.K.) of small hand samples suggested an unexpected mid-Proterozoic age. Additional material was collected in 1979 (J.B.). Owing to weight restrictions in the helicopter, some of the samples are smaller (100-500 g) than we would normally use, but we feel justified in presenting the results since they suggest Proterozoic granitic activity in the area, which has implications for the later history of the Archaean block.
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Most of central Africa is underlain by Archaean terrains (mostly below a Phanerozoic cover), represented by high-grade gneissic complexes and by low-grade granite-greenstone belts. The lowermost Proterozoic is represented either by gneisses in mobile zones or low-grade supracrustals in forelands. The remaining Lower Proterozoic is made of low-grade supracrustal metasediments in mobile zones. Such zones developed thus almost immediately after the end-Archaean cratonization. The successive mobile zones appear to have developed in a centrifugal pattern during the Lower-Proterozoic. The mineral wealth is unevenly distributed. Only some greenstone belts have given an appreciable gold output, whereas the gneissic Archaean terrains have proven to be almost barren. Iron remains an important resource of the Archaean, as manganese is for the Lower Proterozoic. Uranium and some Cu, Co has been found in the Lower Proterozoic of respectively Gabon and Uganda.
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