Showing papers in "Brazilian Journal of Geology in 1982"

The geologic evolution of south america during the archaean and early proterozoic

Abstract: ln the South American continent, Archaean and Lower Proterozoic terranes can be found in all tectonic domains, but especially within the Amazonian and Sao Francisco cratons. They can also be found within the Sao Luis, Luis Alves and Rio de La Plata cratonic fragments, as well as within all mobile belts of the Brasiliano orogenic cycle, as reworked basement. The Arequipa massif is lhe only identified Lower Proterozoic unit in the Andean belt. Within the Amazonian craton, the Central Amazonian province is made up of Archaean rocks, exhibiting cratonic conditions since at least the Lower Proterozoic. It is bounded to the north by the Maroni-Itacaiunas mobile belt, in which the major tectonomagmatic episode occurred in the early Proterozoic, the Transamazonico orogenic cycle. It comprises large portions of supracrustal rocks, identified as greenstone belts, associated to gneisses and migmatites, as well as fragments of high-grade polymetamorphic terranes in which Archaean ages were obtained. Within the Sao Francisco craton, three main types of ancient geologic terranes occur: 1) Archaean granite-greenstone terranes, such as the Brumado-Anajo area in Bahia, and the Quadrilatero Ferrifero area in Minas Gerais; 2) Lower Protcrozoic supracrustal belts, such as the Jacobina, Serrinha and Contendas-Mirante sequences in Bahia, and the Minas Group in Minas Gerais; and 3) Medium to high-grade metamorphic terranes, subjected to extensive granitization during the Transamazonic orogeny (Salvador-Juazeiro mobile belt), and including Archaean "cratonic fragments", of grunulitic composition, such as the Jequie-Mututpe complex in Bahia. Ancient terranes were also found within the mobile belts of the Brasiliano cycle, where they make up their ensialic basement. Lower Proterozoic type radiometric ages are predominant, especially within the Borborema province and the Ribeira belt. Taking into account the general distribution of the ancient terranes, it seems that a major part of the South American continental crust was already existent as such just alter the Transamazonico cycle. Moreover, since the known Transamazonico belts are ensialic, it is clear that large portions of continental material were formed earlier, in Archaean times.

Compartimentacao e evolucao tectonica da bacia do parana

Abstract: The Parana Basin was developed on one of the younger areas of the South American platform, and its basement cratonization had continued up till the Eopaleozoic. The Brazilian South-Southeast folded belt, with a NE-SW strike in Upper Pre-Cambrian, probably had watched the development of subsequent aulacogenic trenches with NW-SE trends in the Cambro-Ordovician. extending easterly from the present Andean belt. These "rift" type zones had controlled the pericratonic sedimentation on the western border area of the present basin during the Silurian. During the Devonian, a portion of the future Parana Basin, belonging to the present Andean belt, had been divided into sections by the uplift of the Asuncion arch. However, the NW·SE weakness zones, which were previously impressed on the basin basement, had strongly controlled the Devonian sedimentation, the Lower Carboniferous tectonic reactivation and erosion, and the very early Permocarboniferous sedimentation. In these espisodes, the vertical movements had been controlled by the old "rift" pattern which is related to the aulacogenic channels in the basement. The basin had acquired an intracratonic sineclise character, which was clearly defined during the Middle-Upper Permian and which was maintened up till the Lower Triassic. In southern Brazil, a wide uplift of the crust, which had begun in the Jurassic-Cretaceous, had caused the vulcanism phenomenon and the old aulacogen zones of weekness and fault had been the tracks for the extrusion of the lava. The consequent shrinking of this crustal uplift area had controlled, subsequently, the subsidence process of the coastal basins, such as Campos and Santos, as well as the reactivation and the sedimentation of the Bauru Group at the continent. The successive boundaries of this contracting movement in the domic area were controlled by the NE-SW alignments. The rift phase of the coastal basins of that time had caused the elevation of the coastal margin of the old NE-SW folded belt and step faults were formed in both direction through the marginal basins and through the continental interior. The structures of Piratininga. Agudos, Artemis, Anhembi, etc, are examples of these step fault structures. Contemporaneously, the Upper Cretaceous sedimentation had begun in the northern area of the basin. The zones of NW-SE alignments had subdivided the Parana Basin, and had controlled the areas of maximum sedimentation, until the Tertiary, when the basin had been even more subdivided by the reactivation of these zones. This subdivision included the northern area which is still receiving sedimentation at the present time. The "riff" control of the basement, especially during the early Paleozoic sedimentation and reactivation and the "rift" strong influence during the Mesozoic evolution, allows the classification of the Parana Basin as an intracontinental, cratonic, type 2A complex basin, similar to the west Siberian basins.

The gravimetric information and the Archean-Proterozoic structural framework of Eastern Brazil

Abstract: AB8TRACT The major gravimetric anomalies emerging from the Bouguer map are described in this paper and interpreted in the light of the geologic-tectonic information. A block-structure mostly due to the Jequié Event (2.7 ±0.2 Ga) is recognized. These blocks are constituted of granite-gneiss terrains, with associated greenstone belts, and are separated by high-grade belts. The contact is marked by a discontinuity besides a zoneof crustal thickening; lhe high-grade belts correspond to lhe lower portion of the uplifted block. The contact zones and block borders were the sites for the Proterozoic supracrustal formations, thermo-tectonic processes, reworking and faulting. The São Francisco Craton is the stable block of the Brasiliano Cycle; except in the limited areas of regeneration and reworking by Proterozoic cvcnrs. it preserves Archean terrains. Some geometric suggestions in the reconstructed Jequié structural framework may be envisaged for the continent collision model, but no petrotectonic evidence is known. The sarne framework and the younger geologic evolution picture impose restrictions upon plate convergence models when applied to Proterozoic reconstructions.

The Pernambuco-Alagoas Massif, Northeast Brazil

Abstract: The Pernambuco-Alagoas median massif which runs E-W in the states of Pernambuco and Alagoas, Northeast Brazil, constitutes a large segment of Archean to Early Proterozoic rocks, surrounded by younger fold systems of Middle to Late Proterozoic age. Its litho-structural context is rather complex, comprising several gneiss-migmatitic suites associated with mafic to ultramafic rocks, and large gneiss-granitic batholiths. Granitic to syenitic stocks of Brasiliano age pierced locally the ancient rocks of the massif. Apparently, the strucrural trends of the Sao Francisco craton, to the south, continue pervasively in this median massif. The schistosity of the surrounding fold systems typically verges onto this unit. A late phase of local deformation is represented by granitic masses which overthrust the southern and northern borders and partially masked the original limits of the massif. Geophysical surveys helped to precise its limits. lmportant gravimetric and magnetometric anomalies shed some light on the geodynamic evolution of the massif plus its surrounding systems.

Geologic and metallogenetic patterns in the Archean and Early Proterozoic of Bahia State, Eastern Brazil

Abstract: The territory of Bahia State is part of the Sao Francisco structural province. The basement of the province is characterized by granite-greenstone and high grade metamorphic terrains developed during the Archean and Early Proterozoic. The Early Archean is represented by small tonalitic-granodioritic and enderbitic-charuockitic nuclei 3,500 Ma old. Several portions of gneissic-migmatitic and granulitie terrains have ages of 3,000 Ma and Rb/Sr isochronic values of 2,700 Ma that represent the principal phase of the Jequie geotectonic cycle. During the Early Proterozoic, extensive areas of the basement of the Sao Francisco province underwent intensive remobilization, through granitization and K-metasomatism related to the Transamazonico geotectonic cycle (2,200-1,800 Ma). Besides greenstone belts the granite-greenstcne terrains contain volcano-sedimentary (greenstone belt like) and platform sequences developed since the Archean to the Early Proterozoic. The high grade metamorphic terrains contain remnants of supracrustal sequences and basic-ultrabasic complexes. The metallogenic Framework of Bahia shows that more important mineral deposits such as Au, Cr , Cu, Fe-Ti-V, Pb-Zn-Mn, magnesite and tale are related to Archean and Early Proterozoic evolution. Gold deposits are located in volcanic rocks of greenstone belt environments, and in conglomerates of Witwatersrand type. Economic deposits of Cr, Cu, Fe-Ti-V are associated with mafic-ultramafic complexes in high grade metamorphic terrains. Pb-Zn and Mn mineralizations occur in volcano-sedimentary (greenstone belt like) sequences and the platform sequences contain large deposits of magnesite and talc.

Distribuição espacial e temporal das unidades litoestratigráficas paleozóicas na parte central da Bacia do Paraná

Abstract: "Paulipetro" has drilled about a score of stratigraphic and wildcat wells in the Parana Basin getting unique informations of the interrelations and distributions of the rock units in its sedimentary filling. This paper presents the new stratigraphic chart demonstrating time-space relationships of the rock units and a revised chrono-stratigraphic classification for the Paleozoic sedimentary section of the Parana Basin. As an initial approach, the proposed chrono-stratigraphic column presents only three units (Serie Campos Gerais, Serie Rio Tiete and Serie Serra do Espigao). However, based on the distinctive geological evolution of the Parana Basin, it has essentially a regional character. The introduction or a new time-space diagram and simplified chrono-stratigraphic column are justified by the demand of improved stratigraphic correlations for petroleum exploration.

Geochronology of the southern part of the São Francisco Craton

Abstract: The Sao Francisco Craton, a large tectonically stable area in Late Proterozoic times, is still poorly known, in respect to its geologic evolution. This work deals with the geochronological interpretation of all available data for the southern part of the cratonic area, about 150 radiometric results, many of which were added to recently, by the work of the Centro de Pesquisa Geocronologica (CPGeo) do Instituto de Geociencias da Universidade de Sao Paulo (IG-USP). Several nuclei, with Late Archean ages, were brought to light by Rb-Sr whole rock measurements. The values range between 3,000 and 2,750 Ma indicating the existence of an ancient terrain formed mainly of granitoid rocks (gneisses, migmatites, granites, amphibolites), in part associated to supracrustals interpreted as greenstone belts (Nova Lima, Piui, etc). Some other Rb/Sr isochron diagrams, and many of the K-Ar apparent ages indicate that the Transamazonico orogeny was the last important tectonothermal event, which overprinted the Archean terrain. A relation was also noted between the radiometric patterns and its geographic situation. The older rocks are situated mainly in the western part of the studied area, where the last regional cooling is related to the Minas diastrophism, while only in the southeastern corner, the Quadrilatero Ferrifero area, a Late Proterozoic thermal episode is charactcrized by the K-Ar radiometric data.

Geological and geochemical evolution of the Archean continental crust

Abstract: The main geological features of Archaean crust are outlined in relation to the geochemical characteristics of the various Archaean crustal components. High-grade gneiss terrains are predominantly made up of a bimodal suite oftonalitic and (subsidiary) basic gneisses, with local intercalations of layered ultramafic-mafic-anorthosite complexes and a metasedimentary assemblage of quartzite-pelite-marble-banded ironstone. Major and trace element compositions of the tonalitic suite are best accounted for through melting of a mafic source under hydrous high pressure conditions whereas compositional variations within the basic gneisses have resulted from low-pressure fractionation. The layered complexes have also evolved through low pressure fractionation and probably represent fragments of ocean crust emplaced through lateral accretion, along with continental margin sediments, in an environment analogous to a subducticn zone. The formation of granulites in the lower crust is related to the removal of heat-producing elements (K, Rb, Th, U) by a CO 2 -rich fluid phase; they do not represent the residues from intracrustal partial melting. Perhaps the best modern analogue of Archaean high-grade terrains is the Cordilleran margin of southern South America made up of deep seated tonalitic plutons with mafic inclusions and intercalated metasediments. Greenstone belts also have a dose analogue in the ensialic marginal basins which occur at active continental margins. Differences between Archaean and modern crustal generation processes are more in degree than in kind.

Archean to Early Proterozoic tectonics and crustal evolution: a review

Abstract: Hypothesis on the formation and evolution of the Earth's continental crust in the Early Precambrian have to account for the independent and directionally consistent motion of crustal segments of considerable dimensions since at least 3,5 Ga ago, and at minimum velocities comparable to those of today. It seems established, therefore, that plate tectonics has governed lithospheric evolution since the generation of rigid crustal segmenta, and it is speculated that a change in style of plate interaction, rather than a change in fundamental mechanisms, has determined non-uniformitarian crustal evolution towards the present Wilson cycle. Uncertainties in the reliable reconstruction of Archaean tectonic settings are caused by low-temperature rock alteration that may obliterate primary magma tic trends, by the variable possible sources and melting processes for the generation of bimodal and calc-alkaline associations, and by disagreement on the interpretation of rocks with "primitive" isotopic characteristics. It is suggested that magmatic underplating played a major role in lithospheric growth and that reworking of continental crust during large-scale crustal differentiation was more important in the Archaean than recognized by most currently popular models. The generation and survival of first continental crust is seen in analogy with the evolution of Iceland, and later greenstone belt development was probably largely intracontinental, following a rift-and-sag mede in response to crustal rearrangements. Most high-grade terrains are considered to be older than neighbouring or overlying greenstone associations and were brought to the surface through low-angle thrusting. Archaean lithospheric growth built stable cratons that experienced little internal deformation in early Proterozoic times. Less stabilized crust reacts to large-scale distortions predominantly by stretching, elongate basin-formation and "ensialic" orogeny through mechanisms ranging from crust restacking to transform shearing, finally generating mobile belts. Locally, modern-type convergent plate margins become recognizable at about 2.2 Ga ago while the full Wilson cycle is finally established in the late Proterozoic.

Archean and early proterozoic complexes of santa catarina, parana and sao paulo states, south-southeastern brazil : an outline of their geological evolution

Abstract: The Luis Alves, Serra Negra and ltatins complexes are Arehean to early Proterozoic metamorphic units in the Brazilian States of Santa Catarina, Parana and Sao Paulo, south of the 24°S parallel. This paper summarizes their geological evolution based on petrographic, chemical, and mainly geochronological data. The three complexes consist of metamorphic rocks of the granulite facies, dominantly: norites, enderbites. charnoenderbites, ultramafites of granulite facies, kinzigites, quartzites, biotite-hornblende gneisses, meta-quartz diorites, metadiorites, metagabbros, serpentinites and talcites. Petrographic and chemical data support an igneous origin for most of these rocks. The radiometric age determinations made bythe Rb-Sr and the K-Ar methods on different materiais gave results related, for the most part, to the Pre-Jequie (about 3,100 m.y.), Jequie (2,600 ± 200 m.y.) and Transamazonico (2,000 ± 200 m.y.) events. Geochronological data suggest that the three complexes were formed predominantly by crustal accretion-differentiation processes.

Crustal evolution and precambrian metallogenesis in western india

Abstract: Proterozoic supracrustal rocks of continental derivation form the major rock groups in Rajasthan and adjacent states in western lndia and act as repositories for conspicuous base metal mineralization. The continental crust in this region evolved through three main cycles: the Bundelkhand-Berach granite pluton intruded during the Archaean-Prcterozoic transition and formed thc continental nuclei. Extensive migmatization of early volcanics and associated sedimenta during the process of cratonization brought about the evolution of the Banded Gneissic Complex. Stabilization of this basemcnt took place around 2.00a. when two slowly sinking major littoral basins received the huge pile lhe Aravalli sedimentary package through almost a billion years and the subsequent c1astogenic Delhi sedimentation. The recurrent syn-sedimentary mctallogencsis in the Precambrians of western India has been examined by presenting the salient characteristics of six, economically important, metamorphosed, sediment-hosted, stratiform, base metal sulfide deposits: one massive, early Proterozoic, Zn-sulfide deposit in lhe migmatized sediments, probably related to initial basement volcanism: two mid-Proterozoic Pb-Zn-(Cu) deposits emplaced in the Aravalli sequencc through rift-fed exhalations related to basic-felsic volcanism ; threc late Proterozoic Pb-Zn-Cu/Cu/Fe sulfide deposits which formed through sea water - basalt interaction or througb basinal brines enriched by convective circulation.

Archaean greenstone terranes : geologic evolution and metallogenesis

Abstract: The principal components of Archaean granite-greenstone terranes worldwide are briefly outlined. Despite a wealth of local detail that bas emerged from these regions over the past two decades there remain to be resolved many fundamental issues. This papel attempts to highlight Archaean complexity and stresses the intimate interrelationships of processes and responses in the geochemical, lithological, structural and metallogenetic history or granite-greenstone evolution. Although many instances exist where sequential development of greenstone belts can be observed it nevertheless appears that, in terms of geologic and geotectonic evolution, inadequate constraints still provide wide scope for conjecture and speculation as to the events that led up to lhe final construction of the Archaean shield areas.

Geology and copper mineralization of curaçá river valley, bahia

Abstract: The high grade terrains of the N-S trending polycyclic belt known in Northeastern Bahia have been investigated along the Curaca Valley region, as important copper mineralizations (180 millions tons of ore with 1,0% Cu, mostly in the Caraiba district) were discovered. The Curaca Valley region is essentially formed by granodioritic to tonalitic gneisses (older basement?), Archean metasediments (quartz-feldspathic gneisses, leptynites, cordierite-sillimanite-garnet-biotite gneisses, quartzites, magnetite quartzites, calc-silicate rocks), and associated mafic-ultramafic intrusions. This lithologic assemblage was affected by rnetamorphism of granulite facies and tight folding apparently related to the Jequie event, and was reworked later during the Transamazonian event (retrometamorphic transformations, migmatization, granitization, deformation). The mafic-ultramafic intrusions (hyperstenite-norite-gabbro-anorthosite) are interpreted as being derived from differentiated tholeiitic magma and hostwell known mineralizations (magnetite-chalcopyrite-bornite-ilmenite, sorne pyrrhotite and pyrite, scarce pentlandite, violarite, millerite, tetrahedrite, hematite).

The lower precambrian of china

Abstract: The Lower Precambrian of China consists of Archean and Lower Proterozoic formations formed probably prior to ca. 1,800-1,900 Ma. They are exposed chiefly in the North China Platform. Archean rocks are composed mainly of gneisses, granulitite and plagloclase-amphibolite of the amphibolite facies, with the lower part containing pyroxene-gneiss and granulite of the granulite fades. The parent rocks were not well differentiated sedimentaries and volcanics, forming two volcano-sedimentary cycles. During the Archean (before 2,500-2,600 Ma), the tectonic environment over an extensive area, was quite uniform yet fairly active. Towards the end of Archean there prevailed median- to high-grade metamorphism often accompanied by rather intensive migmatization. In the first Early Proterozoic epoch, a thick sequence of volcano-sedimentaries were accumulated in some marine troughs regarded as eugeosynclinal and developed on the Archean sialic basement, such as the Wutai Group. The protoliths were lhe rather widespread volcanics, the "semipelitic" and pelitic types and turbidites, mainly of greenschist fades and partly amphibolite facies, occasionally accompanied by migmatization probably not later than 2,300 Ma. After that, a stratigraphic pile accumulated in the miogeosynclinal basins or troughs as represented by lhe HutuoGroup, and was composed of coarser clastics, pelitics and stromatolite-bearing Mg-rich carbonates which show rhythmic deposition. Their greenschist metamorphism probably occurred during ca. 1,800-1,900 Ma and this marked the end of the Early Precambrian history.

Geologic evolution of the Serra dos Carajás, Pará, Brazil

Abstract: The Serra dos Carajas is located in southern Para, in the region between the Araguaia and Xingu rivers, and has been regarded very recently as the most promising and potentially interesting metallogenic province in South America. ln order to characterize its geologic evolution, a geochronological program was carried out by a cooperative effort of Docegeo and the CPGeo-USP, with the help of structural, petrologic and geological information. In the region, a polymetamorphic basement occurs in which gneisses arid related rocks are associated to the supracrustal rocks of the Salobo Formation. The iron formations and low grade metamorphics of the Grao-Para Group overlie the basement, and are covered by the sedimentary rocks of the Rio Fresco Formation. Post-orogenic and cratogenic granites occur, cutting through all the above mentioned units. The granitic rocks indicated Rb-Sr whole rock ages up to 1,850 m.y., K-Ar determinations on micas from the metamorphic indicated that the regional cooling occurred at about 1.950 m.y., after the main events of the Transamazonian orogeny. Some transamazonian results were also obtained on micaceous schists of the Salobo Formation. However, several analytical points, in a Rb-Sr isochron diagram, indicated a Late Archean age for their main rnetamorphic episode.

Precambrian mafic and ultramafic rocks of the cana brava complex, brazil - mineral compositions and evolution

Abstract: The Cana Brava mafic-ultramafic complex, Goias State, Brazil, is an Early Precambrian differentiated massif formed by intrusion and differentiation of a presumably basaltic parental material. Rock types cover a continuous sequence ranging from harzburgites to pyroxenites to ferronorites and ferrogabbros, as indicated by changes in mineral associations and mineral compositions. After the emplacement, the mafic-ultramafic suite was affected by several sub-solidus reequilibrations. The first one took place at approximately 900°C and pressures of 6-7 kb. Textural features as well as mineral distribution of certain elements show that complete equilibrium was not achieved during this event. A successive partial recrystallization converted some of the rocks on an high-grade amphibolite assemblage. A late low-temperature event caused the formation of serpentinites, rodingites and tale schists. P-T estimates for the high-temperature reequilibration at Cana Brava coincide broadly with results obtained from other Early Precambrian Brazilian mafic-ultramafic complexes. The values thus estimated were plotted to obtain geothermal gradients to be compared with those presented in the literature for other Precambrian occurrences.

Contribuição da geoquímica das terras raras à caracterização dos carbonatitos da fazenda varela, lages, sc

Abstract: As a part of a petrological and geochemical study the distribution of the rare earth elements in Fazenda Varela carbonatites was discussed. These carbonatites were the first described in Lages Dome. Santa Catarina State, Brazil. They intruded and fenitized sedimentites of the Tubarao Group; the fenitizing fluids are dominantly potassic. The main constituents of the carbonatites are ankerite, barite, K feldspar, and synchisite; secondary apatite is rare. High Ba/Sr, Nb/Ta, and Th/U ratios are conspicuous. The concentration of the rare earth elements is very high relative to the concentration in chondrites, the mean in the earth crust and even in most carbonatites; normalized distribution curves show higher concentration of the light lanthanides (Ce group) in comparison with the heavy lanthanides (Y group). In addition to other petrological and geochemical features, high absolute and relative concentrations of the light lanthanides indicate that these carbonatites represent the last phases of carbonatitic fractionation and reinforce, the possibility of occurence of carbonatites from the early phases of crystallization at the Lages alkaline district.

Geochemical and stable isotopic studies of the Crown and Mararoa reefs, Norseman, Western Australia

Abstract: The quartz reefs of Archean age at Norseman, Western Australia are restricted to the flatter-dipping sections of reverse faults and the gold mineralization is mainly stratabound. Structural and petrographic data indicate that the reefs were emplaced prior to the peak of metamorphism, but gold distribution and wallrock mineralogy were modified by metamorphism and subsequent deformation events. "Bucky' quartz crosscuts laminated quartz and was deposited during an episode of hydraulic fracturing. Gold and sulfides are concentrated on fractures in the laminated quartz reefs. Wallrock assemblages of the Crown and Mararoa reefs require two temporally distinct phases of alteration synchronous with the emplacement of laminated and "bucky" quartz reef. The first involved the deposition of hydrothermal actinolite and biotite, whereas chloritic overprinting of these assemblages characterizes the second phase of alterarion. Both potassic and chloritic assemblages were modified by the crystallization of metamorphic actinolite and tremolite. In general, single stage alteration is characterized by the addition of volatiles, K 2 0 and CaO, and the loss of Na 2 O. Changes in whole-rock geochemistry during chloritic overprinting of the potassic assemblages include losses of CaO, Al 2 0 3 , Na 2 0 and K 2 0, and a gain of MgO. The δ 18 O values of the Norseman wallrooks range from 5.8 to 9.1 ‰ with a mean value of 7.2 ±0.9 ‰ (16 sampes ). Reef quartzes also exhibit a narrow range of δ 18 O values (11.7 ± 0.4°‰, 26 samples). The wallrock assemblages are cornpatible with a mineralization temperature ≥ 400°C. The calculated δ 18 O value of the water in equilibrium with the quartzes at these temperatures is 7‰. Waters involved in the subsequent metamorphism have identical δ 18 O values which suggest they have a common source, connate fluids and hydrous minerals in the greenstone pile. The observed timing relations and isotope systcmatics suggest that the ore fluids were connate brincs which were mobilized during tectonic dewatering of the volcano-sedimentary pile and channelled up early faults.

Um novo tipo de estrutura em lavas ultramáficas: greenstone belt crixás, go

Abstract: This paper describes a new type of ultramafic flow at the basal portion of Crixas Greenstone Belt. It is characterised by randomly distributed orbicular structures of various dimensions. The structures show a carbonate nucleus and an external shell of talc, serpentine, and chlorite. Sometimes, centrally structured pseudomorphs of pyroxene and olivine are observed. The structures are thought to be originated from local turbulent flows provoked by lamellar flow of ultramafic lava according to the model of Alexander and Heinrich (1978) to orbicular lamprophyres of the alkaline mafic intrusion of Freemont County, Colorado. Occurrence of such structures in ultramafic lavas reinforces the possibility that liquid migration by lamellar flow, and consequent local turbulences along its trajectory, may originate orbicular structures, Besides, the presence of pseudomorphs found in the center of the orbicules suggests the previous existence of olivine crystals in suspension in the ultramafic magma.

The Archaean and Lower Proterozoic of Central Africa

Abstract: 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.