West African Shield Research Articles

A COMPARATIVE ANALYSIS OF DIAMONDS IN NON-KIMBERLITIC ROCKS OF THE WORLD AND NEOGENE SANDS OF UKRAINE

The diamond mineralogy from a number of different non-kimberlitic occurrences of the world was analyzed and a comparison to diamonds from Neogene placers of Ukraine was made. Diamonds from lamprophyres of the Canadian Shield, metakomatiites of the Guiana and West African Shields, and ultra-metamorphic rocks in Asian, European and African continental occurrences were considered. In general, Ukrainian Neogene placer diamonds have many similar mineralogical features to diamonds from lamprophyres and metakomatiites, which differ little from diamonds from kimberlites and lamproites. Ukrainian placer diamonds are characterized by their micron sizes (≤0.5 mm), many of them are colored and have a cubic habit and nitrogen-free compositions, relatively frequent spectral Ib and Iаb types, and orange photoluminescence behavior. In addition, many diamonds are characterized by a relatively high content of hydrocarbons (i.e., CH2, CH3 groups and bonds >C=CH2), OH groups, also C=O, N-H, CO3, NO3-groups. This is evidence for a volatile-rich environment during the diamond crystallization. The formation of diamonds from lamprophyres and metakomatiites, as well as from kimberlites and lamproites, occurred under mantle temperatures and pressures. The relatively recently discovered diamond-bearing lamprophyres and metakomatiites have a number of similarities. They are: i) both formed on the edges of Archean cratonic structures, ii) their old ages (2.7 billion years ago and 1.83 billion years ago for the lamprophyres and 2.2 billion years ago for the metakomatiites), iii) both are strongly metamorphosed, iv) both contain many fragments of rocks of various origins, v) both contain rare xenograins of mantle minerals having diamond-bearing peridotite and eclogite associations, and vi) the diamonds themselves are often microcrystals and many are colored and have a cubic habit. The main difference between diamonds from the lamprophyres and metakomatiites is in their carbon isotopic signatures and their thermal history in the mantle. This is expressed by their different nitrogen contents and the different degree of its aggregation. Diamond-bearing ultrametamorphic rocks are mainly Paleozoic-Mesozoic in age (i.e., 531-92 million years) and occupy a tectonic setting at convergent plate boundaries unlike diamond-bearing lamprophyres and metakomatiites. Diamonds from ultrametamorphic rocks differ from diamonds from lamprophyres and metakomatiites as well as from Ukrainian placer diamonds from Neogene sands in a number of ways.

Geochemical reconnaissance of the Guéra and Ouaddaï Massifs in Chad: evolution of Proterozoic crust in the Central Sahara Shield

Abstract In 1964, W.Q. Kennedy suggested that the crust of Saharan Africa is different from the rest of Africa. To date, the geologic evolution of this region remains obscure because the age and composition of crystalline basement are unknown across large sectors of the Sahara. Most of Africa comprises Archaean cratons surrounded by Palaeo- to Mesoproterozoic orogenic belts, which together constitute Africa’s three major shields (the Southern, Central and West African Shields), finally assembled along belts of Pan-African rocks. By contrast, central Saharan Africa (5.3x106 km2), an area just over half the size of Europe, is considered either as a Neoproterozoic region constructed of relatively juvenile crust (0.5 to 1.0 Ga), or as an older (North African) shield that was reactivated and re-stabilized during that time, a period commonly referred to as “Pan African”. Here, using U-Pb zircon age determinations and Nd isotopic data, we show that remote areas in Chad, part of the undated Darfur Plateau stretching across ¾ million km2 of the central Sahara, comprise an extensive Neoproterozoic crystalline basement of pre-tectonic gabbro-tonalite-granodiorite and predominantly post-tectonic alkali feldspar granites and syenites that intruded between ca. 550 to 1050 Ma. This basement is flanked along its western margin by a Neoproterozoic continental calc-alkaline magmatic arc coupled to a cryptic suture zone that can be traced for ~2400 km from Tibesti through western Darfur into Cameroon. We refer to this as the Central Saharan Belt. This, in a Gondwana framework, is part of a greater arc structure, which we here term the Great Central Gondwana Arc (GCGA). Inherited zircons and Nd isotopic ratios indicate the Neoproterozoic magmas in the central Sahara were predominantly derived from Mesoproterozoic continental lithosphere. Regional deformation between 613 to 623 Ma marks the onset of late alkaline granite magmatism that was widespread across a much larger area of North Africa until about 550 Ma. During this magmatism, the region was exhumed and eroded, leaving a regional peneplain on which early Palaeozoic (Lower-Middle Cambrian) siliciclastic sediments were subsequently deposited, as part of a thick and widespread cover that stretched across much of North Africa and the Arabian Peninsula. Detrital zircons in these cover sequences provide evidence that a substantial volume of detritus was derived from the central Sahara region, because these sequences include ‘Kibaran-age’ zircons (ca. 1000 Ma) for which a source terrain has hitherto been lacking. We propose that, in preference to calling the central Sahara a “ghost” or “meta” craton, it should be called the Central Sahara Shield.

Field Geology and Structures of Migmatitic Gneisses Around Ibillo-Okene Area, Southwest Nigeria

Ibillo-Okene migmatites are a typical example of the Basement Complex rocks of southwest Nigerian, which is part of the larger West African shield. They lie directly north and constitute the northern closure of the Igarra Schist Belt. They are interbanded with relics of metasedimentary units called older metasediments as well as overlain by younger metasediments. These units are affected by co-genetic or similar episodes of deformation that has both petrogenetic and economic significance as observed in Igarra Belt south of this area. Though this area is interesting in terms of geology and tectonic setting, detailed geological and structural interpretation of features in the area is lacking due to limited accessibility. Land Sat 8 and shuttle radar topographic mission (SRTM), data constitute useful tools for preliminary reconnaissance survey as well as structural interpretation. Based image interpretation, a minimum of three deformation events were identified: The first event (D1) which is essentially a ductile deformation produced foliations and isoclinal folds which are axial planar such that both the eastern and western limbs of the folds are parallel with regional metamorphic foliation trend S1 (NNW-SSE) and dominant lineament trends (ENE-WSW). The second event (D2) is believed to have produced a progressive reactivation of the D1 event with accentuation of the E-W compression leading to folding of the axes of F1 folds. The third event (D3) has two deformational stages: a ductile deformation at the onset at a higher temperature and pressure and a brittle deformation at the later stages with lower temperature and pressure. Geologic fieldwork which was further conducted to validate structures and units identified from image processing revealed that the migmatites are essentially metatexites which to the northwest is dialational, dialational net-structured in the northeast and straumatic in the southeast. The southwest of the area constitute the basinal closure of the Igarra area overlain by relics of the metasedimentary units. This research underscore the relevance of remotely sensed data in geological investigation whose results have important implications in terms of regional tectonics and geological mapping as well as in land-use planning and other areas such as hydrogeology or geotechnics. Keywords: Migmatite, Ibillo, Okene, metasediments, Remote sensing. DOI : 10.7176/JEES/9-2-08

Litho-structural analysis of eastern part of Ilesha schist belt, Southwestern Nigeria

The Ilesha schist belt is an excellent example of high strain shear belt within basement complex of southwestern Nigeria which is part of the larger West African Shield. The Ilesha schist belt is characterised by metasediment-metavolcanic, migmatite-gneiss and older granite rocks and the occurrence of a Shear zone which has been traced to and correlated with the central Hoggar Neoproterozoic shear zone as part of the Trans-Saharan Belt. Although the area is interesting in terms of geologic-tectonic setting, however, detailed geological assessment and structural interpretation of features in this area is lacking due accessibility problem.For these reasons we applied principal component analysis (PCA) and band ratio (BR) techniques on Landsat 8 OLI data for lithological discrimination while for structural interpretation, filtering techniques of edge enhancement and edge detection was applied on digital elevation model (DEM) acquired by shuttle radar topographic mission (SRTM) sensor. The PCA outperform BR for discrimination between quartzite and granite which are the most exposed rock units in the area. For structural interpretation, DEM was used to generate shaded relief model and edge maps which enable detailed structural interpretation. Geologic fieldwork was further conducted to validate structures and units identified from image processing. Based image interpretation, three deformation events were identified. The first event (D1) which is majorly a ductile deformation produced foliations and folds whose axial planes trend in NNE-SSW. The second event (D2) resulted in reactivation and rotation of the D1 structures particularly the folds in the NE-SW. The third event (D3) produced a transgressive deformation starting with the ductile deformation resulting in the development of sigmoidal structures oriented in NE-SW to E-W direction and the brittle deformation occurring at later stages producing fractures oriented in the E-W to NE-SW directions.These results have important implications in terms of regional tectonics and geological mapping as well as in land-use planning and other areas such as hydrogeology or geotechnics.

Charnockites and UHT metamorphism in the Bakhuis Granulite Belt, western Suriname: Evidence for two separate UHT events

The Bakhuis Granulite Belt in western Suriname is an ultrahigh-temperature (UHT) metamorphic terrain in the centre of the Paleoproterozoic (Transamazonian) Guiana Shield. Next to the UHT granulites, the belt contains a 30 by 30km body of orthopyroxene-bearing granitoids: the Kabalebo charnockites. This setting offers an excellent opportunity to investigate the source and origin of charnockite magmatism and the common association of charnockites with (ultra)high-temperature metamorphic terrains. We present a detailed geochemical dataset and LA-ICPMS zircon U/Pb ages with the aim to investigate the geochemical and geochronological relationship between charnockite magmatism and UHT metamorphism in the Bakhuis Granulite Belt. The Kabalebo charnockites have a characteristic trace element signature with elevated K2O, P2O5, Zr, REE and Ba coupled with mobile element depletion, which is a consequence of high-temperature melting of anhydrous but fertile granulitic crust. Field and geochemical evidence suggests that the intermediate granulites in the Bakhuis Granulite Belt are the source of the Kabalebo charnockites. The new U/Pb zircon ages indicate that charnockite magmatism (1993–1984Ma) postdates UHT metamorphism (2.07–2.05Ga) by at least 60Myr. We argue that it is not possible to maintain a thermal anomaly >200°C in excess of a normal geothermal gradient for such a prolonged period and hence conclude that the Bakhuis Granulite Belt has experienced two distinct periods in which temperatures >950°C were reached in the lower crust.The presence of comagmatic metadolerite enclaves in the charnockites establishes that mafic magmatism occurred contemporaneously with, and was the likely heat source for, charnockite magmatism at 1.99–1.98Ga. In contrast, the 2.07–2.05Ga UHT metamorphic event is not associated with felsic or mafic magmatism in the Bakhuis Granulite Belt or nearby Guiana Shield and postdates the suturing of the juvenile North Guiana TTG-greenstone belt with the West African Shield by at least 10Myr. We postulate that the UHT metamorphism at 2.07–2.05Ga is the result of mantle upwelling in a slab tear in the subducted West African slab that formed as the result of crustal scale shearing and boudinage. Prior to the final stabilisation of the Amazonian–West African Shield at 1.90Ga, northward subduction at 1.99–1.98Ga caused the emplacement of voluminous hot, mafic magma, resulting in partial melting of the Bakhuis granulite suite to form the Kabalebo charnockites. Charnockite magmatism was roughly contemporaneous with the emplacement of a large belt of shallow granites and felsic volcanic rocks in the SW Guiana Shield. Despite their similar age, the inherited zircon populations suggest that the charnockites are derived from a distinct, juvenile source while the felsic volcanic rocks include an Archaean protolith.

Pangaean megalineaments: geophysical indications for the existence of Laurentian counterparts of the 6000 km Trans-Saharan Tibesti Lineament, and implications for lithospheric tectonics and mineral deposits

A major 6000-km long west-northwest to east-southeast-trending Trans-Saharan lineament, extending from Kenya to Morocco, referred to as the Tibesti Lineament, has been described by Guiraud et al. (2000). This lineament shows up on satellite images, air-photo mosaics and terrain elevation models, as swarms of parallel faults. It corresponds to a major discontinuity of surface wave velocities in the mantle, suggesting that it deeply affects the lithosphere. The highest heat flow measurements in the West African shield are found adjacent to the Tibesti Lineament in the northern Hoggar uplift. Geophysical, geological, and geomorphological evidence suggests that the Tibesti lineament may have continued into North America (Laurentia), which collided with northwest Africa during the Appalachian-Mauritanide orogeny, coinciding with the late Palaeozoic assembly of the supercontinent Pangaea. The contiguous Laurentian lineaments which may be extensions of the Tibesti Lineament, are the here newly recognised Chesterfield Inlet Lineament and the Cabot Strait Lineament, both of which are situated in Canada. These lineaments are defined as linear pattern breaks in processed aeromagnetic anomaly maps of Canada. Some portions of them also can be detected in refraction and reflection seismic survey data ( e.g. , under Hudon Bay), and to a lesser extent, in the Bouguer gravity anomaly map of Canada. The lineaments have present-day (neotectonic) surface expressions in the form of inlets, submarine channels, straight river segments, and strings of linear lakes, extending over 3700 km, from Nunavut Province northwest of Hudson Bay, across Quebec and Newfoundland, and to the Cabot Strait separating Newfoundland from Nova Scotia. The highest heat flow in the Canadian Maritimes is found in the Cabot Strait. The Tibesti-Cabot-Strait-Chesterfield Inlet lineaments reflect deep-seated fracturing of the North African and North American lithosphere, involving both the crust and rigid upper mantle. These lineaments cut right across the grain of numerous orogenic belts dating from the Palaeoproterozoic, Late Mesoproterozoic, and Palaeozoic. By analogy with other large transcontinental lineaments which contain ore deposits, these newly identified megalineaments may be prospective for economic mineral deposits, such as hydrothermal gold and base metal mineralization, and diamondiferous kimberlites.

The Moroccan Hercynides

In Northern Africa deformed Paleozic rocks are observed, mainly in Morocco where they are present in two structural domains: (i) the southern domain, i.e. the Anti-Atlas belt. In this northern rim of the West African craton, the Paleozoic sequences are mildly deformed; (ii) the northern Mesetan domain, itself subdivided into several structural subzones, corresponds—with its eastern prolongation in Algeria- to the Hercynian/Variscan belt of Northern Africa. In the Meseta, the study of the sedimentological history and the structural analysis of the various subzones allow to discuss the tectonic characters of the Hercynian/Variscan orogeny in Northern Africa: – The deformation was realized through three main events: (i) the Late Devonian, Eovariscan Phase, which developed in the eastern zones of the Moroccan Meseta, northwestern Algeria and in the presently Alpine allochthonous domains of the Rif and Kabylias; (ii) the Visean Phase, which was restricted at the limit between the Moroccan eastern and western Meseta. The two preceding compressional events were contemporaneous with the development of Late Devonian-Early Carboniferous transtensive sedimentary basins; (iii) the Late Carboniferous Phase, during which the regional shortening affected the entire Hercynian domain, including the Moroccan western Meseta and Anti-Atlas. – The Anti-Atlas differs fundamentally from the Mesetan domains by: (i) its relatively mild deformation; (ii) the vergence of its regional structures, directed toward the West African shield, whereas the main vergence is toward the west in the Meseta domains. – In the Anti-Atlas like in the weakly deformed zones of the Meseta, the deformation is heterogeneous, being concentrated along several regional shear zones. Sedimentological and stratigraphic evidences point out the control exerted by these long-lived weakness zones through the sedimentation, and the deformation. – No direct evidence can be found to invoke the development of a Paleozoic oceanic lithosphere in the considered domains. Therefore, the Hercynides/Variscides of North Africa are considered to be outside the inner zones of the European belt, at the northern border of the Paleo-Gondwana, and probably related to the Appalachian belt of North America.

The Guiana and the West African Shield Palaeoproterozoic grouping: new palaeomagnetic data for French Guiana and the Ivory Coast

The aim of this study is to document the Palaeoproterozoic palaeomagnetic database for the Guiana and West African Shields in order to better understand the paleogeographic evolution of these two cratons. A total of 59 sites were sampled (33 in French Guiana and 26 in the Ivory Coast) in granites and metavolcanic rocks. Magnetic, petrographic and palaeomagnetic investigations were carried out on these rocks. Magnetic experiments and petrographic observations show that undeformed magnetite is the main magnetic remanent carrier in granites of French Guiana and both magnetite and haematite in rocks from the Ivory Coast. Both thermal and magnetic alternating-field demagnetizations were applied to the rocks. Four high-temperature magnetic remanent directions were isolated in French Guiana and the Ivory Coast. These directions are distinct from the present Earth's field and to the local Early Jurassic palaeomagnetic components. Reversal and contact tests were obtained for the collection from French Guiana. Based on these arguments and mineralogical investigations, we propose that the magnetic remanence represent a Palaeoproterozoic magnetization. Four virtual palaeomagnetic poles were calculated: GUI1, GUI2 for French Guiana; IC1 and IC2 for the Ivory Coast with their corresponding coordinates: GUI1: GUI1=-62°N, GUI1= 61°E, k= 18, A95= 10°, N= 15 ; GUI2: GUI2=-5°N, GUI2= 50°E, k= 26, A95= 18°, N= 5 ; IC1: IC1=-82°N, IC1= 292°N, k= 28, A95= 13°, N= 6 ; IC2: IC2=-25°N, IC2= 83°E, k= 11, A95= 16°, N= 9 . The magnetization age ranged from 2.04 to 1.97 Ga for the French Guiana poles (40Ar/39Ar) and between 2.10 to 2.00 Ga for the Ivory Coast poles (startigraphic ages). Combining these new palaeomagnetic poles and previously published data, two apparent polar wander paths were proposed for these two shields. The comparison of these two Palaeoproterozoic paths seems to indicate that the two cratons belonged to the same block at about 2.00 Ga but separated prior to 2.02 Ga. Although this hypothesis is supported by geological and tectonic observations in both shields, further palaeomagnetic, geochronological and petrographic constraints are needs between 2.04 and 2.10 Ga.

The Oldest Rocks in West Africa: SHRIMP Zircon Age for Early Archean Migmatitic Orthogneiss at Kaduna, Northern Nigeria

Zircons extracted from a migmatitic orthogneiss of granodioritic composition from Kabala, Kaduna area of northern Nigeria, yielded a SHRIMP 207Pb/206Pb zircon age of $$3571\pm 3$$ Ma, indicating that Early Archean crust is exposed in the Nigerian basement complex. The Kabala gneisses are the oldest rocks known so far from the West African shield, and indications for rocks of similar age have recently been reported from the Reguibat and Kenema‐Man shields in West Africa, suggesting a possible link between these terrains in Early Archean time.

Mantle heat flow and geotherms for the main geologic domains in Morocco

Thermal and deep seismic soundings data are used to study the dependence between the compressional Pn velocity and the surface heat flow or the temperature at the Moho discontinuity in Morocco. This correlation indicates a significant decrease in Pn velocity where high heat flow and Moho temperature are observed. This result is consistent with respect to other regions of the world. Crustal heat generation models and geotherms are constructed for the major Moroccan geological domains extending from the Precambrian units in the south to the Alpine units in the north. The crustal contribution in surface heat flow is on average 35 mWm–2, with high values of 41–42 mWm–2 in the western and eastern Meseta where Hercynian granite intrusions could enrich the crust in radioactive heat sources. High mantle heat flow values are obtained beneath the Alboran neogene basin (62 mWm–2), the Rif (47 mWm–2), the Middle Atlas (41 mWm–2), and the south Atlantic margin (40 mWm–2) where the crust is thinned by an extensional tectonic regime. Despite their similar formation context, the intra-continental belts of the Middle and the High Atlas show different geothermal field components. A lithospheric heating process in the Middle Atlas could be the result of a Plio-Quaternary basaltic volcanism. Finally, the Precambrian basement of the Anti-Atlas like all the West African shield is a stable domain showing the lowest subsurface temperatures.

Gaston Beaudet (1934-1994). Le parcours interrompu d'un géomorphologue

Abstract. - With the death of G. Beaudet, Professor of Physical Geography at the University of Paris I, the French geomorphologists lost one of the most outstanding members. He worked on Morocco and the Mediterranean Basin, and then he was interested in studies on the tropical world: Andean and central American mountains and piedmonts, the West African shield and of Central Namibia. His researches concerned the evolution of reliefs and forms of the Neogene at the Quaternary. More generally he insisted on the interactive influence of the rocky substrate, of the great morphoclimatic crisis, of the neotectonic, of the eustatic variations, of the old slope systems that shape condition the recent morphogenesis and he also analysed the part played by man. He compared the organisation in latitudinal and altitudinal zones of the recent morphoclimatic data and the distribution of the shapes and the relict deposits and explained on smaller scale the relations between pedology alteration and morphogenesis

Jurassic Hydrocarbon Exploration of Southern Florida: ABSTRACT

South Florida Jurassic exploration has been overlooked as a viable exploration target due to lack of data and plate-tectonics application. In Florida, {open_quotes}basement{close_quotes} is defined as crystalline, igneous, metamorphic, and unmetamorphosed sediments of Paleozoic age. Age-dating of zircons has proven that the Florida lower Paleozoic terrane is not akin to that of North America but is part of the West African Guinean shield. Previous published reconstructions of late Paleozoic fits of crustal plates and continents have failed to account for the differences in peninsula Florida basement and the geologic and tectonic continuities of peninsula Florida, Yucatan, Cuba, Hispaniola, and Bahamas. Pre-Atlantic reconstruction of the Gulf of Mexico in this study proposes that there was a Florida connection to Yucatan-Cuba-Africa during the Triassic. This reconstruction also shows that the Jurassic sediments that are well known in the northern Gulf Coast should have been deposited in similar depositional environments in southern Florida. Deep drilling on the Florida peninsula has confirmed this hypothesis. By using plate tectonic reconstruction based on the rising of the North Atlantic Ocean and evidence from petrology of basement samples from deep wells together with petrographic analyses of Jurassic sediments, a Smackover-equivalent exploration play can be developed. Petrographic andmore » petrophysical analysis of these wells that have encountered Jurassic marine shales, anhydrite, dolomite, carbonate, and elastic sediments has determined that these sediments are from shallow-water subtidal, tidal, intertidal, and supratidal environments. Excellent gas shows, oil stain in the pores and high TOC values in the marine shales, indicate that large accumulations of hydrocarbon are present.« less

Lead isotope signature of early Proterozoic ore deposits in western Africa; comparison with gold deposits in French Guiana

Lead isotope analyses of samples from ore deposits of the West African shield trace the metallogenic evolution of the Early Proterozoic or Birimian period. The selected deposits, which include the Loulo, Poura, and Banora gold deposits, the Faleme iron deposit, and the Perkoa, Gan, Hanhui, and Pilimpikou base metal deposits, are representative of each stage of the so-called Eburnean evolution. The study was completed by an analysis of gold deposits in the French Guianan Shield, which is a rifted remnant correlative of the West African craton.The isotopic compositions of most of the galenas cluster in a narrow field ( 206 Pb/ 204 Pb = 14.60-15.15, 207 Pb/ 204 Pb = 14.96-15.23, 208 Pb/ 204 Pb = 34.34-34.71) corresponding to the initial lead-isotopic composition of Birimian mineralization. Secondary isochrons, obtained on cogenetic lead-depleted sulfides, all converge toward the same narrow field; it is likely that they reflect an in situ addition of radiogenic lead to older lead from the Eburnean event. The calculated ages (2236-2741 Ma) have no clear geologic significance.The elongated field of initial lead compositions requires a mixing between juvenile, mantle-derived lead as expressed in the syngenetic deposits (Perkoa, mu = 9.11), and a more radiogenic component, such as old (>2.150 Ma) crustal lead, as expressed in the late orogenic deposits (Dorlin-THR stockwork, mu = 10.06). This increase in mu values reflects a gradual increase in the older recycled crustal lead component, favored by the tectonic change from D 1 thrusting to D 2 gold-bearing vertical strike-slip faulting. The age of the old recycled crust would almost certainly be earliest Proterozoic (2300-2145 Ma, Dabakalian); no perceptible Archcan lead influx is required to explain the genesis of these deposits. Geochronological data from other sources bracket most of the Eburnean metallogenesis between 2153 + or - 13 Ma (2Sigma ) age of the Dabakalian metamorphism, and 1949 + or - 55 Ma (2Sigma ) age of the last post-tectonic granites. Pb/Pb model ages, respectively 2113 Ma for the early so-called syngenetic ore deposits and 2014 Ma for the late-orogenic mesothermal gold deposits, are consistent with these independent geochronologieal data and probably reflect fairly well the approximate geologic ages of these deposits.

Formation and transformation processes of iron duricrust systems in tropical humid environment

Formation and transformation processes of iron duricrust developed under tropical humid conditions in the southeastern part of the Central African Republic have been investigated in terms of geomorphological, petrological and geochemical pathways. The study of five typical weathering profiles of the most extensive iron duricrust system now known in the world indicates that saprolitization and ferruginization are sequentially involved in such processes, as reflected by kaolinite-gibbsite and hematite-goethite replacements. Alkali earths, rare earths, metals and elements belonging to chemically inert minerals segregate during saprolite and iron duricrust formation and degradation. Mass-balance calculations performed on a typical weathering system enable us to quantify net mass element transfers deriving from lateritic weathering processes. Volumetric change, collapse or dilation, as well as Al and Ti clearly discriminate between the saprolitization and ferruginization processes. Saprolite development preserves the geochemical inheritance of parent rocks, whereas ferruginization depletes it and mostly reflects, through mineralogical pathways, pedoclimatic conditions and global climatic changes. The global iron duricrust system of the southern part of Central Africa derived from previous hematite-rich crust, and not from ancient bauxites as it is generally believed for iron crusts, formed on West African shields.

The Proterozoic sulphide alteration pipe of Sidi Flah and its host series. New data for the geotectonic evolution of the Pan-African Belt in the eastern Anti-Atlas (Morocco)

Detailed mapping of the Sidi Flah-Bou Skour inlier in the central Saghro, of which the initial aim was to determine the metallogenic potential of the area, enabled us also to challenge the former tectono-lithostratigraphic classifications on the basement units immediately underneath the P.III cover, as well as their relationships with successive magmatic events (effusive and/or intrusive). These new results from this area, compared with data from the literature (and sometimes from additional field trips to other inliers in the central and eastern Anti-Atlas), allow us to revise the tectono-lithostratigraphic classifications of most of the basement units immediately underneath the P.III-cover in the eastern Anti-Atlas. In actual fact, they were generally found to differ only slightly from those in the central Anti-Atlas: prior to the deposition of the P.III-cover, the basement unit of both areas experienced two similar tectonic events (same types of folding: sub-parallel axial trends, foliations, schistosities—with or without associated metamorphism—similar or identical magmatic series, successions and dates of emplacement. Thus, as in the central Anti-Atlas, most of these basement unit must be classified as belonging to the P.II-2 unit and not to the P.II-3 as was previously thought (Choubert's nomenclature, 1963–1979): only rarely do inliers contain P.II-3 basement units. During the period of extension, prior to the first Pan-African compressive phase, mafic volcanics were poured out onto the floor of subsiding basins characterized by detrital terrigenous sedimentation (deltaic fans). Locally, the volcanics are associated with sulfide mineralisation, which show features (alteration pipes and feeding stockworks) suggestive of massive sulfide deposits; similar geometrical relationships with the host rocks, hydrothermal alteration and sulphide parageneses characterise these showings. The presence of such a type of mineralisation is an additional argument for classifying their host series as P.II-2 (by comparison with that of Bleida in the central Anti-Atlas). Moreover, this revision leads us to propose a new palaeogeographical scheme for the evolution of the Middle- to Late-Proterozoic and, subsequently, a new geotectonic model. The central and eastern Anti-Atlas forming, during this period, a diamond-shape, mainly intra-continental, subsiding basin with internal shoals and troughs whose directions were determined by those of pre-existing fractures or zones of weakness: mostly N130° and N70°, (sometimes N110°). These same trends were re-activated again during the Upper- and the Late-Proterozoic and controlled the two-stage pre-P.III Pan-African tectogenesis. The diamond-shape of the basin, including its shoals and troughs, was caused, through wrench-faulting, by transtensional movements inside a multiphase approximately NS compressive stress field generating a mainly SWNE extension. In the earliest phase (Middle- to Upper-Proterozoic), the distension reached a maximum (associated with a thinning of the continental crust), creating an epicontinental ensialic basin between two passive margins (the West African Shield to the southeast and southwest, and an ill-defined continent to the northeast and the northwest: the “European” continental plate. The thinning of the continental crust caused it to rupture locally, and here the upper-mantle came to form the bottom of the basin; this occurred along the Anti-Atlas Major Fault ( =AAMF) in the central Anti-Atlas, e.g., the Bou Azzer-El Graara and Siroua areas. However, the southwestern and northeastern zones remained controlled by the continental crust. Then, during the Upper-Proterozoic, compressive events (the two-stage Pan-African tectogenesis) the basins progressively closed, folding the volcano-sediments contained in them and causing local “intrusions” of remnants of oceanic crust as subvertical slabs (ultramafic rocks at Bou Azzer-El Graara and Siroua). The basins were completely closed at the end of the Upper-Proterozoic first Pan-African event; the second Pan-African phase and the subsequent Upper-Proterozoic events were governed by strictly continental conditions throughout the Anti-Atlas. Thus, the proposed geotectonic model consists of early wrench-faulting and the subsequent creation, though transtension of successive, more or less deep and active grabens and rifts (the present-day inliers), separated by horsts, within a continental environment. This extension is bound to an approximately NS compressive stress-field acting along inherited zones of weakness (mainly N70° and N130° faults). Only one rift opening reached the upper-mantle, the others remained confined to a continental crust thinned to a greater or lesser degree. During the later compressions (approx. EW stress-fields), which led to the pre-P.III tecto-orogenesis of the Anti-Atlas, no subduction of the sea-floor is though to have occurred, but the thinned continental crusts (passive margins) faced each other along pre-existing transform and/or strikeslip faults, after an early and local obduction of small upper-mantle (sea-floor) slices along the northern margin of the West African Shield. This hypothesis, which is an alternative to subduction (either under active margins or under island arcs) for calc-alkaline magmatic generation, the type to which the late- and post-Pan African magmatisms belong, recalls the one proposed for the Neogene evolution of the zone of the Rif-Betic Cordilleras-Alboran Sea and has also been suggested for other Upper-Proterozoic Belts in Central Africa.

Telluride mineralization at Ashanti gold mine, Ghana

AbstractGold mineralization at the Ashanti Mine occurs in shear zones which are second-order components of a major transcrustal shear zone. However, gold-rich telluride mineralization in veins in the hangingwall of the Obuasi ore zone at Ashanti appears to have post-dated development of the major gold-bearing shear zone. Complex assemblages of goethite, chalcocite, coloradoite, calaverite, sylvanite, kostovite, petzite, stutzite, hessite, altaite, rickardite, weissite and henryite were succeeded by a relatively simple assemblage of altaite-petzite-hessite ± sylvanite ± coloradoite. Precipitation occurred over a temperature range of approximately 220°C to 165°C from a CO2-bearing, low salinity (<5 wt. % NaCl equiv.) fluid under conditions of high ƒo2 and high ƒTe2/ƒS2. Replacement of earlier gold-sulphide mineralization is indicated.The fluids were introduced during late activation of the Obuasi shear zone, which evolved during the c.2000 Ma Eburnean tectonothermal event that marked the onset of cratonization of the West African shield. The localized and time-specific distribution of the telluride mineralization may be indicative of a late influx of tellurium-rich fluid linked to emplacement of a synorogenic, Cape Coast-type granitoid intrusion.

40Ar/ 39Ar polyorogenic mineral age record in the northern Mauritanide orogen, West Africa

Northern segments of the Mauritanide orogen in the Akjoujt region are dominated by suprastructural nappe complexes. These structurally overlie other tectonic elements, including: (1) autochthonous/parautochthonous foreland units composed of late Proterozoic-late Paleozoic (Frasnian) sedimentary cover sequences, and unconformably underlying crystalline basement (late Archean-middle Proterozoic) of the West African Shield, (2) parautochthonous successions largely represented by low-grade metasedimentary rocks, and (3) allochthonous infrastructural complexes with characteristics similar to those of late Proterozoic-early Paleozoic (“Pan-African”) tectonic elements exposed in central and southern parts of the Mauritanide orogen. Amphibole from various structural levels of the Ijibiten and Akjoujt suprastructural nappe complexes generally display internally discordant, “saddle-shaped” 40Ar/ 39Ar release spectra suggestive of extraneous (excess) argon contamination. One sample records a well-defined, intermediate-temperature plateau of ~2420 Ma. Muscovite within lower and intermediate structural levels of the complexes displays variably discordant 40Ar/ 39Ar age spectra which suggest that variable thermal rejuvenation of older (up to ~2600 Ma) mineral systems occurred during a ca. 310-300 Ma metamorphism. Uppermost structural levels of the complexes are represented, in part, by penetratively mylonitic metagranite. Muscovite within these rocks displays internally concordant 40Ar/ 39Ar age spectra which define plateau ages of ~300–310 Ma. There is no indication in either amphibole or muscovite results of any late Proterozoic-early Paleozoic (Pan-African) thermal record. Together with field relationships, the 40Ar/ 39Ar results suggest that structural units of the suprastructural nappe complexes are, in part, composed of variably overprinted basement rocks. These structurally overlie allochthonous Pan-African sequences and therefore must have been derived west of and now largely conceal, the Pan-African orogen. The complexes were transported eastward to higher structural levels during late Paleozoic orogenesis. The extent of late Paleozoic thrusting within the Akjoujt region contrasts with more limited transport distances documented southward in the orogen. This contrast probably reflects a kinematic influence of the Reguibat promentory of the West African Shield, which during the late Paleozoic collision of Gondwana and Laurentia effected extensive compression in proximal sequences.

Kilbuck terrane: Oldest known rocks in Alaska

The Kilbuck terrane in southwestern Alaska is a narrow, thin crustal sliver or flake of amphibolite facies orthogneiss. The igneous protolith of this gneiss was a suite of subduction-related plutonic rocks. U-Pb data on zircons from trondhjemitic and granitic samples yield upper-intercept (igneous) ages of 2,070 {plus minus}16 and 2,040 {plus minus}74 Ma, respectively. Nd isotope data from these rocks suggest that a diorite-tonalite-trondhjemite suite ({epsilon}{sub Nd}(T) = +2.1 to +2.7; T is time of crystallization) evolved from partial melts of depleted mantle with no discernible contamination by older crust, whereas a coeval granitic pluton ({epsilon}{sub Nd}(T) = {minus}5.7) contains a significant component derived from Archean crust. Orthogneisses with similar age and Nd isotope characteristics are found in the Idono complex 250 km to the north. Early Proterozoic rocks are unknown elsewhere in Alaska. However, Phanerozoic plutons cutting several continental terranes in Alaska (southern Brooks Range and Ruby, Seward, and Yukon-Tanana terranes) have Nd isotope compositions indicative of Early Proterozoic (or older) crustal components that could be correlative with rocks of the Kilbuck terrane. Rocks with similar igneous ages in cratonal North America are rare, and those few that are known have Nd isotope compositions distinct from those of themore » Kilbuck terrane. Conversely, provinces with Nd model ages of 2.0-2.1 Ga are characterized by extensive 1.8 Ga or younger plutonism, which is unknown in the Kilbuck terrane. At present the case for a North American parentage of the Kilbuck terrane is not compelling. The possibility that the Kilbuck terrane was displaced from provinces of similar age in other cratons (e.g., Australian, Baltic, Guiana, and west African shields), or from the poorly dated Siberian craton, cannot be excluded.« less

Relationship between tectonics, metamorphism and metamorphic textures in Oban Massif (Nigeria), southeastern sector of the West African shield

Field and petrographic investigations of parts of the Oban Massif, which itself constitutes part of the southeastern margin of the mobile zone separating West Africa Shield, reveals that the metamorphic rocks which consist a thick succession of metapelites, semipelites and metabasic rocks have undergone progressive Dalradian type metamorphism recorded by the index minerals chlorite, biotite and almandine garnet. The distribution of the isogrades indicates a N-S increase in the intensity of metamorphism from greenschist to lower amphibolite facies. The metasediments represent remnants of Proterozoic supracrustal cover rocks infolded within the reactivated Archean migmatite-gneiss complex and were subsequently affected by the Pan African orogeny. Rock microfabrics and textures indicate that the area had been affected by three phases of deformation which are correlatable with episodes of metamorphism. It is obvious from the texturaland metamorphic evidence that the Pan-African event reached a status of an orogeny in southeastern basement complex - Orb

40Ar/ 39Ar and paleomagnetic results from Liberia and the Precambrian APW data base for the West African Shield

Lower amphibolite to granulite facies metamorphic rocks in Nimba County, Liberia have yielded 2.2–2.9 Ga RbSr whole rock ages, indicating that they are part of the Archean Liberian age province. We report a 2040 Ma 40 40Ar/ 39Ar plateau date on hornblende from an amphibolite in this region, and suggest that these rocks were also severelyreworked during the Eburnean (∼2.0 Ga) metamorphic episode. 40Ar/ 39Ar analyses of biotite and feldspars from neighboring schists also indicate the presence of two mild thermal events, at 1.5 Ga and 0.6 Ga. Paleomagnetic analyses of samples from these same metamorphic rocks reveal three components of magnetization. The predominant and most stable component (273°E, 21°N) is considered to have been acquired as a result of pos Eburnean uplift and cooling at ∼ 2.0 Ga, whereas the two less stable components with poles at 235°E, 43°N and 16°E, 36°N, probably correlate with the 1.5 Ga and 0.6 Ga thermal pulses, respectively. Rock units from southern Liberia also yield two secondary magnetizations, one at 247°E, 37°N and the other at 104°E, 5°N, and a 1.5 Ga 40Ar/ 39Ar date on plagioclase. Comparison of the paleomagnetic poles corresponding to the ∼2.0 Ga Eburnean component with published paleomagnetic data for West Africa is not consistent with prior interpretations of the polar wander path for West Africa. Our paleomagnetic data, when compared to poles of comparable age from the Kalahari Shield, still suggest that some form of displacement has occurred between the Kalahari and West African Shields since 2.0 Ga.