Ore mineralization related to geological evolution of the Karkonosze–Izera Massif (the Sudetes,Poland) — Towards a model

The Karkonosze–Izera Massif is a large tectonic unit located in the northern periphery of the Bohemian Massif. It includes the Variscan Karkonosze Granite (about 328–304 Ma) surrounded by the following four older units:

• -Izera–Kowary (the Early Paleozoic continental crust of the Saxothuringian Basin),
• -Ještĕd (the Middle Devonian to Lower Viséan sedimentary succession deposited on the NE passive margin of the Saxothuringian Terrane), out of the present study area,
• -Southern Karkonosze (metamorphosed sediments and volcanics filling the Saxothuringian Basin), out of the present study area,
• -Leszczyniec (Early Ordovician, obducted fragment of Saxothuringian Basin sea floor).

The authors present a genetic model of ore mineralization in the Karkonosze–Izera Massif, in which ore deposits and ore minerals occurrences are related to the successive episodes of the geological history of the Karkonosze–Izera Massif:

• -formation of the Saxothuringian Basin and its passive continental margin (about 500–490 Ma)
• -Variscan thermal events:
  • -regional metamorphism (360–340 Ma)
  • -Karkonosze Granite intrusion (328–304 Ma)
• -Late Cretaceous and Neogene-to-Recent hypergenic processes.

The oldest ore deposits and ore minerals occurrences of the Karkonosze–Izera Massif are represented by pyrite and magnetite deposits hosted in the Leszczyniec Unit as well as by magnetite deposit and, presumably, by a small part of tin mineralization hosted in the Izera–Kowary Unit. All these deposits and occurrences were subjected to the pre-Variscan regional metamorphism.Most of the Karkonosze–Izera Massif ore deposits and occurrences are related to the Karkonosze Granite intrusion. This group includes a spatially diversified assemblage of small ore deposits and ore mineral occurrences of: Fe, Cu, Sn, As, U, Co, Au, Ag, Pb, Ni, Bi, Zn, Sb, Se, S, Th, REE, Mo, W and Hg located within the granite and in granite-related pegmatites, in the close contact aureole of the granite and within the metamorphic envelope, at various distances from the granite. Assuming world standards, all these deposits are now uneconomic. Various age determinations indicated that ore formation connected with the Karkonosze Granite might have taken place mostly between about 326 and 270 Ma.The last ore-forming episode in the Karkonosze–Izera Massif is related to hypergenic processes, particularly important in the northern part of the massif, in the Izera–Kowary Unit where some uranium deposits and occurrences resulted from the infiltration of ore solutions that originated from the weathering of pre-existing accumulations of uranium minerals. A separate problem is the presence of oxidation zones of ore deposits and occurrences, both the fossil and the recent.A full list of ore minerals identified in described deposits and occurrences of the Karkonosze–Izera Massif together with relevant, key references is presented in the form of an appendix.     

Antimoniferous mineralization from the Mid-European Saxothuringian Zone: mineralogy,geology, geochemistry and ensialic origin

In the North Bavarian Basement (FR Germany) as well as in the Thuringian Forest (GDR) both of which belonging to the Saxothuringian Zone antimony mineralizations have been worked up to the early fifties. Mineralogical and geological investigations led to the following classification of antimony ore deposits and -concentrations:

1. Stratabound and stratiform fahlore (Fahlerz) mineralization in Silurian and Lower Devonian black shales.
2. Sulphosalt- and stibnite-bearing veins (partly associated with native gold) in wall rocks of the Late Variscan granites.
3. Monometallic and monomineralic antimony lodes along anticlinal structures.
4. Sulphosalt-bearing lead-zinc veins.

Antimony in the black shales was concentrated during late diagenetic cementation along with copper. The sulpho-salt-bearing stibnite veins in the metamorphic country rocks of the granites are genetically associated with the granitic intrusions nearby. This is proved by trace element chemistry and structural as well as mineralogical features of those veins. U/Pb age dating of contemporaneous pitchblende from the »polymetallic uranium paragenesis« from Hoehensteinweg uranium deposit yielded a late Variscan age of formation for these veins of type 2. Early Paleozoic rocks (metavolcanics) are suggested to have been the parent material for these granite-related Sb concentrations. However the monotonous Sb veins are more akin to the basic protore in deeper crustal sections. The thermal aureole of a deep-seated heat source is preserved by the Ag/Sb ratio of galena in Pb-Zn veins (type 4), which shows a marked variation along the SW plunging Berga Anticline from increased Ag/Sb ratios near the core of the anticline towards reduced values in mining sites more peripheral to this fold structure. Pb isotopes of stibnite as well as Pb sulphides point to a crustal derivation of both elements. An in-situ re-deposition and leaching of Sb from the wall rocks as well as antimoniferous black shales may be ruled out. It has to be emphasized that intra-crustal mass movements (A subduction) and the late Vanscan igneous activity are the major factors controling release of Sb from the Early Paleozoic low metal concentrations within metavolcanic rocks and subsequent discharge of Sb-bearing fluids within joints and fractures related to late Variscan tectonic movements. A schematic exploration concept is outlined.     

Environmental impacts by input of substances in landscapes of the District (Bezirk) of Leipzig,Germany

Problems of landscape and resource protection resulting from the intensification of land-use can be mastered only by intersectoral planning and a land management considering (landscape-)ecological principles right from the beginning. In the district of Leipzig ecological studies in the '80ies have focussed on:

1. Determination of the regional pattern of atmospheric immissions;
2. Registration of heavy metals in soil and vegetation;
3. Soil compaction, soil erosion;
4. Study of stress indicators in the aeration zone and in the top-most aquifer in order to examine barrier effects in the percolation process.

First results are discussed.     

Some examples of the Middle-Triassic Marine transgression in South-Western Mediterranean Europe

In Southwestern Mediterranean Europe (NE Spain, Sardinia and NW Apennines e. g. Monti Pisani and Punta Bianca) the Middle-Triassic transgression on a margin of an intracratonic basin appears to be controlled by the different morphology and tectonic activity of the margin itself. The analysis of some sequences leads to recognize three different kinds of margins:

1. margin with a narrow shelf sloping toward a very shallow restricted basin (Central and Southwestern Sardinia);
2. margin with a broad shelf gently sloping toward a shallow basin (NE Spain, Monti Pisani, NW Sardinia);
3. margin with a narrow shelf and steep slope marked by tectonic and volcanic activity (Punta Bianca).

     

Phase equilibria among pumpellyite,lawsonite, epidote and associated minerals in low grade metamorphic rocks

Phase relations of pumpellyite, epidote, lawsonite, CaCO₃, paragonite, actinolite, crossite and iron oxide are analysed on an Al-Ca-Fe³⁺ diagram in which all minerals are projected from quartz, albite or Jadeite, chlorite and fluid. Fe²⁺ and Mg are treated as a single component because variation in Fe²⁺/Mg has little effect on the stability of phases on the diagram. Comparison of assemblages in the Franciscan, Shuksan, Sanbagawa, New Caledonia, Southern Italian, and Otago metamorphic terranes reveals several reactions, useful for construction of a petrogenetic grid:

1. lawsonite+crossite + paragonite = epidote+chlorite + albite + quartz + H₂O
2. lawsonite + crossite = pumpellyite + epidote + chlorite + albite+ quartz + H₂O
3. crossite + pumpellyite + quartz = epidote + actinolite + albite + chlorite + H₂O
4. crossite + epidote + quartz = actinolite + hematite + albite + chlorite + H₂O
5. calcite + epidote + chlorite + quartz = pumpellyite + actinolite + H₂O + CO₂
6. pumpellyite + chlorite + quartz = epidote + actinolite + H₂O

     

Charriage crustal,anatexie et decrochements ductiles dans les Maures orientales (Var,France) au cours de l'orogenese varisque

The kinematics of the deformational events recorded in the catazonal gneisses within the eastern part of the Maures massif (Variscan basement of Provence, southeastern France) has been established. These events can be correlated with both the metamorphic and the magmatic evolution, and the orogenic history of the eastern Maures then consists of the following stages:

1. -HP-metamorphism preserved only in relict eclogites and quartzites rich in calc-silicates,
2. -Catazonal metamorphism involving anatectic melting, broadly contemporaneous with large-scale horizontal transport towards the NNE. During this event, the lithologic units were disrupted and intensively mylonitized.
3. -Intrusion of a first generation of anatectic granitoids.
4. -A second tectonic event under epi-to mesozonal metamorphism conditions is responsible for the northward displacement of the eastern Maures relative to the western part along a sinistral strike-slip fault 4 km wide (Ramatuelle — Plan de la Tour fault) in which the early anatectic granitoids have been mylonitized. Outside the fault zone, this event is marked by upright to W-vergent open folds trending N-S, i.e. parallel to the transport direction.
5. -A moderate cataclastic reactivation of the Ramatuelle — Plan de la Tour fault with a dextral sense of shear, locally accompanied in the eastern part of the area by minor lowtemperature thrusting towards the south.
6. -Intrusion of a second generation of anatectic granites about 320 Ma ago.

The metamorphic, magmatic and tectonic evolution of the eastern Maures suggests a continuous orogenic history in an area of rapid crustal thickening by large-scale thrusting within the continental crust. This evolution may be related to the development of continental subduction during the continent/continent collision responsible for the Variscan orogeny in southern Europe.     

Mesozoic volcanics of western sicily

The Mesozoic lavas and minor intrusions in the thrust sheets of western Sicily have the following characteristics:

1. The lavas in the Triassic Mufara Formation in the north were broken into fragments which rotated independently within the incompetent strata that enclose them. This behavior is characteristic of igneous rocks found within the more internal (northerly) thrust units.
2. The Jurassic lavas in the more external (southerly) units have consistent directions which agree with those of the Ammonitico Rosso limestones in the same zone and lie about 30° clockwise from those of coeval autochthonous formations in Tunisia.Schult's presumed Cretaceous directions from Custonaci on the north coast (similar to those found in the Cretaceous Scaglia Rossa at Terrasini to the east byChannel et al., 1980) are rotated still more (140°) with respect to those of the autochthonous Iblean platform of SE Sicily. These differences are believed to reflect rotation of the thrust sheets during tectonic transport in Cenozoic times, the internal units being the most strongly rotated.
3. All the igneous rocks are highly altered: generally the original mineralogy cannot be completely determined. Relative abundances of some of the less mobile elements (Ti, Sr, Y) suggest that they are intraplate basalts.

     

Field and petrological evidence for the development of an ensialic marginal basin related to the Hercynian orogeny in the Massif Central,France

Six lithologic units in tectonic contact with each other have been defined during mapping of the Devonian in the Beaujolais area of the northeastern Massif Central. Five main igneous suites have been recognized:

1. A transitional basaltic suite restricted to a single unit.
2. An acid volcanic-plutonic suite the members of which are related by fractional crystallization and magma mixing.
3. Low-TiO₂ volcanic rocks with calc-alkaline affinities.
4. A TiO₂-rich tholeiitic suite related to an ophiolitic complex.
5. A plutonic suite with close resemblances to Alaskantype intrusions.

The transitional metabasalts (1) form the oldest igneous suite and could represent either an intraplate magmatic forerunner of rifting or tectonic slices of weakly metamorphosed rocks representing a pre-Acadian event. The next three suites may be related to a short-lived ensialic marginal basin that developed between the Acadian and Bretonian orogenies. The basin is asymmetric, with the ophiolite of the central part flanked by an acid ridge on one side and a passive continental margin on the other. Quartz-keratophyres (2) and calc-alkalic basic volcanic rocks (3) were intercalated in varying proportions to form a bimodal volcanic pile before the rifting event that led to the formation of the ophiolites (4). The acid ridge (2) may be due to the reactivation of a continental basement. Cumulate rocks with Alaskan-type affinities occur as olistoliths, emplaced before the formation of the rift basin, supporting a comparison of the Hercynian belt with accreted magmatic arc terranes.     

Looking at computer cartography

Computer Cartography has gone through many changes during its short life. This article tries to document some of the results of the development. The major conclusions are:

- whereas the application of computer mappins is fluurishing, conceptual development is slow;

- as a result the applications are simpler than they could be, given the development of computer science, and brute force reigns over elegance.

To exemplify these points, the main types of Geographic Information Systems are discussed with respect to their recent achievements: Catastral mapping, Thematic mapping, Topographic mapping, Resource Information Systems, and Digital Terrain Models.     

Essai d'organisation paléogéographique et structurale du PaléozoÏque des Andes Méridionales

Hercynian absolute ages obtained from many plutonites and metamorphites of the Sierras Pampeanas compel to consider this polymetamorphic domain rather as an intermediate hinterland and axis of divergent symmetry between the two elementary geosyncline ranges (Cuyanides and Bolivianides) of the south america hercynian orogen. Attempt of Cuyanides paleogeographic organization indice to define during the geosyncline period (Cambrian to Devonian):

the Pampean zone which consist of:

1. an internal sub-zone (eastern) or pampean s. st. with hinterland significance
2. a more external sub-zone (western) or transpampean or Umango sub-zone with intermediate features between eugeanticlinal ridge and eugeosynclinal furrow;
3. the Calingasta zone as a typical eugeosynclinal furrow with pelagic-terrigenous ordovician sedimentation and ophiolitic magmatism followed by early silurian than devonian Flyschs;
4. the Zonda zone as an external ridge with neritic cambro-ordovician facies and devonian flyschs.

The unwonted position of Zonda no metamorphic Facies surrounded by Calingasta and Umango metamorphic Facies, suggest its window situation and wide allochthonous structures for the internal zones. An eastward origin of the nappes is discussed in relation with pampean syntectonic metamorphism and plutonism. Essentially, structures are referred to the Acadian phase (= bretonic) but early paleo-hercynian phases are also emphasized to explain Flyschs sequences and some pampean plutonic cycles. Tardi- and Post-hercynian organization is briefly sketched with related molasses, neomolasses and magmatism. The remarquable geosyncline pattern of the Cuyanides is emphasizes in all stages of its evolution and under aspects however tectonic, magmatic and metamorphic. Finally, the fundamentally western polarity of the Cuyanides is replaced into the most general framework of Gondwanides organization.     

Rare earth element geochemistry of the Scourian complex N.W. Scotland — Evidence for the granite-granulite link

Medium-to high-pressure granulite facies complexes represent samples of lower crustal material and are, therefore, important in the study of crustal processes. New rare earth element data for the Scourian granulite facies terrain of the Precambrian Lewisian complex of N.W. Scotland indicate that:

1. Overall, the Scourian complex has a light rare earth enriched pattern with a small but distinct positive Eu anomaly;
2. While some rare earth element trends for the complex as a whole (e.g., Σ REE vs SiO₂) are similar to those observed in upper crustal cogenetic sequences, others (e.g., Eu/Eu* vs SiO₂) are reversed;
3. Compared to average upper crust, the Scourian complex is depleted in REE (except Eu) by a factor of 2 to 3.

These new data, along with previously reported major and trace element data, isotopic abundances, and trace element modelling support the hypothesis that the Scourian terrain is the residuum left after genesis and removal of granitic melts.     

The transformation of amphibolite facies gneiss to charnockite in southern Karnataka and northern Tamil Nadu,India

Amphibolite facies metamorphic grade gives way southward to the granulite grade in southern Karnataka, as acid gneisses develop charnockite patches and streaks and basic enclaves develop pyroxenes. Petrologic investigations in the transitional zone south of Mysore have established the following points:

1. The transition is prograde. Amphibole-bearing gneisses intimately associated with charnockite at Kabbal and several similar localities are not retrogressive after charnockite, as proved by patchy obliteration of their foliation by transgressive, very coarse-grained charnockite, high fluorine content of biotite and amphibole in gneisses, and high large-ion lithophile element contents in gneisses and charnockites. These features are in contrast to very low fluorine in retrogressive amphiboles and biotites, very low large-ion lithophile element contents, and zonal bleaching of charnockite, in clearly retrogressive areas, as at Bhavani Sagar, Tamil Nadu.
2. Metamorphic temperatures in the transitional areas were 700°–800° C, pressures were 5–7 kbar, and H₂O pressures were 0.1–0.3 times total pressures, based on thermodynamic calculations using mineral analyses. Dense CO₂-rich fluid inclusions in the Kabbal rocks confirm the low H₂O pressures at the first appearance of orthopyroxene. Farther to the south, in the Nilgiri Hills and adjacent granulite massif areas, peak metamorphic temperatures were 800°–900° C, pressures were 7–9 kbar, and water pressures were very low, so that primary biotites and amphiboles (those with high F contents) are rare.
3. The incipient granulite-grade metamorphism of the transitional areas was introduced by a wave of anatexis and K-metasomatism. This process was arrested by drying out under heavy CO₂ influx. Charnockites so formed are hybrids of anatectic granite and metabasite, of metabasite and immediately adjacent gneiss, or are virtually isochemical with pre-existing gneiss despite gross recrystallization to granulite mineralogy. These features show that partial melting and metasomatism are attendant, rather than causative, in charnockite development. Copious CO₂ from a deep-crustal or mantle source pushed ahead of it a wave of more aqueous solutions which promoted anatexis. Granulite metamorphism of both neosome and paleosome followed. The process is very similar to that deduced for the Madras granulites by Weaver (1980). The massif charnockites, for the most part extremely depleted in lithophile minor elements, show many evidences of having gone through the same process.

A major problem remaining to be solved is the origin of the large amount of CO₂ needed to charnockitize significant portions of the crust. The most important possibilities include CO₂ from carbonate minerals in a mantle “hot spot” or diapir, from emanations from a crystallizing basaltic underplate, or from shelf sediments trapped at the continent-continent interface in continental overthrusting. Ancient granulite massifs may be such suture zones of continental convergence.     

Some Xenoliths from the alkalic rocks of Teneriffe,Canary Islands

1. Xenoliths of ultrabasic, ultramafic, gabbroic or syenitic type occur in Teneriffe: dunites and clino-pyroxenites in the old alkalic basalt formations of Teno and Anaga peninsulas; gabbroic xenoliths in the Pedro Gil region; nepheline-syenite xenoliths in the Las Canadas and Vilaflor regions where intermediate and phonolitic lavas are abundant; ultramafic, clino-pyroxenite and syenitic xenoliths in the Anaga peninsula where there are many intrusions of nepheline-syenite and phonolitic syenite. Several xenoliths show signs of cataclasis, recrystallisation or reaction of their minerals with the host liquids.
2. The ultrabasic, ultramafic and anorthoclase-rich xenoliths appear to be of cumulus origin, subtracted from basic to intermediate alkalic liquids. Major cumulus phases are: magnesium-rich olivine, sub-silicic, aluminous pyroxene, titanomagnetite, sub-silicic potassic kaersutite, and anorthoclase. It is suggested that the xenoliths formed at depths between 11 km and 30 km, largely under wet conditions that helped suppress formation of cumulus plagioclase.
3. The subtraction of kaersutite from liquids of intermediate composition is thought to be a means of producing the gap in silica content between the Teneriffe trachybasalts and the more siliceous trachyphonolites and phonolites. It is also suggested that the subtraction of kaersutite and anorthoclase would considerably deplete residual liquids in alumina whilst enriching then in soda and this might be the means of producing peralkaline liquids.
4. The presence of the xenoliths supports the geophysical data that indicated that Teneriffe has a sub-crustal structure of plutonic rocks. Correlation of the Teneriffe plutonic xenoliths with exposed plutonic basement rocks from other Canary Islands, which are believed to have similar sub-crustal structures, is considered necessary.

     

Untersuchung von suspendiertem Material in den Hydrothermallaugen des Atlantis-II-Tiefs

Suspended matter was separated from the hydrothermal brines of the Atlantis-II Deep in the Red Sea. Contents of iron, manganese, copper, and zink collected on membrane filters were measured by X-ray-flourescence, and the main results were:

1. Metal concentrations in suspended form were lower by 10^?1 to 10^?5 compared to the dissolved concentrations of the brines.
2. Suspended copper and zink were enriched most pronounced in the deeper brine zone — markedly so in the SW-basin, values there ranged between 10 and 30μg/l, one Zn-value was as high as 60μ/l, the other basins contained mostly less than 1 to 10μg/l.
3. Iron, copper, and zink in the deeper brine zone and in few cases also in the 50°-brine were suspended in the form of sulfide compounds. This conclusion is based only on the slightly purple-, green-, and blue-colored gray hues of the material on the filters and its rapid oxidation upon contact with the air.
4. In the transition zone of the 50°/60°-brine iron hydroxides were highly concentrated with values ranging up to 1000μg Fe/l.
5. Suspended manganese is found only within the transition zone of brine to the normal sea water, where up to 200μg Mn/l in form of darkbrown manganese hydroxides were found.
6. Concurrently with the increased thermal activity since 1966 more strongly reducing conditions within the brines seemed apparent which were caused by discharge of higher amounts of Sulfides into the basins. Increased precipitation of heavy metal sulfides was found to be most pronounced in the SW-basin.

     

Origin of coronas in metagabbros of the Adirondack mts., N. Y.

Metagabbros from two widely separated areas in the Adirondacks show development of coronas. In the Southern Adirondacks, these are cored by olivine which is enclosed in a shell of orthopyroxene that is partially, or completely, rimmed by symplectites consisting of clinopyroxene and spinel. Compositions of the corona phases have been determined by electron probe and are consistent with a mechanism involving three partial reactions, thus:

1. Olivine=Orthopyroxene+(Mg, Fe)⁺⁺.
2. Plagioclase+(Mg, Fe)⁺⁺+Ca⁺⁺=Clinopyroxene+Spinel+Na⁺.
3. Plagioclase+(Mg, Fe)⁺⁺+Na⁺=Spinel+more sodic plagioclase+Ca⁺⁺.

Reaction (a) occurs in the inner shell of the corona adjacent to olivine; reaction (b) in the outer shell; and (c) in the surrounding plagioclase, giving rise to the spinel clouding which is characteristic of the plagioclase in these rocks. Alumina and silica remain relatively immobile. These reactions, when balanced, can be generalized to account for the aluminous nature of the pyroxenes and for changing plagioclase composition. Summed together, the partial reactions are equivalent to:

1. Olivine + Anorthite = Aluminous orthopyroxene + Aluminous Clinopyroxene + Spinel (Kushiro and Yoder, 1966).

In the Adirondack Highlands, coronas between olivine and plagioclase commonly have an outer shell of garnet replacing the clinopyroxene/spinel shell. The origin of the garnet can also be explained in terms of three partial reactions:

1. Orthopyroxene+Ca⁺⁺=Clinopyroxene+(Mg, Fe)⁺⁺.
2. Clinopyroxene+Spinel+Plagioclase+(Mg, Fe)⁺⁺=Garnet+Ca⁺⁺+Na⁺.
3. Plagioclase+(Mg, Fe)⁺⁺+Na⁺=Spinel + more sodic plagioclase+Ca⁺⁺.

These occur in the inner and outer corona shell and the surrounding plagioclase, respectively, and involve the products of reactions (a)-(d). Alumina and silica are again relatively immobile. Balanced, and generalized to account for aluminous pyroxenes and variable An content of plagioclase, they are equivalent to:

1. Orthopyroxene+Anorthite+Spinel=Garnet (Green and Ringwood, 1967).

Amphibole coronas about opaque oxides in rocks of both areas are the result of oxide/plagioclase reactions with addition of magnesium from coexisting olivine. Based on published experimental data, pressure and temperature at the time of corona formation were on the order of 8 kb and 800° C for the garnet bearing coronas, with somewhat lower pressures indicated for the clinopyroxene/spinel coronas.     

Space-time analysis of the structural pattern of continents

The Earth's stress field is composed of 4 sub-fields that are induced by

1. the gravitational force (impacts, etc; geodynamic theories on the expansion or contraction of the globe);
2. the centrifugal force of the spinning Earth (models on continental drift explaining the equatorial Alpine-Himalayan collisional mountain belt and longitudinally orientated rifts or oceans);
3. thermal convection (plate tectonic model);
4. tidal forces (extended plate tectonic model).

A standard global stress field results from a combination of these four sub-stress-fields. From the existence of six otherwise inexplicable geodynamic phenomena, it has to be concluded that the standard global stress field of the present can only be an instantaneous (still) photograph of a field that constantly migrates eastwards relative to the Earth's continents. This disclosure can be explained with an extended plate tectonic model, in which the Earth's surface is subdivided by the circum-Pacific ring of subduction zones, into a Pacific area and a continental or Pangaea area with intra-Pangaea oceans (Atlantic, Indian Ocean, etc.). The Pangaea area in turn is subdivided into a North Pangaea area and a South Pangaea area. Due to the off-centre rotation of the spinning Earth around the gravitational centre of the Earth-Moon (-Sun) system (tidal forces), the lower mantle, the Pacific basin, area or state (Pacific crust = lower mantle?), the remaining states that together with the Pacific state compose the Wilson Cycle of ocean opening and closing (Rift/Red Sea state, Atlantic state, Pacific state, Collision/Himalayas state), the ocean sequence of which is permanently arranged from E to W through 360° around the globe, and the standard global stress field as an expression of the Wilson Cycle, are constantly displaced eastwards relative to the upper mantle, the continents or the North and South Pangaea areas with Intra-Pangaea oceans, completing one full turn around the globe in 200 to 250 my (principle of hypocycloid gearing). The continents migrate westwards around the globe and around the Pacific basin in the N and S hemispheres, through sequences of plate tectonic settings of the Oceanic or Wilson Cycle that possess distinct regional stress fields as parts of the standars global stress field, or else the continents are subjected to eastward migrating sequences of settings with distinct regional stress fields as parts of the Wilson Cycle/standard global stress field. By rotations and N-S migrations of the individual continents dissected in all directions by groups of parallel structural planes (fracture systems) through the standard global stress field, the orientation of which is aligned with the spinning Earth's axis and equator and that constantly migrates eastwards relative to the continents, the amount and nature of stress (compression, tension, shearing) a given fracture system is subjected to is constantly altered and the tectonic activity may gradually be transferred from the system under consideration to another fracture system, with slightly different strike directions. Every 400 to 500 my or each Pangaea Cycle (two complete W-E/E-W displacements around the globe between the continents/Pangaea areas with Intra-Pangaea Oceans/upper mantle on the one side and the lower mantle/Pacific basin/ sequence of ocean states and local stress fields of the Wilson Cycle and the standard global stress field on the other) the inhomogeneous standard global stress field is reversed in the N-S direction. Any model proposing the long-time existence of extended lineaments or fracture systems that do not end at the margin of the respective continent or at an orogen/suture zone/former continental margin, in the event of being older than the respective orogenesis, but which cross the surrounding ocean or the younger orogen and continue in the neighbouring continents or former independent continents or even encompass the whole globe, and which puts foreward simultaneous tectonic activity along the whole length of such lineament or fracture system and proposes their longevity or permanent existence, contradicts the physical laws that are the foundation of plate tectonics and mobilism.     

Origin and composition of the ferruginous oolites and laterites of North-Western Nigeria

The ferruginous deposits of north-western Nigeria occur extensively capping sediments which outcrop over thousands of square kilometers. These deposits could be subdivided into 3 major groups:

1. Ferruginous oolites;
2. Crusty/Concretionary laterites;
3. Ferruginous sandstones.

The iron-rich oolites are primary deposits and deposition of ferruginous materials occurred during the late Paleocene. All gradations can be traced between a pure oolitic ironstone and concretionary laterite with scattered oolitic grains of limonite. The crusty laterites and Ferruginous sandstone were formed during the late Tertiary or early Quaternary. They are definitely post Miocene in age and consists of abundant angular quartz grains embedded in a matrix of gothite, haematite, limonite and clay. The laterites and oolites posses different element concentrations which indicate formation under different environmental conditions ranging from deep Marine through shallow near shore marine to continental (fluviatile and lacustrine). The crusty laterites and ferruginous sandstones in northwestern Nigeria attain a thickness of approximately 4 meters or more forming the crust of flat-topped hills or mesas. They are almost certainly equivalent to the laterites covering much of the Northern plains and the Fluvio-volcanic series of the Jos Plateau. In northwestern Nigeria the laterites are restricted to outcrops of sediments of appropriate composition and drainage properties. These ferruginous deposits are usually massive with almost horizontal bedding planes marked by abrupt or gradual changes in morphology and lithology. Cross bedding and ripple-marks can be observed at Gundumi and Imasa as well as in Birnin Kebbi and large scale stratification of the laterites occur at Kalambaina. Laminations are generally localised.     

Gehlenite stability in the system CaO-Al2O3-SiO2-H2O-CO2

P, T, \(X_{{\text{CO}}_{\text{2}} }\) relations of gehlenite, anorthite, grossularite, wollastonite, corundum and calcite have been determined experimentally at P _f =1 and 4 kb. Using synthetic starting minerals the following reactions have been demonstrated reversibly

1. 2 anorthite+3 calcite=gehlenite+grossularite+3 CO₂.
2. anorthite+corundum+3 calcite=2 gehlenite+3 CO₂.
3. 3anorthite+3 calcite=2 grossularite+corundum+3CO₂.
4. grossularite+2 corundum+3 calcite=3 gehlenite+3 CO₂.
5. anorthite+2 calcite=gehlenite+wollastonite+2CO₂.
6. anorthite+wollastonite+calcite=grossularite+CO₂.
7. grossularite+calcite=gehlenite+2 wollastonite+CO₂.

In the T, \(X_{{\text{CO}}_{\text{2}} }\) diagram at P _f =1 kb two isobaric invariant points have been located at 770±10°C, \(X_{{\text{CO}}_{\text{2}} }\) =0.27 and at 840±10°C, \(X_{{\text{CO}}_{\text{2}} }\) =0.55. Formation of gehlenite from low temperature assemblages according to (4) and (2) takes place at 1 kb and 715–855° C, \(X_{{\text{CO}}_{\text{2}} }\) =0.1–1.0. In agreement with experimental results the formation of gehlenite in natural metamorphic rocks is restricted to shallow, high temperature contact aureoles.     

The structure of the guinean continental margin: Implications for the connection between the central and the south atlantic oceans

The results of a recent geological-geophysical survey, conducted off Guinea, are combined with previous data to establish a preliminary stratigraphy and provide a structural sketch of this portion of the West African continental margin. Three sectors are distinguished:

A northwestern portion of the margin comprises a wide and deeply submerged plateau — the Guinean Marginal Plateau underlain by a thick sedimentary sequence and facing westward toward the Gambia Abyssal Plain. Scismic stratigraphy and structures show clear analogies to the Jurassic margins of the central Atlantic. Including the presence of a Cretaceous paleoslope covered by Cenozoic deposits.

A southern area of the margin comprises a series of aligned (W-E trending), acoustic basement features extending along the slope and bounding the Guinean Plateau to the south. These features, basement ridges and volcanic piles are related to a fracture zone system also documented by magnetic anomalies and gravity data. The bordering deep Sierra Leone abyssal plain, also dissected by E-W-trending oceanic fracture zones, contains a sedimentary cover apparently not older than middle Cretaceous.

Between both sectors and between two NW-SE trending scarps lies an intermediate area. The seismic profiles show that here, the margin is dissected by faults creating a series of asymmetric horst and graben features progressively narrowing towards the S-E and covered by untectonized (but partly eroded) Upper Cretaceous to Cenozoic sediments.

The overall structure of the Guinean Margin is interpreted as the result of two major events. During a first phase the margin was created at the southern extremity of the central Jurassic Atlantic and developed like other comparable margins. During a s econd phase (beginning in Early Cretaceous times) the margin was progressively submitted to the opening of the equatorial South Atlantic. This process gave rise to the margin of the southern Guinean plateau (locally injected by volcanics) and generated the tectonic features of the intermediate zone. This protion may thus represent a part of the rifted Jurassic margin discordantly dissected by the oblique opening of the south Atlantic in the area. The oceanic crust of the central and south Atlantic were definitively connected only during Late Albian times as indicated by the end of the tectonic activity and the early Upper Cretaceous unconformity.