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The classical S–I–A-type granites from the Lachlan Orogen, SE Australia, formed as a tectonic end-member of the accretionary orogenic spectrum, the Paleozoic Tasmanides. The sequence of S-to I-to A-type granite is repeated at least three times. All the granites are syn-extensional, formed in a dominantly back-arc setting behind a single, stepwise-retreating arc system between 530 and 230 Ma. Peralkaline granites are rare. Systematic S–I–A progressions indicate the progressive dilution of an old crustal component as magmatism evolved from arc (S-type) to proximal back-arc (I-type) to distal back-arc (A-type) magmatism. The alkaline and peralkaline A-type Younger granites of Nigeria were generally hotter and drier than the Lachlan A-type granites and were emplaced into an anhydrous Precambrian basement during intermittent intracontinental rifting. This geodynamic environment contrasts with the distal back-arc setting of the Lachlan A-type granites, where magmatism migrated rapidly across the orogen. Tectonic discrimination diagrams are inappropriate for the Lachlan granites, placing them in the wrong settings. Only the peralkaline Narraburra suite of the Lachlan Orogen fits the genuine ‘within-plate’ setting of the Nigerian A-type granites. Such discrimination diagrams require re-evaluation in the light of an improved modern understanding of tectonic processes, particularly the role of extensional tectonism and its geodynamic drivers.

International audience; The Tuareg shield in North-West Africa is composed of an assembly of Neoproterozoic continental and oceanic terranes, as well as Archean and Paleoproterozoic terranes separated by major continental shear zones. In the Silet terrane, the igneous suite of Silet-Taourirts is composed of post-collisional ring-complexes of ferroan alkaline and alkali-calcic granites. Six ring-complexes have been dated using the U–Pb SHRIMP technique on zircon. The alkaline Tin Erit (584.8 ± 2.0 Ma), Tihoiiarene (569.8 ± 4.8 Ma) and Tioueine (561 ± 6 Ma) complexes are older than the alkali-calcic Issedienne (538.7 ± 2.9 Ma), Tesnou (536.5 ± 6.7 Ma) and Ait Oklan (529.3 ± 3.1 Ma) complexes. Sharing an enriched mantle source, the complexes display a secular variation of crustal contamination marked by decreasing εHf(t). The oldest alkaline complexes are related to the tectonic escape of the Tuareg terranes to the north after the climax of Pan-African orogeny, while the youngest alkali-calcic complexes were emplaced after the 575-555 Ma intra-continental Murzukian event.

Hydrothermally altered basaltic lava-units in the northern Kerguelen Archipelago contain a wide variety of secondary silicate and carbonate minerals, including zeolites, hydrothermal calcite, dolomite and magnesite, as well as celadonite, orthoclase (adularia) and quartz. Petrography, fluid-inclusion microthermometry, trace-elements geochemistry, Sr isotopes and stable-isotope compositions indicate hydrothermal fluid cells derived from meteoric water interacting with basalts and Rb-rich subvolcanic peralkaline rocks at temperatures ranging from 50 to 200 °C associated with the cooling of the lava pile. The calculated δ 18 O values of meteoric-hydrothermal waters in fossil hydrothermal systems are identical to those in present-day hot springs, suggesting that meteoric recharge was continuous throughout the igneous cooling cycles of the 28–23 Ma older host basalts and the younger 15–5 Ma old peralkaline intrusions. The Kerguelen northern coastline hydrothermal system in the basaltic pile demonstrates that much of the silicate mineralogy and almost all carbonate secondary minerals in altered basalts were derived from meteoric-hydrothermal fluids, rather than products of seawater interaction, or even magmatic-hydrothermal fluids associated with peralkaline intrusions.

Structure- and tectonic-related gas migration into Ordovician sandstone reservoirs and its impact on diagenesis history were reconstructed in two fields in the Sbaa Basin, in SW Algeria. This was accomplished by petrographical observations, fluid inclusion microthermometry and stable isotope geochemistry on quartz, dickite and carbonate cements and veins. Two successive phases of quartz cementation (CQ1 and CQ2) occurred in the reservoirs. Two- phase aqueous inclusions show an increase in temperatures and salinities from the first CQ1 diagenetic phase toward CQ2 in both fields. Microthermometric data on gas inclusions in quartz veins reveal the presence of an average of 92 ± 5 mole% of CH4 considering a CH4-CO2 system, which is similar to the present-day gas composition in the reservoirs. The presence of primary methane inclusions in early quartz overgrowths and in quartz and calcite veins suggests that hydrocarbon migration into the reservoir occurred synchronically with early quartz cementation in the sandstones located near the contact with the Silurian gas source rock at 100-140°C during the Late Carboniferous period and the late Hercynian episode fracturing at temperatures between 117 and 185°C, which increased in the NW-direction of the basin. During the fracture filling, three main types of fluids were identified with different salinities and formation temperatures. A supplementary phase of higher fluid temperature (up to 226°C) recorded in late quartz, and calcite veins is related to a Jurassic thermal event. The occurrence of dickite cements close to the Silurian base near the main fault areas in both fields is mainly correlated with the sandstones where the early gas was charged. It implies that dickite precipitation is related to acidic influx. Late carbonate cements and veins (calcite - siderite - ankerite and strontianite) occurred at the same depths resulting from the same groundwater precipitation. The absence o

A Carbonatite workshop reception and registration was arranged at the Brasserie le Glasgow, Place Hotel de Ville, St Etienne, France on Sunday afternoon and evening, 20* February, with video projections of the 1995 eruption of the active carbonatite volcano, Oldoinyo Lengai, East Africa and a display of posters, followed by an evening meal. It gave all participants attending the workshop the ideal opportunity to meet and discuss topics of mutual interest. The reception was organised by Chantal and Jean-Christophe Perrin and sponsored by the Journal of African Earth Sciences.

The Tertiary (22 Ma) carbonatite centre at Buru hill is located towards the eastern end of the Nyanza rift, western Kenya. As revealed by geochemistry and petrology of drill core samples, the Buru centre ranges in composition from surficial lateritized pyroclastic ferrocarbonatite to recrystallized ferruginous calciocarbonatite at depth.Mineralogically the lateritic cover of the Buru carbonatite centre is characterized by goethite, hematite, psilomelane, baryte, fluorite with minor calcite, bastnäsite and significant amounts of monazite. A transitional middle zone consists of ferrocarbonatite grading into ferruginous calciocarbonatite containing siderite, baryte and fluorite, a greater proportion of bastnäsite replacing calcite, but lesser proportions of monazite. The lowermost zone consists of ferruginous calciocarbonatite containing intimate mixtures of siderite and calcite, with parisite and synchysite existing as major replacive components of calcite down to depths of 200 m. Synchysite appears to be the more stable form of lanthanide fluorcarbonate at depths below 150 m.Oxygen isotope measurements on separated calcite concentrates from three of the Buru drill cores reveal significant deviations from primary igneous carbonatite values suggesting isotopic re-equilibration with meteoric water. Accepting that the African plate may have been stationery during the past 30 Ma, it is possible to use a modern sample of water from the Kenya rift to estimate the equilibration temperature at which Buru carbonatite in western Kenya has recrystallized. Temperatures of ∼60 to 90°C suggest that the hot-spring activity encouraged the formation of replacive lanthanide fluorcarbonate in ferruginous calciocarbonatite. Lanthanide enrichment also occurred closer to the surface within the pyroclastic ferrocarbonatite at temperatures of ∼40 to 60°C corresponding to waning stages of hot-spring activity. Further lanthanide enrichment redistribution and oxide formation occurred at surface supergene temperatures of ∼20 to 30°C forming the lateritized top of the carbonatite.The wider implications from these carbonatite studies are the recognition that lanthanide enrichment can be preserved in volcanic-subvolcanic centres, and that lanthanide fluorcarbonates can replace calcite in response to circulating hot-spring waters. From a classification point of view, ferrocarbonatites should be seen as products of meteoric hydrothermal systems unrelated to carbonatite magmatic processes

The central Damaran (Pan-African) Orogenic Belt in western Namibia is dominated by elongated granite-gneiss domes, surrounded by Neoproterozoic Damaran cover as periclinal synclinoria, mainly composed of amphibolite-facies metasedimentary rocks. The domes consist of remnants of pre-Damaran gneissic basement and/or Damaran granitoids. There is a high strain zone along the margin between the granite-gneiss domes and the Damaran cover, interpreted in terms of a late extensional mid-crustal detachment, following oblique north-south collision of the Kalahari and Congo Cratons (ca 550 Ma). The present paper focusses on two granite-gneiss domes (Khan and Ida) in western-central Namibia, where a previously unrecognised generation of early post-collisional Damaran granitoids (ca 530 Ma old), have invaded the pre-Damaran basement and cross-cut the detachment zone. These granitoids are classified as syeno-monzogranites with peraluminous high-K calc-alkaline compositions and I/S-type signatures. Geochemical data suggest that the syeno-monzogranites were derived from a lower crustal source, tentatively ascribed to the pre-Damaran gneissic basement. The granitoids also show evidence for magma contamination through partial assimilation with dark, amphibolitic cover rocks. As a result of this hybridisation of the syeno-monzogranites, granitoids with more dioritic compositions were developed. The syeno-monzogranites were then cross-cut by Damaran pegmatitic leucogranites (ca 510 Ma). Further evidence of episodic post-collisional magmatic activity is provided by late-stage mineralisation, and by a ca 465 Ma (40Ar-39Ar) thermal overprint in the cover rocks. It is proposed that the post-collisional magmatic activity emanated from the endogenic part of a mid-crustal Damaran granitic batholith, whose cupola-like emplacement in dome structures coincided with regional uplift, and subsequent cooling before ca 465 Ma.

Carbon and O isotopic compositions were determined for calcite and siderite from the Buru and Kuge carbonatite centres. Wide ranges in the isotopic compositions of these minerals were observed with values of δ13C and δ18O for the Buru calcites ranging from +1.3 to −3.2% (PDB) and + 11.3 to +26.2% (SMOW), respectively. The δ13C and δ18O ranges for the Kuge calcites are −3.1 to −8.4% (PDB) and +18.1 to + 25.7% (SMOW). The Buru siderites plot in a narrow and restricted range at −3.1 to −4.4% (PDB) and +12.6 to + 16.1% (SMOW). Calcite and siderite from the Buru and Kuge volcanic carbonatite centres do not retain their isotope signatures expected for primary magmatic carbonatites. Most of the variations in CO isotope composition can be attributed to secondary processes involving low-temperature hydrothermal alteration and isotope exchange between carbonatites and meteoric water. Higher δ18O values (+21.9 to +26.2%) with a significant increase in δ13C values (−1.5 to + 1.3%) shown by the most oxidised samples from the Buru carbonatite may indicate the involvement of supergene exchange with atmospheric CO2 at surface temperatures.

We present a new analysis of the lithospheric architecture of Africa, and its evolution from ca. 3.6 Ga to the present. Upper-lithosphere domains , generated or reworked in different time periods, have been delineated by integrating regional tectonics and geochronology with geophysical data (magnetic, gravity, and seismic). The origins and evolution of lower-lithosphere domains are interpreted from a high-resolution global shear-wave tomographic model, using thermal/compositional modeling and xenolith/xenocryst data from volcanic rocks. These data are integrated to map the distribution of ancient highly depleted subcontinental lithospheric mantle (SCLM), zones of younger or strongly modified SCLM and zones of active mantle upwelling, and to relate these to the evolution of the upper lithosphere domains. The lithospheric architecture of Africa consists of several Archean cratons and smaller cratonic fragments, stitched together and flanked by younger fold belts; the continental assembly as we see it has only existed since lower Paleozoic time. The larger cratons are underlain by geochemically depleted, rigid, and mechanically robust SCLM; these cratonic roots have steep sides, extending in some cases to ≥300-km depth. Beneath smaller cratons (e.g., Kaapvaal) extensive refertilization has reduced the lateral and vertical extent of strongly depleted SCLM. Some cratonic roots extend ≥300 km into the Atlantic Ocean, suggesting that the upper lithosphere may detach during continental breakup, leaving fragments of SCLM scattered in the ocean basin. The cratonic margins, and some intracratonic domain boundaries, have played a major role in the tectonics of Africa. They have repeatedly focused ascending magmas, leading to refertilization and weakening of the SCLM. These boundaries have localized successive cycles of extension, rifting, and renewed accretion; the ongoing development of the East Africa Rift and its branches is only the latest stage in this process. The less depleted SCLM that underlies some accretionary belts may have been generated in Archean time, and repeatedly refertilized by the passage of magmas during younger tectonic events. Our analysis indicates that originally Archean SCLM is far more extensive beneath Africa than previously recognized, and implies that post-Archean SCLM rarely survives the collision/accretion process. Where continental crust and SCLM have remained connected, there is a strong linkage between the tectonic evolution of the crust and the composition and modification of its underlying SCLM.

Combined Pb, Sr and Nd isotope determinations on several Nigerian Mesozoic ( 170 Ma) ring complexes, studied previously by van Breemen and co-workers, indicate a multistage petrogenetic process. Mantle-derived differentiated magmas assimilated crustal basement of average Early Proterozoic age. After crystallisation, some plutons were subjected to a second stage of crustal contamination by circulating hydrothermal fluids. Crustal compositions were constrained by isotopic analysis of the Proterozoic basement of the Jos Plateau. Sm/Nd analysis of six gneisses yielded an average crustal residence age of 2 Ga, corresponding to the Burkinian event recognised elsewhere in western Africa. However, one sample yields a model age of 3 Ga, suggesting the presence of Archean crustal remnants. Pan-African granitoids yield a similar range of Nd model ages to the gneisses, suggesting that they were largely generated by crustal melting. The Zaranda anorogenic complex has relatively radiogenic initial Nd and Pb isotope compositions and unradiogenic Sr ( 0.5126,18.4 and 0.705, respectively), attributed to a mantle-derived differentiated magma which suffered moderate contamination during ascent through the crust. Other ring complexes trend toward less radiogenic Nd and Pb isotope ratios and more radiogenic Sr, indicative of an increasing crustal contribution. Initial Pb isotope compositions yield a well-defined Pb/Pb isotope array with a slope age of1.8 Ga which is consistent with the average Nd crustal residence ages of basement gneisses and granitoids. The arfvedsonite albite apogranite from the Ririwai anorogenic complex has isotope ratios resembling Pan-African basement, probably resulting from hydrothermal overprinting with fluids equilibrated in the continental crust. Other Ririwai intrusions and one unit from the Shere Hills display evidence of hydrothermal overprinting of Sr and to some extent Nd isotope compositions, but only the Ririwai apogranite has been significantly overprinted by hydrothermal Pb. The isotopic evidence supports a model for the Mesozoic anorogenic (“A-type”) granites of Nigeria in which mantlederived magmas suffered crustal contamination during magmatic differentiation to syenitic compositions, followed by sub-solidus hydrothermal alteration in the continental crust.

Field observations, petrological interpretations combined with cationic geochemical data and diagrams illustrate the magmatic and postmagmatic events in the Mesozoic anorogenic complexes of northern Namibia. The magmatic lineage of the Damaraland alkaline centres is cationically defined in a modified Q–P diagram (Debon and Le Fort 1988). Major element concentrations for Brandberg and Erongo supported by chemical data from other Damaraland complexes at Cape Cross, Otjihorongo, Messum, Spitzkoppies, and Paresis, have been converted to millications, to delineate a magmatic lineage which follows an alkaline oversaturated trend. Superimposed on this magmatic lineage is textural and geochemical evidence for hydrothermal metasomatic disequilibrium and partial re-equilibration. Volcanic rocks and their granitic equivalents from the Erongo complex show petrological and geochemical criteria indicating substantial potash and boron metasomatism which has given the Erongo rocks a subalkaline potassic geochemical signature. Similar granite compositional trends are also recorded within the Brandberg massif. However, at Brandberg, there is a later-stage hydrothermal overprint generating a texturally variable peralkaline granite, with spectacular examples of linear zones of aegirine and arfvedsonite apparently constrained to microfractures in marginal granite sheets. The zones of mineral ‘layering’ can be divided into three repetitive units and traced horizontally along the southwestern margin of the Brandberg massif for a distance of 3 km and vertically for 200–300m. These marginal rocks are cross-cut by dykes and sheets of brandbergite, a mesocratic fine-grained peralkaline rock. Fenite veins, described elsewhere from marginal zones of strongly alkaline massifs, are found near the inner brecciated contact zone with the main granitic mass of the Brandberg. These fenite veins are interpreted as fluid-channel ways along which mass transfer of peralkaline components occurred during the final stages of cooling and consolidation.

Most of the African continent consists of various terranes of different Precambrian ages accreted during major orogenies. It is not surprising therefore that the ages and orogenic events throughout Africa cannot always be correlated within the current configuration of the African continent. The Pan-African orogeny in the late Proterozoic played a major role in re-assembling old cratons and terranes to form the supercontinent Pangaea and its southern extremity, Gondwanaland. It was into this basement that the Phanerozoic plutonic complexes were emplaced. As a result of terrane accretion the mineralizing potential of each terrane as a possible source for ore metals must be considerably variable, and perhaps account for part of the irregular distribution of mineralized Phanerozoic complexes on the African plate.

A series of overlapping ring structures each contain varied, pre-caldera volcanic sequences, overlain by massive intra-caldera rhyolitic ignimbrites and enclosed within ring dykes of fayalite-hedenbergite granite-porphyry. Early basalts are hypersthene-normative but may originally have been alkaline or transitional to nepheline-normative. They are linked, through a series of andesine basalts, mugearites and trachyandesites, to aphyric trachytes. Pre-caldera rhyolitic rocks, interbedded with and overlying the basic and intermediate volcanics, are mainly ignimbrites which were perhaps originally peralkaline or near peralkaline, but are often recrystallized to subalkaline compositions. They formed mainly through partial melting with zone refining of crustal rocks, the mantle-derived basic to intermediate magmas providing the heat source. In the early stages of each caldera-forming cycle the ignimbrite eruptions were of small volume and the rocks have varied crystal contents. Later, large high level magma chambers must have developed, from which crystal-rich ignimbrites with volumes in the order of 100 km 3 were erupted, with accompanying caldera collapse. The form of the complex as a whole shows that this cycle occurred repeatedly, but with the location of magma generation migrating progressively westwards, resulting in a chain of rhyolitic cauldrons, into which subvolcanic granites and syenites have been emplaced.

The Streckeisen QAP diagram discriminates four characteristic type series among the large variety of granitoids associated in intrusions. Three of them integrate varied compositions from basic to acid terms: (1) the calcalkaline series with its low K (or trondhiemite), medium K (or granodioritic), high K (or monzonitic) variants; (2) the alkaline series with its characteristic syenitic members; (3) the tholeiitic series, deprived of K feldspars. The fourth (4) corresponds to mobilizates associated with migmatites of varied composition but always quartz rich. The generalization in the QAPF system, allowing the plotting of undersaturated alkaline series, provides an excellent visual display of the major associations and trends for the plutonic rocks.

Sixty-four Rb-Sr and two K-Ar isotopic measurements from seven ring complexes in central Nigeria provide evidence for a systematic age trend along a 200 km zone ranging from 174±5 m.y. in the north to 154±4 m.y. in the south. A peak of anorogenic magmatism occurred in the Jos Plateau region about 164±4 m.y. ago. Although a small syenitetrachyte complex at Zaranda, near Bauchi, gives an age of 190±15 m.y., unpublished ages of 290–330 m.y. for the southern Niger ring complexes confirm the existence of an overall southerly decreasing age trend in the Niger-Nigeria province of West Africa.

The close of the Pan-African was characterized by a change from subduction and arc related magmatism to post tectonic alkaline magmatism in Saudi Arabia and West Africa. For the remainder of the Phanerozoic, folding and tectonism was more restricted in Africa than on any other continent and was limited to the Caledonian, Hercynian and Alpine orogenies in the extreme north, and the Cape fold belt at the southern tip of the continent, with an equivalent orogenic belt on the extreme east of the Arabian peninsula. Thus Phanerozoic magmatism was dominantly anorogenic in nature and alkaline in character throughout the African Plate. Such magmatism was initiated by reactivation of deep-seated lineaments with the peak of activity during the Mesozoic, which can be correlated with fragmentation of Gondwanaland. The Phanerozoic alkaline magmatism is characterised by small centres of subvolanic to plutonic nature often in the form of ring complexes. These can be subdivided into two separate associations consisting of Provinces dominated by centres of oversaturated magmatism such as Nigeria and Sudan. There are also provinces with undersaturated complexes and carbonatites with oversaturated magmatism such as in Namibia, Angola, and the East African rift system. Despite the contrasting nature of undersaturated and oversaturated magmatism many comparisons can be drawn between carbonatites and granitic complexes. Such comparisons include the source of the magmas, the mechanisms of emplacement, the cause and effect of alkali metasomatism and processes of hydrothermal alteration, the trace element geochemistry, the abundance of zircon, uranium, thorium and complex Ti-Zr silicates, the importance of columbite, pyrochlore, fluorine, and rare earth enrichment in mineralization, and in their current and future economic potential. The common feature is the development and reactivation of deep-seated lineaments which were not only controlled the emplacement of anorogenic complexes but also acted as channelways for hydrothermal mineralizing fluids. The development of a fluid phase during crystallisation occurred in both undersaturated and oversaturated alkaline rocks. Subsequent hydrothermal modifications, subsolidus reactions, geochemical variations and mineralization were dependent upon whether this fluid phase was retained or expelled.

Forty Rb-Sr isotopic measurements on minerals and whole rock samples from 11 anorogenic complexes in northern Nigeria complement the evidence for systematic age variations along a series of en-echelon ENE-WSW and N-S lineaments following two major fracture directions of extensional deformation in the Precambrian basement. The new ages presented in this paper confirm our earlier discoveries of a general southerly age trend but substantiate for the first time that major local migrations of magmatic activity were concentrated along at least two ENE-WSW zones. From the overlap in analytical error, the age patterns also suggest that contemporaneous anorogenic magmatism may have been locally derived from several simultaneous high-level magma chambers connected to a common deeper source. Different models that have been proposed for the origin of linear mid-plate magmatic activity are critically reviewed. In our opinion magma generation was most likely initiated in the asthenosphere-lithosphere decoupling zone or...

Twenty-nine Rb-Sr whole-rock isotopic analyses and three U-Pb zircon analyses on foliated granites and largely unfoliated charnockitic rocks indicate that the central part of the Pan-African belt in west Africa was characterised by intense orogenic plutonism. These data and Rb-Sr analyses on muscovite books from late cross-cutting pegmatites indicate that the peak of magmatic activity occurred 610 ± 10 m.y. ago. Initial 87 Sr/ 86 Sr ratios for the granitic and charnockitic rocks are in the range 0.7065–0.7125, and indicate a significantly older crustal component in the magmas.

In Niger, uranium occurs in upper Palaeozoic and lower Mesozoic continental sedimentary basins west of the Aïr mountains, but the source of the uranium had not been identified. Geochemical studies and fissiontrack observations on alkaline ignimbrites preserved in two Palaeozoic anorogenic centres in Aïr show that uranium is concentrated in the matrix and on secondary iron-oxide coatings surrounding lithic and crystal fragments. Based on variable Th/U ratios and degree of oxidation, it is concluded that the original ignimbrite field was enriched in uranium, but that a considerable proportion was leached during the weathering of the volcanic pile. Tectonic uplift, anorogenic magmatism, followed by weathering and erosion of the volcanic cover, with sedimentation in nearby continental basins, have all contributed to the development of uranium mineralization in Niger. The petrological and geochemical similarities between the Palaeozoic ring complexes in Niger and the Nigerian Mesozoic ring structures suggest that sedimentary uranium deposits may also be found in Nigeria if the tectonic and sedimentological controls were favourable.Enriched concentrations of uranium have been discovered in the exposed granitic roof zones of the Nigerian subvolcanic centres, with Th/U ratios approaching unity. Thus local vein deposits of uraninite, as well as dispersed uranium in recent sedimentary horizons, could be discovered particularly in the drainage systems entering the Chad Basin.

Two series of granitoids are present in the Finisterre region. One is formed mainly by syn-collision two-mica leucogranites. The other is composed of syn- and post-collision granodiorites and monzogranites of calc-alkaline affinity. The emplacement of these granitoids took place during and after the main phases of regional deformation. Petrographic and geochemical studies on the leucogranites suggests a common crustal origin with the surrounding metamorphic series, which includes metasediments and orthogneisses of medium to high grade amphibolite facies. The syn- and post-collision "calc-alkaline" series is associated with basic to intermediate rocks, suggesting a lower crustal origin. REE data have been used to characterize the two types of granitoids and to model the source areas of generation. There is no evidence for an origin of the granitoids by melting of mantle-type igneous rocks. Instead, the sequences of granitoids can be explained assuming different degrees of partial melting from a heterogeneo...

Carbonatites are defined as igneous rocks containing more than 50% carbonate minerals. They are divided into four classes: (1) calcic, (2) dolomitic, (3) ferrocarbonatite, and (4) natrocarbonatite. Carbonatites are characteristically associated with ijolites, but not all carbonatites are entirely magmatic. Many centres show metasomatic replacement features, referred to as fenitization. Most subvolcanic and volcanic carbonatite centres are calcic, but trapped fluids as inclusions in apatite crystals indicate that the bulk composition was alkalic and chemically comparable to natrocarbonatite. Experimental evidence suggests that parental carbonatite magma is the product of immiscibility from an alkali silicate liquid of phonolitic or nephelinitic composition. Alkali loss from the bulk carbonatite magma caused fenitization represented by zones of sodic and potassic metasomatism. Sodic metasomatism is more frequently located at deeper eroded centres and is characterized by the occurrence of albitite. Potassic metasomatism is more common in many carbonatite centres and is represented mineralogically by alkali feldspar and phlogopite. Many fenites carry an alkaline ferromagnesian assemblage consisting principally of alkali amphibole (eckermannite-magnesioarfvedsonite) and/or aegirine. Carbonatites have igneous isotopic ratios and distinctive contents of incompatible trace elements. Associated fenites have elevated Sr isotopic ratios and selected TE enrichment pertinent to the dominant metasomatism. Carbonatites exhibit REE, Nb, U, Th mineralization and, more rarely, sphalerite, galena, and chalcopyrite mineralization. Both disseminated and vein-controlled mineralization are found in many carbonatite centres. The most important economic products of carbonatites include pyrochore, columbite, monozite, apatite and zirconium minerals. The alkaline syenites and alkaline granites as ring complexes have textural, petrological and chemical characteristics which are distinctly different from I and S type granitoids. The characteristics of A-type granites are dominated by the separation of an (immiscible?) fluid phase which caused limited sodic and potassic metasomatism. Greisenization by acid metasomatism has played a critical role in several anorogenic provinces producing important UNb, ZnSn mineralization. In the fayalite granites, ferrohedenbergite breaks down to sodic-calcic amphibole as iron-rich olivine is destabilized under the influence of the fluid phase. Facies with aegirine and arfvedsonite can develop to give peralkaline granites. Relative loss of the peralkaline fluid phase results in the development of peraluminous biotite granites. The most significant chemical feature of anorogenic granites is that slight differences in the proportions of Na, K and Al can produce striking changes in the mineralogy of the A-type suite. The most characteristic feature of peralkaline granites is the anomalous enrichment in Zr, Zn, Nb, Y, Th, U, LREE, HREE, coupled with high Rb/Sr ratios. Some of these features are found in biotite granites but with dramatically less Zr, Hf, Nb and HREE. The primary mantle origin of syenitic magmas parental to intraplate alkalien granites is well established from isotope studies. The role of the continental crust is implemented by elevated initial 87Sr/86Sr ratios. The lower crust in Africa is thought to be lower Proterozoic in age from NdSrPb isotopic data. Fenitization is not only associated with carbonatite centres but also to a limited extent with alkaline granites. The evolution and separation of a peralkaline fluid phase from A-type granites has been experimentally demonstrated, and can be recorded by distinctive suites of subsolidus minerals and specific geochemical changes. From the mineralization viewpoint there are many parallels with the carbonatites, particularly with the abundance of sphalerite, rare-earth minerals, zircon and complex titanium-zirconyl silicates, uranium and thorium, columbite and pyrochlore. Such a distinctive alkaline mineralization suite, possibly related to an alkali carbonate fluid phase, is superimposed on a more normal, less alkaline group of ore minerals such as cassiterite, wolframite, chalcopyrite, and galena.

Granitoid rock compositions from a range of tectonic environments are plotted on a multicationic diagram devised by de la Roche and his coworkers. This shows that there is a systematic change through an orogenic cycle which leads progressively to the ultimate development of alkaline magmas. Possible source materials and mechanisms of magma generation are considered from analysis of mineral compositional vectors. These suggest that most granitoid series result from a two-stage process. First, fractional crystallisation of clinopyroxene, olivine and calcic plagioclase from a basic source with tholeiitic affinities produces a magma of intermediate composition. This magma then undergoes periodic mixing with a felsic magma followed by in situ fractionation to generate individual intrusions within granitoid series.

The Nigerian younger granites, as well as the petrologically similar but chronologically older northern centres in Air and south Niger, constitute the best example of epidosically migrating mid-plate magmatism. The Nigerian anorogenic ring complexes provide a classic example of alkaline granitoid evolution. Each anorogenic centre represents the exposed roots of an eroded volcano. Volcanic assemblages preserved by caldera collapse at some centres confirm that early erupted liquids evolved from olivine basalt through hawaiite, mugearite and trachyte. Although there are volumetrically abundant rhyolitic ignimbrites, there are no volcanic equivalents of biotite granite. However, at the subvolcanic level biotite granites are extensively mineralized with rich deposits of columbite, sphalerite, cassiterite and wolframite. Related peralkaline granites contain enriched concentrations of ore elements but apart from pyrochlore, thorium-rich monazite, and topaz, most potential ores are contained in alkali amphiboles or alkali pyroxene. Fluid interaction in the granite cupolas has caused subsolidus mineral growth and modified original element abundances. In some instances fluid reaction has been so intense that original granite has been locally converted to albite, microclinite, and greisen. These reactions can be monitored by the new mineral assemblages, the major element chemistry, trace elements (e.g., the rare earths) and isotopically by the initial strontium isotopic ratios.

Nine rare earths have been determined by Instrumental Neutron Activation Analysis (INAA) in syenites and related granitic rocks from the Nigerian younger granite (Mesozoic) Province. The majority of the granites contain dispersed UNb and vein-controlled ZnSn mineralization. It is inferred from a study of the chondrite-normalized rare-earth patterns that europium depletion in alkaline and sub-alkaline mineralized granites appears to be not just related to magmatic processes but also to hydrothermal changes induced during sub-solidus recrystallization.

The significance of rare earth distribution patterns in trace element geochemistry is briefly reviewed. Established methods for their determination at the Reactor Centre by instrumental and radiochemical techniques are described, and examples of recent applications discussed.

A study of selected rare earths, determined by non-destructive neutron activation analysis, in six Nigerian younger granites, together with previously publisded data on similar granites from northern Nigeria and Russia, reveals that anorogenic granites with peralkaline affinities develop an enrichment in the middle rare-earth group Sm, Gd, Tb and Dy with a corresponding Eu depletion. Such patterns may be related to the precipitation of earlier formed perthitic feldspar and late crystallizing alkaline amphibole from low temperature liquids enriched in volatiles (alkalis and fluorine). Progressively albitized peralkaline riebeckite granites exhibit rare earth enrichments towards Yb until the most albitized peralkaline granite is no longer depleted in Eu and there is a marked upward curvature of the relatively heavier rare-earth patterns. This change could be caused by residual albite-rich hydrothermal fluids which overprinted the earlier rare-earth distribution patterns.

The radioactivity induced in rock slices during fission track analysis for uranium can usefully be employed in instrumental neutron activation analysis for determination of major and trace element distributions in the sample using a collimated gamma spectrometer. A description is given of the application of particle track techniques in conjunction with activation analysis to a geochemical study of element migration in a sample of mineralized granite from Nigeria. The study indicates that U is enriched during the earlier period of rock-fluid interaction (microclinization) and that Th is enriched during greisenization.

The similarity of the lead isotopic composition in older granites, pegmatites and the younger granites, together with the distribution of tin and the absence of tourmaline in the Mesozoic ring-complexes, are utilised as evidence to establish that the Nigerian younger granites were derived from an initial peralkaline melt by fusion of local ‘basement’ rocks. It is also suggested that the associated metaluminous trend is caused simply by more complete melting, and that peraluminous granites developed during subsequent cooling. When feldspar normative compositions of younger granites are plotted in the quartz-saturated ternary feldspar system (James and Hamilton, 1969), they support a cyclic event of fusion and cooling to account for the variation in rock-type. The actual source material has not been established but it could be similar to bauchite, a quartz-fayalite monzonite, which was melted by a linear zone of high heat flow from the mantle during the disruption of Gondwanaland.

Partial melting experiments on Nigerian charnockitic monzonite associated with migmatised gneisses provide consanguineous liquids from which the metaluminous and peraluminous younger granites may be derived. After 40–60% melting at 750–800° C the liquids are metaluminous with hastingsite-fayalite granite affinities. On cooling towards the granite system minimum these liquids can evolve into metaluminous hastingsite-biotite granite and peraluminous biotite granite compositions. These experiments show that partial melting of charnockitic monzonites cannot yield peralkaline liquids at the specified conditions (750–850° C).

From 1960 to August, 1966, the activity of Oldoinyo Lengai took the form of quiet extrusion of carbonatite lava. In August, 1966, the style of activity changed abruptly and violent ash eruptions took place. The activity varied from minor emissions of ash to major Plinian and Vulcanian type eruptions. A new ash-cone built up within the crater and ash was widely distributed on the slopes of the volcano and over the surrounding countryside. The ash consists of sodium carbonate mixed with crystals of nepheline, pyroxene, wollastonite, apatite, melanite and pyrite. Also blocks of ijolite and melteigite were ejected during the activity.

K/Rb ratios have been determined in about 70 granites from Northern Nigeria. It is shown that late stage rocks such as albite-riebeckite-granites have ratios below 30 while early-stage rocks such as Ca-amphibole-granites can have ratios up to 330; biotite-granites have intermediate ratios. In the Liruei complex the ratio decreases progressively with time of emplacement but in the Amo complex the ratio varies irregularly.