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Age tardi-ubendien (Protérozoïque inférieur) des dômes granitiques de l'arc cuprifère zaïro-zambien.

Article in French on a Palaeoproterozoic (U-Pb on zircon age) granitoid from the basement in Katanga (Congo) and the consequences for mineralisations anf geodynamics

A new lithostratigraphic framework for the Anti-Atlas Orogen, Morocco

A new lithostratigraphic nomenclatural framework is proposed for the Anti-Atlas Orogen of Morocco, to replace the previous chronostratigraphic scheme. The oldest, Palaeoproterozoic, rocks of the cratonic basement to the orogen form the northern part of the West African Craton and can be subdivided into a series of Complexes (made up of constituent Groups and Suites etc), depending on geographical outcrop (e.g. the Zenaga and Kerdous Complexes in the Sirwa and Kerdous inliers respectively). The early Neoproterozoic rocks deposited on this basement are termed the Anti-Atlas Supergroup, within which five groups of volcano-sedimentary units (Taghdout, Jbel Lkst, Sarhro, Iriri, and Bou Azzer Groups) and various intrusive igneous rocks are recognised (e.g. Ifzwane and Toudma Suites). These units are related to the earliest passive margin, oceanic and island-arc development phases of the orogen between ~800 and 660 Ma. The earliest Pan-African deformation (at ~660 Ma) resulted from closure of the ocean basin, SW-directed thrusting and accretion of the island-arc remnants. Continued prolonged convergence gave rise to a sinistral transpressional regime in which late syn- to post-tectonic continental volcanic and clastic sequences of the Ouarzazate Supergroup were deposited. The first rocks of this new tectonic regime were deposited in narrow, tectonically-active, strike-slip pull-apart rift basins (Bou Salda, Mgouna, Tafrawt and Anzi Groups), associated with intrusion of high-K calc-alkaline granitoid batholiths (e.g. Assarag, Bardouz Suites) and the juxtaposition of ophiolitic remnants in major strike-slip shear belts. These were followed by the deposition of the much more areally extensive continental volcano-sedimentary molasse, the volcanic components of which were erupted from several interfingering volcanic centres, often associated with caldera complexes with sub-volcanic alkaline granites (Toufhgrane, Tanghourt and Guellaba Suites). The proposed lithostratigraphic scheme encompasses all the major rock units identified in the literature from the orogen and is flexible so that new, lower-ranking, units can be placed at the appropriate stratigraphic level in future. This framework is supported by new high-precision U-Pb zircon dates that have recently become available.

An example of post-collisional mafic magmatism: The gabbro-anorthosite layered complex from the Tin Zebane area (western Hoggar, Algeria).

Abstract The Tin Zebane gabbro-anorthosite layered mafic intrusion represented by plagioclase-rich cumulates forms a set of small lenticular to round-shaped mainly undeformed bodies intruding the Pan-African high-pressure metamorphic rocks from western Hoggar (Tuareg shield, southwest Algeria). The coarse-grained anorthosites are mainly made of slightly zoned bytownite (An86-74) with the higher anorthite content at the cores. Anorthosites are interlayered with leucogabbros and gabbros that show preserved magmatic structures and with olivine gabbros characterised by coronitic textures. The primary assemblage in gabbros includes plagioclase (An93-70), olivine (Fo77-70), zoned clinopyroxene (En43-48Fs05-13Wo41-49 with Al2O3 up to 4.3 wt%) and rare orthopyroxene (En73-78). Pyroxenes and olivine are commonly surrounded by Ca-amphibole. The olivine-plagioclase contact is usually marked by a fine orthopyroxene - Cr-spinel - amphibole symplectite. A magnesian pigeonite (En70-75Fs19-20Wo6-10) is also involved in corona. The coronitic minerals have equilibrated with the primary mineral rims at P-T-aH2O conditions of 797+42°C for aH2O = 0.5 and 808+44°C for aH2O = 0.6 at 6.2+1.4 kbar. The Tin Zebane gabbroic rocks are depleted in REE with a positive Eu anomaly, high Sr (>10*chondrite) and Al2O3 concentrations (17-33%) that support plagioclase accumulation with the extreme case represented by the anorthosites. The REE patterns can be modelized using plagioclase, clinopyroxene and orthopyroxene REE signature, without any role played by accessory minerals. High MgO content points to olivine as a major cumulate phase. Anorthositic gabbros Sr and Nd isotopic initial ratios are typical of a depleted mantle source (Sri= 0.70257-0.70278; Nd= +5.9-+7.8). This isotopic signature is identical to that of the 10-km wide 592 Ma old dyke complex composed of alkaline to peralkaline granites and tholeiitic gabbros and one single bimodal complex can be inferred. The source of the Tin Zebane basic rocks corresponds to the prevalent mantle (PREMA). The Tin Zebane complex was emplaced along the mega-shear zone bounding to the west the Archaean In Ouzzal metacraton. The model proposed suggests a linear lithospheric delamination along this rigid and cold terrane due to post-collisional transtensional movements. This allowed the asthenosphere to rise rapidly and to melt by adiabatic pressure release.

Contrasting origin of post-collisional high-K calc-alkaline and shoshonitic versus alkaline and peralkaline granitoids. The use of sliding normalization

Abstract Abundant high-K calc-alkaline (HKCA) magmatism appears to be post-collisional and often shifts to shoshonitic or alkaline-peralkaline compositions in the final stages of orogeny. The nature and the causes of this transition are studied on the basis of 308 major element and of 86 unpublished trace element (including REE) analyses of the Pan-African granitoids from the Tuareg shield (Adrar des Iforas, Mali and Aïr, Niger). This database covers a wide variety of magmas from subduction-related to intraplate-type including abundant HKCA batholiths. Literature data from geodynamically well-constrained cases are also included. In addition to a conventional geochemical approach of the studied magmatism, the sliding normalization method is proposed. This tool aims at comparing magmatic series: each studied rock is normalized to the interpolated composition of the reference series that has the same SiO2 content as the sample. This method amplifies differences in sources and in fractionation processes and allows comparison of rocks from basic to acid composition. Two distinct juvenile sources are proposed: a previously enriched phlogopite–K richterite bearing lithospheric mantle or a lower juvenile crustal equivalent for HKCA-shoshonitic magmas, and a lowest lithospheric-upper asthenospheric OIB-type mantle for alkaline-peralkaline magmatism. The first source is melted only shortly after its generation when the lithosphere was still hot, which restricts HKCA magmatism mainly to post-collisional settings. The second asthenospheric/lowest lithosphere source is by definition close to its melting temperature and can generate magma ubiquitously both in space and time. The main melting triggers are lithospheric major structures which are not only operative in a post-collisional setting but also in other environments such as intraplate setting. Geochemistry thus gives indications about the nature of the source and on geotectonic settings. However, the latter is a second rank information, which is partly model-dependant. The post-collisional period differs from other settings by a propensity to generate large amounts of magma of various kinds, among which HKCA magmatism is volumetrically the most prominent. Keywords: High-K calc-alkaline, alkaline, geochemistry, sliding normalization, granite, Tuareg shield

Cratons, mobile belts, alkaline rocks and continental lithospheric mantle: the Pan-African testimony

 

Dedication to Louis Latouche

Dedication to Louis Latouche of the Journal of African Earth Sciences special issue on "The Precambrian of the Tuareg shield"

Derivation of the 1.0 - 0.9 Ga ferro-potassic A-type Granitoids of southern Norway by extreme differentiation from basic magmas

Abstract Major and trace elements, Sr and Nd isotopic data as well as mineral compositions are presented for a selection of the 1.0 - 0.9 ferro-potassic A-type granitoids (Bessefjellet, Rustfjellet, Verhuskjerringi, Valle, Holum, Svöfjell, Handeland-Tveit, Åseral, Lyngdal gabbronorites) that occur close to the Mandal-Ustaoset Line of southern Norway. These hornblende biotite granitoids (HBG) define an extensive differentiation trend ranging from gabbronorites (50 wt % SiO2) to granites (77 wt % SiO2). This trend is interpreted as resulting from extreme fractional crystallization of several basaltic magma batches with similar major and trace elements compositions. At 930 Ma, the HBG suite displays a narrower range in ISr (0.7027 - 0.7056) than in Nd(t) (+1.97 down to -4.90) suggesting some assimilation of a Rb-depleted lower crust (AFC process) or/and source variability. An age of 929 ± 47 Ma is given by a Rb-Sr isochron on the Holum granite (Sri = 0.7046 ±0.0006, MSWD = 1.7). Geothermobarometers indicate a low pressure of emplacement (1.3-2.7 kbar) and an oxygen fugacity close to NNO. High liquidus temperatures are given by the apatite saturation thermometer (1005°C to 1054 °C) and are in agreement with results from other studies. The basaltic parent magmas of the HBG suite are partial melts of an hydrous mafic, potassic source lying either in the lithospheric upper mantle or in the mafic lower crust derived from it. This contrasts with the 930 Ma anorthosite-mangerite-charnockite suite (AMC) of the Rogaland Province for which a depleted lower crustal anhydrous gabbronoritic source has been indicated. The present data imply the penecontemporaneous melting of two contrasting sources in southern Norway. The source duality could result from an increasing degree of metamorphism (amphibolite to granulite) from East to West, an horizontal stratification of the lower crust or from the stratification of the lithosphere (melting of the lower crust or upper mantle). It may also indicate that the AMC and HBG suites formed in two distinct crustal segments. The linear alignment of the HBG suite along the Mandal-Ustaoset shear zone suggests that a linear uprise of the asthenosphere, following a lithospheric delamination under this structure, could be the vector of the mantle heat. Keywords: A-type; Granites; Sveconorwegian; Proterozoic; Southern Norway

Early Neoproterozoic magmatism (1000-910 Ma) of the Zadinian and Mayumbian Groups (Bas-Congo): onset of Rodinia rifting at the western edge of the Congo craton

Abstract In Central Africa, the West Congo (or West Congolian) belt extends subparallel to the Atlantic coast between 1° and 12° S along the western edge of the Archaean Congo craton. In the last three decades, the role of this belt in the geodynamic modelling of the Pan-African - Brasiliano belts, in particular since Gondwana and Rodinia supercontinent reconstructions, has been the subject of episodic discussion. We present new ion microprobe (SHRIMP) U-Pb zircon ages as well as some geochemical and isotopic analyses for key igneous units within the central part of the West Congo belt. We integrate these data with revised geological information in an updated geological map of the entire belt (1:1,000,000 scale), including a synthetic type cross-section of the belt and an updated lithostratigraphic chart of the "West Congo Supergroup". From oldest to youngest, the latter comprises the Zadinian, Mayumbian and West Congolian "Groups". Emplacement of early Zadinian peralkaline granites (Noqui massif, 999 ± 7 Ma) and rhyolites (Palabala) is accompanied by incipient rifting sedimentation, corresponding to the onset of transtensional rifting along the craton preferentially in a transverse mega-shear setting. Subsequently, the upper Zadinian series correspond to a thick (1600-2400 m) basaltic sequence (Gangila) with typical geochemical characteristics of continental flood basalts (CFB) and Nd = -2.4 ± 1.2. The Gangila basalts, marking major rifting associated to pull-apart, are rapidly followed by the Mayumbian rhyolitic lavas (3000-4000 m thick sequence; 920 ± 8 Ma for the base of the rhyolites and 912 ± 7 Ma for their top). The felsic lavas are intruded by coeval high-level (micro)granites, whose emplacement is dated respectively at 924 ± 25 Ma (Mativa body) and at 917 ± 14 Ma (Bata Kimenga body) in the Lufu massif (Nd of this felsic volcanic-plutonic sequence = -11 ± 2). This huge bimodal magmatism is similar to the Parana and Deccan provinces, sharing similar lithospheric sources. It corresponds to the initial, transtensional rifting stage along the western edge of the Congo craton before Rodinia breakup. No Mesoproterozoic events are recorded in the area: no Zadinian nor Mayumbian "orogeny" nor "aulacogen" (thus, no local segment of the Kibaran belt), no Mesoproterozoic ophiolites nor suture zone...

Evolution d'un arc insulaire océanique birimien précoce au Liptako nogérien (Sirba): géologie, géochronologie et géochimie.

Abstract The Sirba région (Nigerian Liptako) is made up of volcanics, sedimentary sequences and granitoid batholiths belonging to the Birimian domain of the West African craton (stabilized at c. 1.9 Ga). Field relationships have distinguished three main magmatic groups. The first Group is made up of basic magmatic rocks located below the sediments. Two Subgroups have been set up: 1a (NE Sirba), pillowed basalts (thickness up to 1500m) and 1b (SW Sirba), various basalts, dolerites and gabbros. Group 2 comprises basaltic dykes cutting group 1 (Subgroup 2a) and gabbroic to dioritic sills intruding group 1 and the sediments (Subgroup 2b). Even if rather limited in volume, this group is ubiquitous. Group 3 represents large granitoid batholiths and frequent acid dykes which intrude all other types of rocks. A granodiorite from group 3 has been dated at 2170 ± 9 Ma (zircon U-Pb) giving a minimum age for all the studied groups which are not much older. Subgroup 1a corresponds to a typical oceanic tholeiitic island arc with depleted LREE (LaN/YbN: 0.3 - 0.9); Subgroup 1b is richer in LREE (LaN/YbN: 1.1-2.7) and LIL elements pointing to an island arc low-K tholeiites signature with a calc-alkaline affinity. Group 2 rocks are low-K calc-alkaline and are enriched in LREE (LaN/YbN: 2.3-7.8) and LIL elements. That indicates a transitional island arc environment (transition to a back-arc setting?). Group 3 granitoids are more differentiated and constitute a medium-K calc-alkaline sequence with higher LREE (LaN/YbN: 11.1-25.6) and LIL elements contents but low abundances in Y and Yb typical of volcanic arc continental margins. This group is contemporaneous to large horizontal strike-slip movements and to greenschist to lower amphibolite facies metamorphism. All groups have Sr and Nd initial ratios within the mantle range (Sri: 0.7015 ± 0.0005; Nd= +2.8 ± 0.8) indicating no Archaean crustal participation in their genesis (mean of TDM model ages: 2207 ± 56 Ma). Sirba area displays then a continuous evolution from a primitive (s-Gr 1a) to a more mature (s-Gr 1b) tholeiitic oceanic island arc and to an evolved low-K calc-alkaline island arc (Gr 2) intruded during its accretion at 2.17 Ga by large syn-kinematic and syn-metamorphic medium-K calc-alkaline granitoid batholiths (Gr 3). This preserved early Birimian evolution, the far eastern position of the Sirba belt and the weak intensity of both tectonics and metamorphism suggest an early oblique intra-oceanic colla

Geochemistry and Sr, Nd, Pb isotopic composition of the Central Atlantic Magmatic Province (CAMP) in Guyana and Guinea.

The Central Atlantic Magmatic Province (CAMP) extends over more than 5000 km on both sides of the Atlantic Ocean. Its emplacement records a short period of time (200 ± 5 Ma) and is linked to the initial stage of continental breakup of Pangaea. Two areas on both sides of the Atlantic Ocean are studied herein, French Guyana/Surinam, South America and Guinea, West Africa. The results obtained from these two regions are contrasted and indicate a depleted mantle source close to PREMA-type for French Guyana/Surinam and a more enriched source for Guinea. This geochemical study supports the prevalence in Guinea, as in the majority of CAMP, of a lithospheric mantle source, previously enriched during ancient subduction events, and preferentially reactivated in late Triassic times by edge-driven convection between cratonic and mobile belts domains. Higher contribution of a depleted asthenospheric source is required in Guyana, which may reflect the evolution of CAMP rifting towards the initiation of the central Atlantic oceanic crust.

Isotopic (O, Sr, Nd) and trace element geochemistry of the Laouni layered intrusions (Pan-African belt, Hoggar, Algeria): evidence for post-collisional continental tholeiitic magmas variably contaminated by continental crust

Abstract The three layered intrusions studied in the Laouni area have been emplaced within syn-kinematic Pan-African granites and older metamorphic rocks. They have crystallized at the end of the regional high-temperature metamorphism, but are free from metamorphic recrystallization, revealing a post-collisional character. The cumulate piles can be interpreted in terms of two magmatic liquid lines of descent: one is tholeiitic and marked by plagioclase-olivine-clinopyroxene cumulates (troctolites or olivine bearing gabbros), while the other is calc-alkaline and produced orthopyroxene-plagioclase rich cumulates (norites). One intrusion (WL (West Laouni)-troctolitic massif), shows a Lower Banded Zone where olivine-chromite orthocumulates are interlayered with orthopyroxene-rich and olivine-plagioclase-clinopyroxene cumulates, whereas the Upper Massive Zone consists mainly of troctolitic and gabbroic cumulates. The other two massifs are more homogeneous: the WL-noritic massif has a calc-alkaline differentiation trend whereas the EL (East Laouni)-troctolitic massif has a tholeiitic one. Separated pyroxene and plagioclase display similar incompatible trace element patterns, regardless of the cumulate type. Calculated liquids in equilibrium with the two pyroxenes for both noritic and troctolitic cumulates are characterized by negative Nb, Ta, Zr and Hf anomalies and light REE enrichment inherited from the parental magmas. Troctolitic cumulates have mantle-derived 18O (+5 to +6 ‰), initial 87Sr/86Sr (Sri= 0.7030 to 0.7054), Nd (+5 to –1) values whereas noritic cumulates are variably enriched in 18O (+7 to +9 ‰), show negative Nd (-7 to -12) and slightly higher Sri (0.7040-0.7065). Based on field, petrological and isotopic compositions of Tuareg upper and lower crusts, these Laouni isotopic ratios are interpreted as resulting from a depleted mantle source (Sri= 0.7030; Nd= +5.1; 18O= +5.1 ‰) having experience short term incompatible element enrichment and variable crustal contamination. The mantle magma was slightly contaminated by an Archaean lower crust in troctolitic cumulates, more strongly and with an additional contamination by an Eburnian upper crust in noritic cumulates. Lower crust input is recorded mainly by Sr and Nd isotopes and upper crust input by O isotopes. This is probably due to the different water/rock ratios of these two crust types...

Petrogenesis of the Kabanga-Musongati layered mafic-ultramafic intrusions in Burundi (Kibaran Belt): geochemical, Sr-Nd isotopic constraints and Cr-Ni behaviour

A succession of mafic-ultramafic layered intrusions forms an alignment in the boundary zone between the Kibaran belt and the Tanzania craton. The intrusions represent a continuous series of cumulate rocks. For instance, in the Mukanda-Buhoro and Musongati (MBM) contiguous bodies, the series starts with dunite and passes to lherzolite, pyroxenite, norite, gabbronorite and anorthosite on top. Cumulate textures are conspicuous in all rock types and cryptic layering characterises cumulus mineral compositions, thus evidencing fractional crystallization as a major differentiation mechanism. The increase of Cr in the ultramafic members of the series indicates that chromite was not a liquidus mineral in dunite and lherzolite rocks, thus unable to form chromitite layers. The high Ni-content of dunite seems to preclude the existence of conjugate Ni-rich sulphide deposits. The 87Sr/86Sr initial ratio is relatively constant and averages 0.7087, with some values up to 0.712 due to local assimilation. Fine-grained rocks from the MBM area are isotopically (Nd and Sr) similar to the MBM cumulates. Modelling their crystallization produces cumulus mineral compositions similar to those in the Musongati ultramafic rocks, which suggests a broadly picritic parental magma. On the other hand, fine-grained rocks from the Nyabikere area are not related to the Nyabikere cumulates. Nd and Sr isotope ratios show that the MBM magmatism is related to an enriched source, possibly an old subcontinental lithospheric mantle. The Nyabikere dykes, as well as the Waga dykes, come from a depleted mantle source, as do the A-type granitoids occurring in the same boundary zone. Several lines of evidence point to two types of parental magmas, a picritic magma, and a more evolved magma, broadly similar to the Bushveld Main Zone magma.

Petrology and geochemistry of the Lyngdal granodiorite (Southern Norway) and the role of fractional crystallization in the genesis of Proterozoic ferro-potassic A-type granites.

Abstract In Southwestern Norway, the Sveconorwegian orogenic thickening (1024-970 Ma) is followed by an important post-collisional magmatism (950-930 Ma), divided . in two suites (Vander Auwera et al., this issue): the AMC suite (Anorthosite-Mangerite-Charnockite) and the HBG suite (Hornblende/Biotite Granitoids). The HBG suite displays a continuous trend from gabbronorites to granites. This paper presents the petrography and geochemistry (major and trace elements, Sr-Nd isotopes) of the Lyngdal granodiorite and associated massifs (Tranevåg and Red Granite massifs), which belong to the HBG suite, although being very close to the anorthosite massifs. Mafic microgranular enclaves (MME), resulting from magma mingling, can be abundant and will be demonstrated to correspond to the parent magma of the studied plutons. The latter plutons are subalkaline, metaluminous A-type granitoids with high FeOt/(FeOt+MgO) ratio and K2O content, forming a ferro-potassic A-type continuous trend from quartz monzodiorite to granite (~ 56 to 72 wt% SiO2). In Harker diagrams, the Lyngdal-Tranevåg plutons share the Proterozoic rapakivi granites trend that they extend to lower SiO2 content.. Major and trace element modelling, as well as Sr-Nd isotopes, show that fractional crystallization - without assimilation - is the differentiation process for Lyngdal-Tranevåg. Fractionating minerals are clinopyroxene, hornblende, plagioclase, oxides, biotite, apatite, zircon and allanite. The Red Granite (71-75 wt% SiO2) does not belong to this trend, probably due to a different initial magma composition. This study shows that ferro-potassic A-type granitoidscan be derived by fractional crystallization from mafic magmas. Keywords : Norway, Sveconorwegian, post-collisional, A-type, granitoids, liquid line of descent

Potassic-Ultrapotassic Mafic Rocks Delineate Two Lithospheric Mantle Blocks Beneath The South Peruvian Altiplano

The Altiplano of Southern Peru displays a large spectrum of Cenozoic potassic (K) and ultrapotassic (UK) mafic rocks that delineate two deep lithospheric mantle blocks that have experienced different depletion and enrichment events. Phlogopite lamproites indicate that the Eastern Altiplano block is underlain by a metasomatized, harzburgitic mantle, of Palaeoproterozoic to Archaean age (TDM = 1130-2485 Ma; &#61541;Nd = -5.0 to -11.4; 87Sr/86Sri = 0.7100-0.7159). Beneath the Western Altiplano block, the presence of a younger (TDM = 837-1259 Ma; &#61541;Nd = +0.6 to -6.3; 87Sr/86Sri = 0.7048-0.7069), metasomatized lherzolitic mantle is deduced from multiple occurrences of diopside-rich K-UK lavas (leucitites, leucite-bearing tephrites, olivine and diopside trachybasalts). A third suite of young (< 2 Ma), K-UK rocks outlines the active Cusco-Vilacanota Fault System separating the Western and Eastern Altiplano blocks; this third suite, composed of diopside phlogopite lamproites and augite kersantites, minettes and trachybasalts, sampled a composite mantle source probably including an asthenospheric component (TDM = 612-864 Ma; &#61541;Nd = -1.1 to -3.5; 87Sr/86Sri = 0.7051-0.7062), in addition to lithospheric components inherited from the Eastern and Western Altiplano blocks. The spatial distribution of the south Peruvian K-UK magmatism suggests that K-UK melts reached the surface through re-using older translithospheric weakness zones extending at least to the depth of magma generation. These latter were reactivated by a dextral transpressional regime imposed on the two rigid lithospheric blocks by the Andean orogen.

Shoshonitic liquid line of descent from diorite to granite: the Late Precambrian post-collisional Tismana pluton (South Carpathians, Romania)

Abstract. The post-collision late-kinematic Tismana pluton belongs to the shoshonitic series. It is part of a Late Precambrian basement within the Alpine Danubian nappes of the South Carpathians (Romania). This pluton displays an exceptionally complete range of compositions from ultramafic to felsic rocks (granites). Widespread mingling/mixing relationships at all scales give rise to a variety of facies. A liquid line of descent from the diorites to the granites is reconstructed by considering the variation in major and trace elements (REE, Sr, Rb, Ba, Nb, Zr, Hf, Zn, V, Co, Cr, U, Th, Ga, Pb) from 33 selected samples as well as mineral/melt equilibrium relationships. The first step of fractional crystallization is the separation from a monzodioritic parent magma of a peridotitic cumulate similar to the ultramafic rock found in the massif. A possible contamination by lower crustal mafic component takes place at this stage. The second step marks the appearance of apatite and Fe-Ti oxide minerals as liquidus phases, and the third step, saturation of zircon. Mixing by hybridisation of magmas produced at different stages of the evolution along the liquid line of descent is also operating (endo-hybridisation). As depicted by Nd and Sr isotopes, fractional crystallization was combined to an important early contamination by a mafic lower crust in a deep-seated magma chamber and to a later and mild contamination by a felsic medium crust in an intermediate chamber. The mingling essentially occurred during the final emplacement in the high-level magma chamber. The monzodioritic parent magma, identified by major and trace element modelling, is shown by Sr and Nd isotopes to have its source in the lithospheric mantle or in a juvenile mafic lower crust derived from it. The necessarily recent enrichment in K2O and associated elements of the lithospheric mantle is likely to be related to the preceding Pan-African subduction period. The partial melting of this newly formed deep source has to be linked to a major change in the thermal state of the plate. Keywords: Post-collisional, Shoshonitic, Granitoids, Hybridization, Pan-African, Romania.

Sr, Nd isotopes and geochemistry of the high-grade amphibolites and gneisses from Bayuda Desert (Sudan): an early Pan-African oceanic arc assemblage and not the East Saharan ghost craton.

Abstract High-grade metamorphic basement of the Bayuda Desert is situated at the inferred transition between the juvenile Neoproterozoic mainly greenschist facies Arabian-Nubian Shield (ANS) and pre-Neoproterozoic mainly amphibolite facies domain of the East Saharan Ghost Craton (ESGC). New geochemical and Sr-Nd isotope data reveal that this basement constituted a Neoproterozoic oceanic convergent margin succession with limited and probably late input of old material. Within this series, garnet amphibolites and epidote-biotite gneisses have geochemical characteristics of HFSE-depleted tholeiitic basalts and low- to medium-K dacites and rhyodacites, indicating magmatism in an oceanic island arc or back-arc basin environment. This magmatism occurred at 806 ± 19 Ma (Sm-Nd 11 WR isochron), similar in age to arc magmatism in the ANS. Leucocratic gneisses, muscovite schists and garnet-biotite schists form the dominant metasedimentary rocks of the study area. They were primarily derived from two different sources: volcanogenic sediments from a Neoproterozoic island arc (TDM Nd model ages between 790 and 900 Ma) and terrigeneous sediments from an older continental source (TDM Nd model ages up to 2100 Ma). The volcanosedimentary succession was metamorphosed under amphibolite facies conditions prior to 670 Ma, probably at c.700 Ma. The high-grade metamorphism is related to a frontal collisional event that also produced muscovite-biotite gneisses with geochemical characteristics of syn-collisional peraluminous granites. Meta-igneous rocks from eastern Bayuda have &#61541;Nd values of +5.2 ± 0.4 indicating a less depleted mantle source (crustal contamination is excluded) than the Gabgaba-Gebeit terrane (&#61541;Nd c. +7) from the Arabo-Nubian shield. Less depleted mantle source is alos known more to the south in Gebel Moya. Lithological and structural similarities (dominantly NE striking foliation) with the Bayuda Desert succession occur in many parts of central and western Sudan. It is proposed that a Neoproterozoic oceanic convergent margin collided early (c. 700 Ma) to the NW with the East Saharan ghost craton and that the greenschist facies terranes from the Arabo-Nubian shield accreted later (640-580 Ma) during a more oblique softer collision. Keywords: NE-Africa, Pan-African orogeny, Neoproterozoic, metamorphic rocks, isotopes, geodynamical evolution

Tectonic Setting and Phase Diagram Constraints on the Origin of Proterozoic anorthosites: the Crustal Tongue Melting Model

Abstract Recent detailed field studies in several anorthosite complexes have shown that anorthosites are frequently linked with weakness zones in the crust which favour their emplacement at mid-crust levels. Recent experimental data have also shown that the parent magma compositions of various anorthosite massifs lie on thermal highs in the relevant phase diagrams, indicating that these magmas cannot be derived by fractionation of peridotitic mantle melts but rather are produced by melting of gabbronoritic sources. In the Sveconorwegian province terrane boundaries have been traced in deep seismic profiles to Moho offsets or to tongues of lower crustal material underthrust to depths of 40-50 km. We therefore suggest that the parent magmas of anorthosite massifs are produced by melting of gabbronoritic rocks from the lower crust that had been thrust into the mantle by collision of terranes.

The 1160 ma Hidderskog meta-charnockite: implications of this a-type pluton for the Sveconorwegian belt in Vest Agder (Sw Norway)

The Hidderskog massif is a charnockitic intrusion located in the Rogaland-Vest Agder segment of the Sveconorwegian province of SW Norway. The U-Pb zircon age (1159 ± 5 Ma) and the Rb-Sr whole-rock isochron age (1153 ± 39 Ma) of this pluton are concordant. This age is interpreted as the magmatic emplacement age. The Hidderskog charnockitic intrusion is deformed and at a large scale concordant with amphibolite facies gneisses. It has been partially transformed to amphibole-biotite gneiss during the main Sveconorwegian orogeny (1040 - 980 Ma in Rogaland). Major and trace element composition as well as isotopic systems have been only weakly disturbed during this event. The Hidderskog intrusion displays A-type geochemical features. Most samples could correspond to mixing of a granitic liquid with feldspar crystals. It may be correlated in age with the Gloppurdi and Botnavatnet intrusions of Rogaland and tentatively to the Hovdefjell and Gjeving charnockites of the Bamble sector. This group of charnockitic intrusions defines an intraplate anorogenic geodynamic environment prior to the main Sveconorwegian orogenic phase. These anorogenic charnockites are distinct from the charnockitic magmas spatially related to the Rogaland anorthosite complex, which intruded after the main Sveconorwegian orogeny within a short period of time, from around 950 to 930 Ma.

The alkaline-peralkaline granitic post-collisional Tin Zebane dyke swarm (Pan-African Tuareg shield, Algeria): prevalent mantle signature and late agpaitic differentiation

Abstract The Tin Zebane dyke swarm was emplaced at the end of the Pan-African orogeny along a mega-shear zone separating two contrasting terranes of the Tuareg shield. It is located along the western boundary of the Archaean In Ouzzal rigid terrane, but inside the adjacent Tassendjanet terrane, strongly remobilized at the end of the Precambrian. The Tin Zebane swarm was emplaced during post-collisional sinistral movements along the shear zone at 592.2 ± 5.8 Ma (19 WR Rb-Sr isochron). It is a dyke-on-dyke system consisting of dykes and stocks of gabbros and dykes of metaluminous and peralkaline granites. All rock types have Sr and Nd isotopic initial ratios (Sri= 0.7028 and &#61541;Nd= +6.2) typical of a depleted mantle source, similar to the prevalent mantle (PREMA) at that period. No crustal contamination occurred in the genesis of the Tin Zebane swarm. Even the samples showing evidence of fluid interaction (essentially alkali mobility) have the same isotopic signature. The peralkaline granites have peculiar geochemical characteristics that mimic subduction-related granites: this geochemical signature is interpreted in terms of extensive differentiation effects due to late cumulates comprising aegirine, zircon, titanite, allanite and possibly fergusonite, separated from the liquid in the swarm itself due to magmatic flow turbulence. The Tin Zebane dyke swarm is thus of paramount importance for constraining the differentiation of mantle products to generate highly evolved alkaline granites without continental crust participation, in a post-collisional setting. Keywords: Alkaline granite; Post-collisional; Pan-African; Prevalent mantle; Tuareg shield; Algeria

The Anfeg post-collisional Pan-African high-K calc-alkaline batholith (Central Hoggar, Algeria), result of the LATEA microcontinent metacratonization

Abstract The Anfeg batholith (or composite laccolith) occupies a large surface (2000 km2) at the northern tip of the Laouni terrane, just south of Tamanrasset in Hoggar. It is granodioritic to granitic in composition and comprises abundant enclaves that are either mafic microgranular enclaves (MME) or gneissic xenoliths. It intruded an Eburnian (c. 2 Ga) high-grade basement belonging to the LATEA metacraton at c. 608 Ma (recalculated from the U-Pb dating of Bertrand et al., 1986) and cooled at c. 4 kbar, with a temperature of about 750°C. This emplacement occurred mainly along subhorizontal thrust planes related to Pan-African subvertical mega-shear zones close to the attachment zone of a strike-slip partitioned transpression system. Although affected by some LILE mobility, the Anfeg batholith can be ascribed to a high-K calc-alkaline suite but characterized by low heavy REE contents and high LREE/HREE ratios. The MME belong to the Anfeg magmatic trend while some xenoliths belong to Neoproterozoic island arc rocks. The Anfeg batholith defines a Nd-Sr isotopic initial ratios trend (&#61541;Nd/(87Sr/86Sr)i from &#8211;2.8/0.7068 to &#8211;11.8/0.7111) pointing to a mixing between a depleted mantle and an old Rb-depleted granulitic lower crust. Both sources have been identified within LATEA and elsewhere in the Tuareg shield (&#61541;Nd/87Sr/86Sr)i of +6.2/0.7028 for the depleted mantle, -22/0.708 for the old lower crust. The model proposed relates the above geochemical features to a lithospheric delamination along the subvertical mega-shear zones that dissected the rigid LATEA former passive margin without major crustal thickening (metacratonization) during the general northward tectonic escape of the Tuareg terranes, a consequence of the collision with the West African craton. This delamination allowed the uprise of the asthenosphere. In turn, this induced the melting of the asthenosphere by adiabatic pressure release and of the old felsic and mafic lower crust due to the high heat flow. A gradient in the mantle/crust ratio within the source of the Pan-African magmatism is observed in LATEA from the northeast (Egéré-Aleksod terrane) where rare plutons are rooted within the Archaean/Eburnian basement to the southwest (Laouni terrane) where abundant batholiths, including Anfeg, have a mixed signature.

The Hoggar swell and volcanism: Reactivation of the Precambrian Tuareg shield during Alpine convergence and West African Cenozoic volcanism

We review the NW African Cenozoic volcanic fields, including their regional geology. This forms a basis for understanding the relations between Hoggar volcanism and the Africa &#8211; Europe collision. Volcanic alignments are related to structural features and no spatial age trend exists. In Hoggar, a close link is established between the volcanism and Pan-African structure. During the Mesozoic rifting period, the Hoggar area was already a topographic high, well before any volcanism, which began at c. 35 Ma, just after the initiation of the Africa &#8211; Europe collision at ca. 38 Ma. Hoggar volcanism continued episodically until now, as did the collision. We describe the Hoggar volcanic province based on available field, petrological, geochemical isotopic data and geophysical data, including gravimetry, heat flow and seismic tomography. The latter suggests that NW African volcanism is linked to mantle structure down to 150 km but not deeper, implying a shallow mantle source. In Hoggar, lithospheric structures deduced from the seismic tomographic model and from geology are compatible when their respective resolutions are taken into account. The above considerations cannot be reconciled with a plume model. We propose instead that intraplate stress induced by the Africa-Europe collision reactivated the Pan-African mega-shear zones mainly in metacratonic terranes inducing linear lithospheric delamination, rapid asthenosphere upwelling and melting due to pressure release. Edge-driven convection may contribute. The surface location of the volcanism is influenced by Paleozoic and Mesozoic brittle faults.

The LATEA metacraton (Central Hoggar, Tuareg shield, Algeria): behaviour of an old passive margin during the Pan-African orogeny

Historically, the Tuareg shield is divided into three parts bordered by mega-shear zones with the centre, the Central Polycyclic Hoggar, characterized by Archaean and Palaeoproterozoic lithologies. Nearly 10 years ago, the Tuareg shield was shown to be composed of 23 displaced terranes (Black et al., 1994) whose relationships were deciphered in Aïr to the SE (Liégeois et al., 1994). The Polycyclic Central Hoggar terranes were characterized by the presence of well preserved Archaean / Palaeoproterozoic and Neoproterozoic lithologies. We show here that the terranes from Central Hoggar (Laouni, Azrou-n-Fad, Tefedest, Egéré-Aleksod) belonged to a single old passive margin, to which we gave the acronym name LATEA, which behaved as a craton during the Mesoproterozoic and the Early-Middle Neoproterozoic but was partly destabilized and dissected during the Late Neoproterozoic as a consequence of its involvement as a passive margin in the Pan-African orogeny. The early Pan-African phase consisted of thrust sheets including garnet-bearing lithologies (eclogite, amphibolite, gneiss) that can be mapped and correlated in three LATEA terranes. In the Tin Begane area, P-T-t paths have been established from >15 kbar &#8211; 790°C (eclogite) to 4 kbar &#8211; 500°C (greenschist retrogression) through 12 kbar &#8211; 830°C (garnet amphibolite) and 8 kbar &#8211; 700°C (garnet gneiss), corresponding to the retrograde path of a Franciscan-type loop. Sm-Nd geochronology on minerals and laser ablation ICP-MS on garnet show the mobility of REE, particularly LREE, during the retrograde greenschist facies that affects, although slightly, some of these rocks. The amphibolite facies metamorphism has been dated at 685 ± 19 Ma and the greenschist facies at 522 ± 27 Ma. During the thrust phase, the Archaean-Palaeoproterozoic basement was only locally affected by the Pan-African tectonics. LATEA behaved as a craton. Other juvenile terranes were also thrust early onto LATEA: the Iskel island arc at c. 850 Ma to the west of LATEA, the Serouenout terrane in the 700 &#8211; 620 Ma age range to the east. No subduction-related magmas have intruded LATEA during this epoch, which behaved as a passive margin. During the main Pan-African phase (625&#8211;580 Ma), LATEA was dissected by mega-shear zones that induced several hundreds km of relative displacement and allowed the emplacement of high-K calc-alkaline batholiths.

The Moroccan Anti-Atlas: the West African craton passive margin with limited Pan-African activity. Implications for the northern limit of the craton.

Abstract The Moroccan Anti-Atlas region, located south of the South Atlas Fault, has been traditionally viewed as containing two segments separated by the Anti-Atlas Major Fault. These two segments are said to consist of: a) 600-700 Ma Pan-African segment located in the northeast; and b) ~2 Ga Eburnian segment situated to the southwest. On the basis of observations in the Zenaga and Saghro inliers and of a recent literature review, we suggest that this subdivision is inappropriate in that Eburnian and Pan-African materials occur throughout the Anti-Atlas region: the entire Anti-Atlas is underlain by Eburnian crust, unconformably overlain by a Lower Neoproterozoic passive margin; allochthonous Pan-African ocean crustal slices were thrust onto the West African Craton passive margin sequence ~685 Ma ago as a result of Pan-African accretion tectonics; high-level high-K calc-alkaline and alkaline granitoids locally intruded the Anti-Atlas sequence as a whole at the end of the Pan-African orogeny at 585-560 Ma; the intervening 100 m.y. interval was marked by quiescence. This succession of events can be related to the behaviour of one single rigid cratonic passive margin during an orogeny and correlated to the Pan-African events that occurred to the east in the Tuareg shield and to the north in Avalonian terranes. This model implies that the actual northern limit of the West African craton is located at the South Atlas Fault (SAF) and not at the Anti-Atlas Major Fault (AAMF). We propose that the AAMF corresponds to the southwestern boundary of an aulacogen that formed along the northern margin of the West African craton during early Neoproterozoic times. This is consistent with the development of the Gourma aulacogen on the eastern side of the West African craton. This model further suggests that the northeastern boundary of the West African craton occurs north, and not south, of Ougarta (Algeria). Support for this model is provided by geological and geophysical evidence. Keywords: Anti-Atlas, Morocco, West African craton, Tuareg shield, Eburnian, Pan-African

The Moroccan Anti-Atlas: the West African craton passive margin with limited Pan-African activity. Implications for the northern limit of the craton:

We agree with El Hafid Bouougri (EHB) that Anti-Atlas geology is complex and that only a multidisplinary approach can bring new constraints. In our paper (Ennih and Liégeois, 2001), we tried to show that this partly results from a particular localization of Anti-Atlas on the shoulder of a craton namely the West African craton (WAC). During the Pan-African orogeny, the latter acted as a rigid body but was partly destabilized at its margin, allowing a major but late high-K calc-alkaline plutonism and volcanism to be emplaced (610-560 Ma; Thomas et al., 2002). Before that event, an oceanic terrane built at around 750 Ma accreted at 685-665 Ma (Leblanc and Lancelot, 1980; de Wall et al., 2001; Thomas et al., 2002; Admou et al., 2002), generating a major thrust event affecting also the passive margin series. No continental collision affected Anti-Atlas during the Pan-African orogeny, rendering its geological interpretation uneasy through usual interpretations, explaining the diversity of geological models proposed for Anti-Atlas, as remarked by EHB. We thank EHB for his comments, this will allow us to precise some points that were misunderstood and to answer well-taken points. This will also allow us to make reference to studies published after the writing of our paper and to include an updated version of the figure summarizing our model.

The Neoproterozoic Pan-African basement from the Alpine Lower Danubian nappe system (South Carpathians, Romania).

Abstract The South Carpathians, which were thrust to the Moesian platform in the Alpine orogeny (Late Cretaceous to Tertiary), include the Danubian nappe system. The Danubian pre-Alpine basement comprises two Variscan nappes, each one made up of partially retrogressed amphibolite facies rocks intruded by granitoids and capped by an Ordovician-Devonian volcano-sedimentary cover. No lithological correlation can be established between the pre-Ordovician basements of these two units. The metamorphic basement from the first Variscan nappe, the Drãgºan Group, is composed of banded amphibolites with some augen and aluminous gneisses intruded by granodioritic to tonalitic plutons. The banded structure of the amphibolites, together with their geochemistry, suggests a volcano-sedimentary sequence. Zircon U-Pb data on an intercalated augen gneiss have given an age of 777 ± 3 Ma for the emplacement of the protolith of this gneiss. Nd model ages (TDM) for the amphibolites range from 717 Ma to 817 Ma. At 777 Ma, &#61541;Nd values cluster is at +8.3 to +9.8 and Sr initial ratios range between 0.7007 and 0.7023, indicating an oceanic origin without continental crust contamination. Major and trace elements from the Drãgºan amphibolites consistently display an island arc signature, with three differentiation trends evolving from an early tholeiitic trend to a more differentiated low-K calc-alkaline one. The Drãgºan terrane is similar to the Early Pan-African juvenile terranes of the Sahara. The basement of the second Variscan nappe, the Lainici-Pãiuº Group, is made up of metasedimentary rocks (mainly quartzites, marbles and graphitic mica gneisses) cut by early leucogranitic dykes, medium-K calc-alkaline and alkali-calcic (mainly granitic) plutons, and late medium-K porphyry diorite dykes. This magmatism can be bracketed between 588 Ma and 567 Ma (U-Pb zircon ages). The best preserved pluton (Tismana, 567 Ma old), displays a composite alkali-calcic (very high-K calc-alkaline) magmatic sequence, ranging from gabbro-diorite to monzogranite, including an ultramafic pod. Ages and geochemical signatures resemble the Saharan Late Pan-African granitoids. The existence of Late Precambrian partly juvenile terrains is thus confirmed within the basement of South Carpathians, which renders then a segment of the European Alpine belt that can be successfully compared to the Pan-African Trans-Saharan belt.

The Precambrian of Hoggar, Tuareg shield: history and perspective

Preface of the Journal of African Earth Sciences Special Issue on the Precambrian of the Tuareg shield (vol 37:3-4, 2003)

The Saharan Metacraton

Abstract - This article introduces the name "Saharan Metacraton" to refer to the pre-Neoproterozoic &#8211; but sometimes highly remobilized during Neoproterozoic time - continental crust which occupies the north-central part of Africa and extends in the Saharan Desert in Egypt, Libya, Sudan, Chad and Niger and the Savannah belt in Sudan, Kenya, Uganda, Congo, Central African Republic and Cameroon. This poorly known tract of continental crust occupies ~5,000,000 km2 and extends from the Arabian-Nubian Shield in the east to the Tuareg Shield to the west and from the Congo craton in the south to the Phanerozoic cover of the northern margin of the African continent in southern Egypt and Libya. The term "metacraton" refers to a craton that has been remobilized during an orogenic event but is still recognizable dominantly through its rheological, geochronological and isotopic characteristics. Neoproterozoic remobilization of the Saharan Metacraton was in the forms of deformation, metamorphism, emplacement of igneous bodies, and probably local episodes of crust formation related to rifting and oceanic basins development. Relics of unaffected or weakly remobilized old lithosphere are known. The article explains why the name "Saharan Metacraton" should be used, defines the boundaries of the metacraton, reviews geochronological and isotopic data as evidence for the presence of a pre-Neoproterozoic continental crust, and discusses what happen to the Saharan Metacraton during the Neoproterozoic. A model combining collisional processes, lithospheric delamination, regional extension, and post-collisional dismembering by horizontal shearing is proposed.

The "Taourirt" magmatic province, a marker of the closing stage of the Pan-African orogeny in the Tuareg Shield: review of available data and Sr-Nd isotope evidence.

Abstract The Tuareg Shield, located between the Archaean to Palaeoproterozoic Saharan metacraton and the West African craton, is composed of 23 recognized terranes that welded together during the Neoproterozoic Pan-African orogeny (750-520 Ma). Final convergence occurred mainly during the 620-580 Ma period with the emplacement of high-K calc-alkaline batholiths, but continued until 520 Ma with the emplacement of alkali-calcic and alkaline high-level complexes. The last plutons emplaced in central Hoggar at 539-523 Ma are known as the "Taourirt" province. This expression is redefined and three geographical groups are identified: the Silet-, Laouni- and Tamanrasset-Taourirts. The Silet-Taourirts are cross-cutting Pan-African island arc assemblages while the two others intrude the Archaean-Palaeoproterozoic LATEA metacraton. The Taourirts are high-level subcircular often nested alkali-calcic, sometimes alkaline, complexes. They are aligned along mega-shear zones often delimiting terranes. Mainly granitic, they comprise highly differentiated varieties such as alaskite (Silet-Taourirts) and topaz-albite leucogranite (Tamanrasset-Taourirts). Different subgroups were defined on the basis of REE patterns and major and other trace elements. The Taourirt province displays a wide transition from dominant alkali-calcic to minor alkaline granite varieties. Sr isotopes indicate that these complexes were affected by fluid circulation during the Ordovician along shear zones probably contemporaneous to the beginning of the Tassilis sandstone deposition. Nd isotope systematic indicates a major interaction with the upper crust during the emplacement of highly differentiated melts, particularly in samples showing seagull wing-shaped REE patterns. On the other hand, all Taourirt plutons are strongly contaminated by the lower crust: &#61541;Nd vary from -2 to -8 and TDM from 1200 Ma to 1700 Ma. This implies the presence of an old crust at depth, also below the Silet-Taourirts, which are emplaced within Pan-African island arc assemblages. A model is proposed for the genesis of the Taourirt province where reworking of the mega-shear zones, which dissected the LATEA metacraton, provoked a linear delamination of the lithospheric mantle, asthenosphere uprise and partial melting of the lower crust (or strong interaction with), giving rise to a mixed source. Keywords: Granite; Taourirt; Pan-African; Hoggar; Tuareg Shield

Timing of continental building in the Sveconorwegian orogen, SW Scandinavia

The timing of continental building in the Sveconorwegian orogen of SW Scandinavia is evaluated with zircon U-Pb geochronology. ID-TIMS, LA-ICPMS and SIMS data are reported for 21 samples of orthogneiss, metarhyolite and metasandstone in S Norway, with emphasis on the Suldal area. The Sveconorwegian orogen is divided into one Fennoscandian 1.80&#8211;1.64 Ga parautochtonous segment, the Eastern Segment, and two allochthonous terranes. The Idefjorden terrane is interpreted as a composite 1.66&#8211;1.52 Ga arc formed at the margin or near the margin of Fennoscandia. The western terrane, including the Telemark, Hardangervidda, Suldal and Rogaland-Vest Agder sectors, is named Telemarkia. U-Pb zircon data indicate that Telemarkia was build during a short magmatic event between 1.52 and 1.48 Ga, and was located at the margin of a Palaeoproterozoic craton, possibly Fennoscandia. No basement older than 1.5 Ga can be positively identified. In the early stage of the Sveconorwegian orogeny, Telemarkia collided with the Idefjorden terrane. The Bamble-Kongsberg sector, characterized by a mixed lithology and 1.13&#8211;1.10 Ga early-Sveconorwegian high-grade metamorphism, is interpreted as the original collision zone between these terranes.

TTG magmatism in the Congo craton; a view from major and trace element geochemistry, Rb-Sr and Sm-Nd systematics: case of the Sangmelima region, Ntem complex, southern Cameroon

TTG rocks of the Sangmelima region (Ntem complex, Congo craton) in southern Cameroon range in modal composition from rocks of the charnockitic suite, tonalites to granodiorites. They display a wide spectrum of SiO2 composition (54 &#8211; 76 wt%), defining intermediate varieties that tend to be metaluminous (0.56<A/CNK<1) and acid varieties that are slightly peraluminous (1<A/CNK<1.08). Members of the charnockitic and tonalitic suites represent a typical trondhjemitic differentiation trend while granodiorites portray a slight potassic calc-alkaline affinity. Their trace element spidergrams are characterized by negative anomalies in Nb-Ta, suggesting a subduction related signature. More siliceous members of the rock suites are enriched in LILE. REE degree of fractionation varies from 2<LaN/YbN<28 in the charnockitic suite while higher ratios of 12<LaN/YbN<114 in the tonalitic suite and 18<LaN/YbN<99 in granodiorites indicate melting of the protolithic source rock with garnet as a restite phase. Samples with overall lowest abundance in REE show a positive Eu anomaly and represent smaller amounts of melts with much feldspar contribution. Sr and Nd systematics coupled with Mg#-SiO2 compositions are consistent with derivation by the partial melting in a subduction context or merely from a thickened Archaean crust of eclogite facies basaltic proto-crust with mantle material input. From Rb-Sr and Sm-Nd whole rock data, intrusion timing is defined around 2900 Ma for rocks of the charnockitic suite. The Rb-Sr system in biotite portrays post emplacement reheating during the Eburnean orogeny. TTG intrusion in the Sangmelima region represents a major Archaean accretion event in the Congo craton.

Key Points on African Geology (KPAG) preface

The scope of this special issue, that was named "Key Points on African Geology" (KPAG), was to highlight the current state-of-the-art knowledge of various topics in African Geology through a series of short, high quality papers that present important new data or ideas that enhance our knowledge of the geology of Africa and to stimulate discussion and research on a wide series of fronts. The success of the venture can be measured in the 114 manuscripts that were submitted to KPAG, of which the 55 papers that passed peer review are published here.

Empreinte pan-africaine majeure dans la chaîne ubendienne de Tanzanie sud-occidentale: géochronologie U-Pb sur zircon et contexte structural.

 

Dedication to Russell Black

Dedication to Russell Black of the Lithos Special Issue 45 (1998) entitled "Post-collisional magmatism"

Some words on the post-collisional magmatism

Preface of the Lithos special issue 45 (1998) entitled "Post-collisional magmatism"

Permian alkaline undersaturated and carbonatite province and rifting along the West African craton

 

Pan-African displaced terranes in the Tuareg shield (central Sahara).

 

Les microcontinents cachés du Sahara

 

La province des complexes annulaires alcalins sursaturés de l'Adrar des Iforas, Mali

 

Pan-African plutonism of the Damagaram inlier, Niger Republic

 

Late Pan-African tectonics marking the transition from subduction-related calc-alkaline magmatism to within-plate alkaline granitoids (Adrar des Iforas, Mali)

 

Geochemical evidence for a non-alkalic origin for the carbonatic bodies of Kibuye, Rwanda

 

Contribution to the geochronology of the basement of the Central African Republic

 

The Palaeoproterozoic Ubendian shear belt in Tanzania: geochronology and structure.

 

The Lac Cornu retrograded eclogites (Aiguilles Rouges massif, Western Alps, France): evidence of crustal origin and metasomatic alteration

 

The Permo-Jurassic alkaline Province of Tadhak, Mali: geology, geochronology and tectonic significance.

 

Short-lived Eburnian orogeny in southern Mali. Geology, tectonics, U-Pb and Rb-Sr geochronology

 

Early and late Pan-African orogenies in the Aïr assembly of terranes (Tuareg shield, Niger)

 

Late Kibaran magmatism in Burundi.

 

Kibaran A-type granitoids and mafic rocks generated by two mantle sources in a late orogenic setting (Burundi)

 

Tadhak alkaline ring-complex (Mali): existence of U-Pb isochrons and "Dupal" signature 270 Ma ago.

 

The Timedjelalen alkaline ring-complex and related N-S dyke swarms (Adrar des Iforas, Mali) - a Pb-Sr-O isotopic study

 

Isotopic and geochemical evidence of Proterozoic episodic crustal reworking within the Irumide belt of south-central Africa, the southern metacratonic boundary of an Archaean Bangweulu craton.

Whole-rock geochemistry and Sr-Nd isotopic data for granitoids and volcanic rocks of four main different igneous phases, the Usagaran phase (2.05-1.93 Ga), the Ubendian phase (1.88-1.85 Ga), the Lukamfwa phase (1.65-1.55 Ga) and the Irumide phase (1.05-0.95 Ga), recognised along the southern margin of the Congo Craton in the Bangweulu Block and Irumide Belt of Zambia, demonstrate a long history of crustal recycling of a cryptic Archaean basement complex. The isotopic record indicates that a largely similar crustal source can be assigned to all these magmatic phases, with subtle differences in isotopic and geochemical record reflecting varying distance to orogenic activity during each of these episodes, and varying, but always minor amounts of juvenile mantle input. TDM model ages, ranging between 3.3 and 2.8 Ga for the granitoids, and between 2.9 and 2.4 Ga for volcanic rocks, indicate preponderant Archaean crust within the Bangweulu Block and within the Irumide Belt. The corresponding initial &#949;Nd values vary between -6 and -15 for the granitoids, and between -2 and -7 for the felsic volcanic rocks. Only some of the c. 1.85 Ga mafic volcanic units record a more juvenile character (&#949;Nd(T) between -5 and 0 and TDM model ages between 3.2 and 2.4 Ga) but still with a significant old crust input. Combining all available constraints, we propose that the Irumide Belt corresponds to the recurrently destabilised (at 2, 1.85, 1.6, 1 Ga) southern boundary of the Bangweulu Block, whose preserved nucleus appears to be an Archaean craton covered by Palaeoproterozoic sediments. There are no juvenile subduction-related rocks in the Irumide Belt s.s., but such rocks are reported to the south, across the Mwembeshi Dislocation Zone (MDZ), in a region referred to as the Southern Irumide Belt. Processes of endo-destabilisation in response to external forces (orogenies acting close or elsewhere), such as that recorded in the Bangweulu Block and the Irumide Belt s.s., correspond to a metacratonic evolution. Within the Irumide Belt itself, tectonic movements were mainly vertical, with horizontal movements restricted mainly to the supracrustal sequences.

Archaean high-K granitoids produced by remelting of earlier Tonalite-Trondhjemite-Granodiorite (TTG) in the Sangmelima region of the Ntem Complex of the Congo craton, southern Cameroon

We present a geochemical and isotopic study that, consistent with observed field relations, suggest Sangmelima late Archaean high-K granite was derived by partial melting of older Archaean TTG. The TTG formations are sodic-trondhjemitic, showing calcic and calc-alkalic trends and are metaluminous to peraluminous. High-K granites in contrast show a potassic calc-alkaline affinity that spans the calcic, calc-alkalic, alkali-calcic and alkalic compositions. The two rock groups (TTG and high-K granites) on the other hand are both ferroan and magnesian. They have a similar degree of fractionation for LREE but a different one for HREE. Nd model ages and Sr/Y ratios define Mesoarchaean and slab-mantle derived magma compositions respectively, with Nb and Ti anomalies indicating a subduction setting for the TTG. Major and trace element in addition to Sr and Nd isotopic compositions support field observations that indicate the derivation of the high-K granitic group from the partial melting of the older TTG equivalent at depth. Geochemical characteristics of the high-K granitic group are therefore inherited features from the TTG protolith and cannot be used for determining their tectonic setting. The heat budget required for TTG partial melting is ascribed to the upwelling of the mantle marked by a doleritic event of identical age as the generated high-K granite melts. The cause of this upwelling is related to linear delamination along mega-shear zones in an intracontinental setting.

Geochemical constraints of the petrogenesis of the O&#8217;okiep Koperberg Suite and granitic plutons in Namaqualand, South Africa: a crustal source in Namaquan (Grenville) times

The Namaquan (Grenville) Orogeny (late Mesoproterozoic) in the O&#8217;okiep District is characterized by two tectono-magmatic episodes: the O&#8217;okiepian Episode (1210-1180 Ma) with the intrusion of batholitic granites, and the Klondikean Episode (1040-1020 Ma), which includes the intrusion of the copper-bearing Koperberg Suite and the Rietberg Granite. This study focuses on the geochemistry (major and trace elements, Sr and Nd isotopes) of intrusive rocks of the O&#8217;okiep terrane to provide better constraints on their source rock characteristics and on their petrogenesis. The O'okiepian Granites belong to the K-rich granite kindred, with shoshonitic affinities. The Concordia Granite results from dehydration melting of a pelitic source. The Rietberg Granite shows geochemical similarities with post-collisional magmatic series. The anorthosites and related rocks of the Koperberg Suite are cumulates; their REE distribution is controlled by their apatite content. A new rock type, jotunite, has been identified in the Koperberg Suite; it is analogous to the Rogaland chilled jotunite, a characteristic which gives strong evidence that the Koperberg rocks belong to the massif-type anorthosite suite. Inversion modelling of plagioclase REE compositions from anorthosite permits the reconstruction of melt compositions and places constraints on the melting process and on the characteristics of the source rocks. The occurrence of jotunite in the Koperberg Suite is strong additional evidence for a crustal source, because jotunite is produced by remelting of gabbronorite under dry conditions. The various intrusions in the Koperberg Suite show distinct isotopic signatures, which resulted from isotopic heterogeneities of the crustal source and from minor contamination with the country rocks. The characteristic negative &#61541;&#61518;d(1030 Ma) values (-5 to -11) can be explained by remelting at 1030 Ma of a 1900 Ma-old oceanic crust protolith with an enriched MORB REE distribution. The large range in Sr initial ratios (0.709 &#8211; 0.748) may reflect hydrothermal alteration of the oceanic crust, a process which may also explain the Cu enrichment. The Koperberg intrusions were produced by forceful injection of cumulate crystal mush, which were differentiated in deeper magma chambers or in conduits.

Post-collisional melting of crustal sources: constraints from geochronology, petrology and Sr, Nd isotope geochemistry of the Variscan Sichevita and Poniasca granitoid plutons (South Carpathians, Romania).

 

The Temaguessine Fe-cordierite orbicular granite (Central Hoggar, Algeria): U-Pb SHRIMP age, petrology, origin and geodynamical consequences for the late Pan-African magmatism of the Tuareg shield.

The Temaguessine high-level subcircular pluton is intrusive into the LATEA metacraton (Central Hoggar) Eburnian (2 Ga) basement and in the Pan-African (615 Ma) granitic batholiths along a major NW-SE oriented major shear zone. It is dated here (SHRIMP U-Pb on zircon) at 582 ±5 Ma. Composed of amphibole-biotite granite and biotite syenogranite, it comprises abundant enclaves: mafic magmatic enclaves, country-rock xenoliths and remarkable Fe-cordierite (#Fe= 0.87) orbicules. The orbicules have a core rich in cordierite (40%) and a leucocratic quartz-feldspar rim. They are interpreted as resulting from the incongruent melting of the meta-wacke xenoliths collapsed into the magma: the breakdown of the biotite + quartz assemblage produced the cordierite and a quartz-feldspar minimum melt that is expelled, forming the leucocratic rim. The orbicule generation occurred at T< 850° and P< 0.3 GPa. The Fe-rich character of the cordierite resulted from the Fe-rich protolith (wacke with 4% Fe203 for 72% SiO2). Strongly negative &#61541;Nd (-9.6 to -11.2), Nd TDM model ages between 1.64 to 1.92 Ga, inherited zircons between 1.76 and 2.04 Ga and low to moderately high ISr (0.704 &#8211; 0.710) indicate a Rb-depleted lower continental crust source for the Temaguessine pluton; regional considerations impose however also the participation of asthenospheric material. The Temaguessine pluton, together with other high-level subcircular pluton, is considered as marking the end of the Pan-African magma generation in the LATEA metacraton, resulting from the linear delamination along mega-shear zones, allowing asthenospheric uprise and melting of the lower continental crust. This implies that the younger Taourirt granitic province (535-520 Ma) should be considered as a Cambrian intraplate anorogenic event and not as a very late Pan-African event. Keywords: Fe-cordierite; orbicule; U-Pb zircon; Pan-African; Tuareg shield

Tertiary alkaline volcanism of the Atakor Massif (Hoggar, Algeria): field, petrological, mineralogical, geochemical, and geodynamical features

The Atakor massif is a part of the Hoggar volcanic province, which was emplaced on top of a swell initiated during the Cretaceous. Three main episodes of volcanic activity, separated by long-lasting periods of quiescence, are identified since the Miocene. The lava flows and domes were emitted along lithosphere-scale fault zones. With its famous scenery, the Atakor massif is one of the largest (2150 km2) volcanic districts of the province. The distribution of the various rock types is not random. Mafic rocks, abundant in the centre of the massif, become scarce to the south, where only few scarps can be observed. Phonolite occurs only in the Assekrem area, whereas trachyte can be seen everywhere, with a marked enrichment in quartz to the south and the southeast (Tahifet area), where rhyolite is also exposed. From the field and petrological evidence, two magmatic groups are identified. The mafic group is made up of basanite &#8211; phonotephrite associations, forming uplifted plateaus, scoria cones and valley-filling lava flows. Mantle amphibole ± mica megacrysts and peridotite enclaves, together with the non-primary chemical compositions, evidence that magmas originate in a HIMU source and that differentiation took place within the upper mantle. The felsic group comprises two diverging trends, a silica-saturated benmoreite &#8211; trachyte &#8211; rhyolite trend and a silica-undersaturated trachyte &#8211; phonolite trend. Sources of the magmas are also of HIMU affinity, but original incompatible trace element compositions are largely modified by feldspar + accessory minerals fractionation. Primary magmas were produced through minor linear delamination along mega-shear zones, mantle decompression and low degrees partial melting at variable depths (110 &#8211; 40 km) of upwelling asthenosphere. Discrete volcanic episodes correspond to discrete events of re-activation of fault zones, in response to discrete extensional Neogene events in the ongoing Alpine orogeny in W. Mediterranean induced by Africa-Eurasia collision.

Alkaline magmatism subsequent to collision in the Pan-African belt of the Adrar des Iforas (Mali)

The Pan-African Trans-Saharan belt in the Iforas displays a rapid switch from subduction and collision-related calc-alkaline to typical A-type magmatism, which is accompanied by transcurrent movements along major shear zones inducing weak distension. Detailed Rb-Sr geochronology and geochemical data point to different mantle sources for orogenic (lithospheric depleted mantle + oceanic crust) and within-platge magmatism (more primitive asthenospheric mantle). Both groups suffer lower-crustal contamination. A model is proposed whereby asthenospheric mantle originally underlying the subducted plate has risen to shallow depth beneath the continental lithosphere after the rupture of the cold plunging plate. This source, which is often proposed for alkaline rocks, explains the great similarity of oversaturated alakline ring-complexes whatever their environment. The peculiarities of the alkaline province, for example the lack of Sn mineralization when compared to the Niger-Nigerian province, may be related to the nature and the composition of the basement.

Zircon geochronology and Sr, Nd, Pb isotope geochemistry of granitoids from Bayuda Desert and Sabaloka (Sudan): evidence for a Bayudian event (920-900 Ma) preceding the Pan-African orogenic cycle (860 &#8211; 590 Ma) at the eastern boundary of the Saharan Metac

Results of zircon geochronology and Sr, Nd and Pb isotope investigations carried out on granitoid rocks from the boundary of the Saharan Metacraton with the Arabian-Nubian Shield (ANS) reveal a protracted, partly pre-Pan-African geodynamic evolution and the existence of crustal terranes of different ancestries. (Meta)granitoids and gneisses in north-central Bayuda Desert record a 920-900Ma orogenic event, hitherto unrecognized in northeastern Africa. This early Neoproterozoic Bayudian event is restricted to a pre-Pan-African crustal terrane which appears only slightly affected by Pan-African tectogenesis and deformation, named here the Rahaba-Absol terrane. Within this terrane, amphibolite-facies metamorphism occurred at 921±10Ma in the El Melagi muscovite-biotite gneiss of the Rahaba Series, probably during a collisional phase; followed by the intrusion of the Absol medium-K granite-granodiorite pluton discordantly emplaced at 900±9Ma into high-grade schists and amphibolites of the metavolcanosedimentary Absol Series. Nd TDM model ages (2040-2430 Ma) and ages of rounded zircon cores (1060, 1980, 2540 and 2675 Ma) of the El Melagi gneiss indicate a predominantly late Archaean to Palaeoproterozoic source region and suggest a latest Mesoproterozoic to early Neoproterozoic depositional age of its pelitic precursor sediment. The post-collisional Absol pluton has isotope characteristics (Nd-Sr-Pb)) indicating assimilation of old pre-Neoproterozoic crust. The Rahaba-Absol terrane is part of the Saharan Metacraton and is in tectonic contact with the high-grade metamorphic Kurmut terrane of eastern Bayuda Desert. The Kurmut terrane has juvenile Neoproterozoic isotope characteristics, its granitoids record only Pan-African orogenic events and it is therefore considered a part of the Arabian-Nubian Shield. Both the Rahaba-Absol and the Kurmut terrane of the Bayuda Desert are intruded by post-collisional high-K granitoid magmatism of late Pan-African age, with no intermediate events recorded.

Pb-Zn mineralization in a Miocene regional extensional context: The case of the Sidi Driss and the Douahria ore deposits (Nefza mining district, northern Tunisia).

 

The boundaries of the West African craton, with a special reference to the basement of the Moroccan metacratonic Anti-Atlas belt.

 

Geochronology and metamorphic P-T-X evolution of the Eburnean granulite-facies metapelites of Tidjenouine (Central Hoggar, Algeria): witness of the LATEA metacratonic evolution.

 

A complex multi-chamber magmatic system beneath a late Cenozoic volcanic field: evidence from CSDs and thermobarometry of clinopyroxene from a single nephelinite flow (Djbel Saghro, Morocco).

 

The geology of Morocco: The Pan-African Belt.

 

The geology of Morocco: continental evolution In Western Maghreb

 

Dehydration, melting and related garnet growth in the deep root of the Amalaoulaou Neoproterozoic magmatic arc (Gourma, NE Mali)

TheAmalaoulaouNeoproterozoic island-arcmassif belongs to theGourmabelt in Mali. The metagabbros and pyroxenites forming the main body of this arc root show the pervasive development of garnet. In the pyroxenites, the latter has grown by reaction between pyroxene and spinel during isobaric cooling. By contrast, in the metagabbros, garnet textures and relations to felsic veins exclude an origin through solid-state reactions only. It is proposed that garnet has grown following dehydration and localized melting of amphibole-bearing gabbros at the base of the arc. The plagioclase-saturated melts represented by anorthositic veins in the metagabbros and by trondhjemites in the upper part of the massif provide evidence for melting in the deep arc crust, which locally generated high-density garnet&#8211;clinopyroxene&#8211;rutile residues. Garnet growth and melting began around 850 &#9702;C at 10 kbar and the tonalitic melts were most probably generated around 1050 &#9702;C at P &#8805; 10 kbar. This HT granulitic imprint can be related to arc maturation, leading to a P&#8211;T increase in the deep arc root and dehydration and/or dehydration-melting of amphibole-bearing gabbros. Observation of such features in the root of this Neoproterozoic island arc has important consequences, as it provides a link to models concerning the early generation of continental crust.

The Tessalit ring complex (Adrar des Iforas, Malian Tuareg shield): a Pan-African, post-collisional, syn-shear, alkaline granite intrusion

The Tessalit ring complex (600 km2) belongs to the Adrar des Iforas late Pan-African alkaline province. This paper provides the field lithologies, geochemistry of minerals and of whole-rocks, Rb-Sr geochronology. The Tessalit ring complex, an early post-collisional Pan-African pluton, possesses different structural features recording the effects of the still mobile, recently amalgamated terranes of the Tuareg shield, in particular movements along mega-shear zones. The local extension shown by the permissive type of emplacement can easily be integrated in the regional transpressive context recently proposed forthe Tuareg shield formation

The subduction- and collision-related Pan-African composite batholith of the Adrar des Iforas (Mali): a review

A large composite calc-alkaline batholith, in the Iforas region, Mali, occurs close to the Pan-African suture between the 2000 Ma old West African craton and the Trans-Saharan mobile belt. Its location in an embayment of the West African craton is probably responsible for the important production of magma. The Iforas batholith intrudes the western border of an old continental segment affected by early nappe tectonics (Dl event) and is flanked to the west by the Tilemsi palaeo-island arc. The batholith comprises several successive stages. The cordillera (>620 Ma), probably post-dating the Dl event, is essentially composed of volcanosedimentary sequences. The collision (620-580 Ma) is marked by the production of abundant granitoids mostly emplaced by the end of the D2 EW compressional event. The post-collision tectonic stages (D3 and D4; 580-540 Ma) are characterized by strike-slip movements, reversals in the stress field, and a rapid switch from calc-alkaline to alkaline magmatism. Magmas corresponding to each step show distinctive geochemical trends but all share low 87Sr/86Sr initial ratios (0.7035-0.706l). The possible successive sources have been evaluated from different entities in the Iforas region: Eburnean granulites for lower crust, Tilemsi palaeo-island arc for depleted subduction source and the Tadhak undersaturated province for asthenospheric more primitive mantle. A geodynamic model is proposed where all the calc-alkaline groups originated from a classical subduction source (depleted upper mantle modified by hydrous fluids from the subducted oceanic plate) which, some fifty million years after the beginning of the collision, was taken over by an asthenospheric source producing the alkaline province.

The role of fractional crystallization and late-stage peralkaline melt segregation in the mineralogical evolution of Cenozoic nephelinites/phonolites from Saghro (SE Morocco)

The Saghro Cenozoic lavas form a bimodal suite of nephelinites (with carbonatite xenoliths) and phonolites emplaced in the Anti-Atlas belt of Morocco. Despite the paucity of samples with intermediate composition between the two main types of lava (only one phonotephrite flow is reported in this area), whole-rock major element modelling shows that the two main lithologies can be linked by fractional crystallization. The most primitive modelled cumulates are calcite-bearing olivine clinopyroxenites, whereas the final stages of differentiation are characterized by the formation of nepheline-syenite cumulates. This evolution trend is classically observed in plutonic alkaline massifs associated with carbonatites. Late-stage evolution is responsible for the crystallization of hainite- and delhayelite-bearing microdomains, for the transformation of aegirine-augite into aegirine (or augite into aegirine-augite), and for the crystallization of lorenzenite and a eudialyte-group mineral as replacement products of titanite. These phases were probably formed, either by crystallization from late residual peralkaline melts, or by reaction of pre-existing minerals with such melt, or hydrothermal peralkaline fluid.

Repeated granitoid intrusions during the Neoproterozoic along the western boundary of the Saharan metacraton, Eastern Hoggar, Tuareg shield, Algeria: An AMS and U-Pb zircon age study

The N-S oriented Raghane shear zone (8°30&#8242;) delineates the western boundary of the Saharan metacraton and is, with the 4°50'; shear zone, the most important shear zone in the Tuareg shield. It can be followed on 1000 km in the basement from southern Aïr, Niger to NE Hoggar, Algeria. Large subhorizontal movements have occurred during the Pan-African orogeny and several groups of granitoids intruded during the Neoproterozoic. We report U-Pb zircon datings (laser ICP-MS) showing that three magmatic suites of granitoids emplaced close to the Raghane shear zone at c. 790 Ma, c. 590 and c. 550 Ma. A comprehensive and detailed (158 sites, more than 1000 cores) magnetic fabric study was performed on 8 plutons belonging to the three magmatic suites and distributed on 200 km along the Raghane shear zone. The main minerals in all the target plutons do not show visible preferential magmatic orientation except in narrow shear zones. The AMS study shows that all plutons have a magnetic lineation and foliation compatible with the deformed zones that are zones deformed lately in post-solidus conditions. These structures are related to the nearby mega-shear zones, the Raghane shear zone for most of them. The old c. 793 Ma Touffok granite preserved locally its original structures. The magnetic structures of the c. 593 Ma Ohergehem pluton, intruded in the Aouzegueur terrane, are related to thrust structures generated by the Raghane shear zone while it is not the case of the contemporaneous plutons in the Assodé-Issalane terrane whose structures are only related to the subvertical shear zones. Finally, the c. 550 Ma granite group has magnetic structure related to the N-S oriented Raghane shear zone and its associated NNE-SSW structures when close to them, but NW-SE oriented when further. These NW-SE oriented structures appear to be characteristic of the late Neoproterozoic evolution of the Saharan metacraton and are in relation to the convergence with the Murzuq craton. This evolution reflects the rheological contrast existing along the Raghane shear zone marking the western boundary of the Saharan metacraton.

Sr-Nd isotopes and geochemistry of granite-gneiss complexes from the Meatiq and Hafafit domes, Eastern Desert, Egypt: No evidence for pre-Neoproterozoic crust

Neoproterozoic gneisses at Meatiq and Hafafit in the Eastern Desert of Egypt give Rb-Sr and U-Pb zircon ages of 600-750 Ma. These gneisses are interpreted by different workers to represent deeper levels of juvenile Neoproterozoic crust or Archaean/Palaeoproterozoic crust that was remobilized during Neoproterozoic time. Geochemical and Sr-Nd isotope compositions for these gneisses reported here are remarkably homogeneous: Initial 87Sr/86Sr (0.70252 ± 0.00056) and eNd (+6.4 ± 1.0). These values are best explained as reflecting derivation from depleted asthenospheric mantle sources during Neoproterozoic time, consistent with mean Nd model ages of 0.70 ± 0.06 Ga. The increasing recognition of old, xenocrystic zircons in juvenile ANS igneous rocks can be explained in several different ways. The participation of ancient crust is allowed as one of the explanations, but it is the isotopic composition of radiogenic elements such as Sr and Nd for whole-rock specimens that are the most reliable indicators of whether or not a given crustal tract is juvenile or reworked older crust. These isotopic data indicate that the protolith for the Meatiq and Hafafit gneisses were juvenile Neoproterozoic igneous rocks and sediments derived from them. There is no support in the isotopic data for any significant contribution of pre-Neoproterozoic crust in these two sections of Eastern Desert crustal infrastructure

Flow of Canary mantle plume material through a subcontinental lithospheric corridor beneath Africa to the Mediterranean: Comment

We contend that fi eld and geochemical evidence does not favor mantle plume fl ow under the Atlas mountain range. Early Paleocene to late Pleistocene alkaline volcanism and post-Miocene Atlas thermal uplift, though partly disconnected in time, resulted from both the interplay between reactivation of inherited geological structures and the thermal erosion of the metasomatized lithosphere (Raffone et al., 2009) during Africa- Europe convergence, not from a mantle plume fl ow process. The existence of the Canary mantle plume itself is strongly questionable ( Lustrino and Wilson, 2007).

Sr&#8211;Nd isotopes and geochemistry of granite-gneiss complexes from the Meatiq and Hafafit domes, Eastern Desert, Egypt: No evidence for pre-Neoproterozoic crust

Neoproterozoic gneisses at Meatiq and Hafafit in the Eastern Desert of Egypt give Rb&#8211;Sr and U&#8211;Pb zircon ages of 600&#8211;750 Ma. These gneisses are interpreted by different workers to represent deeper levels of juvenile Neoproterozoic crust or Archaean/Palaeoproterozoic crust that was remobilized during Neoproterozoic time. Geochemical and Sr&#8211;Nd isotope compositions for these gneisses reported here are remarkably homogeneous: Initial 87Sr/86Sr (0.70252 ± 0.00056) and eNd (+6.4 ± 1.0). These values are best explained as reflecting derivation from depleted asthenospheric mantle sources during Neoproterozoic time, consistent with mean Nd model ages of 0.70 ± 0.06 Ga. The increasing recognition of old, xenocrystic zircons in juvenile ANS igneous rocks can be explained in several different ways. The participation of ancient crust is allowed as one of the explanations, but it is the isotopic composition of radiogenic elements such as Sr and Nd for whole-rock specimens that are the most reliable indicators of whether or not a given crustal tract is juvenile or reworked older crust. These isotopic data indicate that the protolith for the Meatiq and Hafafit gneisses were juvenile Neoproterozoic igneous rocks and sediments derived from them. There is no support in the isotopic data for any significant contribution of pre-Neoproterozoic crust in these two sections of Eastern Desert crustal infrastructure.

Nature, origin and significance of the Fomopéa Pan-African high-K calc-alkaline plutonic complex in the Central African fold belt (Cameroon)

The Fomopéa plutonic complex in West Cameroon comprises three petrographical units: a biotite&#8211;hornblende granitoid (BHG) including some diorite, a biotite monzogranite (BmG) and an edenite syenogranite (EsG). Amphibolites occur within each unit. The massif was emplaced into a Pan-African amphibolite-facies metamorphic basement. All rocks display igneous textures and are chemically calcalkaline, the BHG being metaluminous (ASI < 1) the BmG (ASI = 0.98&#8211;1.06) and EsG (ASI = 0.94&#8211;1.1) being metaluminous to weakly peraluminous. U&#8211;Pb dates on zircon give a Pan-African age of 620 ± 3 Ma for a diorite and 613 ± 2 Ma for a quartz&#8211;monzodiorite, both belonging to the BHG. Sr&#8211;Nd isotopic data indicate the mixing between a juvenile source, probably the mantle (nearest Fomopéa pole: eNd(620 Ma) = +4 and 87Sr/86Sr(620Ma) = 0.703) and a Palaeoproterozoic to Archaean lower continental crust (nearest Fomopéa pole: eNd(620Ma) = 16 and 87Sr/86Sr(620Ma) = 0.709; Nd TDM = 2.9 Ga) through a contamination process or through a bulk mixing event at the base of the crust. Evidence for both processes is provided by the coexistence of mafic enclaves and gneissic xenoliths within the granitoids. We propose a model whereby linear lithospheric delamination occurred along the Central Cameroon shear zone (CCSZ) in response to post-collisional transpression. This delamination event induced the partial melting of the mantle and old lower crust, and facilitated the ascent of the magmas. The emplacement of numerous post-collisional Neoproterozoic plutons along the CCSZ during the Pan-African orogeny indicates that this process was of paramount importance. The continental signature and geophysical data from the area indicate that the CCSZ corresponds to the northern lithospheric boundary of the Archaean Congo craton, and that the events recorded here correspond to the metacratonic evolution of the northern boundary of the Congo craton.

Madagascar volcanic provinces linked to the Gondwana break-up: geochemical and isotopic evidences for contrasting mantle sources

After the Gondwana break-up and Greater India splitting off, Madagascar suffered volcanic episodes. The Cretaceous Morondava flood basalt (CFB) province was emplaced ca. 93 Ma ago. Two coeval high-Ti&#8211;P (HTP) and low-Ti&#8211;P (LTP) suites are distributed within the alkaline Ankilizato (central) and tholeiitic Manamana (southern) sub-provinces. Sr&#8211;Nd isotope compositions display a trend from depleted (&#949;Nd=+7.5; Sri=0.7030; Antsoha end-member) to highly enriched products (&#949;Nd=&#8722;17; Sri=0.7228; Manamana end-member). Antsoha end-member is considered to be located at the lithosphere&#8211;asthenosphere boundary while the composition of Manamana end-member implies a location within the Archean to Proterozoic continental lithosphere. Oligocene Ankaratra HTP alkaline province, Miocene Ankilioaka transitional province, Pliocene Ambre Mountain and Pleistocene Nosy Be Island alkaline volcanoes display a restricted isotopic range (&#949;Nd=+4 to 0; Sri=0.7032 to 0.7048) forming an array distinct from the Cretaceous trend, implying the enriched pole did not correspond to Manamana end-member. This Cenozoic array could result from mixing between Antsoha end-member and BSE or a pole beyond. The exceptional variation of mantle sources involved in the Cretaceous episode contrasts with the more homogeneous near-BSE source during the Cenozoic. Several lines of evidence dismiss the mantle plume model. Cretaceous volcanism resulted from reactivation of a lithospheric scale shear zone due to plate reorganisation that eventually led to the Madagascar&#8211;India continental break-up. The lower lithospheric/asthenospheric melt (Antshoha endmember) was able to melt the most enriched/fusible parts of an upper lithospheric mantle (Manamana end-member); these two melts partly mixed. Long-lasting Neogene volcanic activity was less voluminous. The melting source was restricted to a lower part of the lithospheric mantle.