Pan-African Mobile Belt Research Articles

Petrography and geochemistry of the granitoids and associated volcanic rocks of the northern part of Kushaka and Birnin Gwari schist belts, NW Nigeria: implications for provenance and geological setting

The granitoids and the associated volcanic rocks of the northern part of Kushaka and Birnin Gwari schist belts were emplaced in the ca. 3.5 – 1. 0 Ga remobilized basement complex terrain composed of metasedimentary and metaigneous rocks that later underwent medium- to high-grade metamorphism during the Pan-African thermo-tectonic event. They comprise dominantly of diorite, granodiorite, granite, granite gneiss and basalt, and are product of metasomatism and injections. The diorite and granodiorite occur as paleosome and the granite as leucosome with the development of high temperature minerals, locally attaining granulite facie metamorphism. Plagioclase, biotite, hornblende, pyroxene and olivine fractionation played an important role during their genesis through fractional crystallization of basaltic magma and partial melting of older dioritic-granodioritic source rock in the deep crust which were themselves ultimately derived through fusion of mantle materials contaminated by continental crust and enriched by fluids derived from oceanic crust in an arc setting. Geochemical characteristics have revealed different chemical trends in granitoids and basalts. The granitoids are calc-alkaline, ferroan and magnesian, metaluminous and peraluminous in character. They also exhibit I- and S-type signatures with enrichment in LILE, radioelements (Th and U), depletion in Nb, Sr, P and Ti, high LREE fractionation factors (La/Yb) (1.05 to 77.20), and pronounced negative Eu anomalies (Eu/Eu* = 0.34 to 1.10). Similar patterns of spidergrams show that the rocks are genetically related and were emplaced in a volcanic arc and syn-collisional setting. The basalt is tholeiitic, metaluminous and high in Fe and Mg with relative enrichment in LILE, HFSE, low and near flat LREE and HREE, low fractionation [(La/Yb)N = 1.4] with Eu/Eu* value of 1.10. It is evidently a back arc cum mid-ocean ridge (MORB) basalt. The consistent decrease in the content of MgO, Fe2O3 MnO, CaO, Sc, Cr and V of the basalt, diorites, granodiorites, and granites indicates continuous igneous crystallization process. It seems that extrusion of basaltic magmas from the sub-circular Kushaka Complex derived from subduction of oceanic crust resulted in complete change in the genesis of the magmas at the time, in this region. The granitoids and the basalt may have formed behind subducted Pan-African plate due to effects of compressional and tensional forces caused by oceanic plate roll-back which resulted to a zone of extension, parallel to the island arc. The granitoids present similar chemical characteristics to those in the other areas underlain by the basement complex and schist belts in the north and eastern parts of the Pan-African mobile belt, while basalts are similar to ophiolites and amphibolites in other schist belts of Nigeria forming a lateral continuation of the same mobile belt.Â

Kinematic evolution of the Pan-African shear zones of South Maradi, southern Niger

The Pan-African Province (PAP) of south Maradi (Niger) corresponds to the northeastern part of the Benin-Nigerian Shield, which is part of the Pan-African Mobile Belt, located to the East of the West African Craton. This study focuses on the microstructural study supported by geochronological data of the South Maradi Pan-African shear zones. The structural analysis of the Pan-African shear bands, combined with the previous radiometric data allowed to highlight a polyphase history of this northeastern part of the Benin-Nigerian Shield. Two major tectonic periods are highlighted: A ductile to brittle Pan-African deformation stage (D1) relayed by a brittle post-Pan-African deformation stage (D2). The first phase D1 consists of two episodes noted D1a and D1b: (1) the D1a episode was a ductile pan-African deformation stage with a strong coaxial component, which favoured the development of symmetric fabrics of foliation. The tectonic regime was characterized, first by a NW-SE (~N140°) trending pure-shear-dominated shortening episode, which prevailed between 617.9 ± 2.8 to 589 ± 1.9 Ma. Then during episode D1a, the shortening direction evolved gradually from NW-SE to ~E-W (N90°-N70°). This change of direction is due to the constraint coming from the Pan-African orogeny. The deformation had progressively a rotational component and this is how D1b episode began. This latter was characterized by a brittle-ductile deformation, associated with the formation of mylonitic gneiss and micaschist. So, the counter clockwise sinistral rotation of the shortening axis Z (from N140° to N90°- N70°) reveals a Pan-African continuum of deformation between D1a and D1b episode. (2). Subsequently, a stage of brittle deformation D2 affected all the previous structures. This D2 stage is marked by a continental-scale system of conjugated strike-slip faults, NE-SW and NW-SE trending. This investigation contribute to knowledge of Pan-African kinematic evolution of Benin-Nigerian Shield, particularly its northeastern part (South Maradi).

Rayleigh wave group velocity dispersion tomography of West Africa using regional earthquakes and ambient seismic noise

West Africa could teach us much about the early tectonic history of Earth, but current seismic models of the regional crustal and lithospheric structure lack the resolution required to answer all but the most basic research questions. We have improved the resolution of group velocity maps of the West African Craton by complementing the uneven path distribution of earthquake-generated surface waves with surface waves reconstructed from ambient noise cross-correlations. Our joint dataset provides good spatial coverage of group velocity measurements from 20- to 100-s period, enabling us to reduce artifacts in our group velocity maps and improve their resolution. Our maps correlate well with regional geological features. At short periods, they highlight differences in crustal thickness, recent tectonic activity, and thick sediments. At long periods, we found lower velocities due to hot, thin lithosphere under the Pan-African mobile belt and faster velocities due to cold, thick lithosphere under the Man-Leo and Reguibat shields. Our higher resolution maps advance us a step towards revealing the detailed lithospheric structure and tectonic processes of West Africa.

A comparison between the sub-continental lithospheric mantle of Libya, Morocco and Cameroon: Evidences from structural data and trace element of mantle xenolith Cr-diopsides

We analysed the crystallography and the chemistry (major and trace elements) of 16 Cr-diopsides belonging to mantle xenoliths from northern (Waw-En-Namus, Libya; Bou-Ibalrhatene and Tafraoute, Morocco) and central Africa (Nyos and Barombi maars, Cameroon). These mantle xenoliths were extracted from mantle domains underlying a metacraton (Libya), a pericraton (Morocco), and a Pan-African mobile belt (Cameroon). The clinopyroxenes from pericratonic and mobile belt occurrences show similar geochemical features. They usually have high R3+ content and low Ca. When normalized to chondrite abundances, their incompatible trace elements show a wide spectrum of patterns, from slightly depleted to slightly enriched and U-shaped patterns. Such features may indicate interaction with carbonatitic and silicic melts. Their Vcell suggest different equilibrature pressures that are compatible with an ascending diapir and/or entrapment by magmas at different depths within the lithospheric mantle. It is important to stress that variable cell and tetrahedron volumes suggest that Libyan clinopyroxenes formed at variable depths. Those arising from shallower depth are less depleted and may display enriched and U-shaped patterns, while clinopyroxenes deriving from deeper zones display depleted and flat incompatible element patterns, possibly suggesting a melt extraction preceding the delamination of the SCLM.

Quartzite crests in Paleoproterozoic granites (Anti-Atlas, Morocco); a hint to Pan-African deformation of the West African Craton margin

South of the Pan-African suture zone of the Anti-Atlas, the Agadir Melloul inlier exhibits several crests of nearly vertical conglomeratic quartzite beds inserted in Paleoproterozoic granites. These enigmatic rocky walls have been considered as Early Neoproterozoic platform sediments pinched within the 2.03 Ga-old granites during brittle faulting events at ca. 570 Ma. The data presented here contradict this interpretation. The quartzite crests correspond to parts of tight synclines pinched along brittle-ductile, strike-slip shear zones involving the granitic basement itself. This deformation occurred in low-grade greenschist facies conditions at T = 260–280 °C. The detrital zircon grains from the vertical quartzite beds are mostly grouped around 2 Ga, with the youngest grains at 1.8 Ga. Thus, the studied quartzites exhibit the same detrital zircon barcode as the other siliciclastic platform formations of the southwestern Anti-Atlas (Taghdout Group) whose age has been recently constrained between 1.8 and 1.750 Ga. 40Ar-39Ar age measurements were unsuccessful to determine the age of crystallization of small syn-tectonic white mica grains within the quartzite samples, but the large white mica grains of granite samples outside the quartzite-bearing shear zones yielded ages of ∼1.777 and 1.830 Ga. We interpret the enigmatic quartzite structures as the result of two major events that the Paleoproterozoic foreland of the Pan-African mobile belt likely suffered, i.e., an extensional event around 800-750 Ma, coeval with the opening of the Pan-African Ocean, and a compressional event during its final closure at about 650–600 Ma. We propose that the quartzite formations detached then from the granitic basement of the Pan-African foreland in the frame of a complex, thick- and thin-skinned tectonics.

Origin and tectonic implications of ferroan alkali-calcic granitoids from the Hawal Massif, east-eastern Nigeria terrane: clues from geochemistry and zircon U-Pb-Hf isotopes

ABSTRACTLate Cryogenian to Ediacaran magmatism of the East-Eastern Nigerian Terrane (E-ENT) in the Pan-African mobile belt provides a critical geological record that is important for unravelling regional tectonic and geodynamic setting. A detailed study that utilizes whole-rock elemental, in-situ zircon U-Pb and Lu-Hf isotopic geochemistry are conducted on the post-collisional granitoids from the Hawal Massif in the E-ENT. These granitoids are metaluminous to slightly peraluminous, alkali-calcic to calc-alkalic and ferroan in composition. U-Pb zircon ages suggest two granitic plutons were emplaced at 627 ± 4 Ma and ca. 580 Ma. Typically, these rocks are characterized by ƐHf(t) values ranging from +3 to -9.03 and TCDM ages range from 1.3 to 2.1 Ga, suggesting mixing of juvenile mafic melts with the older continental crust at different stages of the Pan-African Orogeny. Slab break-off most likely induced upwelling of the asthenosphere which in turn cause partial melting of the sub-continental lithospheric mantle (SCLM) and the lower crust at the early stage of Late-orogenic to generate 627 ± 4 Ma granites. The magmatism at ca. 580 Ma may have resulted from vertical planar lithosphere delamination as a consequence of transpressive movement along lithospheric scale shear zones which triggered asthenosphere upwelling that facilitated significant crustal melting at the waning stage of the Pan-African orogeny. The presence of post-collisional granite at 627 Ma and distinct isotopic signatures of the E-ENT granitoids compared to those from the NW-Cameroon and Mayo Kebbi domain suggest that E-ENT may have undergone different Pan-African evolution.

Long-term reactivation and morphotectonic history of the Zambezi Belt, northern Zimbabwe, revealed by multi-method thermochronometry

Neoproterozoic-early Paleozoic Pan-African mobile belts that formed during the amalgamation of Gondwana, such as the Zambezi Belt, are inherently weak zones that are susceptible to reactivation by later tectonism. With the exception of Karoo rifting, however, the post-Pan-African morphotectonic history of the Zambezi Belt is poorly constrained. Here, we use multiple low-temperature thermochronometers on samples collected across major structures in northern Zimbabwe to reveal the temporal and spatial pattern of tectonism and denudation in this portion of the Zambezi Belt. Thermal history modelling suggests that a large crustal block encompassing part of the Zambezi Belt and northern margin of the Zimbabwe Craton experienced differential denudation during three main Phanerozoic episodes. This denudation of the Archean-Proterozoic basement was associated with reactivation of the Zambezi Escarpment Fault, which demarcates the southern margin of the Karoo Cabora Bassa Basin. In contrast, other major structures within the region remained relatively stable. The results highlight the value of using a multi-thermochronometer approach, where essentially zircon (U-Th)/He data preserve evidence of late Carboniferous Karoo rifting, apatite fission track data record the thermal effects of Jurassic tectonism associated with Gondwana breakup, and the apatite (U-Th-Sm)/He data reveal Paleogene reactivation of the basin-bounding fault. Thermal history modelling also suggests that the Cabora Bassa Basin experienced Gondwana breakup-related denudation, potentially associated with a proposed concurrent major regional drainage reversal. Since the Cretaceous, the basin has experienced limited sedimentation and erosion.

Strain analysis and tectonite classification using polymictic metaconglomerates in the Igarra schist belt, southwestern Nigeria

Deformed conglomerates in the Igarra schist belt display contrasting strains between different lithological clast populations. We analysed three different clast populations (pegmatite, metasediment, quartz) across three sites in the Igarra metaconglomerates of Edo state, Nigeria. We calculated finite strain using the Rf/o method and Flinn graph. Quartz clasts exhibited the least amount of strain, while the pegmatite and metasedimentary clasts had greater strains (pegmatite > metasediment > quartz). The variability in the finite strains gotten from the three sites is controlled by clast composition and probably grain size. The differences in finite strain and maximum elongation direction (λ1) in the three sites indicates that the Igarra metaconglomerates was subjected to a heterogenous simple shear deformation which is probably associated with the general transpressional deformation that affected the Pan-African mobile belt considering its (Igarra schist belt) spatial proximity with regional dextral shear zones. Three-dimensional strain analysis in site one indicates a constrictive deformation with the dominance of L tectonites. Spatial analysis of two-dimensional strain suggests a strain gradient where finite strain decreases from north to south.

Geology and U/PB geochronology of the Gamtoos Complex and lower Paleozoic Table Mountain Group, Cape Fold Belt, Eastern Cape, South Africa

The Cape Fold Belt (CFB) along the southern coast of South Africa contains several tectonic windows that expose low-grade “basement” rocks as inliers that are inferred to be late Neoproterozoic in age subjected to tectono-metamorphism during the Pan African Saldanian Orogeny (ca. 650 to 550 Ma). Whilst a Saldanian tectonic history along the northwest to southeast trending western branch of the CFB has been well documented, such Neoproterozoic deformation along the east-west trending southern branch of the CFB is not established. This work presents new field observations and SHRIMP U/Pb geochronologic data from the Kleinrivier Sequence within the Gamtoos Inlier exposed along the eastern-most known tectonic window, near Port Elizabeth, and which is here renamed the Gamtoos Complex. The study area includes 3 tectonic sequences from the Gamtoos Complex (“pre-Cape”) and 2 sequences from the overlying Table Mountain Group of the Cape Supergroup. The lower sequences of the Gamtoos Complex comprise north-east verging thrust packages within a regional antiform, all of which have a common oriented planar tectonic fabric (S 2 ) coincident with those in the lower Paleozoic rocks. Phyllites from the Kleinrivier and upper Sardinia Bay sequences (spanning the pre-Cape and Cape boundary) are similarly affected by a slaty cleavage (S 1 ), crenulation cleavage (S 2 ) and multiple generations of quartz veins. U/Pb data are from zircons of metasediments and igneous rocks from the Gamtoos Complex and from the overlying rocks of the Table Mountain Group. Meta-greywackes of the Kleinrivier Sequence (youngest zircon: 523 ± 6 Ma) are probably Cambrian in age. However, some units from the Kleinrivier Sequence are intruded by felsic sills with a Concordia age: 530.2 ± 4.4 Ma and therefore require further U/Pb detrital zircon analyses. Mafic sills and dykes also intrude the Kleinrivier Sequence and were subsequently deformed and contain S 2 . Boulder conglomerates in the Sardinia Bay Sequence (youngest matrix zircon: 521 ± 6 Ma) are separated from the Kleinrivier Sequence by an angular unconformity. It also contains granite-boulders (Concordia age: 530.2 ± 4.7 Ma), and is overlain by the feldspathic psammites and quartzites of the Sardinia Bay Sequence with zircons as young as 510 ± 7 Ma (e.g. middle Cambrian). In total, 75% of the detrital zircons from all sequences date between early-Mesoproterozoic and early-Neoproterozoic (ca. 1455 to 852 Ma) and are likely sourced from the gneisses of the Namaqua-Natal province that are known to underlie the CFB, whilst the Neoproterozoic to mid-Cambrian zircons (ca. 828 to 431 Ma) have been sourced from Pan African mobile belts, possibly the Mozambican and/or Saldanian Belts to the east and west, respectively. Such “Pan African” detrital zircons are more prevalent in the Sardinia Bay Sequence and form the dominant component in the Lower Table Mountain Group, suggesting a lesser source influence from the Namaqua Natal Mobile Belt. The conglomerates at Sardinia Bay possibly represent mid-Cambrian rift sequences, similar to the Klipheuwel Group that overlie the Malmesbury Sequence and Cape granites along the western branch of the CFB. The Sardinia Bay Sequence has zircons equivalent to those of the Peninsula Formation (youngest zircon: 516 ± 5 Ma), and therefore should be linked to represent the lower Table Mountain Group.

Mantle dynamics and secular variations beneath the East African Rift: Insights from peridotite xenoliths (Mega, Ethiopia)

Segments of the Main Ethiopian Rift (MER), a key sector of the East African Rift (EAR) linking the Afar and Turkana depressions, record different stages of lithospheric evolution from initiation to continental break-up and incipient oceanic spreading. Although EAR tectonic and magmatic activity is generally ascribed to a manifestation of sublithospheric mantle processes, the number, depth of provenance and triggering mechanisms of the related mantle upwellings remain topics of debate. We present new Hf and Pb isotope data for EAR mantle xenolith clinopyroxenes from Southern Ethiopia (Mega, the Sidamo region) that compellingly testify to multiple episodes of mantle depletion and metasomatic enrichment. Radiogenic values (εHf up to +1076) suggest that present-day MER lithospheric mantle domains underwent melting, possibly in the presence of residual garnet, billions of years ago, thereby fractionating (increasing) the Lu/Hf ratio and ultimately leading to extremely high 176Hf/177Hf. Although the precise dating of these depletion episodes is hampered by possible metasomatic overprinting, positively correlated Lu/Hf and 176Hf/177Hf indicates apparent ingrowth at ca. 1.9Ga, providing a minimum age for the delineated Proterozoic melting events. Our findings of Early Proterozoic melting episodes are complementary to previously determined Os model ages on xenoliths from the same site indicating melt extraction between 2.4 and 2.8Ga (Reisberg et al., 2004; Chem. Geol. 208, 119–140). Taken together, the geochemical and isotopic characteristics of the Mega xenoliths therefore record melt extraction events and preserve memory of ancient mantle dynamics beneath this site, where important lithospheric discontinuities exist between the Archean/Early Proterozoic Tanzanian craton and the Late Proterozoic Panafrican mobile belt. Subsequent metasomatic reactions variably affected the Nd and Hf isotopic compositions of some samples and completely overprinted the Pb isotopic composition of the whole xenolith suite. The persistence of these geochemical heterogeneities within the investigated suites of xenoliths precludes pervasive melt refertilization, which would have homogenized existing compositions and obliterated the record of previous petrologic processes. This suggests that the MER segment considered here developed on a lithospheric section isolated by pre-existing tectonic structures, far from the influence of plumes originating deep in the convecting mantle.

Shallow Reflection Surveys of the East Ongul Island, the Lützow-Holm Complex, East Antarctica

The shallow reflection surveys were carried out in 2007 and 2010 austral summers in East Ongul Island, the Lützow-Holm Complex (LHC), East Antarctica. LHC is identified by geologically as one of the Pan-African terrains of Eastern Dronning Maud Land. The multi-channel reflection surveys targeted to achieve the image of laminated layering of metamorphic rocks near the surface (the depths down to a few hundreds of meters) of the crystalline crust. Two surveys were conducted in total length of the profiles about 500 m along a main traffic load across the East Ongul Island. The multi-channel acquisition systems were utilized with combining the dense geophones along the profiles. Seismic sources were adopted by combining the boom of a power shovel, a weight drop and hammer shots with their intervals in a few tens of meters. The obtained data include clear first P-arrivals in far offset distance. The energy of P-S converted waves was enhanced because of the characteristics of the seismic sources. Pre-stacked images could be expected to the information on metamorphic layering for several lithological structure composed by hornblende gneiss, garnet gneiss and pyroxene gneiss appearing as the surface bedrocks. The conducted shallow reflection surveys would give rise to one step for understanding tectonic formation of LHC, as one of the Pan-African mobile belts in Gondwana super-continent.

Rock magnetic investigation of possible sources of the Bangui magnetic anomaly

The Bangui magnetic anomaly (BMA) is the largest lithospheric magnetic field anomaly on Earth at low latitudes. Previous studies investigated its geological source using constraints from satellite and ground magnetic field measurements, as well as from surface magnetic susceptibility measurements on rocks from the Panafrican Mobile Belt Zone (PMBZ). Here we combine magnetic field data modelling and rock magnetic property measurements (susceptibility and natural remanent magnetization, NRM) on many samples from this PMBZ and the surrounding formations. It reveals that NRM is a significant component of the total magnetization (Mt) of the BMA source, which reaches 4.3A/m with maximum thicknesses of 38 and 54km beneath the western and eastern parts of the BMA. Only the isolated and relatively thin banded iron formations and some migmatites show such Mt values. Thus we suggest that the thick BMA source may be composed either by overlapped slices of such metamorphic rocks, or by an iron-rich mafic source, or by a combination of these two geological structures.

U-Pb Geochronology of the Jebba Granitic Gneiss and Its Implications for the Paleoproterozoic Evolution of Jebba Area, Southwestern Nigeria

Jebba area southwestern Nigeria forms part of the Nigerian basement complex which lies in the Neoproterozoic PanAfrican mobile belt. It is underlain by several lithological units among which is a polydeformed granitic gneiss. This rock has been dated by LA-ICP-MS yielding a concordant U-Pb zircon age of 2207 ± 20 Ma indicating the crystallization age of the granite protolith. This early Rhyacian age and its affinity with within-plate granites indicates emplacement during crustal extension and rifting presceding the main phase of the Eburnean orogeny. The strong, early, shear fabric, S1, in the rock is interpreted to be also of Paleoproterozoic age i.e. imprinted during the Eburnean orogeny. The Jebba granitic gneiss is thus correlatable with the widely abundant Paleoproterozoic granitic magmatism now represented by many orthogneisses and documented in other parts of southwestern Nigeria, the West African craton, the Borborema Province, the Gurupi Belt, Sao Luis craton and Sao Francisco craton in Brazil.

GEOPHYSICAL INVESTIGATION OF THE TRANSITION ZONE BETWEEN THE CONGO CRATON AND THE KRIBI-CAMPO SEDIMENTARY BASIN (SOUTHWESTERN CAMEROON)

Three different sets of geophysical data are used to investigate the structure of the transition zone between the north-western boundary of the Congo Craton (CC) and the Kribi-Campo sedimentary basin. These are shear wave velocity, gravity and magnetotelluric (MT) data, respectively. The combined use of these data helps to reduce the non-uniqueness of solutions inherent to forward models, leading to a better inference of the structure of the transition zone. The interpretation of shear wave velocity models enhanced the difference in composition of upper crustal layers against the similar composition (densities of ~3.0 g/cm 3 ) of the lower crust, with different thicknesses beneath both tectonic regions. The analysis of gravity maps obtained from the area shows the signature of the main geological units and particularly the north-south gradient correlating with the strike direction of the so-called Kribi-Campo fault (KCF). Using constraints from shear wave velocity models together with surface geology observations, a 2D1/2 gravity model was obtained along a profile crossing the CC margin and the Kribi-Campo sedimentary basin. The new model is consistent with the presence of a thick mafic layer (>10 km) at depth below 18 km beneath both geological units, and also with crustal thicknesses of 28 km and 45 km beneath the basin and CC, respectively. The model also suggests that KCF resulted from the intrusion of magmatic rocks during the continental collision, which were later metamorphosed into granulites. The resulting suture may be interpreted from the model as the thrusting of the Panafrican Mobile Belt rocks onto the CC. Similar conclusions are inferred from the MT model and the signature of the above mentioned presence of granulites is interpreted in this case as low resistive rocks emplaced into high resistive materials. This provides additional support for the gravity model.

Geochemistry of the Central Atlantic Magmatic Province (CAMP) in south-western Algeria

In south-western Algeria, dolerite sills and dykes and scarce basalt lava flows occur in the Tindouf, Reggane, Hank basins and Bechar area, and are part of the large Central Atlantic Magmatic Province (CAMP). They represent the north-easternmost witnesses of this province into the African continent. Here, we report the first geochemical data (major, trace and rare-earth elements) for those rocks. Petrographical and chemical compositions of the studied dolerites and basalts are homogeneous and characteristic of continental tholeiites. They are moderately evolved (Mg# 0.66–0.42) quartz-normative low-Ti tholeiites (TiO 2 = 0.86–1.55 wt.%), displaying slight yet variable enrichment in LILE and LREE [(La/Yb) N = 2.18–5.51] and a negative Nb anomaly. Trace element modelling can reproduce the observed variations by non-modal batch melting of a slightly enriched source via various degrees (4–15%) of melting. A similar evolution is displayed by the neighbouring lava flows from Morocco and Ksour Mountains (North Algeria) and by the dyke swarm from Taoudenni (Mali), arguing for a common source presumed to reside within the sub-continental lithospheric mantle. The magmas were probably generated in response to mantle global warming underneath the Pangea supercontinent, and to edge-driven convection between the thick Reguibat craton and adjacent Pan-African mobile belts.

Genesis of wollastonite- and grandite-rich skarns in a suite of marble-calc-silicate rocks from Sittampundi, Tamil Nadu: constraints on the P–T–fluid regime in parts of the Pan-African mobile belt of South India

The Pan-African tectonothermal activities in areas near Sittampundi, south India, are characterized by metamorphic changes in an interlayered sequence of migmatitic metapelites, marble and calc-silicate rocks. This rock sequence underwent multiple episodes of folding, and was intruded by granite batholiths during and subsequent to these folding events. The marble and the calc-silicate rocks develop a variety of skarns, which on the basis of mineralogy; can be divided into the following types: Type I: wollastonite + clinopyroxene (mg# = 71–73) + grandite (16–21 mol% Adr) + quartz ± calcite, Type II: grandite (25–29 mol% Adr ) + clinopyroxene (mg# = 70) + calcite + quartz, and Type III: grandite (36–38 mol% Adr) + clinopyroxene (mg# = 55–65) + epidote + scapolite + calcite + quartz. Type I skarn is 2–10 cm thick, and is dominated by wollastonite (>70 vol%) and commonly occurs as boudinaged layers parallel to the regional foliation Sn1 related to the Fn1 folds. Locally, thin discontinuous lenses and stringers of this skarn develop along the axial planes of Fn2 folds. The Type II skarn, on the other hand, is devoid of wollastonite, rich in grandite garnet (40–70 vol%) and developed preferentially at the interface of clinopyroxene-rich calc-silicates layers and host marble during the later folding event. Reaction textures and the phase compositional data suggest the following reactions in the skarns: 1. calcite + SiO2 → wollastonite + V, 2. calcite + clinopyroxene + O2 → grandite + SiO2 + V, 3. scapolite + calcite + quartz + clinopyroxene + O2 → grandite + V and 4. epidote + calcite + quartz + clinopyroxene + O2 → grandite + V Textural relations and composition of phases demonstrate that (a) silica metasomatism of the host marble by infiltration of aqueous fluids (XCO2 11 kbar and >950°C) inferred for the Pan-African tectonothermal events from the neighboring areas. Field and petrological attributes of these skarn rocks are consistent with the infiltration of aqueous fluid predominantly during the Fn1 folding event at or close to the ‘peak’ metamorphic conditions. Petrological features indicate that the buffering capacity of the rocks was lost during the formation of type I and II skarns. However, the host rock could buffer the composition of the permeated fluids during the formation of type III skarn. Aqueous fluids derived from prograde metamorphism of the metapelites seem to be the likely source for the metasomatic fluids that led to the formation of the skarn rocks.

Provenance of late Ordovician to early Cretaceous sedimentary rocks from southern Ghana, as inferred from Nd isotopes and trace elements

Geochemical and Nd-isotopic data are reported for 24 shale and sandstone samples comprising the Upper Ordovician to Lower Cretaceous Sekondian Group, southern Ghana. The data are interpreted in terms of the provenance of these siliclastic sediments and the Paleozoic and Mesozoic geologic development of southern Ghana. The sandstones and shales generally have trace element characteristics typical of material eroded from the upper continental crust. Cr and Ni concentrations are low suggesting their derivation from dominantly felsic sources. There appear to be chemical and Nd-isotopic differences between the lower formations (i.e., Ajua Shale and Elmina Sandstone) and the uppermost formation (i.e., Essikado Sandstone) on one hand, and the upper formations (i.e., Takoradi Sandstone, Takoradi Shale, Effia Nkwanta Beds and Sekondi Sandstone) on the other. The upper formations are characterized by evolved major element compositions, high large-ion lithophile element abundances, large negative Eu-anomalies (0.58–0.73), and low ε Nd values (−10.8 to −4.1), indicating stable craton settings. On the other hand, the Ajua Shale, Elmina Sandstone, and Essikado Sandstone formations have less evolved major element compositions, lower large-ion lithophile abundances, and higher Eu-anomalies (0.69–0.88), and this may reflect an active margin setting of the source (juvenile-type Birimian felsic volcanics and/or granitoids) rather than Phanerozoic active continental settings. The upper formations have T DM model ages of 1.8–2.3 Ga indicating that the Paleoproterozoic Birimian rocks are the ultimate sources, whereas the lower formations and uppermost formation have slightly younger T DM values of 1.6–1.9 Ga suggesting of a component from the Pan-African mobile belt. These differences in model ages may, however, be related to lithology (rather than stratigraphy) where the shales (1.6–2.3 Ga; mean, 1.9 Ga) typically give older ages than the sandstones (1.6–1.9 Ga; mean 1.7 Ga).

The 2.1 Ga West Central African Belt in Cameroon: extension and evolution

Available isotopic and geochronological data, combined with new petrographic and structural observations in Cameroon, permit discussion of the nature and extension of the Paleoproterozoic West Central African Belt, which resulted from the Eburnean collision between the Congo and São Francisco cratons. The portion of the belt recognized in Cameroon is approximately oriented NNE-SSW and includes the Nyong series along the NW corner of the Congo craton and Paleoproterozoic remnants cropping out further north within the late Neoproterozoic Pan-African belt. The dominant rock units consist of migmatitic orthogneisses associated with amphibolites, felsic gneisses of volcanic to volcano-sedimentary origin, quartzites, and banded iron formations. Orthogneisses are mostly TTG compositions within the Nyong series and metadiorites to metagranodiorites to the north in the Pan-African belt. Paleoproterozoic evolution is characterized by the development of nappe tectonic structures, recognized in the Nyong series, and by high-grade, granulitic facies metamorphism that was associated with arrested charnockite formation. The Paleoproterozoic structures and mineral assemblages were subsequently reworked more severely in the Pan-African mobile belt than in the Nyong series, where they are locally well preserved. Broadly, the Nyong series may be ascribed to a proximal domain characterized by reworking and recycling of the adjacent Archean cratonic crust, while the occurrences farther north represent a more distal domain characterized by newly formed Paleoproterozoic (Birrimian) crust. This is consistent with the distribution of metamorphic ages, which display a polarity from the internal zones (ca. 2.1 Ga) to the external zones (ca. 2.03 Ga) and suggest origin of the metamorphic rocks in a modern-type collisional belt during the Paleoproterozoic (Eburnean).

Pleistocene magmatism in a lithospheric transition area: petrogenesis of alkaline and peralkaline lavas from the Baringo–Bogoria Basin, central Kenya Rift

New petrological, geochronological, and geochemical data on basalts, mugearites, peralkaline trachytes, and phonolites from the Baringo–Bogoria Basin, central Kenya Rift, are presented. K–Ar dating indicates that the volcanic rocks were emplaced between 894 ± 13 and 92 ± 5 ka. 87Sr/86Sr ranges from 0.70304 to 0.70692, 143Nd/144Nd from 0.51237 to 0.51295, 206Pb/204Pb from 18.4 to 19.8, 207Pb/204Pb from 15.46 to 15.70, and 208Pb/204Pb from 38.2 to 40.5. Despite a rather restricted sampling area and a relatively short time span ([Formula: see text]820 ka), the mineralogical and geochemical variations are not consistent with a simple cogenetic link between the lavas. The studied area is located in a transition zone between two different lithospheric domains (Tanzanian Craton and Panafrican Mobile Belt). We propose that the petrological and geochemical variations of the studied lavas are essentially linked to the nature of the underlying lithosphere. Some basaltic products underwent carbonate contamination, possibly within the crust. Trachytes and phonolites are derived from different basaltic parents through crustal assimilation coupled with fractional crystallization. One phonolite sample contains primary calcite-rich veinlets. Textural relations and geochemical evidence suggest that there is a direct cogenetic link between these carbonate and phonolite melts. The veinlets are the modal expression of a carbonate component included in all the phonolites from the Baringo–Bogoria Basin.

Variation in effective elastic plate thickness of the East Africa lithosphere

We use new and existing Bouguer gravity data to characterize the long‐term flexural rigidity of the lithosphere beneath East Africa. Four tectonic provinces are characterized by distinct effective elastic plate thicknesses. These are the Tanzanian Craton, the Proterozoic Mobile Belts, the Paleogene rift basins in Sudan and Kenya, and the Cenozoic East African Rift System. Beneath the Tanzanian craton the elastic plate thickness is ∼75 km, while the Pan African Mobile Belts and the Ubendian Belts are ∼55 and ∼65 km, respectively. The Paleogene rift basins exhibit an elastic plate thickness varying between 40 and 45 km. The effective elastic plate thickness within the faulted East African Rift System ranges from 14 km in the north to 33 km in the south. The minimum is found beneath the active rift of the Afar depression. The north to south along‐rift axis variation in effective elastic plate thickness can be attributed to contrasts in the mechanical strength of the lithosphere. This is consistent with the north‐south decrease in extensional velocity and the northward decrease in the focal depths of the earthquakes, reflecting a northward thinning in the brittle layer of the crust. The maximum elastic plate thickness is found beneath the Tanzanian craton, while the surrounding Mobile Belts have relatively moderate elastic plate thicknesses. The high effective elastic plate thickness of the Tanzanian craton suggests that this part of the lithosphere is rheologically competent compared to the surrounding Mobile Belts. The craton effectively resists deformation, even though it is located within a broad zone of an east‐west extensional tectonic regime. Consequently, it has altered the direction of the rift propagation into the warmer and weaker lithosphere of the surrounding Mobile Belts.