Petrographic and Geochemical Characteristics of the Pouni Palaeoproterozoic Formations North of the Léo Square Degree (Burkina Faso, West Africa)

The Pouni area is made up of basalts belonging to the Boromo belt, lamprophyres and granitoids. These geological formations are similar to geological formations of the same type in other regions of the Palaeoproterozoic domain of the Man/Leo shield. This study, which focused on the petrographic and geochemical characteristics of these geological formations, led to the following main conclusions: The lamprophyres are basic plutonic rocks that cut through other geological formations. The basalt belongs to the northern part of the Borormo belt and is thought to be a relic of overthickened oceanic plateaus. There are two groups of granitoid rocks. The granodiorite has a geochemical signature close to that of Archean TTGs and is metaluminous in character. It has a low potassium content. The minor element and rare earth element spectra indicate that it could be derived from partial melting of basic magmatic rocks. Biotite granites are peraluminous and highly potassic. Minor element contents and rare earth spectra indicate that they could be derived from partial melting of felsic materials. Geotectonic diagrams show that the granitoids identified in the Pouni zone were emplaced in an active tectonic context, similar to that of present-day subduction zones.

Influence of lithospheric thickness distribution on oil and gas basins,China seas and adjacent areas

The distribution of oil and gas resources is intricately connected to the underlying structure of the lithosphere.Therefore,investigating the characteristics of lithospheric thickness and its correlation with oil and gas basins is highly important.This research utilizes recently enhanced geological–geophysical data,including topographic,geoid,rock layer thickness,variable rock layer density,and interface depth data.Employing the principles of lithospheric isostasy and heat conduction,we compute the laterally varying lithospheric thickness in the China seas and adjacent areas.From these results,two pivotal parameters for different types of oil and gas basins were statistically analyzed:the minimum lithospheric thickness and the relative fluctuation in lithospheric thickness.A semiquantitative analysis was used to explore the connection between these parameters and the hydrocarbon abundance within the oil and gas basins.This study unveils distinct variations in lithospheric thickness among basins,with oil and gas rich basins exhibiting a thicker lithosphere in the superimposed basins of central China and a thinner lithosphere in the rift basins of eastern China.Notably,the relative fluctuations in lithospheric thickness in basins demonstrate significant disparities:basins rich in oil and gas often exhibit greater thickness fluctuations.Additionally,in the offshore basins of China,a conspicuous negative linear correlation is observed between the minimum lithospheric thickness and the relative fluctuation in lithospheric thickness.This study posits that deep-seated thermal upwelling results in lithospheric undulations and extensional thinning in oil and gas basins.Concurrently,sustained deep-seated heat influences sedimentary materials in basins,creating favorable conditions for oil and gas generation.The insights derived from this study contribute to a quantitative understanding of the intricate relationships between deep lithospheric structures and oil and gas basins.These findings provide valuable guidance for future oil and gas exploration in the studied areas.

Insight from Lithostructural and Aeromagnetic Data from the Léo Square Degree, Central-Western Burkina Faso

This study, carried out in the Léo square degree (west-central Burkina Faso) in the Palaeoproterozoic domain of the Man/Léo ridge, aims to define the shapes and positions of granitic plutons and the organisation of deformation structures using aeromagnetic data. These data have shown that there are small sub-circular granitic bodies to the north of the granitic masses in this region, and a large sub-circular granitic body to the south, around which are other smaller granitic bodies. The lineament map shows that the deformation structures are organised along three main directions and largely form these sub-circular plutonic bodies. We suggest that the granitic plutons are coalescent, pending identification of the internal structures of these granites to further refine the geodynamic model.

Gold Mineralisation in the Intiédougou Prospect (Diébougou District) in Southwest Burkina Faso, West African Craton

The Intiédougou located in the Houndé Birimian greenstone belt has been the subject of several mining and geoscience studies that have led to the discovery of mineralized gold targets. One of these mineralized targets has prompted work that raises the issue of control factors for the gold mineralization of the prospect. The methodology used in this study combines a study of core drill hole data located in the area and laboratory studies. The Intiédougou sector is based on andesito-basaltic, andesitic interstratified volcanoclastite rocks and Tarkwaïen type detrital sedimentary rocks caught in a vice in the volcano-sedimentary unit. Lithostructural analysis of the sector shows that the subvolcanic rocks bearing gold mineralization are subjected to heterogeneous ductile to brittle deformations and affected by hydrothermalism evolving at stages marked by large fissure fillings. These hydrothermal phases evolve in the zones of expansion created by the brittle deformations that have contributed to the deposits of different types of gold-enriched sulphides. These different phases of hydrothermal destabilization generally of low degree accompany the tardi to post-eburnean brittle tectonics. This deformation system is favorable to the establishment of gold mineralization in the form of vein bodies. The overimposition of deformed and altered areas suggests a genetic relationship between deformation and hydrothermal activity. In conclusion, the mineralization of Intiédougou in vein styles, set up in a volcanic arc environment with a paragenesis of gold-pyrite deposit ± chalcopyrite would be controlled by the structural aspect and accompanied by hydrothermal alteration.

Mineralogy and Magmatic Processes of Cenozoic Intraplate Alkaline Volcanic Rocks of Bafang and Its Environs (Cameroon Volcanic Line, West Africa)

Alkaline basalts of Bafang and its environs are consisted of feldspars, olivines, pyroxenes and oxides which appear as phenocrysts, microphenocrysts and microcrysts. Feldspars are plagioclases (An67.97-15.84Ab69.19-30.43Or20.59-1.51) and anorthoclases (Ab68.11-61.20Or33.87-20.91An10.98-4.93). Plagioclases are the most abundant amount these feldspars. Anorthoclases appear only in mugearite (BAF 3 and BAF 37) the most differentiated of the studied alkaline-basalts. In High Magnesian basalt, (HMg-B) plagioclases are labradorites (An67.97-59.30Ab38.74-30.43Or2.75-1.60) and sanidine (An45.44-31.82Ab62.66-51.79Or5.52-2.77), whereas in Low Magnesian basalt (LMg-B) there are labrador (An67.4.75-51.96Ab44.98-33.72Or3.06-1.51), andesine (An45.44-31.82Ab62.66-51.79Or5.52-2.77), oligoclase (An26.65-15.84Ab69.19-63.57Or20.59-8.55) and anarthoclase (Ab68.11-61.20Or33.87-20.91An10.98-4.93). Olivines are magnesian (Fo86.7-50.1) and ferriferous (Fo48.8-37.8). In HMg-B, olivine are only magnesian. These olivines are chrysolites and hyalositerite. In LMg-B, olivines are magnesian and ferriferous with the predominance of ferriferous. They are chrysolites, hyalositerite and hortonolite. Pyroxenes are Ca, Mg and Fe clinopyroxenes. There are diopsides (Wo51.94-45.02En44.41-33.16Fs16.42-10.70) and augites (Wo44.88-43.64En41.03-37.04Fs18.25-14.43). Oxides are magnetites represented by ulvospinel (Usp90-75Sp2-7Mt5-23). Fractionation of ferromagnesian minerals (opaque oxide, olivine and pyroxene) is the main differentiation process. Two stages of fractional crystallization can be distinguished: the first stage comes with basanites and the second with hawaiites to mugearites. Chemical compositions of phenocrystals in studied basaltics lavas record signatures of magma recharge by pulsatory intrusions of new magma into the existing magma reservoir before the eruptions.

Occurrence of the Pholopite-Spinel-Corundum-Sapphirine Assemblages in the Contact Between Granulites and Peridotites Massif at the Beni Bousera(Morocco)

The Beni Bousera peridotite massif(Internal Rif, Morocco),5 km in width and 15 km in length,is formed in a major part of spinel-bearing lherzolite rimed by a layer of garnet-bearing peridotite(100 m thick)which is in direct contact with HP-HT granulite metamorphic rocks(16 kbar,860℃).According to recent detailed study,the shearing contact between these two formations shows the presence of serpentinite and

New Petrological and Geochemical Data of the Nephelinitic Lavas and Geodynamic Implications of Mount Etinde (Cameroon)

Mount Etinde is a Recent (<1 Ma) strombolian-type volcano located on the southern flank of Mount Cameroon. Mount Etinde lavas are distinguished on the basis of the mineralogical compositions of their phenocrysts: olivine-pyroxene, olivine-melilite and clinopyroxene-nepheline. Some magnetite and ilmenite occur as inclusions in these early phases. Mafic mineral composition indicates that fractionation involved only limited Fe-enrichment. Oscillatory, normal and sectorial zoning in clinopyroxene relates to the differentiation and fractional crystallization of the magma. Based on the primitive mantle-normalized trace and rare earth element patterns, all nephelinites have high abundances of incompatible elements (103 < La < 281;131 < Ce < 503), with negative anomalies for high field strength element Ti and low Nb/Y (0.1 - 0.2) and Rb/Y (<0.03) ratios, suggesting derivation from a similar source. Textural characteristics and mineral chemical data, as well as whole-rock compositions, suggest that the nephelinitic lavas may have been derived from basaltic magma from a heterogeneous lithospheric mantle. Geochemical modeling of major and trace element variations indicates that the Etinde lavas could not have been produced by only fractional crystallization. Pneumatolytic reaction probably affected the pyromagmas (basaltic magma composition) and was responsible for the nephelinite rocks at lower oxygen fugacity (fO2).

Geological Conditions and Hydrocarbon Accumulation Processes in the Sahul Platform, Northern Bonaparte Basin, Australia

The Sahul Platform where the Sunset-Loxton Shoals and Chuditch gas fields were discovered is located between the Timor Trough to the north and the Malita Graben to the south. These areas are located respectively 440 km and 380 km northwest of Darwin in the northern Bonaparte Basin, Australia. Based on the structural evolution of the northern Bonaparte Basin, data from the wells Loxton Shoals 1, Sunset 1 and Chuditch 1 in the Sahul Platform and Heron 1 in the Malita Graben depocentre, and the Seismic Line N11606 were used to clarify the geological conditions and reconstruct the hydrocarbon accumulation processes in the study area. BasinMod 1-D, 2-D, and 3-D software was used for modeling. The Plover Formation source rock was a poor-to-good hydrocarbon generative potential and reached the middle to late mature oil window in the Sunset-Loxton Shoals field whereas in the Chuditch field, it was an overall fair-to-good hydrocarbon generative potential, and attained the Late mature oil window. The Flamingo, and the Echuca Shoals formations source rocks in the same field were a fair and good hydrocarbon generation potential respectively, and both reached mid-mature oil window. In the Malita Graben depocentre, the Petrel (Frigate) and the Echuca Shoals formations source rocks were a poor-to-very good hydrocarbon generating potential, and had attained wet gas window at the present day. The analyses of organic matter showed that the source rocks in the study area and Malita Graben were gas prone with kerogen types II2 & III and III predominantly. The Middle Jurassic Plover Formation sandstone reservoir in the Sunset-Loxton field was a poor-to-very good quality and potential for gas beds, and it was a very poor-to-very good quality and potential for gas beds in the Chuditch field. The intensities of gas generation and expulsion were more than of oil ones either in the Sahul Platform or in the Malita Graben. The Plover, Petrel (Frigate) and Echuca Shoals formations source rocks in the wells Chuditch 1 and Heron 1, except for the Flamingo Formation in the well Chuditch 1, had higher gas and oil expelling efficiencies than the Plover Formation source rock of the wells in the Sunset-Loxton Shoals field. The hydrocarbon migrated mainly from the Upper Jurassic Frigate Shale source rock in the Malita Graben depocentre (structurally lower) to the Plover Formation sandstone reservoir in the Sunset-Loxton Shoals field during the Late Cretaceous at 66 Ma. In the Chuditch field, the hydrocarbon migration to the Plover Formation sandstone reservoir was initiated during the Late Miocene at 7.5 Ma from the Middle Jurassic Plover Formation source rock in the well Chuditch 1. Nowadays, the main migration pathways are from the southeastward and southward of the Sunset-Loxton Shoals field, and from southward and eastward of the Chuditch field, precisely from the hydrocarbon source kitchens of the Malita Graben depocentre. The traps in the Sahul Platform have been effective to receive the migrated hydrocarbon.

Implication of Paleoproterozoic Basalt Fertility Related to Mantle Plume Activity in Nassara Gold Mineralization (Burkina Faso, West Africa)

The Birimian Nassara volcanic formations are located south of Gaoua in the southern part of the Boromo belt. Within these formations is the Nassara gold deposit where mineralization is hosted at the contact between basaltic volcanic rocks and sedimentary rocks. It is with the aim of understanding the geodynamic context of the basaltic rocks and the implication of their primary gold potential in the Nassara gold deposit that this work is carried out. To achieve our objectives, 28 samples of fresh basaltic rocks were geochemically analyzed for their major and trace element compositions. These analyses show that the Nassara basalts are Fe-rich tholeiitic basalts. Rare earth profiles (La/SmN = 0.75 - 1.50;La/YbN = 0.65 - 2.18) are fairly flat and without europium anomaly (Eu/Eu* = 0.90 - 1.09), nor niobium. In the Zr/Nb vs. Nb/Th and Nb/Y vs. Zr/Y binary diagrams, the Fe-rich tholeiitic basalts of Nassara, as well as those of the Houndé and Boromo belts, are placed in the field of oceanic plateau basalts related to a mantle plume system. A gold fertility test carried out on these basalts was positive. As other studies have already shown, the genetic link between gold deposits and mantle plumes appears to be a general rule. The scenario for the Nassara gold deposit is that it is the source magma that was already more or less enriched in gold and other related elements on its way up. The remobilization of this gold would have occurred during the Eburnean orogeny with the help of metamorphic, hydrothermal and deformation phenomena to be redeposited at the level of shear zones with economic grades. Through this analysis, we show that the fertility of the initial lithologies is very important for the formation of economic size deposits in the proximal shear zones. Exploration work should now integrate this dimension to define the best targets.

Petrostructural and Geochemical Characteristics of the Metamagmatites in the External Zone of the Dahomeyides Belt: Case of the KantèSerpentinites (Northern Togo)

Thanks to detailed field investigations, microstructural and geochemical analysis and relationship with enclosing rocks, microfabrics, magmatic typology and metamorphic evolution of the Kantè sepentinites have been specified for the first time. The Kantè serpentinites in northern Togo constitute a mega-lens of ultrabasic rocks tectonically intercalated in the sericite chlorite schists of the Atacora structural unit. The brecciated, schitotose or massive rock facies are strongly marked by an S1 schistocity plane superimposed by a flat C shear plane linked to a west vergence thrusting movement. The parageneses that compose the metamagmatites are essentially serpentinous, containing plagioclase, opaque minerals (magnetite, chromite, spinel) and pyroxene porphyroblasts. These microfabrics represent relics of a probable gabbroic protolith. In fact, the geochemical characteristics of the Kantè serpentinites suggest that their magmatic typology is that of komatiites or tholeiitic basalts with oceanic arc affinities. They would have been emplaced in an active margin environment. The retromorphic evolution of the protolith corresponds to the phase of involvement in a major tangential contact during the panafrican tectogenesis.

Geochemical Mobility Associated to Gold and Base Metal Occurrences of Mangodara Sector, in Southern Burkina Faso, Banfora Greenstone Belts (West African Craton)

In the Mangodara area within the Banfora greenstone belts (Baoulé-Mossi domain of the West African Craton), our study focused on geochemical assessment of the mobility of major and trace elements. Gold and base metal occurrences are hosted in highly metamorphic felsic (metarhyolite) and intermediate (metadacite and metaandesite) formations. Common mineral assemblages made up of staurolite - kyanite - pyrophyllite are interpreted to represent the metamorphosed equivalent of aluminous hydrothermal alteration. Associated felsic and intermediate volcanic rocks are enriched in Fe2O3, K2O (metaandesite, metarhyolite) and depleted in MgO, Al2O3, CaO, P2O5, Na2O (metarhyolite) and Fe2O3, MgO, CaO (metaandesite). Al2O3 depletion in mineralized kyanite-staurotide bearing metarhyolites suggests corroded minerals. Mineralized metarhyolites show enrichment in Au, Ag, Ba, Bi, Cr, Cu, Eu, La, Mo, Ni, Pb, S, Sc, V and depletion in As Sb Co, Sn, Zn while mineralized metaandesites show enrichment in Au, Ag, As, Mo, S, Sb and depletion in Co, Sn, Zn, Bi, Cr, Cu, Eu, Ni, Pb, Sc. Ba, La, V are immobile in metaandesites. Finally, Ag, As, Sn appear as geochemical vectors for gold exploration in the study area since gold mineralization is characterized by Au + Ba + Cu + Eu + La + Mo + Ni + S association in metarhyolites and Au + S + Sb + As + Ag + Bi in metaandesites.

Identification of Groundwater Potential Zones in Samendeni Watershed in Sedimentary and Semi-Arid Contexts of Burkina Faso, Using Analytic Hierarchy Process (AHP) Method and GIS

Demand for water increases in Samendeni regarding the undertaken agricultural projects while pressure on surface water from global warming/evapotranspiration also increases. Thus, the need to evaluate the groundwater potential in the catchment is crucial as alternative supplier of water and resilience to climate hazards. The AHP was performed integrating ten influencing factors such as geomorphology, geology, soil, land use/land cover (lulc), slope, rainfall, drainage density, borehole rate & depth and piezometric level to generate groundwater potential zones (GWPZs) in Samendeni watershed (4420 km2). All the factors were processed and ranged into five (5) classes. Weight was assigned to each class of thematic layer. These thematic layers were then reclassified based on the normalized weight to be used in the calculation of groundwater potential zones (GWPZ). The final output, groundwater potential map, revealed a significant groundwater potential with very good (11%), good (31%), moderate (30%), poor (20%), and very poor (8%) of proportion. The interesting (very good, good) GWPZs in the study area are mostly in the central towards the east. The poor zones in term of groundwater potential are concentrated in the upper west region of the watershed. Besides the cross-validation with the relationship between different groundwater potential zones and the wells available in the study area, the overall accuracy was estimated to 88% provided from the result of the similarity analysis where 22 out of the 25 validation wells match with the expected yield classes of GWPZs. The statistics from that validation revealed the performance of AHP method to delineate groundwater potential zones at catchment level.

Alteration Typology and Geochemical Signatures of the Napélépéra Gold-Bearing Granodiorite in South-West Burkina Faso (West Africa)

The characterization of the relationships between mineralization and hydrothermal alteration is an essential element in understanding gold deposits. In south-west Burkina Faso, the Napélépéra mineralisation, the mobility of chemical elements and alteration-mineralization relationships were studied by means of selected core drilling and geochemical analyses using ICP-MS (Inductively Coupled Plasma Mass Spectrometry) and ICP-AES (Inductively coupled plasma atomic emission spectroscopy). The mineralised granodiorite is grey porphyroid with quartz, plagioclase, biotite and amphibole. It is metaluminous and located in the tholeiitic series. The Na2O + CaO versus Fe2O3 + MgO alteration diagram divides the samples according to alteration dominance. Chloritisation and carbonation are the main alterations. There is a relationship between gold mineralisation at Napélépéra and alteration, and the paragenesis of gold + pyrite ± carbonate ± silica ± sericite is the main characteristic. Carbonation is the result of fluid input in the shear corridor of the mineralised zone. The mass balance of comparative metals in the proximal and distal zones of the mineralisation shows the absence of metals, while As, Hg, Ag and Bi are strongly enriched from the distal zone to the mineralised zone. The oxides associated with the mineralisation are mainly NaO, SrO and CaO.

Effect of the Number and Orientation of Fractures on the P-Wave Velocity Diminution: Application on the Building Stones of the Rabat Area (Morocco)

This study is focused on two types of Moroccan rocks, among the most widely used as building stones: the calcarenite of Salé (CS) and the marble of oued Akrech (MA). The two rocks, lithologically different, show a clear contrast of their P-wave velocities (Vp): 3.90 vs 5.10 km/s at dry state and 4.29 vs 5.64 km/s at saturation. The “Artificial fractures” created in the two rock types reveal that their Vp undergo diminutions which the rates vary depending of the number and the plane orientation of the fractures. In the CS, Vp shows an increasing of cumulative diminution (Dc) according to the number of fractures, but with a variable rate of unitary diminution (Du) from one fracture to the other. This defines a linear regression with a low coefficient of determination (Dc = 10.18NbFr + 10.96;r2 = 0.87). The mode of the Vp evolution would be related to the roughness of fractures surface, which itself depends upon the petrographic nature of the calcarenite (friable structure, high porosity and heterogenous composition). The MA manifested an increasing Dc with a fairly constant rate of Du from a fracture to another, giving a regression line with a high coefficient of determination (Dc = 12.17NbFr – 10.69;r2 = 0.99). This steady diminution of Vp would be related to the granoblastic texture and the monomineral composition of the marble, which engender smoother fracture surfaces. The rates of Vp diminution also depend on the orientation plane of the fractures relative to the direction of wave propagation. The fractures parallel (θ = 0°) amplify slightly the Vp, playing a significant role of “waveguide”. The fractures oriented at 45° lead to a diminution lower than those of fractures oriented at 25° and 90°. The same trend of diminution, but at variable rates, appears on the samples of the two types of stones at dry and saturated state. This can be explained by the compressive nature of P-waves, which obey the physic laws of the transmission of the constraints in the solid mediums.

Olive Mill Waste Water Management Study by Using Principal Component Analysis

Olive mill waste water (OMWW) is a by-product issued after triturating olives. In Sfax, its management is different from urban to farming area. In this paper we treat it through a statistical analysis study during the season 2005-2006. Principal Component Analysis (PCA) and Hierarchical Classification (HC) methods are carried out on this work. Applied to variables issued from an exhaustive questionnaire including 274 mills, four Principal Components (PCs) are found to be significant, explaining 67% of the total variance. The coordinates of the 13 active variables retained by PCA were used to create a typology relative to the OMWW management and offered 7 groups of individuals which have the same characteristics, explaining 70% of the total inter-variance. This study showed that OMWW management in farming area could causes environmental problems because oleifactors haven’t controlled tanks and could evacuated OMWW on soil (causing oil deposit, waterproofing and possible asphyxia) or on public sewage network (causing corrosion, flow reduction). So, mills transfer from urban to farming areas in the form of agro-industrial complex is needed in the Sfax region.

Evaluation of Hydrocarbon Generated and Expelled from the Jurassic to Early Cretaceous Source Rocks in the Lynedoch Field, Northern Bonaparte Basin, Australia

The Lynedoch field is located on the west flank of the Calder Graben in the north-eastern Bonaparte Basin, Australia. The data from the wells Lynedoch 1 and Lynedoch 2 and Seismic Line N11809 were used to reconstruct the burial and thermal histories and evaluate the hydrocarbon generated and expelled from the Jurassic to Early Cretaceous source rocks of the study area. Basin Mod 1-D and 2-D softwares were used for modeling. The Upper Jurassic Cleia (Lower Frigate) and Lower Cretaceous Echuca Shoals formations source rocks in the well Lynedoch 1 were a fair-to-good source richness with poor hydrocarbon generating potential, showing kerogen type III and gas prone. The Middle Jurassic Plover Formation source rock in the well Lynedoch 2 was a good organic matter richness with poor hydrocarbon generative potential, the late Middle Jurassic (Callovian) Elang Formation source rock in the same well was a fair source rock with poor hydrocarbon generation potential, and the Lower Cretaceous Echuca Shoals Formation source rock in the same well was a fair-to-very good organic richness with poor-to-fair hydrocarbon generating potential, gas prone with kerogen type III, and reaching wet gas window at present day. These previous formations of the both wells generated oil at the Late Cretaceous and gas at the Early Neogene. But, only Echuca Shoals Formation source rock in the well Lynedoch 2 was able to expelled hydrocarbon at the Middle Paleogene and continued up to present day. This Formation represents fair to slightly good potential source rock in the Lynedoch field.

Pressure and Temperature Conditions Recorded by the Beni-Bousera Ultramafic Body and the Overlying Lower-crustal Rocks(Rif,Morocco)

The Beni Bousera massif(Rifean belt,northern Morocco)is predominantly composed of spinel lherzolite with subordinate garnet pyroxenite and garnet peridotite layers.It formed an antiformal dome which was overlain by graphite-sillimanite-garnet gneisses(kinzigites)equilibrated at around 1 GPa and 750℃.Within these kinzigites,kyanite-bearing basic granulites record somewhat higher PT conditions of around 1.6-2.0 GPa and 760-820℃.Garnet clinopyroxenite(either graphite-bearing or graphi-

The Bafoussam volcanic series:origin and evolution of the volcanism along the Cameroon volcanic line

The Bafoussam area in western Cameroon is part of the central Cameroon Volcanic Ligne(CVL).This study presents the mineralogy,major and trace element compositions,Sr-Pb-Hf isotopes,and new K-Ar geochronological data about mafic and felsic volcanic rocks.These rocks belong to two different series:A transitional series made of basalts,basaltic andesite,and trachytes and an alkaline mafic series with basalts,hawaiites,and basanites.New age data show that the transitional series belongs to the oldest part of the CVL and was emplaced between 47 and 35 Ma.The alkaline volcanism is younger,with ages ranging from10 to 4.5 Ma.Magmatic evolution in both series is accomplished through a fractional crystallization process,with the removal of olivine and clinopyroxene,while plagioclase does not seem to be a major crystallizing phase.All the samples are enriched in incompatible trace elements,but the rocks from the alkaline series have more fractionated REE patterns and high Nb content compared to the transitional mafic lavas.Alkaline lavas have lower initial^(87)Sr/^(86)Sr and higher^(176)Hf/^(177)Hf and Pb isotopic ratios than the transitional lavas.Low La/Nb and high^(87)Sr/^(86)Sriratio are among chemical characteristics that show that some samples from the transitional series have interacted with a crustal component during their evolution in the crust.They cannot be used for discussing the mantle source of the volcanic rocks from this series.Trace elements show that primary magmas for both series formed in a garnet-bearing mantle source,with higher partial melting degrees(3-5%)for the transitional magmas than for the alkaline magmas(<2.5%).Combining trace elements and isotopic ratios,we show that the Bafoussam lavas formed from two different mantle sources.Transitional magmas formed from a pyroxenite-bearing enriched mantle with low Pb isotopic composition.This mantle source is present in all the oldest lavas from the CVL.Alkaline magmas formed from an HIMU-like mantle source,different from the Mt Cameroon HIMU mantle source.The depleted asthenospheric mantle is not involved in the Bafoussam magmatism and the two mantle sources are probably located in the lithospheric mantle,in agreement with recent geophysical models presenting the CVL as a consequence of the partial melting of the lithospheric mantle in response to edge convection along the margin of the Congo craton.

A Typical Sublithospheric Mantle beneath the Tello Region, SE-Ngaoundéré (Cameroon Line)

Peridotite xenoliths,raised to the surface by alkaline basalts or kimberlites,provide us direct information on the processes and composition of the upper mantle.They are the major source of information on the state of stress,pressure and temperature in the deep mantle.They are thus a source of petrological and geochemical information that is generally not available on the Earth’s surface.Fresh spinel-lherzolite xenoliths exhibit a protogranular components of the Tello volcano.The Tello is the continental sector of the Cameroon Line,located in the South East of the town of Ngaoundéréat 75 km approximately between(N7°13’,N7°14’)and(E13°40’and E13°60’).Minerals’composition of the xenoliths is~64%olivine,~24%orthopyroxene,~11%clinopyroxene and~1%spinel.Significant variation in(Cr/Cr+Al)of the system shows the reciprocal nature of the spinel solution.The Tello spinel lherzolites show internal chemical homogeneity and represent a normal upper mantle.Their mineral chemistries suggest equilibrium condition of 830°-925°and 1.4 GPa-2.3 GPa.These data suggest that there is good correlation bracket between increasing activity of Al2O3 and decreasing of practionning of TiO2 into spinel.The AlIV and AlVI contents vary by 0.05-0.2 and 0.03-0.2 respectively.The majority of samples caracterise the lithospheric mantle.

郯庐断裂带(安徽段)及邻区的动力学分析与区域构造演化

依据区域构造层次划分,采用构造筛分法,层层深入,层层筛分,确定发生于各个不同时代地层/岩层内的断裂活动的同期及叠加的应力场特征。综合所有的同期应力场特征及辅以叠加的应力场特征来验证,从而确定了一个连续的、完整的断裂活动的应力场演化序列;结合区域构造变形特征分析,阐明郯庐断裂带(安徽段)的构造演化。应力场分析显示:晚三叠-早侏罗世应力场为北北西—南南东或近南北向挤压,属古特提斯构造域,断裂发生同造山走滑;早白垩世早期,应力场为北西—南东向挤压,断裂发生左行走滑运动,中国东部处于西环太平洋构造域;早白垩世晚期—古新世(始新世),区域发生北西—南东向伸展作用,断裂处于伸展断陷作用阶段;新生代,受区域上近东西向的挤压作用影响,断裂发生挤压逆冲兼右行走滑作用。