Petrography, Geochemistry and Relative Chronology of Quaternary Volcanic Formations in the Mermoz and Fann Sectors, West Senegal

Detailed work on Quaternary volcanism has been carried out in the Mermoz and Fann sectors of western Senegal. In the Mermoz sector, the main emission zone is a collapsed crater located at the intersection of three major fractures: NE-SW, NW-SE and N-S. The lithological succession in this Mermoz sector comprises, from bottom to top: 1) a substratum with at its base Eocene limestones on which lie Quaternary sands surmounted by stratified tuffs; 2) a vesicular ball dolerite which deforms the stratified tuffs; 3) a dark early brec-cia; 4) two generations of basanites: the first is vesicular, the second non-vesicular; 5) a clear intermediate breccia and finally 6) a late breccia. The Fann sector contains several emission zones, most of which are currently located in the ocean. The lavas may have reached the coast through E-W and NE-SW faults. The lithological succession includes from bottom to top: 1) scoria-rich early volcanic breccias; 2) a first generation of non-vesicular me-socrate dolerite (D1); 3) a second generation of melanocrate vesicular dolerite (D2); 4) basanites and finally 5) a late breccia. The geochemical characteristics of the lavas studied are compatible with a very enriched and very deep magmatic source of the garnet lherzolite type located in the lower mantle. The magma from this source would have risen in the form of mantle plumes through major NE-SW and E-W faults in a continental intraplate context.


Introduction
The west extremity of Senegal (Figure 1) was, at the end of the Tertiary (Miocene) International Journal of Geosciences and at the beginning of the Quaternary, the seat of an important basic magmatism.This magmatism is closely related to the N-S oriented brittle tectonics whose manifestations, are linked to the opening of the Atlantic [1].
Quaternary magmatism, the subject of this article, outcrops only in the Dakar region, in its northwestern part, and is represented by two craters: a main crater located at Mamelles and another secondary located at Mermoz ( [6] [13] [16] [19]).
However, these authors also suggested the existence of other exit points at "Toundis", at the tip of Almadies-Yoff and at Fann, but no research has been carried out to confirm or refute this hypothesis.
Thus, the aim of this article is to study in detail the sectors of Mermoz-Plage de Fann résidence (Mermoz sector) and Place du Souvenir-Université (Fann sector) in order to characterize the different volcanic formations, establish a relative chronology and highlight a secondary exit point at Fann.
In addition, new geochemical data have been implemented in the two sectors to better characterize origin of the magmas and to propose a geotectonic context.

Geological Context
The west part of the Senegalese-Mauritanian basin has known during geological history, three major periods of volcanic activity: -The first period, from the Upper Cretaceous to the Paleocene, is recognized by drilling [20].It is represented by hornfels with enclaves of microsyenits of the Léona dome, trochoid tuffs in the Dakar region.The Léona dome was set up at International Journal of Geosciences the beginning of the Maastrichtian [2].The trochoid tuffs would come from probably submarine volcanic projections.
-The second period of the Tertiary age is represented by around thirty volcanic bodies, most often small in size, and at depth by numerous vein bodies encountered in wells and boreholes ([2] [3] [10] [21]).There are no longer any parts of well-preserved aerial apparatus, and numerous outcrops in fact correspond to ancient sub-surface intrusions.About ten diatremes and numerous sills and tuff dykes have been recognized [22].Witnesses of Tertiary volcanism fall into four geographical groups (Figure 2(a)): • Dakar system, where the most extensive basaltic outcrops that may correspond to flows are found (Cap Manuel, Gorée, Madeleine Islands); • Rufisque unit, where tuffs are abundant; • Ndiass dome unit, which includes many small lava and tuff intrusions, linked to faults; • Thiès region unit, where the main outcrop, that of Diack, probably corresponds to the remains of a lava lake established in a maar [16].
-The third period is of the Quaternary age and is located only in the head of the Cap-Vert peninsula (Figure 2(b)).This period gave birth to the Mamelles volcano which is a polygenic device built over several phases of activity of different dynamism.It shows successively: • The oldest known episode of the Mamelles: it is a basalt flow whose top is particularly coarse; which gives it a doleritic appearance macroscopically; • The Strombolian episode whose products are tuffs and cinerites, surmounted

Materials and Working Method
Several field trips during low tides were carried out on the basis of available maps and geological sections.The works carried out on outcrops of excellent quality consisted of identifying the different lithologies, analyzing the geometric relationships between facies in order to define a relative chronology, making macroscopic petrographic descriptions of the samples, making structural measurements and finally taking samples for making thin sections and for geochemical analyses.The microscopic petrographic studies were made using a microscope equipped with a camera at the Department of Geology and at the Fundamental Institute of Black Africa (IFAN) of Cheikh Anta Diop University of Dakar (Senegal).Analyzes of major and trace elements on whole rocks were carried out in Canada by the ACTLABS laboratory (Laboratory No.A21-02232).The major elements were analyzed by the FUS-ICP method and the trace elements by the FUS-ICP and FUS-MS methods.
• Marl limestones These rocks (Figure 3(a)) outcropping at the northern end of the "Water front" hotel mark the end of the Tertiary.They underwent thermal metamorphism in contact with a dolerite intrusion.They are also observed as enclaves in dolerites and basanites.
• Infra-basaltic sands About 5m thick (Figures 3(a)-(c)), these sands dating from the Quaternary [23] overlie the marly limestone.The contact between these two formations is highlighted for the first time in the west coast of Dakar.Infra-basaltic sands are a typical formation of a high-energy littoral environment [23].on either side of the volcanic apparatus of Mermoz and around the Radisson hotel.These rocks top the infra-basaltic sands and show well-stratified levels with alternating clear beds and brown beds.They are made up of small elements joined together by very fine cement, with a sedimentary appearance.To the north of the Water Front hotel, the tuffs show intersecting stratifications in their upper part.These cinerites are sometimes reddened and their thickness decreases towards the south.

• Dolerite in balls
The dolerite of the Mermoz volcano is cut into balls of various sizes that can exceed 3 m in diameter.The best outcrops are located near the "Mermoz swimming pool" and north of the Water Front Hotel (Figures 3(a)-(c)).Other outcrops are located in the cove of Ouakam.[16] locates this dolerite under the infra-basaltic sands.They dated it at 1.4 ± 2 Ma and named it a medium volcanic unit, which would correspond to a single flow having undergone lateritic alteration before its burial under the infra-basaltic sands.Our new observations do not militate in favor of this conclusion.Indeed, we noted that the dolerite in balls intersects the Tertiary carbonate formations, the infra-basaltic sands and the stratified tuffs.The bedding planes of these tuffs have been uplifted and deformed by dolerite into balls (Figure 3(b)).This dolerite contains enclaves of metamorphosed sedimentary rocks.It is observed itself in the form of an enclave in the basanites of the Mermoz volcano.Under the microscope, the balled dolerite shows an intergranular doleritic texture marked by contiguous laths of plagioclase what create spaces occupied by pyroxenes (Figure 3(d)).Plagioclases are present inmacrocrystals and microcrystals.Plagioclase macrocrystals show clear polysynthetic twins.Interstitial pyroxenes also occur in macrocrystals and microcrystals.Less abundant pyroxene macrocrystals are automorphic to sub-automorphic and sometimes altered to opaque minerals.

• The scoriaceous breccia
The scoriaceous breccia forms a thick layer marked by a very advanced alteration, which gives it a characteristic red color.It consists of fragments of scoria and large-grained dolerite (dolerite in balls).The explosions at the origin of the scoriaceous breccia preceded the placement of the basanites.Indeed, the breccia is cut by numerous basanite dykes and sills (Figure 3(e), Figure 3(f)).

• Basanites
They were set up from several exit points that are difficult to highlight.Some of these emission points are located in the ocean and the lava could only reach the mainland through fractures, especially those oriented NE-SW and E-W.At the level of the Mermoz volcano, at least two generations of basanites have been clearly identified (Figure 4(a), Figure 4(b)).The basanites cut the dolerite in balls, the infra-basaltic sands and the stratitified tuffs.These tuffs come from the Mamelles volcanic apparatus.The first generation of basanite (basanite1) is vesicular and cut into vertical prisms and elongated slabs.The main outcrop of this  This breccia mainly outcrops in the eastern part of the Mermoz volcano in the form of a small mound about 4 m high (Figure 5(c)).These breccias are the result of phreatic explosions and are located at the limit of the stratified tuffs of the Mamelles volcano.Clay cement gives it a characteristic beige color.This cement brings together various elements whose size can reach 120 cm.Among these elements, we distinguish coarse-grained dolerites, vesicular basanites, massive basanites and basaltic slags.The breccia cuts very clearly the infra-basaltic sands and the stratified tuffs.On the other hand, its chronological relations with the basanite2 with which it is in contact are not very clear.But we can think that the inclination of this basanite towards the SW was caused by the explosions at the origin of the breccia.This interpretation is supported by the presence in the breccia of basanite elements whose prismatic flow resembles that presented by basanite 2.
• The late breccia It represents the ultimate manifestation of the Mermoz volcano.The explosions occurred within the basanites themselves along fractures N140 (Figure 5(d)) and N50.The breccia is generally monogenic and essentially consists of basanite fragments joined by tuffaceous cement.However, there are small fragments of coarse-grained dolerite in places.

Fann Sector (Remembrance Square-IFAN Cheikh Anta DIOP)
All the formations outcropping in this sector, which show from bottom to top: stratified tuffs, an early volcanic breccia, and dolerites belonging to two generations (D1 then D2), basanites and finally a late volcanic breccia.

• Stratified tuffs
These tuffs outcrop a few meters away, at the height of the Embassy of Mali (Figure 6      The fragments present the same mineralogical composition consisting of olivine, pyroxenes, plagioclases and opaque minerals.

Tectonic Structures
The placement of volcanic rocks in the Mermoz and Fann sectors is intimately linked to tectonic structures.

The Mermoz Sector
The main emission zone is a collapsed crater located at the intersection of three International Journal of Geosciences large fractures: NE-SW (N40), NW-SE (N140) and N-S (Figure 11(a), Figure 11(b)).The NE-SW fractures allowed the placement of basanites, which are moreover oriented in this direction (Figure 11(a)).In the crater, the NE-SW alignment of breccias within the basanites themselves undoubtedly indicates that the fractures oriented in this direction were the site of an explosion.In addition, in the eastern part of the volcano, the explosions at the origin of the clear volcanic breccia also occurred along a NE-SW axis.The NW-SE fractures, for their part, are essentially associated with the setting up of the late breccias (Figure 11(c)), which can be followed over several tens of meters in the N140 direction.
All these fractures are vertical to subvertical.Their directions of movement are not always easy to determine.However, dextral and sinistral movements were noted in NE-SW shear corridors (Figure 11

The Fann Sector
No emerged crater related to volcanic products from the Fann sector was observed.The main exit points for basanites are currently in the ocean.These were only able to reach the coast via the E-W faults (Place du Souvenir, Figure 13(a)) or the NE-SW faults (Residence of the Embassy of Mali, Figure 13(b)).The mode of emplacement of the D1 dolerite is not always clear, but the way in which it extends offshore seems to indicate that these are veins emplaced in NE-SW and E-W fractures.The second generation dolerites are also emplaced through the fractures oriented NE-SW, E-W and N-S, which mainly affect the D1 dolerite (Figure 13(d)).These fractures reappeared later and mainly affected the walls of the D2 dolerite veins (Figure 13(c)).A statistical study of the main fracture directions indicates a predominance of those oriented NE-SW and NW-SE (Figure 14(a)) and a predominance NE-SW (Figure 14(b)) for the veins.

Geochemistry
Geochemical analyzes in major and trace elements were carried out on 9 samples, 6 of which belong to the Fann sector and 3 to the Mermoz sector (Table 1).
The quaternary basanites in this study show the same geochemical characteristics as the quaternary basanite analyzed by [26] in the Mermoz sector (Figure 16, Figure 18(a), Figure 18(b)).
Furthermore, the Quaternary lavas from this study were compared to the Quaternary lavas from well-known sites such as Fogo Island in Cape Verde [27] and Réunion Island in France [28].

Magmatic Affinity
The dolerites and basanites of the Fann and Mermoz sectors are located in the alkaline basalt domain in the diagrams of [29] (Figure 19(a)) and [30] (Figure 19(b)).

Discussion
The Regarding geochemistry, our study confirms the alkaline affinity of the quaternary volcanic rocks suggested by former authors but underlines a heterogeneity of magmatic source for the basanites and the dolerites.The characteristics of the studied lavas are compatible with a very deep and enriched magmatic source.This is of the garnet lherzolite type located in the lower mantle.The magma would have risen in the form of mantle plumes in favor of the NE-SW and E-W regional faults.This proposal is opposed to that of [19] who had suggested a rise to the surface of material resulting from the partial melting of the upper mantle.On the other hand, it is close to the conclusions issued by [18] based on the Th/Ta and 4He/3He ratios, which locate the source of the quaternary lavas in the lower mantle.
According to the work of [18] the quaternary basanites of Senegal and Cape Verde were set up thanks to the same hot spot located vertically to Cape Verde.However, according to our study, the lavas of Senegal are less enriched in rare earths.The question that arises is: how can lavas from the same source have differences in rare earth compositions?Before examining this question, we will compare the values of the ratios of the highly hygromagmaphilic elements of the basanites of Cape Verde and Senegal.These values are preserved during fractional crystallization processes and partial melting processes: they are therefore characteristics both of the initial liquids and of the mantle sources ( [37] [38]).The Th/U, Th/La, La/Ta and Th/Hf ratios (Table 2) of the quaternary basanites of Cape Verde and Senegal are very similar.This seems to show that they come from sources of globally identical chemical composition.The greater enrichment in REE of the quaternary lavas of Cape Verde cannot be explained by the classical magmatic processes of the genesis of oceanic basalts.Indeed, the genesis of basalts rich in rare earths by partial melting, at low rates, of a mantle source of the same composition as that at the origin of "normal" basalts, can be excluded [39].Several fluid-based hypotheses have been proposed to explain the high rare earth contents of basalts around the world.In Hawaii, the abnormal rare earth contents would be the effect of the metasomatic fusion of mineral phases rich in halogens + CO 3 + S + K [40].At Tahaa (Polynesia), the successive crystallizations Table 2. Ratio of the hygromagmaphile elements of the basanites of dakar and those of the cape verde islands. of phosphates with no Ce anomaly, of hydroxides with a positive Ce anomaly, then of carbonates, which would result from the percolation of fluids during an advanced stage of magmatic crystallization, would make it possible to explain the high rare earth contents of basalts [41].Furthermore, ( [42] [43]) proposed for the basalts of Hawaii and those of Polynesia, the hypothesis of a prolonged subaerial alteration accompanied by pedogenesis to explain the enrichment in rare earths of these lavas.
In conclusion, the similarity of the ratios of the hygromagmaphilic elements of the basanites of Cape Verde and Senegal could suggest that these lavas come from a single source.Rare earth enrichment of Cape Verde basanites could be linked to the action of metasomatic fluids as for basalts of Hawaii and Polynesia.

Conclusions
The activities of the quaternary magmatism of the Mermoz-Fann zone are associated with tectonic structures having facilitated their establishment.
The substratum of Quaternary volcanism in the Mermoz-Fann zone is made up of: Tertiary marly limestones at the base, covered by Quaternary sands (infra-basaltic sands) which are surmounted by stratified tuffs.
However, it should be noted that this is the first time that the contact between marly limestone and infra-basaltic sands has been highlighted by this study.
In the Mermoz sector, magmatic activity begins with the formation of vesicular dolerite flowing in balls, which deforms the stratified tuffs.This is followed by the emplacement of a scoriaceous breccia.Then, two basanite flows were emplaced in two episodes separated by a period marked by explosions and the emplacement of intermediate breccias.
The Fann sector is largely dominated by the intrusion of two generations of dolerites: the first generation of dolerite (D1) is mesocrate in color and devoid of vesicles, while the second generation of dolerite (D2) is melanocrate and vesicular.
Volcanic activity in the Fann sector is also marked by the establishment of basanite flows with numerous vesicles and vacuoles sometimes filled with carbonates.
Finally, the volcanic activity ends with explosions that affected all the sectors studied.This activity is at the origin of the brecciation of basanites and dolerites.
In addition, the stratified tuffs are the only rocks of the Mamelles volcano intersected by the basanites of Mermoz-Fann.We believe that volcanic products of the Mamelles and those of Mermoz-Fann belong to the same quaternary eruptive system with a main emission zone located at the Mamelles and several

Figure 1 .
Figure 1.Location maps: (a) Location map of the West African continental margin (after [2]).(b) Location of Senegal on the map of Africa.

Figure 3 .
Figure 3. Lithology and petrography of the Mermoz sector: (a) tertiary limestone (Cal.) at marly levels covered by infra-basaltic sands (San.), all intersected by a dolerite intrusion; (b) intrusive ball dolerite in an assemblage consisting of infra-basaltic sands (bottom) and stratified cineritic tuffs; the stratification plan of the tuffs are deformed by the intrusion.(c) covered infra-basaltic sands and cinerites, cut by a basanite flow (Bas.);(d) texture of balled dolerite showing strongly weathered pyroxene (Px) phenocrysts and plagioclase (Pl) and (e) and (f) basanite in sill in the breccia (Br.).

Figure 5 .
Figure 5. Lithology and petrography of the Mermoz sector: (a) porphyritic microlithic texture of vesicular basanite; (b) porphyritic microlithic texture of massive basanite; (c) clear breccia in contact with massive basanite cut into prisms slightly dipping to the SW and (d) breccia resulting from a late explosion within the massive basanite according to N140 fractures.
(a)).They show stratification with clear beds about 30 cm thick.The strata show alternating yellow to pinkish very fine grained beds.These tuffs are cut by doleritesills.•Scoriaceous brecciasThese breccias outcrop at the "Place du Souvenir" (Figure6(b)).They are monogenic essentially made up of slag elements united by clay cement corresponding to the pulverized substratum (infra-basaltic sands and stratified tuffs).The slag elements are generally angular and their size varies between 5 and 15 cm.The volcanic breccias are cut by dolerite and basanite veins.Microscopic observation of the slag fragments reveals a porphyritic microlithic texture with olivine phenocrysts (Figure6(c)).The primary paragenesis consists of plagioclases, olivines and pyroxenes.The sometimes zoned olivine phenocrysts are very often altered to iddingsite, which forms reddish spots.•DoleritesThey outcrop along the coast, between the "Place du Souvenir" and the Hotel Térou-bi.Two generations of dolerites have been observed.The first named D1 is omnipresent and sometimes extends inside the ocean thus constituting points
are very strongly opacified (Figure 8(c), Figure 8(d)).• Basanites They outcrop between the point of the place of remembrance and that of the residence of the Embassy of Mali.The basanite of the Place du Souvenir was emplaced along an EW axis that could correspond to oceanic transform faults.It is cut into a prism (Figure 9(a)) whose diameter varies between 15 and 80 cm.

Figure 9 .
Figure 9. Lithology in the Fann sector: (a) subvertical basanite prisms observed at Place du Souvenir; (b) dykes (Dy.) of basanite which crosscut the monogenic breccia and intrude as sills (sil.) in the dolerite; (c) enclave of gabbro (Gab.) in prismatic basanite; (d) enclave of tuff in prismatic basanite; (e) intrusion of vacuolar basanite in the D2 dolerite and development of a contact metamorphism aureole in the latter; and (f) intrusion of vacuolar basanite into Dolerite D1.
(c)), while essentially sinistral movement was recorded on NW-SE and NS fractures.Thin sections of intensely fractured rocks show a very clear orientation of feldspars.After the setting of late breccias, the Mermoz volcano underwent a major collapse illustrated by streaks of landslides plunging 50˚ at N140 (Figure 11(c)).This collapse is at the origin of a depression at the current location of the crater.The volcanic rocks of the Mermoz sector recorded a final phase of brittle tectonics at the origin of E-W structures which intersect the N40, N140 and NS fractures.A statistical study of the main fracture directions indicates a predominance of those oriented NE-SW and NW-SE (Figure 12(a)) and a predominance NE-SW for the veins (Figure 12(b)).

Figure 11 .
Figure 11.Deformations in the Mermoz sector: (a) N-S trending fracture at the eastern end of the crater; (b) N10 trending fracture zone at the eastern edge of the crater.(c) Fracture N140 that facilitated the placement of the late breach; (d) the elongated massive basanite flow along the N40 direction; (e) Sinistral N40 strike-slip observed in a thin section of massive basalt with plagioclase oriented in the same direction and (f) sinistral N40 shear observed in a thin section with olivines and plagioclases oriented in the same direction.

Figure 12 .
Figure 12.(a) Rose diagram showing the preferential orientation of the fracture planes in the Mermoz sector and (b) rosette showing the preferential orientation of the directions of the veins in the Mermoz sector.

Figure 13 .
Figure 13.Deformations in the Fann sector: (a) outcrop of prismatic basanite (Bas.)extending into the ocean in an E-W direction; (b) extension into the ocean of the point of the Embassy of Mali oriented in a direction N40; (c) faulted contact between dolerite D1 and dolerite D2 and (d) extension into the ocean of a dolerite vein oriented N40.

Figure 14 .
Figure 14.(a) Rose diagram showing the preferential orientation of the planes of the diaclases in the Fann sector and (b) rose diagram showing the preferential orientation of the vein directions in the Fann sector.4.3.1.The Major Elements In the Fann sector, the SiO 2 , Al 2 O 3 , TiO 2 , MgO, Fe 2 O 3 (T), CaO, Na 2 O, K 2 O, P 2 O 5 (a), Figure 15(b), Figure 15(e), Figure 15(f)).

Table 1 .
Majors and traces elements of the totals dolerites and basanites rocks of the Fann and Mermoz sectors.International Journal of Geosciences

Figure 15 .
Figure 15.Diagrams of SiO 2 , TiO 2 , Al 2 O 3 , CaO, Na 2 O, K 2 O and P 2 O 5 as a function of Mg# showing the variations in composition of the major elements of the dolerites and basanites of the Fann and Mermoz sectors.
Figure 16.Diagrams of SiO 2 , TiO 2 , CaO, Na 2 O, FeO 3 (T), K 2 O and P 2 O 5 as a function of Mg# showing the comparison of Quaternary lava from Senegal with that of equivalent lava from the Fogo islands (Cape Verde) and Réunion (France).

4. 3 . 5 .
Figure 18.(a) Spectra of rare earths normalized to NWA974 chondrites[26] and (b) multi-element diagrams of incompatible elements normalized to the primitive mantle[27] of the quaternary lavas of this study compared to those of equivalent lavas in the islands of Fogo in Cape Verde and Réunion in France.

Figure 20 .
Figure 20.Diagrams: (a) La/Yb (ppm) as a function of La (ppm) from[31] and (b) TiO 2 /Yb (ppm) as a function of Nb/Yb (ppm) from[32] showing the nature, heterogeneity and depth of the magmatic source of the magmatic rocks of the Fann and Mermoz sectors.