The geometrical and chronological relationships between the different formations of the Mermoz sector show from bottom to top: 1: marly limestone; 2: infra-basaltic sands; 3: bedded tuffs; 4: balled dolerite; 5: scoriaceous polygenic breccia; 6: basanites; 7: clear polygenic breccia rich in volcanic bombs and clayey cement and 8: late breccia most often monogenic.

· 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] .

· Bedded tuffs

These are cinerites, which outcrop along the coast (Figure 3(b), Figure 3(c)) 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

basanite is located in the western part of the Mermoz volcano and continues into the ocean in a NE-SW direction.

The second generation of basanite outcrops in the eastern part of the Mermoz volcano where it is oriented NE-SW. This second generation of basanite (basanite2) is globally non-vesicular. It is presented in vertical prisms whose superficial parts are fractured and cut into horizontal plates. In the western part of the volcano, the second basanite can be injected in the form of veins and sills into the first basanite (Figure 4(a), Figure 4(b)) and into the tuffs. Basanites may contain enclaves of peridotites (Figure 4(c)) dolerites (Figure 4(d)) volcanic bombs (Figure 4(e)) and cinerite (Figure 4(f)). Microscopic observation of the vesicular basanite shows a porphyritic microlithic texture (Figure 5(a)).The mineralogical composition consists of a primary paragenesis with plagioclase, pyroxene and olivine and a secondary paragenesis with opaque minerals. Plagioclases form microlites that define a discrete planar orientation. Pyroxenes occur in microlites. Olivine occurs in phenocrystals and microcrystals. Olivine phenocrysts are automorphic to sub-automorphic and sometimes completely altered to iddingsite (Figure 5(a)). Microscopic observation of the massive basanite shows the same texture and the same mineralogical composition as the vesicular one. The minerals of this basanite are little altered and show no gaps (Figure 5(b)).

· The clear breccia with clay cement

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.

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(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 breccias

These breccias outcrop at the “Place du Souvenir” (Figure 6(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 (Figure 6(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.

· Dolerites

They 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

like the one in front of the residence of the Embassy of Mali. Dolerite D1 shows a lower part cut into a prism (Figure 6(d)) and a superficial part altered into a ball. She is often affected by numerous fractures. Very rarely vesicular, D1 dolerite always shows a micrograined texture with a medium grain and a light gray appearance which contrasts with the melanocratic aspect of the second generation of Dolerite (D2) which is vesicular everywhere. This D2 dolerite intersects D1 in the form of tubes (Figure 6(e)), veins (Figure 7(a)), stringers (Figure 7(b))

and sills (Figure 7(c)).

The contact between D1 and D2 is marked by a halo of contact metamorphism. Between the “Place du Souvenir” and the tip of the residence of the Embassy of Mali, dolerites D1 and D2 are cut by a vacuolar basanite. A halo of thermal metamorphism has developed in these dolerites (Figure 7(d)). The IFAN Cheikh Anta Diop beach offers many outcrops of D2 dolerite, which can also occur in the form of corded lava (Figure 7(e)). There are also late injections of less vesicular D2 dolerite (Figure 7(f)). Under the microscope, D1 dolerite presents an inter-granular doleritic texture marked by contiguous plagioclase laths making between them interstices occupied by pyroxene and olivine minerals (Figure 8(a)) and a sub-doleritic texture with laths of partially contiguous plagioclases. D2 dolerite shows under the microscope a coarser intergranular doleritic texture (Figure 8(b)) and a subophitic texture. The mineralogical composition of the D1 and D2 dolerites is identical. The primary paragenesis consists of plagioclases, pyroxenes and olivines. The secondary alteration paragenesisis formed of iddingsite, chlorite, epidote and opaque minerals. The mineral mode varies according to the samples with a predominance of either plagioclase or pyroxene.

Moreover, in the blades cut on the samples of dolerites having undergone thermal metamorphism in contact with the basanite, the ferromagnesian minerals

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.

The exit point is therefore currently inside the ocean. During its advance, the basanite came up against a doleritic relief and spread on both sides of it. However, part of the lava was injected into the underlying tuffs, thus giving rise to veins (Figure 9(b)). This basanite contains enclaves of gabbros (Figure 9(c)) and metamorphosed tuffs (Figure 10(d)). Its superficial part is often very vesicular.

Between the Place du Souvenir and the tip of the residence of the Embassy of Mali, the basanite is very vacuolar (Figure 9(e)). As in the previous case, the outflow point of the flow seems to be at sea. The lava then abutted against the doleritic reliefs (D1 and D2) metamorphosing them (Figure 9(f)). At the tip of the residence of the Embassy of Mali, we note that the placement of the basanite was facilitated by a fault oriented N50 and the lava after having abutted against the hitherto doleritic tip propagated laterally from either side of it. The vacuoles present in this basanite are generally sub-circular but can become elliptical at the level of contact with the dolerite. They are often empty but sometimes filled with carbonate. This carbonate is in the form of a nodule or fine whitish needles. The basanite flow contains enclaves of dolerites and tuffs. Microscopic observation of the basanite in prisms shows a porphyritic microlithic texture (Figure 10(a)). Olivine phenocrysts are observed embedded in a mesostasis consisting of microlites of plagioclases, pyroxenes, olivines and secondary minerals (epidote, opaque minerals). The vacuolar basanite shows under the microscope a porphyritic microlithic texture with a doleritic tendency (Figure 10(b)). The primary paragenesis consists mainly of plagioclase, pyroxene and olivine while the secondary alteration paragenesis is represented by carbonates and opaque minerals. The pyroxene phenocrysts show two very sharp 90˚ cleavage directions and corroded surfaces. Plagioclases, which are very abundant, form laths of microlites with sometimes polysynthetic twins. The very abundant carbonates, recognizable by their diamond-shaped cleavage, are destabilization products of pyroxenes and/or plagioclases. They are often associated with opaque needle-like minerals. The gabbro enclaves present in the prismatic basanite show a grainy texture and a mineralogical composition consisting of clinopyroxenes, plagioclases and olivines (Figure 10(c)).

· The Late breccia

A second generation of volcanic breccia outcrops south of “Place du Souvenir”. This breach could represent the ultimate manifestation of volcanic activity in the Fann sector. It is polygenic and is essentially made up of heterometric fragments of dolerites and basanites united by clay cement (Figure 10(d)).The microscopic study of the dolerite elements shows an intergranular texture (Figure 10(e)). The basanite fragments show a porphyritic microlithic texture (Figure 10(f)).

The fragments present the same mineralogical composition consisting of olivine, pyroxenes, plagioclases and opaque minerals.

The main emission zone is a collapsed crater located at the intersection of three 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(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)).

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.

In the Fann sector, the SiO₂, Al₂O₃, TiO₂, MgO, Fe₂O₃(T), CaO, Na₂O, K₂O, P₂O₅ and Mg# values vary respectively from 52.5% - 53%, 14.6% - 14.8%, 1.5% - 1.6%, 5.2% - 6.3%, 10.7% - 11%, 8.3% - 9%, 3.9% - 4%, 0.5% - 0.7%, 0.31% - 0.41% and 48.5% - 53.9% in dolerites and 48.3% - 49%, 14.2% - 14.8%, 1.7% - 1.9%, 7.1% - 8.2%, 9.8% - 11%, 8.7% - 10.1%, 3.6% - 4.2%, 1.2% - 1.5%, 0.44% - 0.53% and 57.2% - 59.5% in basanites. It should be noted, however, that D1 dolerite (MF177B) and D2 dolerite (MF157B) are slightly richer in SiO₂ (52.9% - 53% compared to 52.5%) and in K₂O (0.6-0.7% against 0.5%) but slightly less rich in TiO₂ (1.5% against 1.6%), Na₂O (3.9% against 4%) and P₂O₅ (0.3% against 0.4%) than D2 dolerite (MF221B) (Figure 15(a), Figure 15(b), Figure 15(e), Figure 15(f)).

In the Mermoz sector, the SiO₂, Al₂O₃, TiO₂, MgO, FeO(T), CaO, Na₂O, K₂O, P₂O₅ and Mg# contents vary respectively from 50.6% - 52.4%, 14% - 15.8%, 1.8% - 2.9%, 3.3% - 5.6%, 12% - 12.9%, 7.3% - 8.1%, 4.3% - 4.8%, 1% - 1.5%, 0.3% - 0.5% and 33.8% - 47.9% in the dolerites and equal respectively to 49.4%, 14.7%, 1.9%, 7.7%, 10.8%, 8.5%, 4.7%, 1.5%, 0.58% and 58.7 in basanite.

Fann dolerites show slightly higher values of SiO₂ (52.5% - 53% versus 50.6% - 52.4%), CaO (8.3% - 9%) and Mg# (48.5 - 53.9 against 33.8 to 47.9) but lower values in TiO₂ (1.5% - 1.6% against 1.8% - 2.9%), Fe₂O₃(T) (10.7% - 11% against 12% - 12.9%), Na₂O (3.9% - 4% versus 4.3% - 4.8%) and K₂O (0.5% - 0.7% versus 1% - 1.5%) than the Mermoz dolerites (Figure 15(a), Figure 15(b), Figure 15(d), Figure 15(e), Figure 15(f)). In addition, dolerites from both sectors show poor correlations with Mg#, negative for TiO₂, Na₂O and K₂O (Figure 15(b), Figure 15(e), Figure 15(f)) and positive for CaO (Figure 15(d)). These

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

geochemical characteristics suggest that the dolerites of the two sectors could come from the same magmatic source. Furthermore, Fann basanites are also slightly richer in CaO (8.7% - 10.1% vs. 8.5%) but slightly poorer in SiO₂ (48.3% - 49% vs. 49.4%), Na₂O (3.6% - 4.2% versus 4.7%), K₂O (1.2% - 1.5% versus 1.53%) and P₂O₅ (0.4% - 0.5% versus 0.6%) than Mermoz basanite (Figure 15(a), Figures 15(d)-(g)). It should also be noted that the basanites of the two sectors show very similar chemical compositions, probably indicating a single magmatic source. Finally, the dolerites of the two sectors compared to the basanites of the two sectors are richer in SiO₂ (50.6% - 53% against 48.3% - 49.4%) but poorer in MgO (3.3% - 6.3% against 7.1% - 8.2%) and Mg# (33.8% - 53.9% versus 57.2% - 59.5%) (Figure 15(a)). These chemical characteristics as well as the absence of correlations between dolerites and basanites and the presence of enclaves of dolerites in basanites suggest different magmatic sources.

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] .

This comparison shows that the lavas of this study have geochemical characteristics (major and traces) very different from those of these oceanic islands (Figure 16, Figure 18(a), Figure 18(b)). Indeed, the major elements are different, the lavas of this study being richer in SiO₂ (48.3% - 49.4% against 40.6% - 42.5%) and Mg# (57.2 - 59.5 against 46.8 - 56.3) but poorer in TiO₂ (1.7% - 1.9% versus 3.5% - 4.4%), CaO (8.5% - 10.1% versus 11.1% - 12.8%), Fe₂O₃(T) (9.8% - 11% versus 13.1% - 14.2%), K₂O (1.2% - 1.5% versus 1.9% - 2.8%), MnO (0.13% - 0.14% vs. 0.18% - 0.21%) and P₂O₅ (0.4% - 0.6% vs. 0.6% - 0.9%) than Fogo lavas. In addition, the lavas in this study are also richer in SiO₂ (48.3% - 49.4% versus 47.1% - 47.6%), Na₂O (3.6% - 4.7% versus 2.2% - 3.4%), K₂O (1.2% - 1.5% vs. 0.5% - 0.9%), MnO (0.13% - 0.14% vs. 0.10% - 0.11%) and P₂O₅ (0.4% - 0.6% against 0.1% - 0.3%) but poorer in TiO₂ (1.7% - 1.9% against 2.6% - 3.5%) and Fe₂O₃(T) (9.8% - 11% against 12.4% - 13.2%) than lava from Réunion.

In the Fann sector, compatible elements such as Ni, Cr, Co and V vary respectively from 120 - 170 ppm, from 260 - 270 ppm, from 36 - 42 ppm and from 143 - 152 ppm in the dolerites and from 230 - 300 ppm, from 300 - 360 ppm, from 40 - 44 ppm and from 150 - 156 ppm in basanites. Incompatible elements such as Nb, Zr and Y vary respectively between 31.9 - 34 ppm, 82 - 89 ppm and 17.5 - 25.2 ppm in the dolerites and between 41.5 - 53.1 ppm, 125 - 145 ppm and 18.1 - 19.9 ppm in basanites.

In the Mermoz sector, the concentrations of compatible elements such as Ni, Cr, Co and V vary greatly respectively from 50 - 200 ppm, from 100 - 280 ppm, from 31 - 52 ppm and from 163 - 222 ppm in the dolerites and are respectively

equal to 250 ppm, 340 ppm, 43 ppm and 147 ppm in the basanites. Incompatible elements like Nb, Zr and Y also vary a lot in the dolerites respectively between 32.3 - 56 ppm, 97 - 157 ppm and 20.5 - 33.2 and equal respectively to 53.9 ppm, 141 ppm and 20.3 ppm in basanites.

In conclusion, the Ni and Cr contents are higher in the basanites than in the dolerites and this is undoubtedly linked to the abundance of olivine crystals in the basanites unlike the dolerites, which contain very little.

Fann dolerites with ƩREE = 652 - 820 ppm are very enriched in LREE by 106 to 215 times the chondrites and very depleted in HREE by 8 to 9 times the chondrites. This results in very steep REE spectra (La_N/Yb_N = 20.4 - 20.8) marked by a progressive depletion of LREE (La_N/Nd_N = 2.8 - 3) towards HREE (Dy_N/Lu_N = 2.3 - 2.5) passing through the MREEs (Sm_N/Gd_N = 1.3 - 1.4) with very slight negative anomalies in europium (Eu/Eu* = 0.88 - 0.94) (Figure 17(a)). However,

it should be noted that the D2 dolerite has an intermediate chemical composition between the two samples of D1 dolerites. On the enlarged diagram normalized to the primitive mantle (Figure 17(b)), all the dolerites present enrichment in LILEs (Large Ion Lithophilic Elements) (Cs, Rb, Ba) and negative anomalies in Pr, Zr and Yb and positive in Sr.

Mermoz dolerites with ƩREE = 618 - 941 ppm show quite similar REE spectra with the same fractionation as Fann dolerites. They are enriched by 135 to 201 times the chondrites in LREE (La_N/Nd_N = 2.2 - 2.3) and depleted by 8 to 13 times the chondrites in HREE (Dy_N/Lu_N= 2.5 - 2.6) with very steep spectra (La_N/Yb_N = 14 - 15.4) and very slight negative anomalies (Eu/Eu* = 0.86 - 0.94) (Figure 17(a)). On the enlarged diagram normalized to the primitive mantle (Figure 17(b)), we observe enrichment in LILEs and very marked negative anomalies in U and Zr and positive anomalies in Nb.

Fann basanites with ƩREE = 719 - 855 ppm are characterized by highly fractionated REE spectra (La_N/Yb_N = 22.1 - 25.8) marked by a very strong LREE enrichment (La_N/Nd_N = 2.5 - 2.6) by 175 to 216 times compared to chondrites and a net HREE depletion (Dy_N/Lu_N = 2.7 - 2.8) by 7 to 8 times compared to chondrites (Figure 17(c)). Europium shows very weak negative anomalies (Eu/Eu* = 0.91 - 0.95). However, the basanite in prisms is very slightly more enriched in REE than the basanites in vacuolars. On the enlarged diagram normalized to the primitive mantle (Figure 17(d)), the Fann basanites present an enrichment in LILEs and negative anomalies in U, Pr, Zr and Yb and positive in Nb and Sr. Mermoz basanite (ƩREE = 873 ppm) on the other hand, has a spectrum identical to the spectrum of Fann basanite. It is enriched in LREE by 223 times the chondrites with a very marked fractionation of the spectrum (La_N/Yb_N = 24.4) and a very slight negative europium anomaly (Eu/Eu* = 0.95) (Figure 17(c)). On the enlarged diagram normalized to the primitive mantle (Figure 17(d)), the Mermoz basanite also shows the same positive and negative anomalies as the Fann basanites. It should be noted that the very pronounced positive anomalies in Sr observed in the basanites of the two sectors are compatible with the abundance of plagioclases, while the negative anomalies in Zr encountered in all the samples would be linked to the fractionation of clinopyroxenes. Moreover, the REE spectra indicate, like the major elements, source heterogeneity between the dolerites and the basanites.

The REES of the Fann and Mermoz lavas were compared with those of the lavas of the Fogo and Réunion islands. The LREEs of the lavas of this study are intermediate between those of Fogo more enriched (285 to 373 times against 175 to 223 times the chondrites) and those of Réunion less enriched (79 to 170 times against 175 to 223 times the chondrites) (Figure 18(a)). On the other hand, with regard to the HREEs, the lavas of this study are less enriched (7 to 8 times against 10 to 13 times the chondrites for Fogo and 11 to 13 times the chondrites for Réunion) with a more marked fractionation (Dy_N/Lu_N = 2.7 - 2.8 against 2.4 - 2.7 for Fogo and 1.9 - 2.6 for Réunion) (Figure 18(a)). In addition, the LILEs of the lavas in this study are intermediate between those of the Fogo and Réunion lavas while the HFSEs are less enriched (Figure 18(b)).

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)).

The position in the La/Yb (ppm) versus La (ppm) diagram of [31] (Figure 20(a)) of dolerites and basanites from the Fann and Mermoz sectors indicates that these rocks come from a very enriched magmatic source. This source which

could be the same as that of the basalts of the oceanic islands (OIB) is above the values of the primitive mantle (MP), the normal residual upper mantle (N-MORB) or enriched (E-MORB) and the arc. It should be noted, however, that this diagram also confirms the heterogeneity of the source, which gave rise to the dolerites and the basanites, the latter coming from an even more enriched source. In addition, the TiO₂/Yb (ppm) versus Nb/Yb (ppm) diagram of [32] (Figure 20(b)) which is made to estimate the depth of the source of lava from oceanic domains not influenced by subduction confirms the alkaline nature of the basanite lavas from both sectors and shows that they are located in the garnet stability field. This result suggests that the source of the basanite magma is very deep, located at the level of the enriched lower mantle and would be of the garnet lherzolite type. The existence of garnet in the magmatic source is further confirmed by spectra very depleted in HREE.

The geochemical characteristics of the Quaternary lavas in the Fann and Mermoz sectors are similar to those of the OIBs (oceanic island basalts) (Figure 20(a), Figure 20(b)) emplaced by a hot spot. However, the flow in prisms observed of all basanite lavas suggests continental volcanism. This continental volcanism is also confirmed by the geotectonic diagrams of [33] (Figures 21(a)-(c)) and [34] (Figure 21(d)) which locate the basanite lavas of the two sectors in thedomain of continental intraplate alkaline basalts.