The Comoros archipelago consists of four volcanic islands ([1][2]): Mayotte, Anjouan, Moheli and Grande Comore. These islands are distributed over an average distance of approximately 275 km along an ESE-WNW oriented submarine plateau ([1][2]). Eastward, the archipelago extends into a series of banks, some of which are emergent (Glorieuses, Geyser) and others submerged (Zélée, Leven, Cordilières), located in the northern part of the Mozambique Channel. The Mozambique Channel itself represents an ancient rift that formed during the separation of the Malagasy microcontinent from the rest of Africa more than 180 million years ago [25]. In this context, [26] proposed two major stages in the tectonic evolution of the Mozambique Channel: (i) an intracontinental rifting phase, from the Permian to the Early Jurassic, leading to the separation of the Indo-Malagasy block from the supercontinent Gondwana [27]; and (ii) the opening of the Somali and Mozambique oceanic basins from the Middle Jurassic to the Early Cretaceous, through a process of oceanic accretion between ~ 150 and 120 Ma. This latter stage involved north-south crust extension along east-west-trending ridges [28], combined with decoupled activity along the Davie and Mozambique ridges.

The island of Anjouan is an extinct and largely barren volcanic island belonging to the Comoros archipelago [29]. It is generally interpreted as a central shield volcano that has been progressively eroded over time, upon which a younger Hawaiian-type rift system has been superimposed, in a N100˚ and 0˚ direction [30]. Previous studies ([29]-[32]) have shown that the island underwent different volcanic phases or periods during its formation (Figure 2). These are:

1) an early shield-building phase, corresponding to the initial stages of volcanism construction, during which the central massif of the island, known as the Ntringui massif, was formed. During this stage, a large circular gabbroic intrusive body was identified within a basin-shaped depression;

2) an eruptive phase along faults forming the island’s three peninsulas, corresponding to the intermediate stage of volcanism;

3) and finally, a rejuvenation or erosional phase, accompanied by the emplacement of the most recent lava flows, corresponding to the island’s recent volcanism.

However, it should be noted that our study area, located in the Nioumakélé region in the southern part of Anjouan island, between latitudes 12˚20'30"S and 12˚23'00"S and longitudes 44˚29'00"E and 44˚32'30"E (Figure 2), has never been the subject of previous studies.

Figure 2. Geological map of Anjouan Island showing the location of the study area [29], modified.

Fieldwork has identified four lithological types (Figure 3): alkaline basalts, basanites, phonolitic nephelinites, and phonolites, displaying both vesicular and non-vesicular structures, as well as porphyritic and fluidal microlitic textures.

Figure 3. Outcrop map of the study area showing the locations of the samples analyzed.

Porphyric vesicular basalts constitute the most common facies in the study area. Based on the nature and distribution of the phenocrysts, these basalts can be are subdivided into three sub-facies: (i) vesicular basalts with olivine phenocrysts, (ii) vesicular basalts with olivine and clinopyroxene phenocrysts, and (iii) vesicular basalts with olivine, clinopyroxene and orthopyroxene phenocrysts. Vesicular basalts containing olivine phenocrysts crop out north-east of the study area, approximately 600 m from the village of Mramani, and to the south of the village of Hantsahy. Basalts bearing olivine and clinopyroxene phenocrysts are located south of the villages of Chaoueni and Hantsahy, north of Hamchaco, and north-east of Mramani. Finally, vésicular basalts with olivine, clinopyroxene and orthopyroxene phenocrysts occur approximately 100 m south of Chaoueni and about 250 m east and north-east of the village of Nounga. All these basalts display similar petrographic characteristics, with outcrops forming polygonal blocks, dark in color, and exhibiting elliptical vesicules of varying sizes (Figure 4(a)).

Under the microscope, the rocks show a porphyritic microlitic texture, with a primary minrealogical composition consisting of plagioclase, olivine, and pyroxene embedded in a relatively abundant mesostasis. Plagioalse (44% - 52%) occurs as abundant microlites. Olivine (13% - 20%) is present as both phenocrysts and microlites. Olivine phenocrysts (0.6 - 4.8 mm) are automorphic to subautomorphic, displaying vivid colors and cracking (Figure 4(b)). These phenocrysts are only rarely altered to iddingsite and opaque minerals. Pyroxenes (10% - 15%), represented by clinopyroxene and/or orthopyroxene, also occur as microlites and phenocrysts (Figures 4(c)-(d)), and are locally altered into opaque minerals. Opaque minerals (7% - 13%), generally xenomorphe in shape, result from the alteration of olivine and pyroxene.

Figure 4. (a) (b) Macroscopic and microscopic characteristics of vesicular basalts (V) showing olivine (Ol) phenocrysts; (c) (d) microscopic characteristics of porphyritic vesicular basalts showing phenocrysts of olivine, clinopyroxene (Cpx), and orthopyroxene (Opx).

Porphyritic non-vesicular basalts are less frequent than their porphyritic vesicular basalts. They cropout to the south and west of the Chaoueni locality in the from of melanocratic polygonal blocks, cut by NNE - SSW-trending fractures (Figures 5(a)-(b)). Under the microscope, these rocks exhibit a porphyritic microlitic texture and a mineralogical composition characterized by a primary paragenesis of plagioclase, olivine and clinopyroxene, set within a relatively sparse mesostasis rich in opaque minerals (5% - 10%). Plagioclase (45% - 52%) occurs predominantly as microlites. Olivine (5% - 12%) and clinopyroxene (15% - 25%) occur both as microlites and as automorphic to subautomorphic phenocrysts, and are sometimes altered into xenomorphic opaque minerals. Clinopyroxene phenocrysts (0.8 - 6 mm) are occasionally twinned (Figure 5(c)) and may contain inclusions of olivine (Figure 5(d)). Olivine phenocrysts commonly display fractures, the margins of which are altered to iddingsite (Figure 5(e)).

Figure 5. (a)-(e) Macroscopic and microscopic characteristics of non-vesicular basalts containing olivine (Ol) and clinopyroxene (Cpx) phenocrysts. The outcrops exhibits NNE-SSW-trending fractures (b). Olivine phenocrysts commonly show fracturing, with margins partially altered to opaque minerals (Op) and reddish iddingsite. Clinopyroxene phenocrysts enclose cracked olivine crystals and opaque minerals.

Basanitescrop out approximately 100 m and north of the villages of Chaoueni and Dziani respectiveley, where they occur as small exposures composed of melanocratic polygonal blocks (Figure 6(a)). Petrographic observations reveal that these the basanites exhibit a porphyritic microlitic texture, with a primary mineral assemblage consisting of plagioclase, olivine, clinopyroxene and orthopyroxene set in a sparse mesostasis. In contrast to basalts, basanites are characterized by a lower abundance of plagioclase microlites (40% - 47%). Olivine (5% - 12%) typically occurs as both microcrystals and phenocrysts. Clinopyroxene (15% - 21%) commonly occurs as automorphic to subautomorphic phenocrysts (1.2 - 4.3 mm), but also as xenomorphic microcrystals. Clinopyroxene phenocrysts are frequently grouped into aggregates that include opaque minerals (Figure 6(c)). In contrast, orthopyroxene represents only about 5% of the rock volume and occurs predominantly as automorphic phenocrysts (1.9 to 2.8 mm) (Figure 6(b)). Opaque minerals (7% - 15%), occur in two forms: automorphic crystals enclosed within olivine, pyroxene, and plagioclase; and xenomorphic grains, resulting from the alteration of ferromagnesian minerals.

Figure 6. (a)-(c) Photographs and photomicrographs of basanite containing olivine (Ol), clinopyroxene (Cpx), and orthopyroxene (Opx). Orthopyroxene are twinned, whereas clinopyroxene occurs in aggregates.

Phonolitic nephelinites crop out at several locations, including approximately 350 m northeast of the village of Mramani, as well as to the west and north, and about 800 m north of the villages of Hantsahy, Dziani and Hamchaco, respectively. In outcrop, these rocks occur as polygonal blocks that are commonly rounded to subrounded. They are melanocratic in color and display small vesicules, generally elliptical in shape (Figure 7(a)). Under the microscope, the phonolitic nephelinites contain rare microphenocrysts of clinopyroxene (Figure 7(b)) and olivine embedded in a very abundant groundmass composed of plagioclase, olivine, clinopyroxene, and opaque mineral microlites. Clinopyroxene microphenocrysts (0.6 - 1.9 mm; 7% - 12%) are subautomorphic to automorphic, in shape and are occasionally altered to opaque minerals. Plagioclase microlites are highly abundant (45% - 50%). Olivine (2% - 5%) is less abundant and occurs mainly as microlites, and only rarely as microphenocrysts (0.3 - 0.8 mm), which are automorphic to subautomorphic, and may be partially altered into opaque minerals. Opaque minerals (25% - 33%) are very abundant and occur as xenomorphic microcrystals (0.3 - 0.6 mm).

Figure 7. (a) (b) Macroscopic and microscopic characteristics of a phonolitic nephelinite showing rare clinopyroxene (Cpx) microphenocrysts within a very abundant groundmass rich in plagioclase (Plg), olivine (Ol), and opaque mineral (Op) microlites.

Phonolites are very rare and have limited spatial extent in our study area. They crop out exclusively to the west of the locality of Hantsahy, where they occur as polygonal blocks (Figure 8(a)). Under the microscopic, the rock reveals a fluidal microlitic texture, characterized by sanidine microlites (60% - 65%) oriented in the direction of lava flow (Figure 8(b)). The rock also contrain microlites and microphenocrysts (0.3 - 0.7 mm) of clinopyroxene (3% - 7%) and nepheline (2%), as well opaque minerals (17% - 25%) within a very abundant groundmass (Figures 8(b)-(c)).

Figure 8.(a)-(c) Macroscopic and microscopic characteristics of a phonolite containing microphenocrysts of clinopyroxene and nepheline (Ne), displaying a fluidal texture marked by the preferred orientation of sanidine microlites (San) along the direction of lava flow.

Eleven (11) rock samples corresponding to the fissural eruption phase were selected from the southern part of Anjoaun Island for geochemical analysis. The concentrations of major and trace elements, along with the lithological characteristics of these samples, are reported in Table 1 below. All samples display very low loss on ignition (LOI) values, ranging from - 0.2 to 1.4wt.%, suggesting minimal alteration during post-magmatic process such as low-grade metamorphism or water-rock interaction. Consequently, large ion lithophile elements (LILE) and high field strength elements (HFSE) are considered reliable for geochemical characterisation in the study. Rare earth element (REE) concentrations are normalized to NWA 974 chondrite values [33], whereas incompatible trace elements are normalized to the primitive mantle composition [34].

Table 1. Concentrations of major and trace elements in lava samples from the southern part of the Anjouan Island.

4.2.1. Classification and Magmatic Affinity

Plotting the samples from the southern part of Anjouan Island on the total alkali-silica (Na₂O + K₂O vs. SiO₂) classification diagram [35] (Figure 9) indicates that the lavas comprise alkali basalts, basanite-tephrites, phonolitic nephelinites, and phonolites, all of which fall within the field of silica-undersaturated alkaline rocks. However, the sample falling within the basanite-tephrite field is classified as a basanite, owing to its olivine content exceeding 10% [36].

Figure 9.Total alkali-silica (Na₂O + K₂O vs. SiO₂) diagram [35] showing the classification and magmatic affinity of lavas from the southern part of Anjouan island.

4.2.2. Major and Trace Elements

Alkaline basalts are characterized by SiO₂ contents ranging from 43.1 to 46.5 wt.%, TiO₂ 1.9 to 3.2 wt.%, Al₂O₃ from 10.8 to 15.1 wt.%, MgO from 6.36 to 11.2wt.%, FeO(t) from 12.7 and 15.1wt.%, CaO from 9.3 and 11 wt.%, Na₂O from 3 and 3.9 wt.%, MnO at 0.2 wt.%, K₂O from 1.0 and 1.5 wt.%, P₂O₅ from 0.5 to 0.7 wt.% (Figure 10). The Mg number (Mg# = [MgO/40.3/(MgO/40.3 + FeO/71.85)] * 100) ranges from 64.3 to 77. Compatible trace elements concentrations show that Ni ranges 110 to 340 ppm, Cr from 120 to 950 ppm, Co from 42 to 52 ppm and V from 193 to 268 ppm. The relatively high contents of MgO, FeO(t), Ni and Co are consistent with olivine fractionation. Similarly, the concentrations of CaO, Cr, and V suggest the involvement of clinopyroxene crystallization of clinopyroxene crystallization, whereas the elevated Al₂O₃, Na₂O and CaO contents are indicative of plagioclase crystallization.

Alkaline basalts (ΣREE = 967 - 1 263 ppm) are characterized by steeply sloping rare earth element (REE) patterns (La_N/Yb_N = 16.9 - 25.3), marked by strong enrichment in light REE (LREE), ranging from 233 to 321 times chondritic values, and depletion in heavy REE (HREE), ranging from 10 to 14 times chondritic values. These patterns are accompanied by very slight negative europium anomalies (Eu/Eu* = 0.88 - 0.91) (Figure 11(a)). On the primitive mantle-normalized multi-élément diagram (Figure 11(b)), the alkaline basalts display positive anomalies in Ba, Nb, and Sr, and negative anomalies in Th, U, Pr, Zr, and Yb. An exception is represented by samples AR-23, which exhibits negative Nb and Sr anomalies and a positive anomaly U anomaly.

Basanite exhibits geochimical characteristics very similar to those of alkali basalts, except for its higher Na₂O (4.5 wt.% compared 3.0 - 3.9 wt.%) and P₂O₅ (0.8 wt.% compared to 0.5 - 0.7 wt.%) contents (Figure 10). It is also generally enriched in rare earth elements (ΣREE = 1419 ppm) and displays a less pronounced negative europium anomaly (Eu/Eu* = 0.94, versus 0.88 - 0.91 in alkali basalts) (Figure 11(a)). On the primitive mantle-normalized multi-element diagram (Figure 11(b)), the basanite exhibits patterns of positive and negative anomalies to those observed in the alkali basalts, except for uranium, which display a positive anomaly.

Phonolitic nephelinite differs from alkali basalts and basanite by its higher Na₂O (6.9 wt.% compared to 3.0 - 3.9 wt.% and 4.5 wt.% respectively) and K₂O contents (2.5 wt.% compared 1.0 - 1.5 wt.% and 1.55 wt.%). In contrast, it exhibits lower MgO (4.8 wt.% compared to 6.4 - 11.2 wt.% and 6.6 wt.%), CaO (7.4 wt.% compared 9.3 - 11 wt.% and 10.5 wt.%) (Figure 10), and Mg # (56.8 compared to 64.3 - 77 and 64.6). It is also strongly enriched in light rare earth elements (LREE), reaching up to 652 times chondritic values, and in heavy rare earth elements (HREE), up to 21 times chondritic values, with a more fractionated REE pattern (La_N/Yb_N = 29.3 versus to 16.9 - 25.3 for alkali basalts and 22.7 for basanite) (Figure 11(a)). On the primitive mantle-normalized multi-element diagram (Figure 11(b)), the phonolitic nephelinite displays patterns of positive and negative anomalies to those of the alkali basalts, except for positive anomalies in Th and U a negative anomaly in Nb.

Finally, phonolite is clearly distinguished from the other lava types by its higher Al₂O₃ (18.18 wt.%), Na₂O (8.1 wt.%), MnO (0.3 wt.%), and K₂O (4.1 wt.%), contents, as well as its lower concentrations of TiO₂, (0.2 wt.%), MgO (0.2 wt.%), FeO(t) (8.8 wt.%), CaO (2.7 wt.%), P₂O₅ (0.12 wt.%) (Figure 10), together with a very low Mg # (7.1). It is also strongly enriched in rare earth elements (ΣREE = 2343 ppm), with LREE concentrations reaching up to 740 times chondritic values and HREE concentrations up to 25 times chondritic values. The REE pattern is characterized by a more pronounced negative europium anomaly (Eu/Eu* = 0.85) (Figure 11(a)). The phonolite is further distinguished by a negative Sr anomaly and a positive Zr anomaly on the primitive mantle-normalized multi-element diagram (Figure 11(b)). These geochemical features, together with the very low MgO content, indicate a magma that has undergone extensive fractionation of olivine, plagioclase, and pyroxene.

In summary, the geochemical characteristics of the lavas from the southern part of the Anjouan island suggest a genetic relationship and a progressive magmatic evolution from alkali basalts to phonolites, through basanites and phonolitic nephelinites. This differentiation trend, driven by fractional crystallization, is reflected in a gradual decrease in TiO₂, MgO, FeO(t), CaO, Ni and Cr, contents, accompanied by a concomitant increase in SiO₂, Al₂O₃, Na₂O, MnO and K₂O (Figure 10). This evolutionary process is further evidenced by the continuous enrichment and sub-parallelism of REE patterns, from the least evolved rocks (alkali basalts) to the most evolved ones (phonolites), passing successively through basanites and phonolitic nephelinites (Figure 11(a)). Finally, it should be noted that all the lava types display positive Nb anomalies, with the exception of the alkali basalt sample AR-23 and the phonolitic nephelinite sample (AR-26) (Figure 11(b)).

Figure 10. Harker diagrams illustrating the variation of major oxide compositions as a function of SiO₂ in lavas from the southern part of Anjouan island.

Figure 11. Diagrams illustrating: (a) REE spectra normalized to NWA974 chondrite values [33], and (b) primitive mantle-normalized multi-element patterns of incompatible trace elements in lavas from the southern part of Anjouan Island [34].

4.2.3. Nature and Depth of the Magmatic Source

To constrain the nature of the magma source, only the mafic lavas (alkali basalts and basanites), which are representative of primary magmatic liquids, were plotted on the Sm/Yb (ppm) versus La/Yb (ppm) diagram [36] (Figure 12(a)). On this diagram, both the alkali basalts and basanite plot within the enriched mantle field and show a close correlation with melting curves of asthenospheric mantle in the garnet lherzolite facies. The same samples were also plotted on the TiO₂/Yb (ppm) versus Nb/Yb (ppm) diagram from [37] (Figure 12(b)), which was developed to estimate the depth of magma sources in oceanic settings unaffected by subduction. This diagram confirms the alkaline nature of the lavas and places them within the garnet stability field, suggesting a deep mantle source of garnet lherzolite composition. The presence of garnet in the source is further supported by the markedly depleted heavy rare earth element (HREE) patterns, which reflect the retention of HREE by residual garnet. In addition, previous studies [38]-[40] have shown that high Ba/Rb ratios associated with low Rb/Sr ratios indicate an amphibole-bearing source, whereas low Ba/Rb ratios suggest the presence of phlogopite. Accordingly, the relatively high Ba/Rb ratios (14.11 - 25) and low Rb/Sr ratios (0.02 - 0.05) observed in the studied mafic lavas are consistent with a magma derived from a source in equilibrium with amphibole-bearing mantle.

Figure 12.Diagrams: (a) Sm/Yb (ppm) vs La/Yb (ppm) [36] and (b) TiO2/Yb (ppm) vs Nb/Yb (ppm) [37], showing the nature and depth of the source lavas in the southern part of Anjouan Island. PM = Primitive Mantle; N-MORB = Normal Mid-Ocean Ridge Basalt; E-MORB = Enriched Mid-Ocean Ridge Basalt; OIB = Ocean Island Basalt. Data from [34].

4.2.4. Geodynamic Context

To constrain the geodynamic setting of the lavas from the southern part of Anjouan Island, triangular and binary diagrams involving elements considered immobile or relatively immobile during alteration and metamorphic processes were employed. Accordingly, the Th - Zr/117 - Nb/16 [41] (Figure 13(a)) and Zr/4 - 2Nb - Y [42] (Figure 13(b)) plot all the lavas from the southern part of Anjouan Island within the field of intraplaque basalts (WPA or AI). These results are further supported by the FeO(t) - MgO - Al₂O₃ diagram [43] (Figure 13(c)), the Ti (ppm) versus V (ppm) [44] (Figure 13(d)), Nb/Y versus Zr/Y [45] (Figure 13(e)), and Th/Yb versus Nb/Yb [37] (Figure 13(f)), all of which place the samples within the domain of alkaline basalts and/or ocean island basalts (OIB-AB or OIB). It should be noted, however, that this geodynamic setting is consistent with the strongly LREE-enriched patterns and the positive Nb anomalies observed in the samples (except for sample AR-23). It also consistent with the position of all the lavas from the southern part of Anjouan Island above the ΔNb line on the Nb/Y versus Zr/Y diagram [45] (Figure 13(e)), indicating that their source is related to a mantle plume originating from an enriched lower mantle. The magma is inferred to have ascended through a hotspot activity mecanism, facilitated by lithospheric fractures.

N-MORB = Normal Mid-Ocean Ridge Basalts; E-MORB = Enriched Mid-Ocean Ridge Basalts; IAT = Island Arc Tholeiites; CAB = Volcanic Arc Basalts; WPT = Tholeiitic Within-Plate basalts; WPA = Alkaline Within-Plate basalts; AI = Within-plate alkali basalt; AII = Within-plate alkali basalt and within-plate tholeiites; B = E-MORB; C = Within-plate tholeiites and volcanic arc basalts; D = N-MORB and volcanic arc basalts; ARC = arc related basalts; OFB = Ocean Flood Basalts; OIB-AB = Ocean Island Basalts and Alkali Basalt; OPB = Oceanic Plateau Basalts. Hypothetical mantle sources: DM = shallow depleted mantle; EN = enriched component; PM = primitive mantle; REC = recycling component; UC = upper continental crust; DEP = depleted plume component; HIMU = high (U/Pb) source; EM1 and EM2 = enriched mantle sources (modified after [45][46]).

Figure 13. Diagrams: (a) Th - Zr/117 - Nb/16 [41], (b) Zr/4 - 2Nb - Y [42], (c) FeO(t) - MgO - Al₂O₃ [43], (d) Ti (ppm) vs V (ppm) [44], (e) Nb/Y vs Zr/Y [45] and (f) Th/Yb vs Nb/Yb [37] showing the composition ranges of various oceanic lavas, within which the lavas from the southern part of Anjouan Island are plotted.