*3.3.4 Clinopyroxenes*

In the caldera of Mount Bambouto, clinopyroxenes in lavas are found in most sub-automorphic to automorphic crystal rocks with an average size of 0.5 × 1 mm. They show two directions of cleavage in some sections. They show gulfs of corrosion

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hedenbergite.

**Figure 8.**

**Figure 7.**

*3.3.5 Feldspars*

*The Caldera of Mount Bambouto: Volcanological Characterization and Classification*

in some sections. They are cracked in the trachytes and show a macle h1

*Classification of clinopyroxènes of lavas of the mount Bambouto caldera in the en-Wo-Fs diagram.*

*Position of oxides of lavas of the Mount Bambouto caldera in the FeO-TiO2-Fe2O3 diagram.*

The classification of [34] has made it possible to identify three types of clinopyroxene in the lavas of the caldera of Mount Bambouto (**Figure 8**); diopside, augite and

Feldspars are the minerals most represented in the lava of the caldera of the Mount Bambouto. Their edges are corroded in certain sections of the phonolites. However, they are sub-automorphic to automorphic, cracked and elongated depending on the flow. They are found in microlites and phenocrystals with sizes ranging from 0.1 × 0.3 to 0.5 × 0.8 mm for plagioclases and from 0.1 × 0.4 to 1 × 2 mm for alkaline feldspars. The latter have a Carlsbad twin, unlike plagioclases with a polysynthetic twin. The most frequent plagioclases (An30-60) in lava are andesine and labrador. In phonolites, the alkaline feldspars are anorthose (Or17 and Or37) and sanidine (Or37 and Or44) (**Figure 9**). However, anorthoses are in the

in the basalts.

*DOI: http://dx.doi.org/10.5772/intechopen.93694*

**Figure 6.** *Evolution of the forsterite content in lavas in the Mount Bambouto caldera.*

*The Caldera of Mount Bambouto: Volcanological Characterization and Classification DOI: http://dx.doi.org/10.5772/intechopen.93694*

**Figure 7.**

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

with forsterite contents between Fo57 and Fo75 (**Figure 6**).

feldspathoid, amphiboles and oxides.

**3.3 Nomenclatures of some rock minerals**

subangular fragments.

*3.3.1 Olivines*

*3.3.2 Oxides*

*3.3.3 Apatite*

feldspars.

*3.3.4 Clinopyroxenes*

texture containing phenocrystals and microcrystals of alkali feldspars, pyroxene,

*Ignimbrites* (**Figures 4** and **5**) have a vitroclastic texture dominated by a faciesdominated matrix and whole or broken sections of alkali feldspar, quartz, pyroxene, and rock enclaves (trachytes and basement) in the form of rounded balls or

In the caldera of the Mount Bambouto, olivine is present in the basalts with an average size of 0.5 × 3 mm. It is automorphic to subautomorphic. Their section is traversed by numerous cracks along which one notes the beginning of iddingsitization and serpentinization. Some sections have a core and borders corroded by mesostase. The olivine in the caldera of the Mount Bambouto is globally magnesian

The lava oxides in the study area are represented by titanomagnetite and ilmenite (**Figure 7**). These two minerals coexist in some lava, notably dolerite mugaearites. They are sometimes automorphic with various shapes (square, rectangular and rod-shaped), with sizes ranging from 0.2 to 1 mm. They occur as phenocrystals and microcrystals either embedded in minerals such as olivine, clinopyroxene and feldspars; or embedded in mesostase. Furthermore, titanomagnetite

appears as the most abundant oxide in basalts, mugaearites and trachyes.

Apatite is observed in almost all the lavas of the caldera of the Mount Bambouto. It occurs as elongated crystals, xenomorphic to sub-automorphic and sometimes with transverse breaks in the intermediate lavas. Their size is between 0.2 and 0.8 mm and is observable as inclusions in olivine, oxides, and alkaline

In the caldera of Mount Bambouto, clinopyroxenes in lavas are found in most sub-automorphic to automorphic crystal rocks with an average size of 0.5 × 1 mm. They show two directions of cleavage in some sections. They show gulfs of corrosion

*Evolution of the forsterite content in lavas in the Mount Bambouto caldera.*

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**Figure 6.**

*Position of oxides of lavas of the Mount Bambouto caldera in the FeO-TiO2-Fe2O3 diagram.*

### **Figure 8.**

*Classification of clinopyroxènes of lavas of the mount Bambouto caldera in the en-Wo-Fs diagram.*

in some sections. They are cracked in the trachytes and show a macle h1 in the basalts. The classification of [34] has made it possible to identify three types of clinopyroxene in the lavas of the caldera of Mount Bambouto (**Figure 8**); diopside, augite and hedenbergite.

### *3.3.5 Feldspars*

Feldspars are the minerals most represented in the lava of the caldera of the Mount Bambouto. Their edges are corroded in certain sections of the phonolites. However, they are sub-automorphic to automorphic, cracked and elongated depending on the flow. They are found in microlites and phenocrystals with sizes ranging from 0.1 × 0.3 to 0.5 × 0.8 mm for plagioclases and from 0.1 × 0.4 to 1 × 2 mm for alkaline feldspars. The latter have a Carlsbad twin, unlike plagioclases with a polysynthetic twin. The most frequent plagioclases (An30-60) in lava are andesine and labrador. In phonolites, the alkaline feldspars are anorthose (Or17 and Or37) and sanidine (Or37 and Or44) (**Figure 9**). However, anorthoses are in the

**Figure 9.** *Evolution of the anorthite content in lavas of the Mount Bambouto caldera.*

majority. In ignimbrites, the composition of alkali feldspars is between Or33 and Or37 and are therefore exclusively anorthoses.

## **3.4 Classification of lava in the study areas**

The lavas in the caldera of Mount Bambouto are alkaline in nature as shown in the following diagrams in **Figure 10**. The data used to make these diagrams have been supplemented by the data in [20, 22].

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**Figure 11.**

*Sketch highlighting the stages of the formation of the Mount Bambouto caldera.*

*The Caldera of Mount Bambouto: Volcanological Characterization and Classification*

**3.5 The volcanological evolution of the caldera of mount Bambouto**

Mount Bambouto is a Hawaiian shield volcano [18]. Its history has been ruled by volcanic and tectonic events that led to the formation of a huge caldera on the Pan-African granitoid basement [35–37]. The Mount Bambouto Caldera formation

The *Precaldera Stage* (Over 19 Ma) is characterized by the tumescence of the volcanic shield due to magma injection giving rise to several annular fissures observed

The *Syncaldera stage* (18–15.28 Ma) is materialized by two features: firstly, explosive eruptions are responsible for scoria, ignimbrites, trachytes and rhyolites; secondly, piecemeal intravolcanic collapse of the magmatic chamber roof is fol-

The *Postcaldera stage* (15–0.5 Ma) is typified by some trachytic and basaltic supplies and the protrusion of phonolitic domes. Activity ends with the explosive eruptions on the northeastern flank of the volcano where is built the multiple

To assign the code of a given caldera, one must use the numbering system developed in the Catalog of Active Volcanoes of the World. In fact, the world is divided in 19 main regions that are subdivided, in turn, in several subregions. Hence, the study area is located in the African Region with the corresponding database code 2. In addition, these calderas are located in the Central African Sub-Region with the

lowed by the protrusion of trachytic domes and some basaltic supplies.

*DOI: http://dx.doi.org/10.5772/intechopen.93694*

in the whole volcano.

scoria cones.

*3.6.1 Location*

**3.6 Classification of calderas**

corresponding database code 203.

(**Figure 11**) included three main stages [38] as follow:

**Figure 10.** *Chemical nature of lavas in the Mount Bambouto caldera.*

*The Caldera of Mount Bambouto: Volcanological Characterization and Classification DOI: http://dx.doi.org/10.5772/intechopen.93694*
