The mineralogical study carried out on the doleritic formations of Mbaoussi shows that the rocks are characterized by a typical mineralogy of tholeiitic and calc-alkaline lavas. The rocks consist of clinopyroxene (diopside and augite), plagioclase (andesine and oligoclase), alkaline feldspar (anorthosite and sanidine), iron-titanium oxides (titano-magnetite and ilmenite), black mica (biotite and phlogopite), amphibole (tschermakite and magnesio-sadanagaite) and quartz (in the dolerites of group 2). The higher values of AlIV (0.13 < AlIV < 0.25) in the dolerites of group I (orientcompared to those of group II (0.06 < AlIV < 0.13), suggest that the clinopyroxenes of Group II dolerites would have crystallized under conditions of high silica activity. The values of higher ratios of AlIV (0.19
Mbaoussi locality belongs to the Adamawa plateau, part of so called Adamawa - Yadé Domain 1 of Central Pan Africa Fold Belt Chaine (Figure 1). This huge domaine is located between the northern and the southern domains, respectively bounded by the Tcholliré-Banyo and Sanaga Pan African faults 1, 2, 3. The Adamawa plateau in it domain shows a dome shape feature due to the reactivation of Pan African strick slip faults which have favored its uplift scenario through a transpressive tectonic event at Tertiary time 4, 5, 6, 7. Central Africa basement is composed of two main geological formations including Archean craton formation and Pan African mobile belt one 8. The former is reputed to be stable and have withstood the historical tectonic events through geological times when the evolution of the letter is directly linked to numerous tectonic events which have affected the central Africa 9. The mechanism of the evolution of the central Pan African mobile basement have been questioned 1, 10. The Cameroon Volcanic Line (CVL), the Benue Through (BT) and the Adamawa Plateau (AP) are well-known tectono-volcanic structures in central Africa which resulted from tectonic events which have affected the central African basement during it evolution. Dolerite lavas are among the volcanic products which occur in those volcanic areas 11, 12, 13, 14, 15, 16, 17. They are acknowledged worldwide as keys for geodynamic comprehension 18. Thus, dolerite dykes are considered as importance geological formations in modelling the geodynamic evolution of central African basement. Petrology studies have been undertaken on dolerite dykes from Mbaoussi region in north-east Ngaoundéré through petrographic and geochemical studies 13, 19. Although those data have raised the important petrological results, the lack of mineralogical contribution does not permit to decipher the role of different petrological processes which have contributed to their formation.
This work is focused on mineralogical data recently obtained on Mbaoussi dolerite dykes in view to provide supplement information on their petrogenesis.
The Adamawa Plateau is a morpho-structural unit with an average altitude of 1100m limited to the north by the Adamawa faults and to the south by the faults of Mbere - Djerem 7. The Adamawa plateau basement is composed of post-orogenic granitoids of 630-620 Ma pre- to syn-D1, 580-600 Ma syn-D2 and 550 Ma 10, 20, 21. More recently, 22 have discovered the intensely deformed and metamorphosed granitoids into gneiss of amphibolite series at the north of Ngaoundere. Syn to post tectonic granitoids of Th-U-Pb age of 615 ± 27 Ma and 575 Ma granitoïds also occur in the same area. Mbaoussi dolerite dykes are intrusive through those granitoids. It is fractured by a network of Pan-African faults following directions N70°E and N130-140°E which cuts the continental crust down to the mantle 5, 7, 6, 23. This plateau is thus assimilated to a Central African volcano-tectonic horst of Cenozoic age and Precambrian basement 6 intersected by the Foumban shear zone, part of the Shear Zone of Central Africa (SZCA) which extends over approximately 2000 km to Sudan 24. It is intensely cut by a beam of major accidents 6 locally masked by Cenozoic basaltic flows. The main fault that crosses it, also called the “Adamawa Fault”, represents one of the directions of the Pan-African foliations. It crosses Africa from West to East 24 and corresponds to the South of the plateau with mylonitic accidents 7. This horst is constituted since the end of the Finite-Precambrian collision by more than 7 inter-cratonic crustal blocks located at different altitudes and delimited by ductile shears of dominant NE-SW directions 25.
A structural study undertaked on the northern and southern edges of the plateau 7 highlights: (1) a phase of shortening (P1) WNW-ESE, in a strike-slip regime which brings into place a dextral sliding the Pan-African mylonitic faults of N70°E corresponding to Central African Shears 26; (2) a distensive phase (P2) NNE-SSW and (3) a compressive phase (P3) NW-SE 5. A submeridian extension in the Cenozoic facilitated the uplift of the horts of the Adamawa with volcanic activity and accentuated the collapse of the Djerem and Mbere trench (N70°E) 5.
Sampled dolerites cropping out in Mbaoussi locality have undergone petrography studies using 15 thin sections prepared at the laboratory of petrography of the Faculty of Science (building 504), University of Paris-sud. Microprobe mineral analyses of Mbaoussi dolerites were performed on Camebax SX100 after preliminary phase of metallization at the service Camparis of the University of Pierre et Marie Curie, Paris Sorbonne, France. The operating conditions were an accelerating voltage and a beam current as follow: olivine and pyroxene: 15 kV and 40 nA, 20 s except Si for olivine (10 s) and Ti for pyroxene (30 s); titanomagnetite: 15 kV and 40 nA, Si, Ca, Ni: 10 s; Mn: 25 s; Cr; 15 s; Al: 30 s; Ti, Fe, Mg: 40 s. Standard used were a combination of natural and synthetic minerals. Data corrections were made using the PAP correction of 27.
Previous petrography and geochemical studies carried out on Mbaoussi dolerites 13, 19, 28 have shown that two groups of dolerite lavas occur in the area according their direction: Group 1 dolerites are directed N100E to N130E and the second group is directed N70E 19. Both groups are composed of more than 43 dykes parallel to Pan African regional lineaments. hand specimen show samples of light to dark grey coated with 2 mm to 2 cm-thick light brow patina, exhibiting a coarse grained texture and composed of whitish feldspar, black crystals of pyroxene or oxides associated to green epidote crystals, while clay mineral and pockets of carbonate in the matrix. Greenish heheudral amphibole crystals and elongated biotite are also presents. Microlithes of all those minerals are intimately linked to the oxides microcrysts in the matrix. ICP-AES and ICP-MS geochemical analyses of Mbaoussi dolerites distinguished the mafic or primitive (SiO2 contents between 44.9 and 47.5 wt%) group I dolerites whit Mg# ≥ 44 (up to 58) and more felsic (SiO2 contents between 49.7 and 53.0 wt%) group II dolerites having Mg# ≤ 44. In TAS (Na2O+K2O) vs. SiO2 diagram, group I dolerites are composed of basalts and trachybasalts, whereas group II dolerites are trachybasalts and basaltic trachyandesites 13, 28. Both groups show slightly alkaline affinities but exhibit the continental tholeiitic signature. CIPW normative compositions shift progressively from silica undersaturated to saturated to oversaturated.
4.2. MineralogyMicroprobe chemical analyses of plagioclase, K-feldspar, clinopyroxene, amphibole, biotite and oxides, main mineral phases are shown in the attached tables. All minerals are presents in both groups excepted K-feldspar which are only found in group 2 dolerites.
Plagioclase crystals of basalts and trachybasalts of group 1 dolerites (Table 1) are composed of albite (An1.41-An7.98) and oligoclase (An10.86-An23.74) for trachybasalts and basaltic trachyandesites, respectively (Figure 2). Rare crystals of andesine (An47.44) composition are found in trachybasalts.
The plagioclase crystals of the dolerites studied (Figure 4, Table 1) mostly composed of albite (An 1.41 -An 7.98) and oligoclase (An 10.86 - An 23.74) for the dolerites of groups II and I, respectively. A plagioclase with an andesine composition (An 47.44) is analyzed in the group I dolerites. The plagioclase crystals of the group II dolerites are more enriched in the albite component, the highest contents of which exceed 98% by weight.
Alkaline feldspar crystals (Figure 5, Table 2) are analyzed only in group II dolerites. They are composed of anorthosite (Gold11 -13.85 Gold13.85) and sanidine (Gold 43.28 - Gold 94.45). FeO contents in alkaline feldspar crystals are between 0.32 and 2.1% by weight. The highest value is found in sanidine crystals.
Clinopyroxene crystals from Mbaoussi dolerites have compositions of diopside (Wo 45.11-48.06, En 35.78-36.42, Fs 18.5-19.06) and augite (Wo 23.12 -Wo 44.58, En 29.07 -En 41.65, Fs 18.16 -Fs 47.81) according to the classification of 29. Diopside crystals are found only in group I dolerites, while augite crystals are present in both dolerite groups (Figure 6, Table 3). The decrease in the calcium content in the studied clinopyroxene crystals is accompanied by an increase in the iron content and a decrease in the magnesium content in their composition. The compositions of the clinopyroxene crystals of the Mbaoussi dolerites are similar to those of the continental tholeiites of Likok 12 by the evolution of their Ca, Fe and Mg contents.
The titanium-iron oxide crystals have compositions of titano-magnetite and ilmenite (Figure 7, Table 4). Ilmenite crystals are present only in group I dolerites, while group II dolerites are composed of both ilmenite and titano-magnetite crystals.
The analyzes of amphibole crystals from the Mbaoussi dolerites correspond to those of the group of calcic amphiboles according to the classification of 30. According to this classification, the amphibole crystals studied project into the field of tschermakite and magnesiosadanagaite. Tschermakite is found in both groups of dolerites, and magnesiosadanagaite is only present in group 1 dolerites (Figure 8, Table 5). The Si contents in amphiboles from group II dolerites are higher than those in amphiboles from group I dolerites.
The compositions of black mica crystals are shown in Table 6. Based on the AlIV cations contents and Mg number contents (Figure 9), the black mica crystals of group I dolerites are composed of biotite, while those of group II dolerites are composed of biotite and phlogopite. The AlIV content of biotites in group I dolerites are higher than those of group II dolerites.
In the diagram of 31, relating the alumina content and the degree of alkalinity, the clinopyroxene crystals of the Mbaoussi dolerites show an evolution of their composition from the domains of sub-alkaline rocks and from alkaline to hyper alkaline rocks (Figure 10). The majority of clinopyroxene crystals from group I dolerites are more Al2O3 rich compared to those from group II dolerites enriched in SiO2 compared to clinopyroxene crystals from group I dolerites.
In the Ti+Cr in function of Ca diagram (Figure 11) of 32, clinopyroxene crystals from Mbaoussi dolerites show a progressive evolution from the field of non-orogenic basalts to that of orogenic basalts. This evolution is marked by a decrease in Ti contents of clinopyroxene crystals from group I dolerites which are relatively high in this element (between 0.04-0.10) towards those of clinopyroxene crystals from group II dolerites, relatively low and between 0.01-0.03 (Table 3).
Similarly, in the Ti vs Ca+Na diagram by the same author (Figure 12), this progressive evolution of clinopyroxene compositions places the group I dolerites in the field of alkaline basalts, while the group I dolerites are located in the field of tholeiitic and calc-alkaline basalts.
4.4. Thermo-barometric Conditions of Magma CrystallizationClinopyroxene crystals from group I dolerites (Figure 13) show higher Al IV values (between 0.13 and 0.25) compared to those of group II dolerite crystals which are relatively low (0. 06 < AlIV < 0.13).
In the AlIV/AlVI diagram (Figure 13) of 33, the clinopyroxene crystals of group II dolerites are positioned in the field of relatively hydrated (<10% H2O) and low pressure (around 5kb), whereas the clinopyroxene crystals of group I dolerites are located in the domain of water-depleted, or even anhydrous, magmas at pressures greater than 10 kb.
The AlIV /AlVI ratio of clinopyroxene crystals from group I dolerites are clearly less than 1 (0.07<AlIV <0.20). The same ratios in the clinopyroxene crystals of group II dolerites gradually evolve until exceeding the value of 1 (0.19<AlIV <1.13).
The Ca content in clinopyroxene crystals has been shown to decrease with increasing temperature 37, 38, 39. Lindsey's thermometer 38 was therefore used to estimate the crystallization temperatures of clinopyroxene crystals in the Mbaoussi dolerites. According to this thermometer (Figure 14), the clinopyroxene crystals of the Mbaoussi dolerites show the following crystallization temperatures:
The clinopyroxene crystals of the Mbaoussi dolerites as a whole mostly show crystallization temperatures between 1100 - 400°C.
Diopside crystals in group I dolerites show lower crystallization temperatures, between 400-500°C:
The augite crystals (less calcic) of the group I dolerites show crystallization temperatures which vary widely between 800-500°C.
Those of clinopyroxene with an augite composition (less calcium) of the dolerites of group II show crystallization temperatures in the interval 1000-800°C. On the other hand, clinopyroxene crystals composed of very weakly calcium augite crystallizes at a temperature ranges which vary between 1100-1050°C.
The total aluminum content is of geobarometric interest 40. In a pressure range comprised between 3.5 and 24 Kb, he proposes the following equation for estimating the crystallization pressure of amphiboles:
P total (± 0.5Kb) = -3.01 + 4.76 Alt (4)
From this calibration, the amphibole crystals of group I dolerites which are rich in Alumina present maximum crystallization pressures which vary between 25-19 kb and those of amphibole of group II dolerites show lower crystallization pressures which vary little, between 7-5 kb.
The estimation of the crystallization temperature of amphiboles in magmas (Table 7) was made using the geothermometer established by 40, based on the titanium content in amphiboles and following the formula: T = 2603 / [1.7-Ln(Ti)].
In this table, the amphibole crystals of the group I dolerites show crystallization temperatures which vary widely between 970-715°C in general. The amphiboles rich in TiO2 (3.50<TiO2 <2.95, cf. Table 5) have crystallization temperatures between 970-918°C, while those of amphiboles with low TiO2 contents (1.35<TiO2 <0.22) have crystallization temperatures range between 715-690°C.
The amphibole crystals of group II dolerites, on the other hand, exhibit almost constant crystallization temperatures, ranging between 750-674°C.
The mineralogical study carried out on the doleritic formations of Mbaoussi shows that the rocks are characterized by a typical mineralogy of tholeiitic and calc-alkaline lavas. The evolution of the compositions of clinopyroxene crystals (diopside – more calcic augite – less calcic augite) in the dolerites of Mbaoussi in general is therefore marked by a progressive decrease in Ca contents and an enrichment of Fe contents accompanied simultaneously by a decrease in Mg contents. The absence and presence of k-feldspar crystals and quartz in the dolerites of groups I and II respectively suggest that the dolerites of group II would be more evolved or differentiated than those of group I. This hypothesis confirms those put forward in the previous work carried out on these doleritic formations in the same locality 13, 19. The coexistence of oligoclase and/or albite with sanidine suggests a crystallization temperature below solvus 42.
Most of the clinopyroxene crystals of group II dolerites have Si cation values greater than 1.90 (see Table 2), which reflects their supersaturated silica character compared to group I dolerites which have Si cations between 1.75 and 1.88, reflecting an alkaline character.
The higher values of AlIV (0.13 < AlIV < 0.25) in group I dolerites compared to those of group II dolerites which are relatively low (0.06 < AlIV < 0.13), suggest that clinopyroxenes from group II dolerites would have crystallized under conditions of high silica activity, unlike clinopyroxenes from group I dolerites where silica activity is low 43, 44.
The values of the AlIV /AlVI ratio much lower than 1 (0.07<AlIV /AlVI <0.20) in the clinopyroxene crystals of the group I dolerites suggest that the clinopyroxenes of the group I dolerites crystallized at pressures greater than 10 kb 35, 45. The clinopyroxenes of group II dolerites, which have higher ratio values of these same elements (0.19<AlIV/AlVI <1.13, would have crystallized under lower pressures than those of the clinopyroxenes of group I dolerites. The contents of these same elements suggest that the clinopyroxene crystals of group II dolerites with tholeiitic affinity would have crystallized under conditions of relatively hydrated magmas (<10% H2O) according to the work of 34, 35, 36. On the other hand, the magma at the origin of the clinopyroxene crystals of the dolerites of group I would be poor in water, even anhydrous.
The different temperature variations in crystallization of the different clinopyroxene minerals suggest that these minerals resulted in an evolution from more calcic diopside crystals to clinopyroxene crystals with a weak calcic augite composition.
According to the calibration of 40, it would likely be that at a maximum pressure between 25-19 kb, the magma at the origin of the Mbaoussi dolerites experienced the crystallization of early amphiboles with a composition of tschermakite and magnesio-sadanagaite rich in alumina (28<Al2O3<33) in the dolerites of group I and that during the evolution of the magma followed by a decrease in pressure (between 7-5 kb), crystallized the amphiboles of the dolerites group II with lower alumina values (10<Al2O3<12) and tschermakitic composition. These results suggest that the crystallization of amphiboles in the Mbaoussi dolerites took place at different depths: 1) the amphibole crystals of group I dolerites would have crystallized at maximum depths between 90 and 70 km and 2) those amphiboles from group II dolerites would have crystallized at shallower depths, between 25 and 18 km (given that the pressure increases by 2.7 kb every 10 km). The low TiO2 contents in the Mbaoussi dolerites would thus be the result of a moderate crystallization temperature and a fugacity of oxygen (fO2) quite high 36.
The mineralogical data carried out on the dolerites of Mbaoussi shows an evolution of the compositions of the minerals in the two groups of dolerite: the clinopyroxene evolves from diopside to augite; plagioclase evolves from albite to andesine and alkali feldspar have compositions that increase from sanidine to anorthosite. The oxide crystals have compositions of ilmenite and titano-magnetite and those of amphibole are of the calcic type with composition of Tschermakitique and magnesio-sadanagaite. Black micas have biotite and phlogopite compositions. The mineralogical data show a typical crystallization of the extension domains of the orogenic contexts. Group I dolerites crystallized under the conditions of water-depleted magmas and at higher pressures (25-19 kbar) and group II dolerites crystallized under the conditions of hydrated magmas and lower pressures (7-5 kbar). The Mbaoussi dolerites were put in place under temperature conditions between 400-800°C for group I dolerites and between 800-1100°C for group II dolerites.
This work benefited from the support of Pr. Jacques-Marie BARDINTZEFF of the University of Paris 11, who took charge of financing all the thin sections made and mineralogical analyzes carried out on an electron microscopes at the University of Paris 6. All reviewers are also thanked.
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| In article | View Article | ||
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| In article | |||
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| [44] | Johan, Z. Pyroxenes des complexes basiques et ultrabasiques, particularités d’évolution magmatique des complexes minéralisés en cuivre. Facteurs contrôlant les minéralisations en nickel. BRGM, Orléans, p.170-218. 1972. | ||
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| [45] | Marcelot, G., Maury, R.C, Lefèvre. Mineralogy of Erromango lavas (New Hebrides): evidence of an early stage of fractionation in island arc basalts. Lithosphere, 16, 135-151. 1983. | ||
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| In article | View Article | ||
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| In article | View Article | ||
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| In article | View Article | ||
| [44] | Johan, Z. Pyroxenes des complexes basiques et ultrabasiques, particularités d’évolution magmatique des complexes minéralisés en cuivre. Facteurs contrôlant les minéralisations en nickel. BRGM, Orléans, p.170-218. 1972. | ||
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| In article | View Article | ||
