Abolo et al. STD2014Vol.15.Full
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Abolo et al. STD2014Vol.15.Full
Sciences, Technologies et Développement, Volume 15, pp32-42, Février 2014 Sciences, Technologies & Développement, ISSN 1029 - 2225 http://www.univ-douala.com/sdt/ E-mail : [email protected] ISSN 1029 - 2225 Petrography and mineralogy of the Nlonako anorogenic complex rocks, Central Africa : petrogenetic implications Martin Guy Aboloa, Daniel Lamilenb, Ismaïla Ngounounoc, Dieudonné Bitomd a Société Nationale des Hydrocarbures, BP 955 Yaoundé Cameroun, Département des Sciences de la Terre, Faculté des Sciences, Université de Yaoundé 1, BP 812 Yaoundé, Cameroun cEcole de Géologie et d’Exploitation Minière de l’Université de Ngaoundéré à Meiganga, BP 454 Meiganga, Cameroun d Département des Sciences de la Terre, Faculté des Sciences, Université de Ngaoundéré, BP 454 Ngaoundéré, Cameroun Received : May 2013 Revised: September 2013 Accepted: September 2013 Available online: Febuary 2014 b Abstract The Nlonako Anorogenic Complex (NAC), emplaced along the Cameroon Hot Line (CHL) is characterized by a wide range of plutonic rocks with minor volcanic rocks. Plutonic rocks include alkali granites, syenites, microsyenites, monzonites, monzodiorites, micromonzodiorites, microdiorites, microgabbros, anorthositic gabbros and gabbros. Volcanic rocks comprise mainly alkali rhyolites and mugearites. In addition to textural variations, these rocks show different mineralogical compositions that could be related to the physical and chemical conditions changes of the magma. Most importantly is the great amount of quartz in most of rocks suggesting the role of the crustal contamination during the differentiation. Mineralogical studies reveal a labradorite + hortonolite + magnesian augite + magnesio-hornblende + hastingsitic magnesiohornblende + titaniferous biotite paragenesis in basic and intermediate rocks, and a perthitic feldspar + fayalite + ferroaugite + augite + aegirine augite + ferrohedenbergite + aenigmatite + riebeckite + magnesian hastingsitic hornblende + ferro-edenitic hornblende paragenesis in acid rocks. Mineralogical evolution of the main mineral phases (feldspars and clinopyroxenes) is progressive and marked by an increase of Fe2+, Mn, Fe3+, Na and by a decrease of Mg, Al and Ca. This mineralogical evolution of the gabbro-diorite-monzodiorite-monzonite-syenite-granite alkali suite is interpreted in terms of fractional crystallization mainly controlled by olivine, clinopyroxene and feldspar. ISSN 1029–2225©2014 Sciences, Technologies et Développement os Keywords: Nlonako Anorogenic Complex, plutonic rocks, volcanic rocks, mineralogical evolution, fractional crystallization, Cameroon Hot Line, Cameroon. Résumé Le complexe anorogénique du Nlonako (CAN), mis en place à la faveur de la Ligne Chaude du Cameroun se caractérise par une grande variété de roches plutoniques largement majoritaires par rapport aux roches volcaniques. Les roches plutoniques comprennent les granites alcalins, les syénites, les microsyénites, les monzonites, les monzodiorites, les micromonzodiorites, les microdiorites, les microgabbros, les gabbros anorthositiques et les gabbros. Les roches volcaniques sont constituées essentiellement de rhyolites alcalines et de mugéarites. En plus des variations texturales, ces roches montrent des variations minéralogiques vraisemblablement induites par des changements des conditions physico-chimiques au cours du refroidissement du magma. La forte proportion du quartz dans la plupart des roches fait penser au rôle de la contamination crustale durant la différenciation. L'étude minéralogique définit la paragénèse à labrador + hortonolite + augite magnésienne + magnésio-hornblende + magnésio-hornblende hastingsitique + biotite titanifère dans les roches basiques et intermédiaires, et la paragénèse à feldspath perthitique + fayalite + ferroaugite + augite + augite aegyrinique + ferrohédenbergite + aenigmatite + riébeckite + hornblende hastingsite magnésienne + hornblende ferro-édenitique dans les roches acides. L’évolution minéralogique continue des principales phases minérales (feldspaths et pyroxènes) est marquée par une augmentation de Fe2+, Mn, Fe3+, Na et par une diminution de Mg, Al et Ca. Cette évolution rappelle celle d’une suite alcaline de type gabbro-diorite-monzodiorite-monzonite-syénite-granite qui est interprétée comme le résultat de la cristallisation fractionnée essentiellement contrôlée par l’olivine, le pyroxène et le feldspath. ISSN 1029–2225©2014 Sciences, Technologies et Développement os Mots clés : Complexe anorogénique du Nlonako, roches plutoniques, roches volcaniques, évolution minéralogique, cristallisation fractionnée, Ligne Chaude du Cameroun, Cameroun. 1. Introduction The Nlonako Massif is one of the over sixty anorogenic complexes encountered along the Cameroon Hot Line (CHL). It reaches an altitude of 1825 m and exhibits an elliptical form that covers a surface area of about 71 km². Apart from a few reconnaissance studies in the 80’s (Dumort, 1968; Tchoua, 1970, 1974; Lasserre, 1978; Cantagrel et al., 1978; Tempier and Lasserre, 1980), no detailed study on petrography and mineralogy has been published on the Nlonako Anorogenic Complex (NAC) so far. 4This investigation is to present the *Corresponding author. E-mail: [email protected] new petrographic and mineralogical data obtained respectively from polarizing microscope descriptions and CAMEBAX electron microprobe analyses with the aim to define the petrogenetic evolution of the Nlonako Massif. 2. Geological setting and previous investigations The Nlonako Massif was emplaced along the CHL. The CHL is an important volcano-tectonic structure which includes volcanic massifs and anorogenic plutons trending N30°E from Pagalu Island to Lake Chad (fig.1). Many hypotheses have been proposed to explain its origin (Mascle, 1976; Fitton and Dunlop, 1985; Lee et al., 1994; Burke, 2001; Montigny et al., 2004 and Déruelle et al., 2007). 32 Abolo et al., Sciences, Technologies et Développement (Février 2014), Volume 15, 32-42 Sciences, Technologies & Développement, ISSN 1029 - 2225 Previous investigations on the NAC remain fragmentary. Dumort (1968) described the hill as a well rounded massif made up of syenites and gabbros. According to that author, peralkali syenites postdate the gabbroic emplacement. Tchoua (1970) emphasized the predominance of syenites over the other rocks. In addition, he suggested that the Nlonako massif was differentiated from peralkali syenitic and granitic magmas (Tchoua, 1974). Geochemical data have confirmed the alkali nature of the magma (Tempier and Lasserre, 1980). Radiogenic dating K/Ar and Rb/Sr methods indicates an age between 45.0±1 and 35±1 Ma (Cantagrel et al., 1978) and an age of 43.3±0.3 (Lasserre, 1978) respectively. 3. Analytical Techniques and Methods Prior to electron microprobe studies, polished thin sections were described under the polarizing microscope. The nomenclature of the rocks is defined from QAP diagram (fig. 2), on both modal and normative compositions (Streckeisen, 1976; Streckeisen, 1980; Müller, 1982). Mineral analyses of rock samples were performed on a CAMEBAX electron microprobe at “Bundesanstaalt für Geowissenschaften und Rohstoffe” (B.G.R), Hanover, Germany. Selections of these analyses are shown in tables 1-6 and are given in terms of oxides of the element (wt. %). Iron content is indicated in form of FeO, which corresponds to the value of total iron. Chemical analyses were performed on olivines in quartz syenites, microgabbros and micromonzodiorites. The pyroxene types were defined after Morimoto (1988). Although the International Mineralogical Association (IMA) classification of Leake (1997) does not adequately address new discoveries of new compositional types of amphibole, this classification has been used for the amphibole nomenclature in this work instead of the recent classification by Hawthorne et al. (2012) which takes into account the crystal-chemical and petrological importance of compositional variables such as Fe2+, Fe3+, Li, and wO2contents. Our choice is based on the fact that some elements (Li, Pb, Be or O2-) which are taken into account by this recent classification were not determined in this study. Figure 1. Location of the Cameroon Hot Line, showing the main plutonic and volcanic massifs. The Nlonako Alkaline Complex is underlined. (After Déruelle et al., 2007). 33 Abolo et al., Sciences, Technologies et Développement (Février 2014), Volume 15, 32-42 Sciences, Technologies & Développement, ISSN 1029 - 2225 4. Petrography The NAC is quite different from other anorogenic complexes of the CHL in the sense that it includes a greater variety of petrographic facies (fig. 3). In accordance with the chemical character of the feldspars, the abundance of dark minerals and the texture, the diagram in figure 2 allows recognition of the following rocks types: fayalite alkali granites; aegirine augite alkali granites; aegirine augite, aenigmatite and riebeckite bearing alkali granites; pyroxene bearing quartz syenites; fayalite and pyroxene bearing quartz syenites; amphibole bearing quartz syenites; amphibole quartz microsyenites; quartz syenites; quartz monzonites; monzonites; quartz monzodiorites; quartz micromonzodiorites; amphibole bearing micromonzodiorites; quartz microdiorites; microdiorites; microgabbros; anorthositic gabbros and gabbros. Beside these plutonic rocks, volcanic rocks comprise of aegirine and arfvedsonite alkali rhyolites and mugearites. Figure 2. Position of plutonic rocks from NAC in Q-A-P diagram, (a): modal compositions, (b): normative compositions. 34 Abolo et al., Sciences, Technologies et Développement (Février 2014), Volume 15, 32-42 Sciences, Technologies & Développement, ISSN 1029 - 2225 Figure 3. Simplified geological map of the Nlonako Alkaline Complex. Syenites occupy 70% of the Nlonako hill. In general, they are grey and heterogranular. Their texture is either porphyritic microgranular or finely granular at the border and porphyritic granular at the centre of the mountain (fig. 4). Quartz content of the syenites ranges from 6 to 15%. Perthitic orthoclase is largely dominant (60 vol. %) and often occurs as thickset or elongated prisms with different sizes. The presence of the plagioclase (An13-An32) depends of facies. Pyroxenes are always present in highly variable amounts and compositions: they include ferroaugite, aegirine augite and ferrohedenbergite. Amphiboles are represented in small amounts with various compositions (arfvedsonite, ferrowinchite, ferrorichterite, kataphorite and hornblende). Other minerals include sporadic fayalite, astrophyllite, aenigmatite, biotite and accessory minerals (apatite, zircon and dark minerals). Relations between these minerals suggest a crystallization order as follows: apatite-dark minerals-colored minerals-feldspars-quartz-astrophyllite and biotite. Granites are homogeneous compared with the syenites. The texture is coarse grained. They are free of plagioclases and contain strongly perthitic alkali feldspars (fig. 5). Quartz appears into two generations. One early generation with variable size; crystals are enclosed within the colored minerals. The microcrystalline generation is made of interstitial crystals and corresponds to the last phase of crystallization. Ferromagnesian minerals include fayalite, aegirine augite, riebeckite and aenigmatite. Light grey or yellow green monzonites are porphyraceous. Plagioclase (An28-An41) and orthoclase remain the primary mineral phase (> 30 vol. %). Other minerals include augite, hornblende, fayalite, biotite, quartz, apatite and ilmenite. Secondary minerals are calcite, epidote and uralite resulting from the alteration of the rock. The relation between minerals suggests that apatite was formed first and was followed respectively by ilmenite, colored minerals, feldspars and quartz. 35 Abolo et al., Sciences, Technologies et Développement (Février 2014), Volume 15, 32-42 Sciences, Technologies & Développement, ISSN 1029 - 2225 OR FA CPX 0,36 mm Figure 4. Granular texture in the quartz syenite. CPX OR QZ AE 0,36 mm Figure 5. Coarse grained alkali granite displaying strongly perthitic alkali feldspars. PL PL QZ 0,36 mm Diorites comprise several facies according to their texture and the abundance of quartz and amphibole. Rocks are mostly fine grained and often slightly deformed (fig. 6). The volume of plagioclase varies from 28.5% to 65.4% while its composition ranges from oligoclase to labradorite (An23-An57). Polysynthetic twins coexist with Carlsbad and/or Pericline twins. Rare orthoclase has crystallized in the same thickset or elongate habit as plagioclase. Colored minerals are present in variable amounts and comprise magnesian hastingsite, magnesio-hornblende, augite, ferroaugite, biotite, chlorite and olivine. Zircon, Fe-Ti oxides and apatite are the accessory minerals. The crystallization sequence may have started with apatite and was followed in order by Ti-Fe oxides, zircon, colored minerals, feldspars and quartz. The presence of orthopyroxene (ferrosilite and enstatite), encountered respectively within quartz micromonzodiorite and quartz diorite, is unusual for alkali series and need to be discussed. Gabbros are commonly grey or dark depending on the proportions of ferromagnesian minerals. Their texture is either microgranular, intergranular or granular (fig. 7). The mineralogy is dominated by labradorite, bytownite, augite and olivine. Olivine is partially transformed into iddingsite along cracks. Other minerals comprise magnesio-hornblende, hastingsitic magnesio-hornblende, tremolite, biotite, kaersutite, chlorite, apatite and ilmenite. In granular gabbro, plagioclase formed first and was followed in order by ilmenite, clinopyroxene, biotite and secondary minerals. In the microgabbro, the crystallization order is ilmenite-olivineplagioclase-clinopyroxene-biotite and amphibole. Rare light grey rhyolite and dark mugearite cut the plutonic rocks. Rhyolite displays a porphyritic microlithic or fluidal texture where phenocrysts of anorthoclase and aegirine are enclosed in a matrix composed of K-feldspars, quartz, aegirine and arfvedsonite. The texture of mugearite is microlitic or microgranular whereas constitutive minerals are phenocrysts of plagioclase (An30) and clinopyroxene and microcrysts of feldspars, quartz, pyroxene and opaques. In both cases, opaques may have crystallized first and were followed by clinopyroxene, amphibole, feldspars and quartz. Figure 6. Slightly deformed quartz microdiorite. 5. Mineralogy CPX CPX PL CPX 0,36 mm Figure 7. Microgranular texture in the gabbro. 5.1. Olivine Olivine occurs in variable proportion in almost all the rock units (up to 10%). The mineral is completely transformed into iddingsite in granites, syenites and monzonites. In addition, olivine is always linked to clinopyroxene. Olivine analyses indicate manganese enrichment within the acid rocks compared to basic rocks and very limited composition variations within the same rock. A compositional gap, probably due to limited number of analyses, is observed between Fo55 and Fo7.6 (fig. 8). Olivine in quartz syenites corresponds to fayalite (87.5-93% of Fe2SiO4). It is typically rich in Mn (2.87%-4.6%). Calcium 36 Abolo et al., Sciences, Technologies et Développement (Février 2014), Volume 15, 32-42 Sciences, Technologies & Développement, ISSN 1029 - 2225 content ranges between 0.25% and 0.38%. Tephroite and larnite mean contents are respectively 4% and 0.5%. Olivine is relatively rich in Mg and poor in Mn and Ca, and corresponds either to hyalosiderite or hortonolite in microgabbros and micromonzodiorites. Fayalite and fosterite contents range between Fa35.9-44.1 and Fo55-63.4 respectively. Table 1. Olivine compositions (wt% and atom per formula unit “a.p.f.u.”). Rock Microgabbros Quartz Micromonzodiorites type syenites SiO2 36.09 36.43 29.08 29.07 34.47 TiO2 0.04 0.02 0.02 0.05 0.04 Al2O3 0.01 0.02 0.00 0.00 0.01 FeO 31.69 31.76 66.15 65.79 38.18 MnO 0.50 0.52 4.61 4.57 0.78 MgO 31.38 31.29 0.05 0.03 27.13 CaO 0.06 0.05 0.31 0.32 0.04 Traces 0.04 0.00 0.01 0.01 0.03 Sum 100.08 100.23 99.83 100.68 Trace elements (ppm) Ni 280.00 0.00 40.00 40.00 200.00 Si 0.99 0.99 0.99 0.99 0.97 Ti 0.00 0.00 0.00 0.00 0.00 Al 0.00 0.00 0.00 0.00 0.00 Fe2+ 0.73 0.72 1.88 1.87 0.90 Mn 0.01 0.01 0.13 0.13 0.02 Ni 0.00 0.00 0.00 0.00 0.00 Mg 1.28 1.27 0.00 0.00 1.14 Ca 0.00 0.00 0.01 0.01 0.00 Somme Y 2.02 2.01 2.02 2.02 2.06 Fosterite 63.39 63.30 0.11 0.07 55.33 Fayalite 35.91 36.04 92.78 92.81 43.68 Tephroite 0.58 0.59 6.55 6.53 0.91 Mg* 0.64 0.64 0.00 0.00 0.56 Fig. 8: Compositions of olivines of plutonic rocks in the Mg-Mn-Fe diagram (cation %). 5.2. Pyroxenes Clinopyroxenes constitute the most important mineral phase after the feldspars. They are euhedral and primary. As typical of alkaline intrusive complexes, clinopyroxene shows a wide range of composition; several types are defined: Ca-Mg-Fe pyroxenes in quartz syenites, micromonzodiorites, microgabbros and gabbros, Ca-Na pyroxenes in alkali granites and quartz syenites, and Na-pyroxenes in alkali granites. Two evolutions are defined on the Di-He-Ac diagram (fig. 9). The first one is continuous from microgabbros to alkali granites and extends from calcic augites to ferro-augites and ferro-hedenbergites. The second evolution stretches along He-Ac join and is characterized by a compositional gap between Di3 Hd40 Ac57 and Di3 Hd72 Ac25; also by Na enrichment at the last stages of the differentiation. Chemical variation of pyroxenes during the differentiation is accompanied by Mg decrease at the expense of Fe2+ and Mn, then by Na enrichment in the last differentiated types. 5.3. Amphiboles Amphiboles are unequally distributed in the Nlonako rocks. They are primary or they result from the pseudomorphosis of pyroxenes. Yellow-green magnesian hastingsite and magnesio-hastingsite occur in the micromonzodiorites. Locally, these minerals form rims around calcic augites. Ferro-edenitic hornblende and magnesian hastingsitic hornblende are intensively corroded by felsic minerals and occur in the quartz microsyenites. Magnesio-hornblende and magnesio-hastingsitic hornblende are secondary minerals encountered as subhexagonal crystals as small amount in microgabbros. Small amount of ferro-winchite, ferro-richterite, katophorite and arfvedsonite occur as euhedral to subhedral and skeletal phenocryts in quartz syenites. Alkali granites and some quartz syenites display euhedral to subhedral riebeckites. These minerals are also interstitial in felsic minerals and are frequently associated with clinopyroxenes. Their green or brown color depends on Ti enrichment. The general evolution of the amphiboles during the differentiation is marked by Ti, Fe2+, Fe3+, Na, Mn increase, and by Mg, AlIV, Ca simultaneous decrease, whilst Si remains constant. 5.4. Micas Micas are Ti-rich (3.7 to 6 wt% TiO2) and their chemical compositions correspond to phlogopite in micromonzodiorites and microdiorites and to biotite in micromonzodiorites, microdiorites and gabbros. Despite the limited number of analyses, a slight progressive evolution with the differentiation is recognized. Fig. 9: Compositional variations of clinopyroxenes from NAC in the Di-He-Ac diagram (diopside-hedenbergite-acmite, % cations). 5.6. Feldspars Feldspars represent more than 50% of the total volume of the rocks except in the amphibole micromonzodiorites (35.5%). They appear as euhedral to subhedral crystals with different sizes. 37 Abolo et al., Sciences, Technologies et Développement (Février 2014), Volume 15, 32-42 Sciences, Technologies & Développement, ISSN 1029 - 2225 Table 2: Clinopyroxene compositions (wt. % and “a.p.f.u.” on the basis of 6 oxygens) Quartz micromonzodiorites SiO2 50.60 49.62 TiO2 0.53 0.38 Al2O3 1.45 0.54 Fe2O3 3.70 3.86 FeO 12.89 27.89 MnO 0.56 0.88 MgO 12.87 16.53 CaO 18.08 1.09 Na2O 0.30 0.00 K2O 0.00 0.00 Traces 0.03 0.00 Sum 100.10 100.80 Trace elements (ppm) Cr 0.00 30.00 Ni 200.00 0.00 Si 1.91 1.92 AlIV 0.06 0.02 CrIV 0.00 0.00 FeIV 0.02 0.05 Sum T 2.00 2.00 Ti 0.01 0.01 Al 0.00 0.00 Cr 0.00 0.00 3+ Fe 0.08 0.06 2+ Fe 0.41 0.90 Mn 0.02 0.03 Ni 0.00 0.00 Mg 0.48 0.95 Ca 0.00 0.05 Na 0.00 0.00 K 0.00 0.00 Sum C 1.00 2.00 Mg 0.25 Ca 0.73 Na 0.02 K 0.00 Sum B 1.00 Factor 2.27 2.33 Q 1.86 1.90 J 0.04 0.00 Fe+Mg 0.89 1.86 Fe/Fe+Mg 0.46 0.49 Rock type Micromonzodiorite 52.25 0.42 1.52 1.66 8.35 0.36 14.30 21.17 0.32 0.00 0.05 100.40 0.00 380.00 1.94 0.06 0.00 0.00 2.00 0.01 0.01 0.00 0.05 0.26 0.01 0.00 0.66 Quartz microdiorites 50.90 51.48 0.24 0.17 0.58 0.53 3.65 2.71 23.15 23.15 0.88 0.85 19.72 20.34 1.34 1.07 0.03 0.00 0.00 0.02 0.01 0.01 100.49 100.32 50.00 0.00 1.93 0.03 0.00 0.04 2.00 0.01 0.00 110.00 1.95 0.02 0.00 0.03 2.00 0.00 1.00 0.13 0.84 0.02 0.06 0.73 0.03 0.00 1.11 0.05 0.00 0.00 2.00 0.00 0.00 0.00 0.05 0.73 0.03 0.00 1.15 0.04 0.00 0.00 2.00 0.00 0.00 0.00 1.00 2.23 1.90 0.05 0.92 0.28 0.00 2.28 1.90 0.01 1.85 0.40 0.00 2.27 1.92 0.00 1.88 0.39 Figure 10. Compositions of feldspars (mol %) from NAC in the AbAn-Or diagram. Alkali feldspar occurs in alkali granites and quartz syenites. This mineral always displays Carlsbad twinning. In addition, it is characterized by a great development of perthitic texture allowing them to be considered as hyposolvus minerals (Tuttle and Bowen, 1958; Parsons, 1978). Chemical analyses define an orthoclase (Or93-96) in granites. In syenites, the composition is heterogeneous and is organized along Or-Ab Microdiorite Microgabbros Alkali granites Quartz syenites 50.76 0.35 1.07 3.19 11.71 0.33 11.13 21.15 0.41 0.05 0.01 100.15 50.51 1.14 2.51 3.22 8.60 0.32 13.74 20.61 0.34 0.00 0.00 101.01 Gabbros 50.43 1.28 2.74 3.22 6.26 0.23 14.79 20.88 0.40 0.00 0.00 100.22 52.43 0.51 1.27 2.95 6.31 0.25 15.61 20.93 0.41 0.00 0.03 100.71 49.36 1.60 3.69 3.68 5.51 0.20 14.46 20.77 0.51 0.01 0.00 99.78 50.76 1.22 0.27 30.24 2.42 0.57 0.10 5.00 11.26 0.00 0.00 101.84 48.77 1.66 0.45 25.37 11.33 2.03 0.47 1.98 8.18 1.67 0.01 110.92 48.70 0.18 0.30 4.01 21.99 1.21 3.79 20.10 0.57 0.01 0.05 100.91 48.15 0.38 0.49 4.21 21.82 1.25 3.90 19.95 0.50 0.01 0.02 100.68 40.00 0.00 1.94 0.05 0.00 0.02 2.00 0.01 0.00 0.00 0.08 0.37 0.01 0.00 0.53 0.00 0.00 1.88 0.11 0.00 0.01 2.00 0.03 0.00 0.00 1.87 0.12 0.00 0.01 2.00 0.04 0.00 80.00 1.92 0.02 0.00 0.06 2.00 0.05 310.00 0.00 1.95 0.01 0.00 0.04 2.00 0.01 130.00 0.00 1.93 0.02 0.00 0.04 2.00 0.01 0.08 0.19 0.01 0.00 0.68 30.00 0.00 1.84 0.16 0.00 0.00 2.00 0.04 0.00 0.00 0.10 0.17 0.01 0.00 0.67 0.00 0.00 1.94 0.01 0.00 0.05 2.00 0.04 0.08 0.27 0.01 0.00 0.61 0.00 250.00 1.93 0.06 0.00 0.01 2.00 0.01 0.00 0.00 0.07 0.19 0.01 0.00 0.71 0.08 0.73 0.04 0.00 0.13 1.00 0.15 0.82 0.02 0.00 1.00 2.24 1.85 0.05 0.88 0.31 1.00 0.14 0.83 0.03 0.00 1.00 2.23 1.84 0.06 0.87 0.22 1.00 0.14 0.83 0.03 0.00 1.00 2.21 1.88 0.06 0.91 0.21 1.00 0.13 0.83 0.04 0.00 1.00 2.24 1.81 0.07 0.84 0.20 0.69 0.37 0.07 0.00 0.03 0.08 0.62 0.08 2.00 0.09 0.74 0.04 0.00 0.13 1.00 0.10 0.86 0.03 0.00 1.00 2.29 1.87 0.06 0.90 0.41 0.82 0.08 0.02 0.00 0.01 0.04 0.00 0.00 1.00 0.17 0.83 0.00 1.00 2.30 0.29 1.67 0.08 0.93 0.00 0.00 0.00 0.00 2.36 0.48 1.25 0.40 0.93 1.00 0.09 0.86 0.04 0.00 1.00 2.40 1.82 0.09 0.87 0.85 1.00 0.10 0.86 0.04 0.00 1.00 2.41 1.82 0.08 0.86 0.85 join but close to the albite end-member. A hiatus is observed between Or35 and Or93 (fig. 10). Apart from alkali granites, plagioclase exists in all rocks, with a wide variation in composition from An12 to An75. Oligoclase predominates in quartz microsyenites, quartz syenites and microdiorites. Its composition varies slightly within the same mineral (An26-An29) and the Or content remains high. Oligoclase and andesine with calcic core (An53-An57) occur in micromonzodiorites. The composition of plagioclase is quite homogeneous (An54-An64) in gabbros. Labradorite, often with bytownite core, occurs in microgabbros. Chemical evolution of plagioclases is progressive and continuous from basic rocks to acid rocks. The differentiation process is characterized by an impoverishment of Ca at the enrichment in Na, in other words, K-feldspar becomes more important than plagioclase. 5.7. Aenigmatite Aenigmatite is present as red prisms with variable dimensions in alkali granites (7%) and in quartz syenites (0.5%). Chemical analyses were carried on quartz syenites. The structural formula indicates aenigmatite (s.s.) composition of 65 to 78.3 mol. %, Fe-aenigmatite composition of 19.8 to 29.6 mol. % and rhoenite composition of 1.9 to 6.4 mol. %. 38 Abolo et al., Sciences, Technologies et Développement (Février 2014), Volume 15, 32-42 Sciences, Technologies & Développement, ISSN 1029 - 2225 Table 4. Mica compositions (wt% and “a.p.f.u.” on the basis of 22 oxygens). Table 3. Amphibole compositions (wt% and “a.p.f.u.” on the basis of 23 oxygens). Rock Micromonzodiorites type SiO2 41.75 TiO2 3.74 Al2O3 10.70 Fe2O3 4.33 FeO 10.61 MnO 0.27 MgO 12.38 CaO 11.25 Na2O 2.68 K2O 0.90 Traces 0.08 Sum 98.70 Trace elements (ppm) Cr 50.00 Ni 580.00 Si 6.16 AlIV 1.84 CrIV 0.00 FeIV 0.00 Sum T 8.00 Ti 0.41 AlIV 0.03 Cr 0.00 3+ Fe 0.48 2+ Fe 1.31 Mn 0.03 Ni 0.01 Mg 2.72 Sum C 5.00 Ca 1.78 Na 0.22 Sum B 2.00 Na 0.55 K 0.17 Sum A 0.72 Factor 8.87 CANA 2.00 NAB 0.22 NAKA 0.72 Si 6.16 AlIV 1.84 MGFE 0.68 42.34 3.26 10.43 4.42 10.24 0.21 12.70 11.15 2.68 0.96 0.03 98.42 190.00 0.00 6.25 1.75 0.00 0.00 8.00 0.36 0.06 0.00 0.49 1.26 0.03 0.00 2.79 5.00 1.76 0.24 2.00 0.53 0.18 0.71 8.87 2.00 0.24 0.71 6.25 1.75 0.69 Quartz micromonzodiorites 44.46 44.52 1.62 1.59 6.86 7.18 10.20 9.39 10.04 11.26 0.38 0.32 11.58 11.19 10.76 10.96 1.50 1.55 1.15 1.12 0.04 0.08 98.58 99.15 Quartz syenites 46.23 47.91 1.69 1.40 1.40 0.86 9.10 6.98 29.26 30.67 1.44 1.32 0.04 0.36 5.27 4.14 4.90 6.35 1.16 1.25 0.02 0.00 100.50 101.24 Quartz microsyenites 42.04 42.33 1.01 1.01 7.18 7.07 8.42 8.68 18.50 17.50 1.79 1.96 6.43 6.69 10.62 10.47 2.46 2.34 1.28 1.21 0.05 0.00 99.78 99.26 0.00 330.00 6.61 1.20 0.00 0.19 8.00 0.18 90.00 50.00 7.35 0.26 0.00 0.38 8.00 0.20 130.00 280.00 6.49 1.31 0.00 0.20 8.00 0.12 0.95 1.25 0.05 0.01 2.57 5.00 1.71 0.29 2.00 0.14 0.22 0.36 8.93 2.00 0.29 0.36 6.61 1.20 0.67 80.00 500.00 6.60 1.25 0.00 0.14 8.00 0.18 0.91 1.40 0.04 0.01 2.47 5.00 1.74 0.26 2.00 0.19 0.21 0.40 8.91 2.00 0.26 0.40 6.60 1.25 0.64 0.70 3.89 0.19 0.00 0.01 5.00 0.90 1.10 2.00 0.41 0.23 0.64 9.55 2.00 1.10 0.64 7.35 0.26 0.00 0.00 0.00 7.55 0.16 0.00 0.29 8.00 0.17 0.53 4.04 0.18 0.00 0.08 5.00 0.70 1.30 2.00 0.64 0.25 0.89 9.46 2.00 1.30 0.89 7.55 0.16 0.02 0.78 2.39 0.23 0.00 1.48 5.00 1.76 0.24 2.00 0.49 0.25 0.74 9.28 2.00 0.24 0.74 6.49 1.31 0.38 0.00 0.00 6.54 1.29 0.00 0.18 8.00 0.12 0.83 2.26 0.26 0.00 1.54 5.00 1.73 0.27 2.00 0.43 0.24 0.67 9.28 2.00 0.27 0.67 6.54 1.29 0.41 Microgabbros Alkali granites 45.65 0.65 9.86 6.33 8.04 0.16 13.91 12.07 2.09 0.08 0.00 98.84 44.24 0.75 10.54 6.23 8.99 0.13 13.13 12.20 2.12 0.09 0.01 98.45 0.00 70.00 90.00 0.00 6.45 6.33 1.55 0.00 0.00 1.66 8.00 8.00 0.08 0.96 0.26 0.00 0.68 1.32 1.10 2.38 0.02 0.34 0.00 2.85 0.01 5.00 5.00 1.91 0.00 0.09 2.00 2.00 2.00 0.51 0.43 0.02 0.00 0.52 0.43 8.76 9.33 2.00 2.00 0.09 2.00 0.52 0.43 6.45 6.33 1.55 0.00 0.72 0.00 6.58 1.42 8.00 0.07 0.26 0.00 0.69 0.97 0.02 2.99 5.00 1.86 0.14 2.00 0.45 0.01 0.46 8.67 2.00 0.14 0.46 6.58 1.42 0.76 40.80 8.18 0.02 25.52 18.31 2.56 0.05 0.01 8.07 0.02 0.01 103.55 40.75 8.18 0.04 24.64 18.50 2.25 0.11 0.01 7.90 0.00 0.00 102.37 0.00 0.00 6.38 0.01 0.00 1.61 8.00 0.96 1.29 2.42 0.30 0.00 0.03 5.00 0.00 2.00 2.00 0.40 0.00 0.40 9.41 2.00 2.00 0.40 6.38 0.01 0.01 Rock Quartz type microdiorites SiO2 37.77 38.31 TiO2 4.94 4.72 Al2O3 12.38 12.31 FeO 14.63 14.46 MnO 0.08 0.03 MgO 15.80 16.03 CaO 0.02 0.00 Na2O 0.08 0.16 K2O 9.67 9.74 Traces 0.05 0.04 Sum 95.43 95.80 Trace elements (ppm) Cr 80.00 240.00 Ni 310.00 60.00 Si 5.60 5.65 Al 2.17 2.14 Cr 0.00 0.00 Mg Fe3+ 0.23 0.20 Sum 8.00 8.00 Al Ti 0.55 0.52 Fe3+ 0.13 0.15 2+ Fe 1.45 1.43 Mg 3.50 3.53 Mn 0.01 0.00 Ni 0.00 0.00 Sum 5.65 5.63 Ca 0.00 0.00 Ba Na 0.02 0.05 K 1.83 1.83 Sum 1.86 1.88 Factor 8.92 8.86 35.44 4.76 13.31 23.65 0.11 10.47 0.02 0.15 9.02 0.02 96.95 36.20 4.92 12.92 22.60 0.04 10.86 0.00 0.21 9.17 0.02 96.94 Quartz micromonzodiorites 39.31 37.34 4.28 4.28 11.93 11.90 20.37 20.60 0.12 0.05 11.72 12.19 0.00 0.00 0.17 0.20 9.18 9.26 0.04 0.01 94.13 95.83 0.00 130.00 5.38 2.38 0.00 20.00 110.00 5.46 2.30 0.00 150.00 180.00 5.60 2.17 0.00 0.24 8.00 0.24 8.00 0.54 0.36 2.40 2.37 0.01 0.00 5.69 0.00 0.04 1.75 1.79 9.12 Microdiorites Micromonzodiorite Gabbros 36.56 4.85 14.43 14.85 0.04 15.36 0.02 0.52 8.86 0.01 95.50 36.31 6.13 13.50 17.64 0.09 12.79 0.00 0.18 9.46 0.01 96.11 35.84 5.83 13.48 17.66 0.12 12.50 0.00 0.36 9.24 0.10 95.14 70.00 0.00 5.65 2.12 0.00 0.00 80.00 5.41 2.52 0.00 50.00 0.00 5.48 2.40 0.00 0.12 0.00 820.00 5.47 2.43 0.00 0.10 0.22 8.00 0.23 8.00 0.07 8.00 0.56 0.34 2.28 2.44 0.01 0.00 5.63 0.00 0.50 0.30 2.10 2.70 0.02 0.00 5.62 0.00 0.49 0.29 2.08 2.75 0.01 0.00 5.62 0.00 0.54 0.29 1.47 3.39 0.01 0.00 5.70 0.00 0.06 1.77 1.83 9.07 0.05 1.81 1.86 9.27 0.06 1.79 1.85 9.09 0.15 1.67 1.82 8.89 8.00 0.00 0.70 0.00 2.23 2.76 0.01 0.00 5.69 0.00 0.00 0.05 1.82 1.87 9.07 8.00 0.00 0.67 0.00 2.25 2.74 0.02 0.01 5.69 0.00 0.00 0.11 1.80 1.91 9.17 39 Abolo et al., Sciences, Technologies et Développement (Février 2014), Volume 15, 32-42 Sciences, Technologies & Développement, ISSN 1029 - 2225 Table 5: Feldspar compositions (wt% and “a.p.f.u.” on the basis of 8 oxygens) Rock Quartz Quartz Quartz Quartz type microsyenites micromonzodiorites Micromonzodiorites microdiorites Microdiorites Alkali granites syenites Microgabbros Gabbros SiO2 64.86 64.79 63.75 64.91 64.64 65.26 53.56 61.65 54.17 55.37 52.91 50.20 61.92 61.61 48.33 48.73 52.18 51.99 Al2O3 17.67 17.76 19.20 21.40 18.46 21.68 29.31 24.31 28.90 27.92 29.35 31.47 23.75 24.05 32.18 32.10 29.75 30.15 Fe2O3 0.84 0.87 0.23 0.17 0.16 0.29 0.35 0.27 0.22 0.44 0.50 0.49 0.42 0.22 0.43 0.36 0.36 0.28 CaO 0.47 2.59 0.00 2.76 11.79 5.86 11.01 10.48 12.33 14.54 5.10 5.07 15.43 15.04 12.78 12.99 Na2O 0.75 0.62 4.83 9.19 0.64 9.68 4.78 8.08 5.12 5.48 4.30 3.15 8.41 8.31 2.64 2.74 4.39 4.15 K2O 15.79 16.20 8.73 1.14 15.79 0.78 0.18 0.51 0.18 0.19 0.41 0.24 0.78 0.91 0.17 0.16 0.28 0.25 Sum 99.90 100.24 97.20 99.38 99.69 100.45 99.97 100.69 99.59 99.88 99.79 100.09 100.37 100.17 99.19 99.13 99.73 99.81 Si 3.00 3.00 2.96 2.88 2.86 2.87 2.42 2.72 2.45 2.50 2.41 2.29 2.75 2.74 2.23 2.25 2.38 2.37 Al 0.96 0.97 1.05 1.12 1.13 1.12 1.56 1.27 1.54 1.49 1.57 1.69 1.24 1.26 1.75 1.74 1.60 1.62 3+ Fe 0.03 0.03 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 Sum 4.00 4.00 4.02 4.01 4.00 4.00 4.00 4.00 4.01 4.00 4.00 4.00 4.00 4.00 4.00 4.00 3.99 3.99 Ca 0.02 0.12 0.13 0.13 0.57 0.28 0.53 0.51 0.60 0.71 0.24 0.24 0.76 0.74 0.62 0.63 Na 0.07 0.06 0.43 0.79 0.82 0.83 0.42 0.69 0.45 0.48 0.38 0.28 0.72 0.72 0.24 0.24 0.39 0.37 K 0.93 0.96 0.52 0.06 0.04 0.04 0.01 0.03 0.01 0.01 0.02 0.01 0.04 0.05 0.01 0.01 0.02 0.01 Sum 1.00 1.01 0.97 0.98 1.00 1.00 1.00 1.00 0.99 1.00 1.00 1.00 1.01 1.01 1.01 1.00 1.03 1.01 Factor 2.78 2.78 2.79 2.67 2.65 2.64 2.72 2.66 2.72 2.71 2.73 2.74 2.66 2.67 2.77 2.77 2.74 2.74 Or 93.31 94.50 53.05 6.58 4.33 4.37 1.03 2.87 1.03 1.09 2.35 1.39 4.37 5.13 0.99 0.93 1.57 1.41 Ab 6.69 5.50 44.57 80.84 82.64 82.63 41.86 69.33 45.23 48.07 37.79 27.80 71.63 70.96 23.41 24.56 37.76 36.09 An 0.00 0.00 2.38 12.58 13.03 13.00 57.12 27.80 53.74 50.84 59.85 70.81 24.00 23.90 75.60 74.51 60.68 62.50 Table 6: Ilmenite (a), magnetite (b) and aenigmatite (c) compositions (wt%) Rock Qz Qz type microdiorite Microdiorite syenite Gabbro TiO2 49.96 51.84 50.91 48.72 Al2O3 0.00 0.00 Fe2O3 4.95 1.07 3.16 6.56 FeO 42.26 45.47 42.90 42.74 MnO 1.02 1.01 2.85 0.90 MgO 0.91 0.05 0.00 0.08 Traces 0.06 0.04 0.04 0.05 Sum 99.15 99.49 99.86 99.04 Trace elements (ppm) Cr 250.00 0.00 250.00 20.00 Ni 50.00 0.00 0.00 0.00 Zn 100.00 360.00 0.00 380.00 2+ Fe 1.79 1.93 1.82 1.83 Mg 0.07 0.00 0.00 0.01 Mn 0.04 0.04 0.12 0.04 Zn 0.00 0.00 0.00 0.00 Ni 0.00 0.00 0.00 0.00 Fe3+ 0.10 0.02 0.06 0.13 Sum A 2.00 2.00 2.00 2.00 Ti 1.90 1.98 1.94 1.87 Al 0.00 0.00 Cr 0.00 0.00 0.00 0.00 Fe3+ 0.09 0.02 0.06 0.13 Sum B 2.00 2.00 2.00 2.00 Factor 3.05 3.05 3.04 3.07 a Rock Qz Qz Qz type micromonzodiorite microsyenite Gabbro microdiorite TiO2 2.51 8.48 1.44 1.94 Al2O3 0.78 0.37 1.46 1.26 Fe2O3 63.33 52.12 63.54 63.47 FeO 33.46 36.76 32.33 32.48 MnO 0.13 1.92 0.03 0.08 MgO 0.02 0.02 0.18 0.25 Traces 0.68 0.25 1.35 0.60 Sum 100.91 99.92 100.32 100.08 Trace elements (ppm) Cr 70.00 10.00 510.00 180.00 Ni 260.00 300.00 40.00 410.00 V 3230.00 0.00 7800.00 2800.00 Zn 1270.00 1660.00 1000.00 900.00 Mg 0.00 0.00 0.01 0.01 Fe2+ 0.92 0.69 0.94 0.92 Ni 0.00 0.00 0.00 0.00 Mn 0.00 0.06 0.00 0.00 Ti 0.07 0.24 0.04 0.06 Zn 0.00 0.01 0.00 0.00 Sum A 1.00 1.00 1.00 1.00 Al 0.03 0.07 0.07 0.06 Cr 0.00 0.00 0.00 0.00 Fe2+ 0.14 0.08 0.08 0.11 Fe3+ 1.81 1.82 1.82 1.82 V 0.01 0.03 0.03 0.01 Sum B 2.00 2.00 2.00 2.00 Factor 2.28 2.28 2.28 2.29 b 5.8. Fe-Ti oxides Absent from alkali granites, Fe-Ti oxides occur in the rest of the rocks in amounts up to 10%. The minerals crystallized after the plagioclase in gabbros, elsewhere, it is primary. Chemical analyses indicate the presence of ilmenite (46.8 < TiO2 % <51.8) and magnetite (88.8 <Fe2O3+FeO % < 97). 6. Discussion The primary magma source of the Nlonako rocks is basaltic, as demonstrated by the presence of gabbros and Qz syenite Rock type SiO2 38.90 TiO2 6.63 Al2O3 0.69 Fe2O3 15.45 FeO 31.08 MnO 1.21 MgO 0.00 CaO 0.69 Na2O 7.82 K2O 0.00 Traces 0.04 Sum 102.50 Trace elements (ppm) Cr 0.00 Ni 330.00 Si 5.49 Al 0.11 Cr 0.00 3+ Fe 0.40 Sum T 6.00 Ti 0.70 Fe3+ 0.30 Sum Ti 1.00 Fe3+ 0.95 Fe2+ 3.67 Mn 0.14 Mg 0.00 Sum Y 4.76 Ca 0.10 Na 2.14 K 0.00 Sum X 2.24 Facteur 8.48 Rhoenite 5.19 Aenigmatite 65.21 Feaenigmatite 29.60 c microgabbros. These rocks include metaluminous (Na2O+K2O<Al2O3) and peralkaline (Na2O+K2O>Al2O3) suites. Sodium in the melt was high enough to allow crystallization of arfvedsonite and aegirine in peralkaline rocks under both reduced and oxidized conditions (Markl et al., 2010, Marks et al., 2011). In particular, the presence of titanite+magnetite +quartz indicates oxidizing oxygen fugacity conditions whereas the presence of aenigmatite and ilmenite indicates that the rocks were relatively reducing (Shellnutt and Lizuka, 2010). In addition to textural variations, these 40 Abolo et al., Sciences, Technologies et Développement (Février 2014), Volume 15, 32-42 Sciences, Technologies & Développement, ISSN 1029 - 2225 rocks display evidence for mineralogical variations which could be related to changes in the physical and chemical conditions of the magma. The presence of perthitic feldspars demonstrate that the rocks were generated at depth from hot and dry magma. Moreover, the absence of free plagioclases in alkali granites is linked to the fact that the rocks have crystallized at relatively high temperature, under hypersolvus conditions (Bard, 1980). Perthitisation took place after the crystallization of alkali feldspars. The occurrence of orthopyroxenes (ferrosilite and enstatite) respectively in quartz micromonzodiorite and quartz microdiorite is unusual in the alkaline series. It indicates the interaction between the magma and the country rocks as suggested in Mount Mboutou (Parsons et al., 1986). The mineralogical study defines in the basic and intermediate rocks a labradorite + hortonolite + magnesian augite + magnesio-hornblende + hastingsitic magnesio-hornblende + titaniferous biotite paragenesis. For the acid rocks, the paragenesis is perthitic feldspar + fayalite + ferroaugite + augite + aegirinic augite + ferrohedenbergite + aenigmatite + riebeckite + magnesian hastingsitic hornblende + ferroedenitic hornblende. The main substitution observed during the evolution of the olivine is Mg 1 Fe2+ exchanges within Fa-Fo series; but it can also be accompanied by Fe2+ 1 Mn substitution within Fa-Te series. Both Fe and Mn increase with the differentiation while Mg decreases. The first evolution of pyroxenes in figure 9 suggests low oxygen fugacity during crystallization (Larsen, 1976) and is controlled by the substitution Ca Mg 1 Ca Fe2+, Mn2+ ; whereas, the second one can be related to peralkalinity. This evolution suggests high oxygen fugacity and is controlled by the substitution Ca Mg Fe2+ Mn 1 Na Fe3+ Al3+ Al Ti (Yagi, 1966). The passage from calcic amphiboles to alkaline amphiboles corresponds to the diminution of the temperature during the crystallization (Fabries, 1978). The main substitution which results is: Na(B) Fe3+(C) 1 Ca(B) Mg(C). The evolution of micas is similar to the one observed in Ntumbaw (Ghogomu, 1984). It shows Fe increase follow by Mg decrease. Fe/Fe+Mg varies from 28% to 54%. The main substitution here is Mg2+1 Fe2+. In general, the mineralogical evolution of the main mineral phases (olivines, feldspars and clinopyroxenes) is progressive and is marked by an increase of Fe2+, Mn, Fe3+, Na and by a decrease of Mg, Al and Ca. This mineralogical evolution and the importance of K-feldspar in comparison with plagioclase during the differentiation are compatible with the gabbro-diorite-monzodiorite-monzonitesyenite-granite alkali suite. Therefore, it appears that the fractional crystallization is the only processes that generated acids rocks from basic rocks in the NAC. However, the mineralogical study has highlighted the quartz enrichment in some rocks suggesting the minor influence of crustal contamination at the end of the differentiation. This is confirmed by initial 87Sr/86Sr values of 0. 7055 and 0.7056 obtained respectively in syenites (Lasserre, 1978) and in granites (analyses from “BGR Laboratory, Germany). In the field, the predominance of felsic rocks over mafic rocks seems to contradict the proposed fractional crystallization. This can be explained either by erosion not yet reaching down into more basic-dominated levels or by density differences promoting the rise of acid more than basic liquids. 7. Conclusions Petrographical and mineralogical topics considered in this paper help to complete earlier, fragmentary investigations on the petrogenesis of Nlonako hill and to discuss some issues that are raised. The NAC comprises wide range of predominantly plutonic rocks with minor volcanic rocks. The plutonic rocks form a complete petrographic suite, some associated with microgranular equivalents, which extend from gabbros to alkali granites through diorites, monzodiorites, monzonites and syenites. Most of these rocks indicate high content in quartz in addition to textural and mineralogical variations. The occurrence of orthopyroxene (ferrosilite and enstatite) in the Nlonako massif suggests an interaction between the magma and the basement during the differentiation. In spite of the wide range of petrographic types with various compositions encountered in the massif, an hypothesis invoking the existence of two magmas (an acid one and a basic one) is to be excluded. The rocks were differentiated at depth from a hot and dry magma by fractional crystallization processes mainly controlled by olivine, clinopyroxene and feldspar. The assimilation of continental crust has affected this process to some extent. Acknowledgements Martin Guy ABOLO gratefully acknowledges all anonymous reviewers for providing constructive comments on the manuscript, the German Academic Exchange Service (Deutscher Akademischer Austausch Dienst “DAAD”) for providing him with a six months grant in Germany and the Bundesanstaalt für Geowissenschaften und Rohstoffe (B.G.R) in Hanover, Germany for performing all the analyses related to this study. References Bard, J. 1980. Microtextures des roches magmatiques et métamorphiques. Masson Ed. Paris. 192 p. Burke, K. 2001. Origin of the Cameroon Line of Volcano-Capped Swells. The Journal of Geology 109: 349-362. Cantagrel, J. M Jamond, C. & Lasserre, M. 1978. Le magmatisme alcalin de la ligne du Cameroun au Tertiaire inférieur : données géochronologiques K/Ar. Comptes rendus bulletin de la société géologique de France 6: 300-303. Carmichael, I. S. F., Turner, F. J. & Verhoogen, J. 1974. Igneous petrology. Mac graw Hill Book co, New York. 789 p. 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