Hostname: page-component-cd9895bd7-jn8rn Total loading time: 0 Render date: 2024-12-25T19:33:57.187Z Has data issue: false hasContentIssue false
  • English
  • Français

The mafic mineralogy of the Pandé massif, Tikar plain, Cameroon: implications for a peralkaline affinity and emplacement from highly evolved alkaline magma

Published online by Cambridge University Press:  05 July 2018

E. Njonfang  and
C. Moreau
E. Njonfang
Affiliation:
Laboratoire de Géologie, École Normale Supérieure, Université de Yaoundé I, BP 47, Cameroun
C. Moreau*
Affiliation:
Département des Sciences de la Terre, Pôle Sciences et Technologie, Université de la Rochelle, Avenue Michel Crépeau, 17402, La Rochelle Cedex 1, France
*
* E-mail: [email protected]
Get access Rights & Permissions [Opens in a new window]

Abstract

The Pandé massif is a small (4.9×63.4 km) subvolcanic complex of the Cameroon Line striking W – E and intrudes a Panafrican granite basement. It comprises a syenite-granite suite, where coarse- to finegrained syenites are predominant and the granites are the product of residual melt after syenite crystallization, and two volcanic (trachyte-rhyolite and trachyte) sequences. Amphibole and pyroxene are the dominant mafic silicates, the first occurring mainly in rhyolites and coarse- to medium-grained syenites, and the second, principally in all syenites, trachytes and granites. Rare biotite flakes are encountered in the coarse-grained or alkaline syenites and fayalite rimmed with oxides occurs in trachyte from the first volcanic sequence (T1). Apatite and zircon are common accessories, whereas some titanite occurs in the medium-grained syenites. The plutonic rocks are drusy, intrude the first volcanic sequence but pre-date the second (T2).

All the mafic minerals are Fe-rich. Detailed studies of amphibole and pyroxene show that their compositions define relatively limited trends, amphibole varying from ferro-richterite to arfvedsonite and pyroxenes along the acmite-hedenbergite join of the Ac-Hd-Di diagram, in both the intrusive suite and volcanic rocks. Where the two minerals coexist, pyroxene crystallized subsequent to amphibole, a situation generally found in late-stage or subsolidus aegirines. The overlap in plutonic and volcanic pyroxene trends suggests their crystallization from magmas of the same composition. However, the presence of quartz and fayalite in T1 and of pure aegirine in T2 and the occurrence of Zr-bearing aegirine (NaZr0.5Fe0.52+Si2O6) in the early crystallizing alkaline syenites evolving towards pure aegirine from medium- to fine-grained quartz syenites and granites, are consistent with changes in oxygen fugacities during magmatic differentiation. Two stages are distinguished: fO2 increasingly decreased from T1 to alkaline syenite emplacement (from 10−16 to 10−24 bracketed by WM and QFM buffers) where a disequilibrium, probably caused by water dissociation with volatile loss (H2) during magma degassing, favoured crystallization of Zr-bearing aegirine; a decrease in amphibole proportions towards medium-grained quartz syenites and an increase in fO2 from the medium-grained quartz syenites to granites and T2 sequence.

The Mg-poor nature of all the mafic silicates, subsolidus origin of amphiboles, crystallization of pyroxene subsequent to amphibole and subsolidus trends defined by pyroxenes are compatible with the parental magma having itself been a late-stage derivative magma, e.g. the last product of an alkaline melt from which the voluminous Mayo Darlé granite bodies crystallized.

Keywords

Plutono-volcanismPandéTikar plain,Cameroon Linesyenite-granite, trachyte-rhyoliteamphibolesclinopyroxenesperalkaline affinity
Type
Research Article
Information
Mineralogical Magazine , Volume 64 , Issue 3 , June 2000 , pp. 525 - 537
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2000

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Carmichael, I.S.E. and Nicholls, J. (1967) Iron-titanium oxides and oxygen fugacities in volcanic rocks. J. Geophys. Res., 72, 4665–87.CrossRefGoogle Scholar
Charles, R.W. (1975) The phase equilibria of richterite and ferrorichterite. Amer. Mineral., 60, 367–74.Google Scholar
Czamanske, G.K. and Dillet, B. (1988) Alkali amphibole, tetrasilicic mica and sodic pyroxene in peralkaline siliceous rocks, Questa Caldera, New Mexico. Amer. J. Sci., 288-A, 358–92.Google Scholar
Deer, W.A., Howie, R.A. and Zussman, J. (1962) Rock-Forming Minerals, Vol. 3, Sheet Silicates. Longmans, London.Google Scholar
Déruelle, B., Moreau, C., Nkoumbou, C., Nkambou, R., Lissom, J., Njonfang, E., Ghogomu, T.R. and Nono, A. (1991) The Cameroon Line: a review. Pp.274–327 in: Magmatism in Extensional Structural Settings; The Phanerozoic African Plate (Kampunzu, A.B. and Lubala, R.T., editors). Springer-Verlag, Berlin.CrossRefGoogle Scholar
Droop, G.T.R. (1987) A general equation for estimating Fe3+ concentrations in ferromagnesian silicates and oxides from microprobe analyses, using stoichiometric criteria. Mineral. Mag., 51, 431–5.CrossRefGoogle Scholar
Duggan, M.B. (1988) Zirconium-rich sodic pyroxene in felsic volcanics from the Warrumbungle volcano, Central New South Wales, Australia. Mineral. Mag., 52, 491–6.CrossRefGoogle Scholar
Eby, G.N. (1990) The A-type granitoids: A review of their occurrence and chemical characteristics and speculations on their petrogenesis. Lithos, 26, 115–34.CrossRefGoogle Scholar
Engell, J. (1973) A closed system crystal-fractionation model for the agpaitic Ilimaussaq intrusion, South Greeland, with special reference to the lujavrites. Bull. Geol. Soc. Denmark, 22, 334–62.Google Scholar
Fabriès, J. and Rocci, G. (1972) Évolution cristallochimique des amphiboles dans la série de Fort-Trinquet (Mauritanie). Contrib. Mineral. Petrol., 35, 215–25.CrossRefGoogle Scholar
Fudali, R.F. (1965) Oxygen fugacities of basaltic and andesitic magmas. Geochim. Cosmochim. Acta, 29, 1063–75.CrossRefGoogle Scholar
Giret, A., Bonin, B. and Leger, J.M. (1980) Amphibole compositional trends in oversaturated and undersaturated alkaline plutonic ring-complexes. Canad. Mineral., 18, 481–95.Google Scholar
Hawthorne, F.C., Ungaretti, L., Oberti, R. and Bottazzi, P. (1993) Li: An important component in igneous alkali amphiboles. Amer. Mineral., 78, 733–45.Google Scholar
Jones, A.P. and Peckett, A. (1980) Zirconium-bearing aegirine from Motzfeldt, South Greenland. Contrib. Mineral. Petrol., 75, 251–5.CrossRefGoogle Scholar
Koch, P. (1953) Notice explicative sur la feuille Banyo + carte géologique de reconnaissance au 1/500000. Imp. Nat. Paris.Google Scholar
Larsen, L.M. (1976) Clinopyroxene and coexisting mafic minerals from the alkaline Ilimaussaq intrusion, South Greenland. J. Petrol., 17, 258–90.CrossRefGoogle Scholar
Leake, B.E. (1978) Nomenclature of amphiboles. Canad. Mineral., 16, 501–20.Google Scholar
Martin, D.J. (1984) Titanian aegirine in a teschenite sill. Mineral. Mag., 48, 529–31.CrossRefGoogle Scholar
Mian, I. and Le Bas, M.J. (1986) Sodic amphiboles in fenites from the Loe Shilman carbonatite complex, NW Pakistan. Mineral. Mag., 50, 187–97.CrossRefGoogle Scholar
Mitchell, R.H. (1990) A review of the compositional variation of amphiboles on alkaline plutonic complexes. Lithos, 26, 135–56.CrossRefGoogle Scholar
Mitchell, R.H. and Platt, R.G. (1978) Mafic mineralogy of ferroaugite syenite from the Coldwell alkaline complex, Ontario, Canada. J. Petrol., 19, 627–51.CrossRefGoogle Scholar
Morimoto, N., Fabriès, J., Ferguson, A.K., Ginzburt, I.V., Ross, M., Seifert, F.A. and Zussman, J. (1988) Nomenclature of pyroxenes. Mineral. Mag., 52, 535–50.CrossRefGoogle Scholar
Mungall, J.E. and Martin, R.F. (1995) Petrogenesis of basalt-comendite and basalt-pantell erite suites, Terceira, Azores, and some implications for the origin of ocean-island rhyolites. Contrib. Mineral. Petrol., 119, 43–55.CrossRefGoogle Scholar
Nash, W.P. and Wilkinson, J.F.G. (1970) Shonkin Sag laccolith, Montana. 1. Mafic minerals and estimates of temperature, pressure, oxygen fugacity and silica activity. Contrib. Mineral. Petrol., 25, 241–69.CrossRefGoogle Scholar
Nguéné, F.R. (1982) Geology and geochemistry of the Mayo-Darlé tin deposit, West-central Cameroon, Central Africa. PhD thesis, New Mexico Institute.Google Scholar
Nielsen, T.F.D. (1979) The occurrence and formation of Ti-aegirine in peralkaline syenites: an example from the Tertiary ultramafic alkaline Gardiner complex, East Greenland. Contrib. Mineral. Petrol., 69, 235–45.CrossRefGoogle Scholar
Njonfang, E. and Moreau, C. (1996) The mineralogy and geochemistry of a subvolcanic alkaline complex from the Cameroon Line: The Nda Ali massif, Southwest Cameroon. J. Afr. Earth Sci., 22, 113–32.CrossRefGoogle Scholar
Nkoumbou, C. (1990) Etude géologique des Monts Roumpis: un ensemble plutonique et volcanique de “la Ligne du Cameroun”; données pétrologiques sur les néphé linites du Mont Etindé (Cameroun). Thèse Doct. Univ. Nancy I, France.Google Scholar
Nolan, J. (1969) Physical properties of synthetic and natural pyroxene in the system diopside-hedenbergite-acmite. Mineral. Mag., 37, 216–29.CrossRefGoogle Scholar
O'Halloran, D.A. (1985) Ras Ed Dom migrating ring complex: A-type granites and syenites from the Bayuda Desert, Sudan. J. Afr. Earth Sci., 3, 61–75.Google Scholar
Platt, R.G. and Woolley, A.R. (1986) The ma. c mineralogy of the peralkaline syenites and granites of the Mulange complex, Malawi. Mineral. Mag., 50, 85–99.CrossRefGoogle Scholar
Poldervaart, A. and Hess, H.H. (1951) Pyroxenes in the crystallization of basaltic magma. J. Geol., 59, 472–89.CrossRefGoogle Scholar
Stephenson, D. (1974) Mn and Ca enriched olivines from nepheline syenites of the South Qôroq Centre, South Greenland. Lithos, 7, 35–41.CrossRefGoogle Scholar
Stephenson, D. and Upton, B.G.J. (1982) Ferromagnesian silicates in a differentiated alkaline complex: Kûngnât Fjeld, South Greenland. Mineral. Mag., 46, 283–300.CrossRefGoogle Scholar
Stormer, J.C. Jr. (1973) Calcium zoning in olivine and its relationship to silica activity and pressure. Geochim. Cosmochim. Acta, 37, 1815–21.CrossRefGoogle Scholar
Streckeisen, A.L. (1976) To each plutonic rock, its proper name. Earth Sci. Rev. 12, 1–33.CrossRefGoogle Scholar
Strong, D.H. and Taylor, R.P. (1984) Magmatic-subsolidus and oxidation trends in composition of amphibole from silica-saturated peralkaline igneous rocks. Tschermaks Mineral. Petrog. Mitt., 32, 211–22.CrossRefGoogle Scholar
Velde, D. (1978) An ñ nigmatite-richteri te-olivine trachyte from Puu Koae, West Maui, Hawaii. Amer. Mineral., 63, 771–8.Google Scholar
Wones, D.R. and Gilbert, M.C. (1969) The fayalite-magnetite-quartz assemblage between 600° and 800°C. Amer. J. Sci., 267-A, 480–8.Google Scholar
Woolley, A.R. and Jones, G.C. (1992) The alkaline/peralkaline syenite-granite complex of Zomba-Malossa, Malawi: mafic mineralogy and genesis. J. Afr. Earth Sci., 14, 1–12.CrossRefGoogle Scholar
Woolley, A.R. and Platt, R.G. (1988) The peralkaline nepheline syenites of the Junguni intrusion, Chilwa province, Malawi. Mineral. Mag., 52, 425–33.CrossRefGoogle Scholar