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Clinopyroxene from some felsic alkaline rocks of the Cameroon Line, Central Africa

Emmanuel Njonfang and Alexandre Nono
Clinopyroxene from some felsic alkaline rocks of the Cameroon Line, Central Africa
European Journal of Mineralogy (June 2003) 15 (3): 527-542

Abstract

This paper studies in detail the composition and evolutionary trends of pyroxenes (salite to aegirine) from some felsic alkaline rocks belonging to the continental part of the Cameroon Line (CL). The investigated samples include both SiO (sub 2) -oversaturated (trachytes, rhyolites, syenites and granites) and -undersaturated (mainly phonolites) types. Pyroxene compositions vary from calcic to sodic in most massifs, and straddle the salite-augite-aegirine-augite-aegirine series. Ti-aegirine (TiO (sub 2) : 4.0-6.5 wt.%) is ubiquitous, and indicates the predominance of NaTi (sub 0.5) (FM) (sub 0.5) Si (sub 2) O (sub 6) (referred to as NAT components) where FM = Fe (super 2+) +Mn+Mg, both in plutonic and volcanic rocks. High ZrO (sub 2) contents (1.7-5.6 wt.%) are more common in volcanic rocks and similarly suggest the presence of NaZr (sub 0.5) (FM) (sub 0.5) Si (sub 2) O (sub 6) (referred to as FM-NAZ components). Zr and Ti sometimes show concomitant enrichment. Ca-rich (Na< or =25%) and Na-rich (Na> or =55%) pyroxenes are generally separated by a gap where the coexisting amphiboles plot. In the kaersutite-bearing peralkaline phonolite of Nganha volcano, however, pyroxenes are rather poor in Na and consequently plot in the pyroxene gap. The low Na content of these pyroxenes is due to hauyne crystallization (4 vol.%) after amphibole and before pyroxene. In the Na-(Fe (super 2+) +Mn)-Mg diagram, trends of the studied pyroxenes (Kapsiki rhyolites and Pande syenites excepted), though different from each other, are those of Na-enrichment at relatively high Mg/Fe ratios. These trends are therefore inferred to different batches of compositionally similar parental magmas evolving under different magmatic conditions. As already shown in other areas, oxygen fugacity and peralkalinity of the evolved liquids played important roles. The high Zr content of the studied felsic rocks, the ubiquity of aegirine and/or sodic amphibole in all the massifs and the presence of zircon crystals only in the peralkaline granite of Pande, are consistent with the peralkaline affinity of the melts from which the rocks crystallized. Low oxygen fugacity (fO (sub 2) ) favours crystallization of Ti-aegirine and/or Zr-aegirine, whereas fairly high fO (sub 2) , above quartz-fayalite-magnetite (QFM) buffer, favours the crystallization of aegirine-rich pyroxenes. On the other hand, the similar (super 87) Sr/ (super 86) Sr initial ratios (0.7084 and 0.7082), but distinct pyroxene trends in trachytes from Nganha (Adamawa plateau) and Kapsiki (northernmost plateau), respectively, do not point to a significant role of crustal contamination on the observed trends.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 15
Serial Issue: 3
Title: Clinopyroxene from some felsic alkaline rocks of the Cameroon Line, Central Africa
Affiliation: Universite de Yaounde I, Laboratoire de Geologie, Yaounde, Cameroon
Pages: 527-542
Published: 200306
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 72
Accession Number: 2003-055037
Categories: Mineralogy of silicatesIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables, sketch map
N04°00'00" - N12°00'00", E10°00'00" - E14°00'00"
Secondary Affiliation: Universite de Dscjhang, CMR, Cameroon
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200318
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