Alexandre Nono

Clinopyroxene from some felsic alkaline rocks of the Cameroon Line, central Africa: petrological implications

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 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 2 :4.0 — 6.5 wt.%) is ubiquitous, and indicates the predominance of NaTi 0.5 (FM) 0.5 Si 2 O 6 (refered to as NAT components) where FM = Fe 2+ + Mn + Mg, both in plutonic and volcanic rocks. High ZrO 2 contents (1.7–5.6 wt.%) are more common in volcanic rocks and similarly suggest the presence of NaZr 0.5 (FM) 0.5 Si 2 O 6 (referred to as FM-NAZ components). Zr and Ti sometimes show concomitant enrichment. Ca-rich (Na ≤ 25%) and Na-rich (Na ≥ 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 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 2 ) favours crystallization of Ti-aegirine and/or Zr-aegirine, whereas fairly high fO 2 , above quartz-fayalitemagnetite (QFM) buffer, favours the crystallization of aegirine-rich pyroxenes. On the other hand, the similar 87 Sr/ 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.