Emmanuel Njonfang

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.

The mineralogy and geochemistry of a subvolcanic alkaline complex from the Cameroon Line: The Nda Ali massif, southwest Cameroon

Abstract The Nda Ali massif is one of the 60 Tertiary pluntonic anorogenic complexes known as ‘granites ultimes’ of the Cameroon Line. It forms a plutono-volcanic complex comprising a gabbro-diorite-monzonite-syenite suite and associated basanite-trachyte-phonolite volcanic dykes and lava flows. The mineralogical and geochemical data indicate: 1. i) Alkaline affinity of both plutonic and volcanic rocks. 2. ii) Early development of peralkalinity in syenites marked by the presence of aegirine (only one sample contains normative acmite); its crystallization after alkali amphibole indicates that the high activity of sodium may be the controlling factor. 3. iii) Penecontemporaneity of trachytes and basanite dykes with plutonic rocks; both are chemically similar. The other lavas form a later volcanic sequence with both undersaturated (tephri-phonolite to phonolites) and saturated (only represented by trachyte domes) trends. 4. iv) Evolution from an undersaturated alkaline magma near syeno-monzonitic or basanitic composition; both rocks have 100× Rb Ba ratios (6.7 and 6.5) in the range of those obtained for mantle liquids. This evolution is dominated by fractional crystallization, the process involving feldspars, pyroxenes, amphiboles and oxides together with apatite for plutonic rocks and sphene for the volcanic sequence. 5. v) Moderate and negligible crustal contamination, respectively, for plutonic rocks and associated dykes and for the undersaturated volcanic trend. Isotopic data, not yet available, are needed here to quantify the effect of crust involvement.

Mapping and Assessment of Volcanic Hazards Related to the Ignimbritic Eruption by AMS in Bambouto Volcano (Cameroon Volcanic Line)

Pyroclastic deposits constitute major components of explosive volcanic activity. To help improve the safety of the population faced with natural disasters, a study is carried out at Bambouto volcano with a view to map potential hazards related to pyroclastic flows. The Bambouto volcano is indeed considered to be still active since the recent discovery of Quaternary basalts (0.5 Ma) at Totap, a locality situated near the Bambouto Caldera. This discovery has led to reclassify Mount Bambouto among active volcanoes of Cameroon and, therefore, considered as potentially dangerous. The dangerousness of this volcano is accentuated by the presence of ignimbrites that are witnesses of ancient pyroclastic flows. Because a map of volcanic hazards is non-existent on the volcano, anisotropy of magnetic susceptibility (AMS) is the method used in this paper to characterize magnetic fabrics and provides an estimate of flow direction of each ignimbrite sheet (represented by massive lapilli tuff and massive lithic breccia facies). Inferred transport directions based on the AMS data and field indicators show that Bambouto Caldera is the source of main pyroclastic deposits of Mount Bambouto. These results have enabled us to produce a new hazard map related to potential future pyroclastic flows.

Transpressional granite-emplacement model: Structural and magnetic study of the Pan-African Bandja granitic pluton (West Cameroon)

The Pan-African NE–SW elongated Bandja granitic pluton, located at the western part of the Pan-African belt in Cameroon, is a K-feldspar megacryst granite. It is emplaced in banded gneiss and its NW border underwent mylonitization. The magmatic foliation shows NE–SW and NNE–SSW strike directions with moderate to strong dip respectively in its northern and central parts. This mostly, ferromagnetic granite displays magnetic fabrics carried by magnetite and characterized by (i) magnetic foliation with best poles at 295/34, 283/33 and 35/59 respectively in its northern, central and southern parts and (ii) a subhorizontal magnetic lineation with best line at 37/8, 191/9 and 267/22 respectively in the northern, central and southern parts. Magnetic lineation shows an ‘S’ shape trend that allows to (1) consider the complete emplacement and deformation of the pluton during the Pan-African D 2 and D 3 events which occurred in the Pan-African belt in Cameroon and (2) reorganize Pan-African ages from Nguiessi Tchakam et al. (1997) compared with those of the other granitic plutons in the belt as: 686 ±17 Ma (Rb/Sr) for D 1 age of metamorphism recorded in gneiss; and the period between 604–557 Ma for D 2–D 3 emplacement and deformation age of the granitic pluton in a dextral ENE–WSW shear movement.

Petro-geochemistry, Genesis and Economic Aspects of Mafic Volcanic Rocks in the West and Southern Part of The Mamfe Basin (SW Cameroon, Central Africa)

Geologic prospecting, petrographic and geochemical analyses of mafic volcanic exposures in the west and southern part of the Mamfe Basin (SW Cameroon) distinguishes: basanites, picro-basalts, alkali basalts and tholeiitic basalts. They are relatively LREE-enriched, undersaturated, saturated or oversaturated due to presence or absence of normative nepheline, hypersthene or quartz. Basanites mainly form pillow-like lavas, and are aphyric or porphyritic. They have significant concentration of Ni (up to 387 ppm) and Ba (up to 436 ppm). These alkaline rocks cooled from less evolved mantle source magma. Picro-basaltic fragments exclusively found in the western part of the basin are Ni (up to 259 ppm) Ba (up to 2090 ppm) -enriched porphyritic, alkaline or subalkaline rocks. They also cooled from less evolved mantle source magma. Basalts form volcanoclasts, flow and dykes. They are aphyric or porphyritic, alkaline, transitional or subalkaline. Some of these rocks are Al-enriched. They crystallized from variably evolved mantle source magma within the Oceanic Island Basalt and Continental Rift Basalt tectonic settings.