The textural characteristics of felsic lavas and ignimbrites in the Sabga area along the continental segment of the Came roon Volcanic Line (CVL) are documented in this study. Two rhyolitic lava flows separated by mafic and rhyodacitic lava flows were dated by zircon LA-ICP-MS technique in order to constrain the timescales of successive flow emplace ment as well as magma chamber processes in this volcanic field. The studied samples have rheomorphic characteristics such as eutaxitic textures defined by elongated fiamme as well as deformed and welded glass shards. They exhibit flow banding and contain spherulitic groundmass. Rhyolitic autobreccias and ignimbrites are widespread in the Sabga area, are peralkaline in nature and their ages are broadly similar at ~23.0 ± 0.3 Ma (23.34 ± 0.34 Ma for the ignimbrite and 22.98 ± 0.28 Ma for the rhyolitic autobreccia unit). These ages suggest rapid recharge of magma into a periodically replenished and open chamber. The felsic rocks in the Sabga area and around the Bamenda Highlands are also younger compared to other felsic units along the CVL (29 Ma to 69.4 ± 0.4 Ma).

No abstract

This paper presents principally the textural and geothermometric evidence of polymagmatic activity at a monogenetic volcano along the Cameroon Volcanic Line (CVL) in a bid to contribute to the global understanding of how such volume-limited basaltic magmatic systems operate. Preliminary geochemical data presented show that the lavas are alkali basalt and basanite (SiO 2 : 40.42 -47.59 wt%, MgO: 7.61 -9.13 wt%) and are petrographically indistinguishable. They outcrop as low mounds of columnar basalts associated with sparse pyroclastic materials. The lavas are fine, porphyritic with phenocrysts of olivine and clinopyroxene set in a plagioclase microlite-dominated hypocrystalline groundmass. Olivine has two distinct populations: grains that are anhedral to glomerocrystic with spinel and plagioclase inclusions and usually have resorbed margins (olivine 1); and grains that are euhedral, sometimes skeletal, with a thin and well preserved rim (olivine 2). Similarly two clinopyroxene crystal populations are recognizable: clinopyroxene grains that are resorbed, dismembered, with sieve-textured cores and irregular core-rim margins (clinopyroxene 1); and euhedral clinopyroxene grains partially enclosing olivine phenocrysts with sector and hour glass shape zoning (clinopyroxene 2). These textural features suggest that the eruption was caused by an influx of a fresh batch of magma (that crystallized olivine 2 and clinopyroxene 2) into a fractionated crystal mush (olivine 1 and clinopyroxene 1) in the chamber. The crystal mush was re-heated by the intruding basaltic magma resulting in a broad pre-eruption liquidus temperature of 1040 -1156 o C calculated using mineral chemistry of equilibrium olivine-clinopyroxene pairs.

No abstract