Abstract
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 (SiO2: 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 hour glass and sector 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 – 1156oC calculated using mineral chemistry of equilibrium olivine-clinopyroxene pairs. Keywords: Cameroon volcanic line, geothermometry, monogenetic volcano, olivine-clinopyroxene pairs
Highlights
Monogenetic volcanoes are very widespread in continental basaltic volcanic provinces and they were once thought of as simple volcanic edifices resulting from less complicated magmatic processes (e.g., Boyce et al 2015 and references therein)
Three magma batches have been identified at Jeju, South Korea (Sohn et al, 2012) while sequential eruption of alkaline and sub-alkaline magmas have been studied at monogenetic volcanoes in New Zealand (Needham et al, 2011)
The volcanic cones mapped are generally lowlying and 0.5 to 1.5 km in diameter. They are breached by lava flows and are asymmetrical (Warack) to symmetrical in shape (Baossi 1 and Baossi 2) and entirely covered with blocky and isolated or jointed columnar basalt flows (Figure 2 and 3)
Summary
Monogenetic volcanoes are very widespread in continental basaltic volcanic provinces and they were once thought of as simple volcanic edifices resulting from less complicated magmatic processes (e.g., Boyce et al 2015 and references therein). Geochemical and textural data suggest that various mechanisms exist defining the functioning of polymagmatic processes at monogenetic volcanoes (see Brenna et al, 2010, 2011, 2012 for details). Three magma batches have been identified at Jeju, South Korea (Sohn et al, 2012) while sequential eruption of alkaline and sub-alkaline magmas have been studied at monogenetic volcanoes in New Zealand (Needham et al, 2011). These studies have emphasized the need for further investigation of monogenetic volcanoes and this study is aimed at contributing to this theme
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