Petrology and geochemistry of Late Holocene felsic magmas from Rungwe volcano (Tanzania), with implications for trachytic Rungwe Pumice eruption dynamics

• Published in: Lithos Vol. 177; pp. 34 - 53
• Main Authors: Fontijn, Karen, Elburg, Marlina A., Nikogosian, Igor K., van Bergen, Manfred J., Ernst, Gerald G.J.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.09.2013
• Subjects: Alkaline magmas; Magmatic differentiation; Plinian eruptions; Rungwe; Tanzania; Trachyte; Africa, East African Rift; Tanzania; Plinian eruptions; Trachyte; Rungwe; Alkaline magmas; Magmatic differentiation; Tanzania
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2013.05.012

Rungwe in southern Tanzania is an active volcanic centre in the East African Rift System, characterised by Plinian-style explosive eruptions of metaluminous to slightly peralkaline trachytic silica-undersaturated magmas during its late Holocene history. Variations in whole-rock major and trace element compositions of erupted products have been investigated, in combination with electron microprobe data for melt inclusions and phenocrysts comprising sanidine, biotite, clinopyroxene, titanomagnetite, ilmenite, haüyne, titanite, apatite and traces of plagioclase and amphibole. Compositional variations largely reflect fractional crystallisation, with a limited influence of magma mixing. Subtle variations in whole-rock composition and mineralogical characteristics between and within deposits, suggest the existence of a chemically zoned trachytic magma chamber beneath Rungwe. For the two most important studied deposits, the Isongole and Rungwe Pumice, co-existing Fe–Ti oxides constrain pre-eruptive temperature to 915–950°C and oxygen fugacity to NNO+0.25–NNO+0.45. For the Rungwe Pumice, melt inclusions suggest that the melt was water-undersaturated (maximum inferred H2O concentration 5.5wt.%). In the range of the defined pre-eruptive temperatures, this corresponds to melt viscosities as low as 103.3Pa·s, i.e. significantly lower than magmas that typically generate highly explosive eruptions. Because no microlites formed in the conduit during ascent, which would have strongly increased the effective magma viscosity, the highly explosive nature of the eruptions may be attributable to a crucial role of exsolved CO2 and S phases, and very high ascent rates. •Rungwe is characterised by a silica-undersaturated trachytic zone magma chamber.•Rungwe trachyte is dominantly generated by fractional crystallisation.•Pre-eruptive parameters are constrained for Rungwe and Isongole Pumice.