FRACTIONAL CRYSTALLIZATION HISTORY RECORDED BY THE LAVA FLOWS OF KETUMBEINE VOLCANO, NORTH TANZANIA

The East African Rift (EAR) continental extension has been modelled as an active rift related to one or more mantle plumes initiating pre-rift magmatism and causing lithospheric thinning. The North Tanzanian Divergence (NTD) zone is a section of the EAR where early stage rifting is observed. The lavas erupting throughout the NTD since the Miocene (~5.8 Ma) display broad geochemical variations. Our study focuses on Ketumbeine, a shield volcano rising within the NTD.

Samples from the Ketumbeine volcano were collected by Dr. Mana and previously analyzed for ⁴⁰Ar/³⁹Ar dating and whole rock geochemistry. 11 samples from individual lava flows have been selected because of their varying geochemistry and ages. Based on their geochemical compositions they can be classified into 2 magma series: a basalt to trachybasalt magma series (2.20 to 1.94 Ma) and a phonotephrite to tephriphonolite magma series (1.28 to 1.23 Ma). Optical investigations of petrographic thin sections are employed to analyze the mineralogy and texture of these lavas, while Adobe Photoshop was used to obtain modal abundances.

Varying mineral assemblages, modes, and grain sizes are observed. Textures vary from hypocrystalline aphanitic to porphyritic with some samples displaying trachytic texture. A few samples show well defined directional orientation. Phenocrysts of CPX (6-11%), plagioclase (4-11%) and opaques (2-4%) have been identified as common ubiquitous primary mineral phases. The older lava samples tend to be aphanitic (99% groundmass) with abundant fine grained crystals (e.g. 10-KTM-05). The amount of phenocrysts increases with age. In samples KTG-04 the groundmass content decreases to 84% and in KTW-03B to 72%. The youngest sample of the basaltic magma series (e.g. KTG-4) displays subhedral to anhedral phenocrysts dispersed into a glassy groundmass with microlitic plagioclase. The younger magma series presents an increase of subhedral CPX and a decrease of plagioclase phenocrysts. Indications of resorption of both mineral phases are observed suggesting disequilibrium and possible magma chamber recharge (e.g. KTW-2A). Alteration of some individual mineral grains is present.