INTERPRETING THE PROCESS OF FRACTIONAL CRYSTALLIZATION OF GELAI VOLCANO OVER TIME

The East African Rift (EAR) extends from the Afar triple junction to the Mozambique channel and sits above one or more mantle plumes making it a present-day model of active continental rifting. The eruptive style, chemistry and volume of volcanic products exposed along the rift vary in an overall coherent fashion across different segments of the rift. The basalts are vital to understanding the structure of the mantle and the interaction between magmatic activity and rifting which are superimposed upon a pre-existing zone of crustal weakness. The North Tanzanian Divergence zone (NTD) is an early stage rift characterized by the widening of the EAR to over 200km underlined by Plio-Pleistocene magmatism including Gelai, the focus of our study. The NTD magmatism is chemically heterogeneous due to variations in the source and magma differentiation producing basaltic, rhyolitic, trachytic, phonolitic and carbontitic lavas.

Thirteen samples were collected from various individual flows. Whole rock geochemical analyses suggest that fractional crystallization had a major role in the differentiation of this magma chamber over time. Petrographic observations are employed here to obtain insights on mineral assemblages and textures while modal abundances were obtained digitally through Adobe Photoshop. All of the samples are porphyritic with variable amounts of elongated plagioclase, CPX and opaques. Over time we observe an overall increase in plagioclase and decrease in CPX. Amphibole is present in the younger samples, while altered olivine is only found in older samples (e.g. 10-GEL-19) suggesting that the magma chamber is fractionating over time. A few mineral aggregates have been observed implying contributions from mineral settling and/or assimilation.

Texturally, the older samples show an overall lower degree of alignment and the phenocrysts are subhedral to anhedral; while in younger samples are euhedral to subhedrals and the groundmass tends to display flow direction. Some samples present evidences of resorption suggesting disequilibrium and possible magma chamber recharge. The size of the phenocrysts increases over time. For instance, plagioclase phenocryst increase in size from 1 x 0.62mm (10-GEL-19) to 3.4 x 1.4mm (10-GEL-03).