THE INFLUENCE OF FRACTIONAL CRYSTALLIZATION IN THE MODAL ANALYSIS OF GELAI VOLCANO

The East African Rift (EAR) is about 6,400 km long stretching from the Afar triple junction to the Mozambique channel. The rift is a present-day model of active continental rifting due to its geographical location above one or more mantle plumes. The North Tanzanian Divergence Zone (NTD) is located in the southernmost section of the eastern branch of the EAR. This section of the rift is in its early stages of rifting. The magmatism within the NTD dating back to 5.8 Ma displays a wide variety of chemical compositions related to multiple magma sources within the lithosphere and asthenosphere producing basaltic, rhyolitic, trachytic, phonolitic, and carbonatitic lavas. Gelai is a stratovolcano located within the NTD and dated at 1.2 Ma to 2 Ma.

Whole rock geochemical analyses obtained from lava flows collected on Gelai volcano advocate fractional crystallization as a major contributor to the magmas differentiation over time. Observed mineral phases include plagioclase, clinopyroxene, weathered olivine, amphibole, opaques, and minor alkali feldspar. The purpose of this study is to analyze samples from the Gelai volcano to get a better understanding of the process of fractional crystallization occurring in its magma chamber. In particular, this study focuses on previously unidentified mineral phases, as well as the comparison between the composition of phenocrysts and their respective groundmass crystals. The chemical compositions are obtained by Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS).

Previous studies of the plagioclase phenocrysts from samples ranging in age between 1.18-1.13 Ma analyzed on a petrographic microscope with the Michel-Levy Method show a shift from more calcium rich to more sodium rich plagioclase interpreted as progressive fractional crystallization. However, zoned and resorbed phenocrysts are observed indicating a more complex history within the magma chamber. Preliminary SEM-EDS analyses corroborate the hypothesis of magma recharge.