Partial melt generation and evolution of magma reservoir conditions at the Paka volcanic complex in Kenya: Constraints from geochemistry, petrology and geophysics

Abstract

In this study, we determine magma generation and evolution of magma storage conditions beneath Paka volcano located in northern segment of the Kenya Rift. The surface geology of the area is characterised by five main sequences including pre-Paka formations and volcanic sequence 1–4, all built up by basalt, intermediate and trachyte rocks spanning 582–8 ka. Numerical trace element modelling suggests that the primary magma in Paka resulted principally from 5 to 10% garnet–peridotite partial melting and possibly 0.1–1.5% partial melting of spinel-lherzolite. The parental magma was probably held at the Moho (~30 km) and evolved by ~76% gabbro fractional crystallization to generate a liquid of similar chemistry as that of erupted Paka basalts. We propose that under moderately oxidized conditions (∆FMQ +1) the primary magma underwent a further ~70% fractional crystallization and ~ 20% assimilation of crustal syenitic material to generate Paka trachyte lavas. Basaltic and intermediate magmas were held at polybaric storage pressure conditions ranging between 1.4 and 7 kbar (5–25 km), at temperatures between 1086 and 1204 °C with relatively low water content of <1.8 wt%. The trachyte magma resided at low pressures, i.e., 0.3–2.5 kbar (1–9 km) at 901–980 °C with water contents of 3.5–5.1 wt%. Based on our data eruption source depths remained largely the same for a period spanning from 582 to 11 ka. Geophysical data show correlation with magma depths beneath Paka volcano.