Abstract

Silali, 1°10'N, 36°12°E, is the largest Quaternary caldera volcano in the northern Gregory Rift and is composed predominantly of peralkaline trachytic lavas and pyroclastic deposits, and mildly alkaline to transitional basalts. Detailed mapping combined with radiometric 40Ar/39Ar age determinations is used to constrain the evolutionary development of Silali. Activity commenced at c. 400–220 ka with the construction of a low relief lava shield whose summit area was subsequently modified by alternating periods of faulting, subsidence and infilling associated with two major periods of explosive activity. This activity ceased around 133–131 ka and was probably the result of fracturing and decompression of a high level magma chamber by regional extension and the injection of basaltic dykes below the volcano. Later eruptions (c. 120 ka) along the western flanks migrated eastward with time and culminated in the eruption of viscous trachyte lavas from a circumferential fissure zone. The emplacement of a basic dyke swarm to shallow crustal levels beneath Silali resulted in the formation a broad volcanic rift zone within which large volumes of fluid basalts were erupted to mantle the flanks of the volcano. This activity mainly pre-dated, but probably also overlapped with, incremental subsidence and asymmetric downsagging of the summit area and the propagation of a circumferential fissure zone. Basalt and trachyte lavas, erupted along the circumferential fissure zone and from a major meridional summit fissure, moved outward to form a series of flat summit benches, and ponded in summit depressions before overflowing. The continuing inward collapse of the summit area and the lateral drainage of magma from a high-level reservoir finally culminated in the formation of a large caldera at c. 63 ka. Post-caldera activity utilized pre-existing weaknesses within the caldera, erupting basalt and trachyte lavas until around 7 ka, and was contemporaneous with the eruption of trachyte lava domes (9–7 ka) on the eastern flanks.

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