The Journal of Petrology, Volume 38, Issue 12: December 1997.

Comagmatic A-type granophyre and rhyolite from the Alid volcanic center, Eritrea, Northeast Africa

JB Lowenstern^*, MA Clynne and TD Bullen

US Geological Survey, Mail Stop 910, 345 Middlefield Road, Menlo Park, CA 94025, USA, ^*Corresponding author e-mail: jlwnstrn@usgs.gov

Granophyric blocks within late-Pleistocene pyroclastic flow ejecta from the Alid volcanic center, northeast Africa, are the rapidly crystallized, intrusive equivalent of pumice from the pyroclastic flow. Phenocryst compositions and geochemical characteristics of the pumice and granophyre are virtually identical. Silicate melt inclusions and other geochemical and geological constraints reveal those processes leading to development of the granophyric texture. Rhyolitic (A-type) magma with 2.6 wt % dissolved H₂O and a temperature near 870°C was intruded to within 2-4 km of the surface, causing deformation and structural doming of shallow marine and subaerial strata. Eruptions of crystal-poor rhyolite from this shallow magma chamber caused degassing, which forced undercooling and consequent granophyric crystallization of some of the magma remaining in the intrusion. The most recent eruption from Alid excavated the crystallized granitic wall of the magma chamber, bringing the granophyric clasts to the surface.

Key words: Danakil; Afar; granite; rhyolite; melt inclusions

Pages 1707-1721