COMAGMATIC A-TYPE GRANOPHYRE AND RHYOLITE FROM THE ALID VOLCANIC CENTER, ERITREA, NORTHEAST AFRICA

Granophyric blocks within late-Pleistocene pyroclastic flow ejecta fro m the Alid volcanic center, northeast Africa, are the rapidly crystall ized, intrusive equivalent of pumice from the pyroclastic flow. Phenoc ryst compositions and geochemical characteristics of the pumice and gr anophyre are virtually identical. Silicate melt inclusions and other g eochemical and geological constraints reveal those processes leading t o development of the granophyric texture. Rhyolitic (A-type) magma wit h similar to 2.6 wt % dissolved H2O and a temperature near 870 degrees C was intruded to within 2-4 km of the surface, causing deformation a nd structural doming of shallow marine and subaerial strata. Eruptions of crystal-poor rhyolite from this shallow magma chamber caused degas sing, which force undercooling and consequent granophyric crystallizat ion of some of the magma remaining in the intrusion. The most recent e ruption of the Alid excavated the crystallized granitic wall of the ma gma chamber, bringing the granophyric clasts to the surface.