Comagmatic A-type Granophyre and Rhyolite from the Alid Volcanic Center, Eritrea, Northeast Africa

Abstract

vermicular to cuneiform (Barker, 1970). Typically, the Granophyric blocks within late-Pleistocene pyroclastic flow ejecta from the Alid volcanic center, northeast Africa, are the rapidly groundmass feldspars radiate off pre-existing feldspar crystallized, intrusive equivalent of pumice from the pyroclastic flow. phenocrysts, with which they are in optical continuity Phenocryst compositions and geochemical characteristics of the (Dunham, 1965). Some workers prefer the term pumice and granophyre are virtually identical. Silicate melt inclusions ‘micrographic’ for highly regular, interlocking arand other geochemical and geological constraints reveal those processes rangements of quartz and feldspar, as in the coarser leading to development of the granophyric texture. Rhyolitic (Agraphic granite (Fenn, 1986; Lentz & Fowler, 1992). type) magma with ~2·6 wt % dissolved H2O and a temperature Though Schloemer (1964) has shown that some graphic near 870°C was intruded to within 2–4 km of the surface, causing textures can grow by replacement [see also Augustithis deformation and structural doming of shallow marine and subaerial (1973)], most workers agree that phenocryst-bearing strata. Eruptions of crystal-poor rhyolite from this shallow magma granophyres typically form by rapid and simultaneous chamber caused degassing, which forced undercooling and consequent crystallization of quartz and feldspar from a melt (Smith, granophyric crystallization of some of the magma remaining in the 1974). Such crystallization is generally believed to be due intrusion. The most recent eruption from Alid excavated the cryto pronounced undercooling of the silicate liquid (Vogt, stallized granitic wall of the magma chamber, bringing the grano1930; Dunham, 1965); not necessarily at eutectic temphyric clasts to the surface. peratures or compositions (Fenn, 1986; London et al., 1989; Lentz & Fowler, 1992). Granophyric textures are common in epizonal granitic bodies, particularly those associated with volcanic rocks (Buddington, 1959; Dunham, 1965). On occasion, they erupt as comagmatic ejecta in pyroclastic deposits (e.g.