Mineralogical relations and magma mixing in calc-alkaline andesites from lake Atitl�n, Guatemala

Abstract

Quaternary calc-alkaline andesites erupted form three neighboring volcanoes along the Guatemalan volcanic front have mineralogic compositions and textures which show varying degrees of disequilibrium. Basaltic andesites and andesites (SiO2 % = 50−59), erupted from Atitlan volcano located nearer to the trench, have the lowest degree of disequilibrium. These lavas contain an anhydrous phenocryst assemblage of mildly bimodal plagioclase, olivine, augite opx, and magnetite. Orthopyroxene occurs at the expense of olivine with increasing whole rock SiO2. Most pyroxene phenocrysts show a trend of Fe enrichment. Andesites from Toliman (SiO2% = 53−62) and San Pedro (Si02% = 54−67) volcanoes, located further away from the trench, show comparatively high and moderate degrees of disequilibrium, respectively. Toliman andesites have bimodal plagioclase compositions and textures. Olivine persists with increasing whole rock Si02 and lacks clear modal relations with coexisting orthopyroxene and hornblende phenocrysts. When compared to Atitlan andesites, Toliman olivines are more forsteritic and pyroxenes contain higher proportions of Mg-rich rims, though normal zoned phenocrysts occur within the same rock. Toliman andesite also have lower proportions of phenocrysts to microphenocrysts, more calcic plagioclase groundmass compositions, and higher modal phenocrystic magnetite. San Pedro andesites have disequilibrium assemblages similar to Toliman andesites but are not as striking. Magma mixing is proposed as the dominant cause for observed disequilibrium. Disequilibrium features are preserved best in Toliman and San Pedro andesites because inferred durations between mixing and eruption are shortest, and consequently, these mixed andesites more clearly record mafic and silicic endmember compositions. The mafic component is a relatively high temperature, high-Al basalt containing phenocrysts of Mg-rich olivine (Fo = 78−80), calcic plagioclase (An 70−80), augite and titanomagnetite. The silicic component contains quartz, sodic plagioclase (An 40−50), Fe-rich orthopyroxene and titanomagnetite. Short durations between mixing and eruption produce petrographic features which, in part, mimic the effects of increasing PH20 in a fractionating magma. Inferred mixing durations for Atitlann andesites are longer and involve a less-silicic composition. The intervolcano disequilibrium relations suggest that as Si02 in a silicic endmember increases, the duration and efficiency of mixing decreases.