Petrogénesis de los thundereggs en domos riolíticos del centro del Chubut, Patagonia

Petrogenesis of thundereggs in Eocene rhyolite domes of central Chubut, Patagonia. Thundereggs at the glass domes of Piedra Parada Caldera are described. Their geological, petrological and geochemical charac- teristics suggest growth from immiscible liquid drops that devitrified as anorthoclase spherulites. The expansion of the segregat- ed vapor phase of the devitrified immiscible drops broke the spherulites and made large vesicles lined with the broken pieces of the spherulite. Further cooling of the rest of the magma yielded a glass dome with perlitic texture. Low temperature water lixivi- ated the perlitic glass and spherulites, filling the cavities within the vesicles and broken spherulite with opal and/or chalcedony.