Tertiary dacites and rhyodacites in SE Spain contain garnets, both as phenocrysts in the lavas and within their enclosed metamorphic xenoliths. Microprobe analyses show that calc-alkaline and metamorphic rocks garnets are quite similar in composition (almandine-rich). However, the zoning patterns displayed in the two types of garnets differ. Garnets included in lavas show normal zoning (or a slight enrichment of Mn at the borders), while garnets in xenoliths have Mn-enriched rims or reverse zoning. The differences in zonation, as well as in petrographic features, indicate that the garnets in volcanic rocks crystallized directly from the calc-alkaline magma at an early stage of its crystallization. On the other hand, the uniform composition of dacitic and rhyodacitic garnets, as well as its low pyrope and grossular content, suggest that the conditions in which the garnets crystallized were rapidly exceeded.

Los auteurs remercient le docteur V. ARAÑA de l'Institut Lucas Mallada (C.S.I.C.) la lecture critique du manuscrit. Le docteur F. DIMANCHE de l'Université de Liége a réalisé la traduction en français du travail.

"Instituto Lucas Mallada" (C.S.I.C.)

Peer reviewed