gems, garnet, Spain

Tertiary dacites and rhyodacites in southeastern Spain contain garnets both as phenocrysts within the volcanic lavas and within enclosed metamorphic xenoliths. Microprobe analyses reveal that garnets from both the calc-alkaline lavas and the metamorphic inclusions are compositionally similar, being rich in almandine. However, their zoning patterns differ: garnets in the lavas exhibit normal zoning or slight manganese enrichment at the rims, while those in xenoliths show Mn-enriched edges or reverse zoning. These differences in zoning and petrographic features suggest that the volcanic garnets crystallized early from the calc-alkaline magma, whereas the more uniform composition and low pyrope and grossular content in the dacitic and rhyodacitic garnets imply that the crystallization conditions were only briefly maintained.