Plagioclase-ultraphyric basalts of the galapagos archipelago

Abstract

Basalts containing as much as 50% plagioclase phenocrysts that attain lengths of several centimeters are common on the northern islands of the Galapagos Archipelago. Mineralogic and geochemical evidence requires that the large anorthitic phenocrysts nucleated and grew from a less evolved magma than that in which they were erupted. We suggest that plagioclase-ultraphyric basalts (PUBs) form when a buoyant phenocryst-laden magma enters a chamber containing a denser, more evolved magma. As the new magma mixes with the old, anorthitic plagioclase phenocrysts may accumulate near the roof in a hybrid liquid. Thinner crust under the northern islands increases the probability that viscous plagioclase-rich magmas can ascend to shallow magma chambers and may account for the fact that PUBs are more common on the northern islands, than in the rest of the Archipelago.