Abstract

Pinta, one of the northern islands in the Galapagos archipelago, is the top of a shield volcano which became emergent ∼0.7 m.y. ago. A regional north-north west-trending fracture system influenced the locus of volcanism. Pinta's geologic development can be divided into an older, shield stage and a younger, fissure stage. Each stage produced a similar, yet distinct, series of evolved olivine tholeiites (magnesium number = 52). The shield series underwent some additional shallow-level fractionation in a periodically replenished chamber. Rare-earth-element data indicate that the mantle source for both of the series has residual garnet. Higher concentrations of incompatible trace elements iri the fissure series imply a lower degree of partial melting. 143Nd/144Nd and 87Sr/86Sr ratios suggest that both series have an enriched, hotspot-type source. Despite close proximity to the Galapagos spreading ridge, Pinta's lavas lack features characteristic of mid-ocean ridge basalts. The isotopic and rare-earth-element data from Pinta and the other northern islands are consistent with previous models for the Galapagos spreading ridge, which require north-south axially directed transfer of enriched asthenosphere and/or melts from the Galapagos hotspot to the Galapagos Ridge.

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