Petrogenesis of oceanic granites from the Aves Ridge in the Caribbean Basin

Abstract

A genetic model proposed for granitic rocks dredged from the Aves Rise involves a sodium-enriched hydrothermal fluid phase, shallow emplacement, and significant derivation of components from the upper mantle. Megascopic observations of serially slabbed sections of the rock samples show that aggregates of plagioclase phenocrysts concentrate into channel-like mats. The presence of a late-stage, hydrothermal fluid is suggested by crystal-lined cavities in the plagioclase aggregates. With progressively decreasing pressure, this low density, sodium-rich, hydrous fluid phase unmixed from the silicate melt and entrained the plagioclase aggregates as it rose. Reaction between this sodium-rich hydrous phase and the plagioclase produced albite rims. Genetic independence of the plagioclase aggregates and the granitic groundmass which surrounds them is implied by the results of a chi square test. Thus, the system consisted of three distinct components — the early-formed calcic plagioclase, a sodium-rich hydrous fluid phase, and a granitic-silicate fluid phase. Shallow intrusion (less than 15 km) is indicated by the structural state of K-feldspars and presence of miarolitic cavities. 87Sr/ 86Sr values, which correspond to ratios obtained from oceanic basalts, suggest that the granitic rocks are primitive and most likely from the upper mantle.