PRIMITIVE INTRATRANSFORM VOLCANISM AT GARRETT TRANSFORM-FAULT (EAST PACIFIC RISE)

The paper presents the results of a petrological study of samples coll ected by submersible from volcanic features that floor the intratransf orm domain of the Garrett Transform Fault. Most intratransform volcani cs are typically highly porphyritic and primitive mid-ocean ridge basa lt (MORE) (glasses have Mg# higher than 0.65), whereas most volcanics close to the East Pacific Rise - transform intersection zone are nearl y aphyric evolved MORE (glasses have Mg# lower than 0.54). In the intr atransform volcanics, phenocryists and megacrysts are plagioclase and olivine and accessory spinel microphenocrysts in the magnesian lavas, and clinopyroxene and plagioclase in ferrobasalts. Variable mineral ch emistry of plagioclase and spinel, chemical disequilibria between thes e phases and surrounding glass, and resorption features and oscillator y zoning in plagioclase suggest that limited magma mixing occurred dur ing genesis of intratransform MORE. Aluminous-to chromian-spinel compo sitions are in agreement with melt interactions in small magma pockets . These processes are superimposed on complex partial melting events o f a heterogeneous source region underlying the intratransform domain. Generation of most ferrobasalts can be explained by crystal fractionat ion of primitive MORE. Some ferrobasalts appear to derive from discret e magma bodies, since they are not simply connected to the magnesian t holeiites through fractional crystallization processes.