The Hole 900A rocks exhibit compositional and textural characteristics of cumulate gabbros formed, metamorphosed, and enriched by seawater alteration at or near a mid-ocean ridge. The major element composition of these rocks is typical of basalt or gabbro enriched in LILE by later seawater interaction. The depleted trace element composition is typical of cumulate gabbros that have lost essentially all of the trapped magma before solidification. Nd isotopic data indicate the cumulate gabbros have a MORB origin and have been enriched in U and Sr from seawater. The metamorphic style, with discontinuous bands of recrystallized plagioclase and pyroxene and incomplete destruction of porphyroclasts, is similar to Hole 735B metamorphosed gabbros. It is also probably typical of high-grade seafloor metamorphism and very atypical of high-grade continental metamorphism. A loosely constrained parental magma calculation yields a REE pattern similar to transitional MORB. Our data and the 40Ar/39Ar age of 136.4 Ma by Feraud et al. (this volume) are consistent with the conclusion that the Hole 900A rocks were formed and synchronously metamorphosed at or near the present site as the Atlantic Ocean started to open.