The occurrence of leucocratic rocks in ophiolites, herein collectively named oceanic plagiogranites, indicates that in some instances, differentiation has taken place during the development of oceanic crust. These plagiogranites consist primarily of quartz and plagioclase with only minor amounts of ferromagnesian minerals. High silica, moderate alumina, low iron-magnesium, and extremely low potassium characterize oceanic plagiogranites and distinguish them from continental granophyres developed from differentiated mafic rocks. The 87Sr/86Sr ratios of the mafic and silicic rocks of the ophiolite suite of Troodos are similar. This similarity is consistent with the idea of low-pressure differentiation of sub-alkaline basaltic magma to form K poor leucocratic rocks. Slow-spreading centers may favor the formation of the oceanic plagiogranotes because crystal fractionation is more likely to occur in such situations.