A low-grade metamorphic rock composed of albite, chlorite, actinolite, sphene, and calcite, with or without epidote, prehnite, and laumonite, and formed by sea-floor metasomatism of mid-oceanic-ridge basalts. Sea water circulating through the oceanic crust is heated by the cooling basalt dykes and lavas and reacts with them, introducing sodium and water into the rock system and converting the basalt mineral assemblage into a typical spilite assemblage.