Two major magma series are recognised in the northeastern Carribean island arcs and correlated with numerous examples elsewhere, mainly in the western Pacific. The Primitive Island Arc (PIA) series is apparently the oldest in each of its island-arc occurrences and is followed by the calcalkaline (CA) series, which locally co-occurs with a high-potassium (HK) variety, especially in topographically larger island areas. Also occurring in the northeastern Caribbean are tectonically emplaced examples of metamorphosed mid-ocean ridge basalt (MORB) and a diabase dike swarm possibly related to a rifting event. PIA series is distinguished from the CA by its lower content of LIL elements, including REE. Th, U, Zr, K, and Ba. The difference is most striking in the abundant siliceous differentiates of this series, in which these elements are scarcely higher in abundance than in mafic examples. The HK series is considered to be a subseries of the CA which is higher in K, Ba, Sr, Mg, and Ni, but has REE, Th, and other element levels very similar to the CA. The PIA series has been confused with MORB but has lower REE (with flatter patterns generally), much lower Ni, lower Ti, Zr, and Mg, and appears to be intrinsically hydrated. The abundance of siliceous differentiates also separates this series from MORB. PIA magma is believed to be generated by massive melting of hydrated mantle which rises in front of the descending snout of a newly subducting slab at the initial stage of island-arc genesis; i.e., before the island-arc platform has formed. CA magma has a different source and is probably the partially fused material of the slab itself, more or less modified by volcanic-sedimentary materials dragged to depth during subduction.