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A suite of West Indian igneous rocks ranging in age from pre-Middle Cretaceous to Recent has been analyzed for major elements, Th, U, and Sr and Pb isotopes. Two major groups of magmas are revealed by the Th, U, and Pb isotopes. The first of these is a “chemically primitive” group generated prior and briefly subsequent to the major unconformity of the Middle Cretaceous, which is in turn related to initial rupture along the Benioff Zone and the initiation of expression of trench-island platform topography. A “chemically more evolved” group was dominant from the Middle Cretaceous to the Recent.

Two explanations for the evolution in radioactive elements and isotope con-tents are suggested: (1) the crustal and upper mantle transported down the Benioff Zone to the locus of fusion varied with time, reflecting either original vertical or lateral variations; or (2) increasing transport of more radioactive sedimentary materials to the locus of fusion during island-arc evolution resulted in increased contamination. Positive evidence for the latter explanation during the Tertiary is presented, but the former effect is also suggested.

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