Isotopic and trace element constraints on the petrogenesis of subduction‐related lavas from Martinique, Lesser Antilles

Abstract

The Sr, Nd, and Pb isotopic compositions of orogenic lavas from Martinique span considerable ranges, from values close to those of mid‐ocean ridge basalts to compositions more characteristic of continental crust. The isotopic compositions, together with rare earth element (REE) and other trace element distributions, are used to characterize a number of distinct volcano‐stratigraphic “centers” on Martinique within which magma evolution is largely controlled by fractional crystallization processes. The range in REE distributions, incompatible trace element ratios, and isotopic compositions between centers on Martinique is, however, not compatible with an origin purely through fractional crystallization. In general, La/Yb ratios increase with decreasing 143Nd/144Nd. Similarly, K/Rb ratios, while decreasing only slightly within individual centers, vary by an order of magnitude for all Martinique volcanics and correlate well with isotopic ratios. These geochemical and isotopic features are thought to reflect contamination by a continentally derived sediment or melt thereof. The behavior of oxygen isotopes relative to radiogenic isotopes cannot be reconciled with a model in which subducted sediments alone are involved, and significant contamination of ascending magmas by terrigenous sediments intercalated within the arc crust is thought to occur. A model is proposed whereby parental magmas are derived from a subduction‐modified mantle wedge. During ascent, derivative magmas assimilate continentally derived sediments intercalated in the arc crust. This process increases δ18O and 87Sr/86Sr, and decreases 143Nd/144Nd. The proportion of assimilated intracrustal sediment involved in producing the most contaminated compositions may be more than 50%.