Abstract

Basaltic lavas from Grenada, Lesser Antilles, may be divided into two series on the basis of CaO-MgO relations. The C-series roughly corresponds to the ankaramites or high-Sr series of the earlier literature and may be distinguished from the M-series basalts by higher CaO at a given MgO and by their strongly augite-phyric nature. Compositional variation within the C-series is dominated by fractional crystallization of augite and plagioclase but this was accompanied by assimilation of sedimentary material from the island arc crust. The assimilation-fractional crystallization process is well illustrated by an increase in 206Pb/204Pb from 18.6 to 19.4 with fractionation, and correlated increases in Nd/Sr, Zr/Sm and Rb/Ba. The most primitive members of the C-series can not be generated from erupted M-series near-primary basalts, but instead show evidence for an origin by olivine fractionation from high-Mg basalts never seen at the surface. These appear to have been derived from a source similar to that of MORB, but enriched in 87Sr and other hydrophile elements by dehydration of subducted lithosphere, and in REE and radiogenic Pb by a small component of subducted sediment.

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