On the island of Mustique, fresh and propylitized olivine-plagioclase-clinopyroxene basalt, plagioclase-clinopyroxene-orthopyroxene and plagioclase-clinopyroxene-amphibole andesite lavas and minor intrusions are interbedded with Oligocene pyroclastic and epiclastic rocks. Chemical data show that two isotopically identical, but chemically different, suites of lava are present: (i) the OPXS (Sr-87/Sr-86 0.70403-0.70454; Nd-143/Nd-144 0.512952-0.512986; delta(18)O(cpx) 5.49 and 5.61), comprising basalts and orthopyroxene-bearing andesites; and (ii) the AMPHS (Sr-87/Sr-86 0.70401-0.70457; Nd-143/Nd-144 0.512981-0.513037; delta(18)O(cpx) 5.54), made up of basalts and amphibole-bearing andesites. The OPXS has higher contents of TiO2, P2O5, light rare earth elements, Sm, Pb, Th, U, Zr, Y and Nb, and higher La/Yb ratios than the AMPHS. The isotopic data suggest that both suites formed from melts derived from the same subduction-modified depleted mantle source as the volcanic rocks of nearby St Vincent and Bequia, and the northern islands of the Lesser Antilles Arc. The immobile trace element contents, and La/Yb ratios, of the OPXS are indicative of similar to10% partial melting of the source, whereas those of the AMPHS are indicative of similar to25% partial melting. The within-suite chemical variation of the OPXS is consistent with similar to45% fractional crystallization of its intratelluric mineral assemblages, and that of the AMPHS is consistent with the removal of similar to65% of its intratelluric assemblages. Experimental evidence suggests that both suites of basalt crystallized at pressures <8 kbar from melts containing 1-2 wt% water. After extensive fractional crystallization, the andesites crystallized at pressures between approximately 5 and 2 kbar. The OPXS magmas appear to have lost more of their water content than the AMPHS magmas. Thus, the OPXS andesites formed from melts with an estimated water content of 2-3 wt%, whereas the AMPHS andesites formed from melts containing at least 4.5 wt% water.
