The geochemistry of the volcanic rocks of Canouan, Grenadine islands, Lesser Antilles arc

Abstract

AbstractIn late Oligocene or early Miocene, a series of basalts and basaltic andesites erupted on the island of Canouan and formed the Old Volcanic Massif (OVM). In a second eruptive phase a series of basalts, basaltic andesites and andesites formed the upper part of the Canouan Formation (CF; lower Middle Miocene; post‐16 Ma) and the Upper Volcanic System (UVS; end‐Miocene; 6.8 Ma). These two eruptive phases are characterized by systematic differences in isotope ratios and chemical composition. The OVM rocks have 87Sr/86Sr and 143Nd/144Nd ratios ranging from 0.70512 to 0.70657 and 0.512415 to 0.512414, whereas in the CF/UVS rocks the ratios range from 0.70454 to 0.70493 and 0.512924 to 0.512843. In addition, the rocks of OVM also have lower ΣFe2O3 and Co contents, and higher La/Yb ratios than those of the CF/UVS. The isotopic and chemical data suggest that both suites were formed from similar partial melts of a subduction‐modified, depleted spinel‐lherzolite source. These data also show that the OVM melts assimilated relatively large amounts of highly radiogenic crustal material, whereas the CF/UVS melts assimilated relatively small amounts of highly radiogenic crustal material. The chemical and isotopic variations of the OVM rocks are consistent with the removal of mixtures of clinopyroxene, plagioclase and magnetite during assimilation‐fractional‐crystallization. Within the CF/UVS, the chemical and isotopic variations can be explained in terms of minor crustal assimilation; the removal of olivine, clinopyroxene, plagioclase and magnetite to form the basalts and basaltic andesites; and the removal of clinopyroxene, plagioclase, magnetite and amphibole to form the andesites. Copyright © 2008 John Wiley & Sons, Ltd.