Abstract
Analyses of young volcanic rocks from the New Hebrides reveal the existence of two geochemical groups which may be identified on the basis of their contents of K 2O and related trace elements. Low K 2O rocks are believed to be comparable with conventional island arc volcanism, whereas the high K 2O rocks are believed to be related to volcanism associated with tensional rifting. By comparison with high pressure experimental data it is concluded that these rocks can be derived by partial melting of hydrous mantle above the Benioff zone. However this mantle must have different concentrations of incompatible elements from the source of ocean ridge tholeiites, and residual minerals must have RE partition coefficients which differ from those of phenocrysts in volcanic rocks.
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