Magma genesis in the lesser Antilles island arc |
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| Authors: | CJ Hawkesworth M Powell |
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| Institution: | Department of Earth Sciences, Leeds University, Leeds LS2 9JT U.K. |
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| Abstract: | 143Nd/144Nd,87Sr/86Sr and REE results are reported on volcanic rocks from the islands of Dominica and St. Kitts in the Lesser Antilles. Particular attention is given to the lavas and xenoliths of the Foundland (basalt-andesite) and the Plat Pays (andesite-dacite) volcanic centres on Dominica. Combined major and trace element 2] and isotope results suggest that the bulk of the andesites and dacites on Dominica, and by analogy in the rest of the arc, are produced by fractional crystallisation of basaltic magma. The differences in the erupted products of the two volcanoes do not appear to be related to any significant differences in the source rocks of the magmas.Along the arc87Sr/86Sr ratios range from 0.7037 on St. Kitts, to 0.7041–0.7047 on Dominica, and 0.7039–0.7058 on Grenada 5], and these are accompanied by a parallel increase in K, Sr, Ba and the light REE's. Moreover, compared with LIL-element-enriched and -depleted rocks from MOR and intraplate environments, the basic rocks from the Lesser Antilles are preferentially enriched in alkaline elements (K, Ba, Rb, Sr) relative to less mobile elements such as the rare earths.143Nd/144Nd varies from 0.51308 on St. Kitts, to 0.51286 on Dominica, and 0.51264–0.51308 on Grenada 5], and all these samples have relatively high87Sr/86Sr ratios compared with the main trend of Nd and Sr isotopes for most mantle-derived volcanic rocks. Alkaline elements and87Sr appear to have been introduced from the subducted ocean crust, but the results on other, less mobile elements are more ambiguous — island arc tholeiites (as on St. Kitts) do not appear to contain significant amounts of REE's, Zr, Y, etc., from the subducted oceanic crust, but such a contribution may be present in more LIL-element-enriched calc-alkaline rock types. |
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