The island of Ischia consists of a young volcano (0.13 Ma), which was built during the activity of the Phlegraean Fields, a part of the Roman Comagmatic Province (RCP). Two main volcanic series can be distinguished, the older consisting of strongly evolved rocks, and the younger of intermediate products with some trachybasalts and latites. Lava samples from both series have been analyzed in order to determine the major and trace elements including REE. It has been found that although all these lavas have undergone a significant evolution process, none of them have been derived from primary liquids. Calculations combining the major and trace elements indicate that simple crystal fractionation cannot account for all the geochemical characteristics observed. This conclusion is also supported by published isotopic data. The chemical and isotopic variations observed are also in agreement with results from quantitative theoretical models involving mixingmagmas and crystal fractionation processes. It would appear that two magmas will mix when one has similar geochemical characteristics to the RCP, and the other is less rich in radiogenic Sr and incompatible elements. The elemental and isotopic distributions of the latter are in fact fairly close to values of a primary liquid generated in a “normal mantle”. The presence of this magma supports the hypothesis that the geochemical and isotopic characteristics of the mantle beneath the RCP is strongly zoned.
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