Geochemical Characteristics of the Late Mesozoic Andean Volcanics
We have evaluated new trace element, Nd and Sr isotope analyses for two late Mesozoic volcanic series located in south Peru: the Jurassic Chocolate Formation and late Cretaceous Toquepala Group. The trace element results confirm the close affinity of the Chocolate extrusives with other calc-alkaline volcanics erupted in mature volcanic arcs or active continental margins. The lavas are divided in four groups; within each group the basaltic andesites and andesites can be generated by fractional crystallization from a basaltic parent enriched in LIL elements. Although Sr isotope data may show some dispersion due to alteration processes, Nd isotopes clearly indicate a depleted source for the entire suite (εNdi = +2.7 to +7.9). The Toquepala rhyolites and dacites display great variation in their trace element ratios, but in general have high LIL element concentrations. Whilst the Nd isotopes indicate a depleted source (εNdi = +3.0 to +6.5) their origin remains unclear. Derivation by crystal fractionation from an andesitic parent or by partial melting of underlying Precambrian crust is not supported by the combined trace element and isotopic data. However, the complete volcanic suite must have undergone a different geochemical evolution to that of the Cenozoic Arequipa and Barosso Volcanics which assimilated Precambrian crustal material during ascent in the upper crust.
KeywordsBasaltic Andesite Crustal Contamination Crystal Fractionation Active Continental Margin Trace Element Abundance
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