Abstract
Abstract Intermediate and silicic volcanics crop out along the western margin of the Canterbury Plains and form part of the basement to the Lyttelton volcano on Banks Peninsula, New Zealand. The Late Cretaceous Mt Somers Volcanics range in composition from basaltic andesite to rhyolite. The rhyolites are peraluminous and commonly contain garnet phenocrysts, features which are typical of S-type silicic magmas. Regression analysis of Sr whole-rock isotope data for rhyolites from Mt Somers indicates an age of 89.3 ± 2.0 Ma, with an initial 87Sr/86Sr ratio of 0.7085. This initial ratio is consistent with derivation by partial melting of quartzo-feldspathic Torlesse sediments. Intermediate Mt Somers Volcanics have variable but lower 87Sr/86Sr ratios which do not form a linear array. Positive correlations between increasing SiO2 content, the abundance of metasedimentary xenoliths, and 87Sr/86Sr ratios indicate that these mantle-derived magmas have been contaminated during ascent through the crust Regression ana...
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