Strontium isotope composition and geochronology of intermediate—silicic volcanics, Mt Somers and Banks Peninsula, New Zealand

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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 analysis of Sr isotope data for rhyolites from the McQueens Valley area on Banks Peninsula give an age of 80.9 ± 1.9 Ma, with an initial 87Sr/86Sr ratio of 0.7090. This initial 87Sr/86Sr ratio is close to that of rhyolites from Mt Somers and suggests a similar origin. The similarity in ages and petrogenesis of the Mt Somers Volcanics and McQueens Valley volcanic rocks indicates that intermediate-silicic volcanism was widespread in Canterbury during the Late Cretaceous. Dacites and rhyolites from the Governors Bay area, which were erupted prior to the construction of the main cone of the Ly ttelton volcano, give an age of 10.8 ± 0.1 Ma, with an initial 87Sr/86Sr ratio of 0.7052. This age is the same as that of the base of the overlying Lyttelton volcano (as defined by K-Ar geochronology) and confirms the presence of two suites of intermediate-silicic volcanics on Banks Peninsula. The Miocene age, initial 87Sr/86Sr ratio (which is similar to that of subalkaline dikes intruding the volcano), and distinctive geochemistry of this suite are all consistent with the interpretation that it represents an early phase of Lyttelton volcanism.

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