A precise Rb‐Sr age for the Mandamus Igneous Complex, North Canterbury, and regional tectonic implications

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Abstract
A precise Rb‐Sr isochron age of 97.0 ± 0.5 Ma is presented for whole‐rock and mineral samples from the Mandamus Igneous Complex, North Canterbury. A low 87Sr/ 86Sr initial ratio of 0.70342 ± 0.00002 implies that the mantle‐derived mafic parent magma underwent high‐level differentiation with no detectable contamination from continental crust. The Mandamus Rb‐Sr age is in close agreement with previously obtained K‐Ar ages for the Tapuaenuku Igneous Complex and basalt lavas of the Gridiron Formation, Marl‐borough. These units form part of a belt of alkaline igneous rocks of Ngaterian age, the emplacement of which accompanied extension of the thin continental crust of the Pahau subterrane following collision and subduction of a spreading ridge. The Mt Somers Volcanics Group of Central Canterbury is probably at least 6–10 Ma younger than the Ngaterian alkaline rocks and is compositionally distinct. Mt Somers magmas were emplaced through thick continental crust of the Rakaia subterrane in the same, postsubduction, extensional tectonic conditions.