A convergent margin magma series with characteristic low Nb and Ta abundances and enrichments in alkalis and alkaline earths is intercalated with typical intraplate alkalic basalts in a back-arc setting, 200–250 km above the Wadati-Benioff zone on the North Island, New Zealand. These two contrasting magma types, together with late-stage K-rich maflc lavas, were erupted over a short time period (1˙60–2˙74 Ma) and constitute the Alexandra Volcanics. Field relationships indicate that these diverse magma types were contemporaneous, and thus their mantle source regions coexisted, in a single tectonic environment.