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 (160-274 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.The convergent margin magma series forms a linear chain of stratovolcanoes aligned at right angles to the present subduction zone. Closed-system polybaric fractional crystallization models can explain the evolution from ankaramites to transitional olivine basalts to olivine tholeiites to high-Al basalts to medium- and high-K andesites. The most primitive lavas have geochemical (high LIL/LREE and LIL/HFS element ratios) and Sr, Nd, and Pb isotopic compositions typical of convergent margin magmas. Calculated source compositions suggest that three components are involved: a MORB component, a component derived from subducted oceanic crust, and a contribution from subducted sediments.The alkalic basalts occur as dispersed monogenetic volcanoes and are intercalated with the larger convergent margin stratovolcanocs. These basalts are enriched in LILE, LREE, Nb, and Ta, and have low Ba/Nb and Ba/La ratios, all of which are characteristic of ocean island (intraplate) basalts (OIBs). Their relatively high εNd (+55 and low 87Sr/86Sr(0703l-07036) are also typical of OIBs. These alkalic magmas were derived from the underlying continental lithospheric mantle that has been enriched by upward-migrating silica-undersaturated melts, probably including volatiles, from the low- velocity zone. A subducted slab component is not required to account for their incompatible element enriched character.The K-rich mafic lavas, basanites, and absarokites are volumetrically minor and cap the largest of the stratovolcanoes, Pirongia. The basanites have geochemical and isotopic compositions which suggest they are mixtures of multiple source components, including the alkalic and convergent margin region.
ASJC Scopus subject areas
- Geochemistry and Petrology