Abstract
Calc-alkaline and alkaline magmatism generally occur separate in space and time at convergent margins, with the latter possibly related to slab retreat or to the formation of a slab window inducing asthenospheric upwelling. Irrespective of the mechanism, how this succession is reflected in the mantle sources is still largely unknown. Some clues may come from ultramafic xenoliths found in pyroclastic deposits of the Alexandra Volcanics in the North Island (New Zealand), where calc-alkaline and alkaline eruptive products are randomly intercalated in the volcanic sequence of the Pliocene-Quaternary Karioi and Pirongia stratovolcanoes. The xenoliths include typical mantle lithotypes (lherzolites and harzburgites, characterized by medium to coarse-grained protogranular, porphyroclastic and equigranular textures) as well as abundant olivine-clinopyroxenites, dunites and wehrlites with cumulitic and protogranular textures. Disseminated and vein amphibole is present in some olivine-clinopyroxenites and dunites. Reaction textures with formation of secondary, newly-generated phases and glass are widespread in all nodules but especially in the cumulates, where distinct metasomatic episodes have been recognized even within a single sample. The lithospheric section represented by Karioi peridotites underwent partial melting (F%) between 5 and 15%, as confirmed by spinel, olivine compositions and clinopyroxene HREE contents. Samples showing the highest F% degree are characterized by orthopyroxenes and clinopyroxenes with the highest LILE, LREE and MREE enrichment. A post-depletion metasomatic event occurred from interaction with a calc-alkaline melt similar in composition to arc lavas found in the Alexandra Volcanic Group. On the other hand, mineral compositions in the olivine-clinopyroxenites and dunites indicate their origin as deep fractional crystallization products of the same calc-alkaline magmas, which most probably also repeatedly infiltrated all lithologies, giving rise to multiple metasomatic pulses. A final metasomatic infiltration occurred, generating amphiboles with trace elements (particularly Nb) suggesting a different magmatic affinity, that was probably transitioning toward the alkaline type.