The origin of plagioclase phenocrysts in basalts from continental monogenetic volcanoes of the Kaikohe-Bay of Islands field, New Zealand: implications for magmatic assembly and ascent

Late Quaternary, porphyritic basalts erupted in the Kaikohe-Bay of Islands area, New Zealand, provide an opportunity to explore the crystallization and ascent history of small volume magmas in an intra-continental monogenetic volcano field. The plagioclase phenocrysts represent a diverse crystal cargo. Most of the crystals have a rim growth that is compositionally similar to groundmass plagioclase (~ An 65 ) and is in equilibrium with the host basalt rock. The rims surround a resorbed core that is either less calcic (~ An 20–45 ) or more calcic (> An 70 ), having crystallized in more differentiated or more primitive melts, respectively. The relic cores, particularly those that are less calcic (