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

Abstract

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 (~ An65) and is in equilibrium with the host basalt rock. The rims surround a resorbed core that is either less calcic (~ An20–45) or more calcic (> An70), having crystallized in more differentiated or more primitive melts, respectively. The relic cores, particularly those that are less calcic (< ~ An45), have 87Sr/86Sr ratios that are either mantle-like (~ 0.7030) or crustal-like (~ 0.7040 to 0.7060), indicating some are antecrysts formed in melts fractionated from plutonic basaltic forerunners, while others are true xenocrysts from greywacke basement and/or Miocene arc volcanics. It is envisaged that intrusive basaltic forerunners produced a zone where various degrees of crustal assimilation and fractional crystallization occurred. The erupted basalts represent mafic recharge of this system, as indicated by the final crystal rim growths around the entrained antecrystic and xenocrystic cargo. The recharge also entrained cognate gabbros that occur as inclusions, and produced mingled groundmasses. Multi-stage magmatic ascent and interaction is indicated, and is consistent with the presence of a partial melt body in the lower crust detected by geophysical methods. This crystallization history contrasts with traditional concepts of low-flux basaltic systems where rapid ascent from the mantle is inferred. From a hazards perspective, the magmatic system inferred here increases the likelihood of detecting eruption precursor phenomena such as seismicity, degassing and surface deformation.