Magmatic processes recorded in crystals from intraplate basalt volcanoes, Kaikohe-Bay of Islands volcanic field, New Zealand

Abstract

Late Quaternary, porphyritic basalts erupted in the Kaikohe-Bay of Islands volcanic field (KBIVF), New Zealand, provide an opportunity to explore the crystallisation and ascent history of small volume magmas in an intra-continental monogenetic volcanic field. The plagioclase, clinopyroxene and olivine phenocrysts all represent a diverse crystal cargo. Plagioclase have relic cores that are more sodic and more calcic than the equilibrium groundmass crystals. They have 87Sr/86Sr ratios that are either mantlelike (~0.7030) or crustal-like (~0.7040-0.7060), 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. The cores of clinopyroxene phenocrysts variously grew in a magma comparable to that of the host rock (cognate) and in melts that were more evolved or more primitive (antecrysts); and display textures indicative of magma mixing. Crystal-melt equilibria indicate formation pressures of 430±170 MPa (2σ) (=16±6 km depth) and 730±160 MPa (=28±6 km depth). The olivine population also comprises cognate crystals and antecrystic/xenocrystic components from both more evolved and more primitive melts. Their δ18O mean values (5.60 ‰; range 5.12-6.20 ‰) are higher than that of olivine from MORB-HIMU sources and overlap those associated with subduction sources. These values could reflect a relic subduction-induced contamination of the shallow mantle. The character of the crystal phases demonstrates that the KBIVF magmatic system is likely to be a crystal mush column extending vertically through the crust. This is surprisingly similar to that proposed for intermediate arc volcanoes, but the KBIVF system would have been ephemeral. Geophysical data indicates the presence of a body of partial melt at 10-19 km and the Moho at ~28 km depths, which coincide with depths calculated from clinopyroxene thermobarometry. Thus, it can be inferred that buoyancy and/or rheology contrasts may have caused basalt magmas to temporarily or permanently stall, promoting crystallisation and interaction with earlier magmas or intrusions. The magmas are derived from the mantle or the deep crust, but their crystals and crystal textures are a product of crustal inheritance and modification.