Abstract
A significant feature of Quaternary volcanism in New Zealand is the presence of several compact fields of monogenetic basaltic volcanism, in many respects comparable to back-arc volcanism of Japan and intra-plate volcanism in S.E. Australia. Volcanism in South Auckland between 1.56 m.y. (B.P.) and 0.51 m.y. has produced some 20 km3 of lava and scoria from at least 70 centres. Older lavas are alkalic (basanite and alkali-olivine basalt), and are associated with maars and small scoria cones and many contain mafic and ultramafic xenoliths and megacrysts. Younger subalkalic lavas (mainly transitional tholeiite) form large scoria cones with extensive lava aprons.
Chemically and mineralogically the two types are distinct and represent separate parent magmas. Alkalic magma probably differentiated near source (<25 kb) by precipitation of kaersutite, olivine, and clinopyroxene. Subalkalic lavas were the product of more voluminous melting and observed differentiation trends may be due to Al-opx and subcalcic Al-cpx precipitation at 10 to 15 kb. Progressive melting of a mantle diapir rising through the low velocity zone can account for the temporal and petrologic relationships between the two magmas.
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Rafferty, W.J., Heming, R.F. Quaternary alkalic and sub-alkalic volcanism in South Auckland, New Zealand. Contrib. Mineral and Petrol. 71, 139–150 (1979). https://doi.org/10.1007/BF00375430
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DOI: https://doi.org/10.1007/BF00375430