Patterns of Quaternary basaltic volcanism in the northern North Island, New Zealand

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Abstract
North of Taupo Volcanic Zone (TVZ) are 4 major Quaternary volcanic fields; South Auckland, Auckland, Whangarei, and Kaikohe-Bay of Islands. Volcanoes in these fields are monogenetic and include maars and scoria cones atop lava pedestals. Volumes are small (3) although a few centres erupted approximately 5 km3 of lava and scoria. Early,, often explosive, undersaturated volcanism is followed by eruption of more voluminous saturated basalts. Differences in chemistry and volcanic style between earlier and later volcanism are matched by differences in texture and mineralogy of the lavas. A similar style of basaltic volcanism occurred throughout the Pliocene and Miocene. The basalts are explained as products of progressive melting of mantle diapirs formed by convection in the mantle wedge to the rear of the TVZ. Melting probably occurs between 70 and 120 km, which is a zone of low Q beneath the northern North Island. Ascent rates of mantle diapirs of the order of 102 cm/year are suggested by the rapid development of tholeiitic basalts during the short (c.60 000 year) history of the Auckland field.