PETROGENESIS OF LATE CENOZOIC VOLCANIC-ROCKS FROM THE TAUPO VOLCANIC ZONE, NEW-ZEALAND, IN THE LIGHT OF NEW LEAD ISOTOPE DATA

Lead isotopes and U/Pb concentrations have been measured for a wide range of volcanic rocks from the Taupo Volcanic Zone (TVZ), New Zealand. Combined geochemical and isotope plots suggest that the majority of mafic-intermediate volcanic rocks result from contamination (AFC) of primitive basalt by Mesozoic metasedimentary basement. Lead isotope data indicate that some previously published petrogenetic models require modification, notably in the choice of parental magma and in the composition and proportion of the crustal assimilant. Evidence is also presented to show that the voluminous ignimbrites and rhyolites of the TVZ could be derived by melting of early-formed andesites (with accompanying minor crustal contamination) and rule out previous models such as direct melting of Mesozoic basement rocks or contamination of mantle derived melts.