Crystallisation history and crustal inheritance in a large silicic magma system: 206Pb/ 238U ion probe dating of zircons from the 1.2 Ma Ongatiti ignimbrite, Taupo Volcanic Zone

Abstract

Cathodoluminescence (CL) images of more than 300 zircons from the 1.21 Ma Ongatiti ignimbrite, Taupo Volcanic Zone (TVZ), reveal rare resorbed zircon cores surrounded by euhedral and zoned magmatic rims. U/Pb ages from in situ spot analyses of magmatic zircon cores and rims range from 1.18 to 1.44 Ma with a peak crystallisation age of 1.31±0.03 Ma ( n=23, MSWD=1.7). In contrast to zircon ages from the large-magnitude Whakamaru system, age differences between magmatic cores and rims cannot be resolved for the Ongatiti system, and there is no evidence for prolonged crystallisation (>200 kyears), nor inheritance from earlier magma batches at the Mangakino caldera (>1.5 Ma). Four resorbed zircon cores produced 206Pb/ 238U ages of 119±2, 231±4, 315±7 and 340±6 Ma, with young (∼1.3 Ma) magmatic rims. These ages broadly correspond to the dominant detrital zircon age populations in the local Cretaceous metasedimentary basement rocks, and provide the first direct evidence for significant contributions of these basement rocks to rhyolite petrogenesis in the early TVZ.