Magma Systems at Taupo Volcano, New Zealand: Insights from U-Th Model-age Spectra in Zircons

In the last 60 kyr, there has been erupted c. 580 cubic kilometres of silicic (overwhelmingly rhyolitic) magma from vents under or just north of Lake Taupo. About 90% of this volume was vented in the 27 ka Oruanui event, but a complex sequence of >12 eruptions before and 28 after the climactic event show how the magmatic systems feeding these eruptions in the general Taupo area have evolved in time and space. Zircon model age spectra from a suite of eruptions prior to the Oruanui show that systems which vented contrasting compositions from sources only 15 km apart underwent independent crystallization (i.e., thermal) histories. Precursor leaks of the system that fed the Oruanui eruption itself are compositionally similar and have a common 90-100 ka peak in their zircon model-age spectra, but differ in the relative proportions and ages of younger (shortly pre-eruptive) crystallization peaks. Field data show that the vent sites for two of these precursor events (Okaia, 30 ka; Tihoi, c. 45 ka) were sited within the modern lake, overlapping with the footprint of the Oruanui chamber. Despite the spatial overlap of vents, however, the three units concerned have different modes in their younger crystallization ages. Their common chemistries and older 90-100 ka model-age peaks imply that they share a common source (mush) zone, but the model-age spectra show that the melt-rich bodies in each eruption were independently extracted. Thus the c. 530 cubic kilometre Oruanui melt-dominant body is implied to have formed in only about 3000 years (from 30 to 27 ka, each +/- 1 kyr). Post-caldera dacites and rhyolites are chemically and isotopically distinct from the Oruanui, with no mixing relationships. The first of the rhyolites (11.8 ka) has a model-age peak identical to the young Oruanui peak, but a slightly older (16 ka) rhyolite erupted nearby, 15 km north of Taupo is dominated by zircons with model ages post-dating the Oruanui. The crust in the Taupo area includes several independent magma generation systems, active over tens of thousands of years or more, that can be provoked into segregating huge volumes of magma extraordinarily rapidly, driven, we infer, by tectonic processes.