High-Si rhyolite magmas of Central North Island New Zealand (Taupo Volcanic Zone) occur as discrete physiochemical batches that are produced both within and between eruption episodes. The processes that create these compositional variations are magma mixing and mingling, crystal fractionation and variation in source parameters (melt proportion, source composition, residual mineral assemblage). Here we use a suite of samples selected to exclude magma mixing and mingling and examine the role of fractionation and variable source parameters in the production of magmas erupted from adjacent active caldera complexes (Okataina and Taupo Volcanoes) that have been active during the last 50 ka. Although both volcanoes have been highly active during this period, eruption rates are different as are phenocryst proportions and assemblages, pre-eruption temperatures and oxygen fugacity. REE abundance patterns all show inclined La-Sm and flat Gd-Lu. Magma batches show different total REE abundances and there are also variations within batches; most show small negative Eu anomalies. Modeling indicates that all have essentially the same origin in amphibole dominated middle crustal (0.3-0.5 Gpa) sources. However, HFS element abundances and ratios demonstrate the presence of source compositions that are discrete and indicate that the proportions of melting vary between magma batches even on short time scales (less than 5 ka). The magmatic systems feeding rhyolite volcanoes of the Taupo Volcanic Zone are characterized by continuous melting and rapid assembly prior to eruption rather than by the long term accumulation of magmas in magma chambers. This dynamic and unstable behavior is related to the thin crust (less than 25 km) and very high heat flows that characterize the central North Island of New Zealand and is in contrast with longer lived systems found in mature continental settings such as Yellowstone in the USA.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 1065 Major and trace element geochemistry;
- 3618 Magma chamber processes (1036);
- 3619 Magma genesis and partial melting (1037);
- 8439 Physics and chemistry of magma bodies