GEOCHEMISTRY AND HEAT-TRANSFER PROCESSES IN QUATERNARY RHYOLITIC SYSTEMS OF THE TAUPO VOLCANIC ZONE, NEW-ZEALAND

The Taupo Volcanic Zone (TVZ) is a Quaternary volcanic arc associated with subduction at the southern end of the Taupo-Kermadec arc-trench s ystem. At least 10,000 km3 (dense equivalent) of volcanic rocks have b een extruded during the last 2 Ma along the 200 km long segment of the exposed arc. These consist dominantly of rhyolite lavas and pyroclast ics (80% by volume) with subordinate andesite, dacite and basalt. The TVZ's crustal heat transfer of about 5650 MW is anomalously high and i s a result of production of large volumes of dominantly rhyolitic magm a by crustal anatexis. About 20% of this magma reaches the surface' Th e erupted rhyolites have high-Si metaluminous compositions showing onl y small variations between and within eruption centres. Magma batches of essentially homogeneous composition have been identified which feed single eruption sequences as well as multiple sequences with timespan s of up to 20 Ka. The chemical composition of the TVZ rhyolites indica tes that they are partial melts derived from an apparently homogeneous lower crust. The TVZ has been classified in the past as a back-arc ba sin heated by ponded mafic melts associated with a concealed, spreadin g asthenosphere plume (exogenous crustal heating). However, geochemica l and geophysical arguments indicate that there is little evidence for any asthenosphere plume beneath the TVZ. An alternative heat source i s proposed in which crustal heating is produced by plastic deformation of the ductile crust concentrated within a narrow tectonic hinge line (endogenous crustal heating). Such deformation promotes crustal fusio n and leads to fractionation of the lower crust into an upper ''felsic layer'' and a lower mafic to ultramafic ''restite layer''.