Abstract:
The melting relations of three analysed greywackes, from the Ikawhenua Ranges, eastern flank of the Taupo Volcanic Zone, have been examined under excess water conditions. Pressures up to 350 MPa and temperatures up to 820°C were used. Pressure-temperature curves were determined for the solidus of each system, the disappearance of alkali feldspar and the disappearance of quartz. The solidus curves for the three greywacke systems were within 25°C of Tuttle and Bowen's (1958) granitic solidus. The 'rhyolitic' greywacke Type A produced a simple phase system predominantly of quartz, plagioclase, mica and titaniferous magnetite. The 'andesitic' greywacke Type C produced a more complex phase system commonly comprising quartz, plagioclase, augite, hornblende, calcic cummingtonite and titaniferous magnetite. Greywacke Type B forms an intermediate system between systems A and C.
Models for the petrogenesis of the Taupo Volcanic Zone andesites, dacites and rhyolites are proposed based on the melting relations of the greywackes and published data. All the models are based on four main assumptions: (l) the source rock is similar to greywacke Type B; (2) a water content of 2-3 weight percent; (3) a geobaric gradient of 27 MPa per km; and (4) a geothermal gradient of 50°C per km, The andesite model involves initial melting at 20 km depth, absorption of basaltic material, a relatively rapid ascent and eruption. The dacite model postulates, initial melting at 17-19 km depth, a rapid first stage ascent, a slower second stage ascent and eruption. The high-temperature pumice model involves initial melting at 17-19km depth, a slow first stage ascent, a rapid second stage ascent and an explosive eruption. The rhyolite model involves initial melting at 15-16 km depth followed by a slow ascent and an eruption less violent that for the pumice model.
