The nature of primary rhyolitic magmas involved in crustal evolution: Evidence from an experimental study of cummingtonite-bearing rhyolites, Taupo Volcanic Zone, New Zealand

Abstract

Many of the rhyolites of the Taupo Volcanic Zone (TVZ), North Island, New Zealand, have been interpreted as representing primary magmas from the melting of crustal sources, especially the greywackes of the TVZ Mesozoic basement. High-silica (~75% SiO 2) rhyolites of the Okataina caldera complex which carry cummingtonite phenocrysts have been considered as products of low-temperature (~750°C) crustal magmas formed under conditions close to saturation in water ( a H2 O ~ 1). Experimental crystallization studies on contrasted metaluminous (2.6% Di) and peraluminous (1.5% C) Okataina rhyolites confirm that cummingtonite could have crystallized only at pressures ≤ 3 kb (depths ≤ 10 km), probably in the roof zone of a shallow crustal magma chamber and that these rhyolites are more likely to represent fractionated magmas. This view is supported by comparison of their major element compositions with those of experimental silicic liquids from the melting of model metaluminous (TVZ dacite) and peraluminous (TVZ “Western Basement” greywacke) crustal compositions. The Okataina rhyolites and also less silicic average TVZ rhyolite compositions, are notably poorer in the An-component of normative feldspar than the experimental melts, particularly those produced under conditions of a H 2 O ~ 1 (which contain up to 18% An). This suggests that they were derived from more primitive magmas (probably the end products of complex processes involving magma sources in both the crust and mantle) by crystal fractionation or assimilation-fractional crystallization (AFC) dominated by plagioclase-rich extract assemblages. In assessing the nature of magmatic processes involved in internal differentiation of the continental crust, it should be noted that primary crustal magmas are likely to be appreciably richer in plagioclase, more mafic and less silicic, than those represented by voluminous rhyolitic extrusive rocks. Normative An and PI contents may be used as an indicator of primitive character for rhyolitic crustal melts in the same way as normative Fo and Ol for picritic-basaltic melts derived from the mantle.