Influence of the crust and crustal structure on the location and composition of high‐alumina basalts of the Taupo Volcanic Zone, New Zealand

Abstract

High‐alumina basalts (HABs) that occur throughout the central part of the Taupo Volcanic Zone (TVZ) are associated particularly with faulting, and many occur where faults intersect caldera margins. For convenience, the basalts are described in terms of three geographic‐tectonic segments: Okataina in the north, Kapenga in the middle, and Taupo in the south. Evidence for mixing and mingling between rising basaltic magmas and rhyolitic rocks and magmas is common, including the frequent occurrence of xenocrysts and xenoliths, quench textures, and melting around the rims of inclusions. Chemically, the basalts are similar in terms of major element compositions, suggesting relatively homogeneous PT conditions in the mantle source, but variation between some trace elements suggests different processes are operating in the crust with variable degrees of contamination. The model presented for HAB generation in the TVZ is for partial melting of mantle peridotite in the upper mantle, with the melt rising into the lower crust via dike swarms. In the upper crust, the distribution of HAB is strongly influenced by location and structure. In the Kapenga segment, there is little evidence for interaction between basaltic and rhyolitic magma, other than at very shallow levels, perhaps because the rhyolitic magma chambers (or pods) were solid, allowing brittle deformation and rapid intrusion of basalt dikes. At Okataina there is much greater mixing and mingling, suggesting there was still partially molten rhyolitic magma chambers beneath this area during basalt intrusion. Basalt in the Taupo segment occurs outside the Taupo caldera complex and may be related to the earlier Whakamaru caldera complex. The basalt is thought to rise through the crust as a network of unrelated melt batches into a plexus of discrete magma chambers and conduits, many of which are sited along fault zones causing fissure eruptions at the surface.