Geology and petrology of the Tauhara volcanic complex, Taupo, New Zealand

Abstract

The Tauhara VolcanicComplex consists of seven volcanic domes with a combined volume of 1.2km3, located on theNE rim of the Taupo Volcanic Centre, Taupo Volcanic Zone (TVZ), New Zealand. A detailed investigation of the structural aspects of Tauhara has revealed features typical of other domes and dome complexes, including seven lava flows, two pyroclastic flows, spines, autobreccias, talus banks and hydrothermal alteration. Structural and stratigraphic observations suggest the sequence of dome extrusion was (from oldest); Western, Central, Breached and Main Domes, with Trig M and Waipahihi Domes preceding Breached Dome, while Hipaua Dome preceded Main Dome. Tephrostratigraphic evidence indicates volcanic activity ceased 15 to 20kyr ago at Main Dome and 19 to 20kyr ago at Hipaua Dome. Tauhara samples contain a diverse phenocryst assemblage of plagioclase (An25-40 and An75-90), orthopyroxene (En45-55 and En70-80), Clinopyroxene (two morphologically distinct augites- large euhedral equant phenocrysts and finegrained spheroidal monomineralic clustersof acicular augite phenocrysts, often with quartz phenocrysts at the core of the cluster), hornblende, quartz, olivine (Fo82) and biotite, with accessory magnetite, apatite and cristobalite. The phenocryst assemblage is not in equilibrium with the host lava and phenocrysts typically exhibit strong zoning (normal or reverse) near their rims, are resorbed or display fusion textures. Groundmass plagioclase compositions are intermediate to those of phenocryst plagioclase (An50-70). Tauhara samples are predominantly calc-alkaline dacites (64.5 to 71.5wt% SiO2), but each Tauhara dome is chemically and modally heterogeneous with respect to other TVZ domes and dome complexes. However, correlation coefficients between modal phenocryst phases and sample SiO2-contents are weak. The range of dome compositions and trends overlap. The origin of the Tauhara dacites was considered in terms of four petrogenetic processes;(i) partial melting of crustal rocks, (ii) fractional crystallisation of a basalt or andesite magma, (iii) assimilation-fractional crystallisation involving a basalt or andesite magma and rhyolitic wallrock, and (iv) magma mixing involving a basalt or andesite magma and a rhyolite magma. To be successful, a petrogenetic model must account for the distinctive chemical and mineralogical features of the Tauhara dacites; (i) their often well-defined (correlation coefficients greater than 0.9) linear element/element oxide versus SiO2trends, (ii) their MgO, Sr and Ni-rich, K2O-poor nature relative to other TVZ dacites, (iii) their linear trend and isolated compositional field on an AFM diagram, (iv) their diverse and compositionally bimodal phenocryst assemblage, and (v) the fusion textures of some phenocrysts and other evidence of disequilibrium. Neither partial melting nor fractional crystallisation models can adequately account for the chemical and mineralogical features of the Tauhara dacites, while assimilation-fractional crystallisation is incapable of reproducing the Tauhara dacite chemistry if the magma and wallrock end-members are among published analyses of TVZ rocks. Magma mixing can reproduce these features if the end-members were similar to Trig 9471 rhyolite (located at the NW base of Tauhara) and either Rolles Peak andesite (located 8km ENE of Tauhara) or an andesitic xenolith (VUW 19544) from Trig 9471 rhyolite. Subtle chemical differences between the Tauhara domes suggest magma mixing may have proceded as one or two general mixing episodes, each punctuated by several mixing events. The Tauhara dacites contain 34 to 87% of the rhyolite end-member. Equilibrium exchange coefficients enable the Tauhara phenocryst assemblage to be separated into three associations; (i) phenocrysts whose compositions indicate crystallisation from a basaltic or andesitic magma, (ii) phenocrysts whose compositions are typical of TVZ rhyolites, and (iii) phenocrysts compositionally similar to those of association (i) thatrecord abrupt changes in the magma towards a more silicic composition. The association (iii) phenocrysts can be used to trace the mixing process and indicate that the basaltic orandesitic magma became supercooled and hydrated at the start of the mixing event.