Plutonic lithics in ignimbrites of Taupo Volcanic Zone, New Zealand; sources and conditions of crystallisation

Abstract

Plutonic rock fragments occur as rare accessory lithics within some of the ignimbrites erupted from calderas and caldera complexes in central Taupo Volcanic Zone (TVZ), New Zealand. The two main suites of plutonic lithics are: (1) medium to fine-grained granitoids and (2) fine-grained dolerites and microdiorites. The granitoids are mostly medium-grained rocks with both equigranular and granophyric intergrowth textures, the latter indicating rapid crystallisation at shallow crustal levels. Biotite is the predominant mafic phase in most granitoid samples, although rare hornblende and orthopyroxene-bearing granitoids also occur. Glass occurs in some granitoid samples from Rotoiti ignimbrite, indicating that these samples were incompletely crystallised prior to eruption. Dolerite and microdiorite fragments typically contain abundant hornblende, and up to 10% vesicular interstitial glass. SiO 2 abundances in the granitoids range between 70.3 and 77.9%. They are weakly metaluminous to weakly peraluminous and show an enrichment in Large Ion Lithophile (LIL) elements (Rb, K, Ba), and a depletion in Nb, relative to MORB, similar to recent TVZ rhyolitic volcanic rocks. Chemically and petrographically the granitoids fall into two distinct groups: high-Sr granodiorites (120–205 ppm Sr), and chemically more evolved low-Sr monzogranites (26–65 ppm Sr), with an apparent compositional gap between the two. Eu/Eu* anomalies and strong correlations between Al 2O 3, CaO and Sr indicate that compositional variation between the two granitoid groups is dominantly controlled by plagioclase fractionation. The high- and low-Sr groups coincide with the highest and lowest Sr concentrations observed in TVZ rhyolites. REE data and Sr, Nd, Pb and O isotope ratios for granitoids are comparable to published data for TVZ rhyolites, and consequently they are considered to represent crystallised portions of zoned silicic magma chambers associated with Quaternary TVZ volcanic activity, and not fragments of older pre-TVZ basement. Most granitoids are regarded as comagmatic with their host ignimbrites, but those from Rotoiti ignimbrite have different mineral assemblages and whole-rock chemistry to the host ignimbrite. These fragments appear to be incompletely crystallised portions of an earlier magma system, possibly the same system that generated the Matahina ignimbrite. Dolerites and microdiorites are geochemically similar to evolved high alumina basalts erupted from the central TVZ and are considered to be intrusive equivalents.