PLUTONIC LITHICS IN IGNIMBRITES OF TAUPO VOLCANIC ZONE, NEW-ZEALAND -SOURCES AND CONDITIONS OF CRYSTALLIZATION

Plutonic rock fragments occur as rare accessory lithics within same of the ignimbrites erupted from calderas and caldera complexes in centra l Taupo Volcanic Zone (TVZ), New Zealand. The two main suites of pluto nic lithics are: (1) medium to fine-grained granitoids and (2) fine-gr ained dolerites and microdiorites. The granitoids are mostly medium-gr ained rocks with both equigranular and granophyric intergrowth texture s, the latter indicating rapid crystallisation at shallow crustal leve ls. Biotite is the predominant mafic phase in most granitoid samples, although rare hornblende and orthopyroxene-bearing granitoids also occ ur. Glass occurs in some granitoid samples from Rotoiti ignimbrite, in dicating that these samples were incompletely crystallised prior to er uption. Dolerite and microdiorite fragments typically contain abundant hornblende, and up to 10% vesicular interstitial glass. SiO2 abundanc es in the granitoids range between 70.3 and 77.9%. They are weakly met aluminous to weakly peraluminous and show an enrichment in Large Ion L ithophile (Ln) elements (Rb, K, Ba), and a depletion in Nb, relative t o MORE; similar to recent TVZ rhyolitic volcanic racks. Chemically and petrographically the granitoids fall into two distinct groups: high-S r granodiorites (120-205 ppm Sr), and chemically more evolved low-Sr m onzogranites (26-65 ppm Sr), with an apparent compositional gap betwee n the two. Eu/Eu anomalies and strong correlations between Al2O3, CaO and Sr indicate that compositional variation between the two granitoi d groups is dominantly controlled by plagioclase fractionation. The hi gh- and low-Sr groups coincide with the highest and lowest Sr concentr ations observed in TVZ rhyolites. REE data and Sr, Nd, Pb and O isotop e ratios for granitoids are comparable to published data for TVZ rhyol ites, and consequently they are considered to represent crystallised p ortions 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, pos sibly the same system that generated the Matahina ignimbrite. Dolerite s and microdiorites are geochemically similar to evolved high alumina basalts erupted from the central TVZ and are considered to be intrusiv e equivalents. (C) 1998 Elsevier Science B.V. All rights reserved.