Voluminous low temperature rhyolitic magmas in New Zealand

The glassy acid pumices, lavas and ignimbrites of the Pleistocene-Recent Central volcanic region of New Zealand contain iron-titanium oxide microphenocrysts, whose composition has been determined. Thirteen coexisting titanomagnetite-ilmenite pairs (all onephase and homogeneous) give two groups of equilibration temperatures: 735–780° C for amphibole-bearing rhyolites, and 860–890° C for very young non-amphibole-bearing pumices. O¹⁸/O¹⁶ analyses of coexisting phenocrysts from five amphibole-bearing rhyolites give estimated temperatures in the range 695–860° C, with an average of approximately 780° C.Using Zen's (1971) thermodynamic data on anthophyllite, the fugacity of water has been calculated for two rhyolites with cummingtonite-orthopyroxene-quartz phenocrysts; at 735° C and 745° C, fH₂O is 1100 and 1300 bars respectively. These values are consistent with that derived for an analysed phenocryst assemblage of biotite-sanidine-magnetite. As all the investigated rhyolites contain phenocrysts of orthopyroxene and magnetite, it is suggested that the small increase in of the low-temperature amphibole assemblages in comparison to the amphibole-free assemblages is caused by higher silica activity, as quartz phenocrysts are absent in the high-temperature amphibole-free rhyolites.The existence of large-scale rhyolitic magmas, with phenocryst contents ranging from 0–40 %, at temperatures in the lower part of the magmatic range, is interpreted to be consistent with previously suggested models of upper crustal anatexis in New Zealand, in which the breakdown of micas contributed the water necessary for partial melting.Details of the occurrence and chemistry of the cummingtonite phenocrysts are given in the appendix.