CORRELATION OF RHYOLITIC PYROCLASTIC ERUPTIVE UNITS FROM THE TAUPO VOLCANIC ZONE BY FE-TI OXIDE COMPOSITIONAL DATA

Grain-specific analyses of Fe-Ti oxides and estimates of eruption temp erature (T) and oxygen fugacity (fO(2)) have been used to fingerprint rhyolitic fall and now deposits that are important for tephrostratigra phic studies in and around the Taupo volcanic zone of North Island, Ne w Zealand. The analysed Fe-Ti oxides commonly occur in the rims of ort hopyroxene crystals and appear to reflect equilibrium immediately prio r to eruption because of geochemical correlation with the co-existing glass phase. The composition of the spinel phase is particularly diagn ostic of eruptive centre for post-65 ka events and can be used to dist inguish many tephra beds from the same volcano. The 29 different units examined were erupted over a wide range in T (690-990 degrees C) and Delta log fO(2) (-0.1 to 2.0). These parameters are closely related to the mafic mineral assemblage, with hydrous mineral-bearing units disp laying higher fO(2). Such trends are superimposed on larger difference s in fO(2) that are related to eruptive centre. At any given temperatu re, all post-65 ka Okataina centre tephra have higher fO(2) values tha n post-65 ka Taupo centre tephra. This provides a useful criterion for identifying the volcanic source. There are no temporal T and fO(2) tr ends in the tephra record; over intervals >20 ka, however, tephra sequ ences from Taupo centre form characteristic T-fO(2) buffer trends mirr oring the glass chemistry. Individual eruptive events display uniform spinel and rhombohedral phase compositions and thus narrow ranges in T (+/- <20 degrees C) and log fO(2) (+/- <0.5), allowing these features to identify individual magma batches. These criteria can help disting uish tephra deposits of similar bulk or glass composition that origina ted from the same volcano. Distal fall deposits record the same T-fO(2 ) conditions as the proxima ignimbrite and enable distal-proximal corr elation. Lateral and vertical compositional and T-fO(2) variability di splayed in large volume (>100 km(3)) ignimbrites, such as the Oruanui, Rotoiti and Ongatiti, is similar to that found in a single pumice cla st and thus mainly reflects analytical error; however, thermal gradien ts of ca. 50 degrees C may occur in some units.