Geochronology, Correlation and Magnetic Studies of Quaternary Ignimbrites in the Taupo Volcanic Zone, New Zealand
Voluminous, rhyolitic ignimbrites erupted from calderas in the Taupo Volcanic Zone (TVZ) of North Island, New Zealand during the last ca. 1.6 Ma, are characterised by geochemical, paleomagnetic, magnetic fabric and isotopic age techniques to determine their stratigraphy and source vent areas. Most of the welded ignimbrites record distinctive thermoremanent magnetism (TRM) directions that can be defined with a precision of less than 5 degrees. On this basis, individual ignimbrites may be identified and correlated. These data indicate that the voluminous Whakamaru group ignimbrites, mapped by various names in different parts of the TVZ, were probably erupted over a period of as little as 100 years. The Kaingaroa and Matahina ignimbrites display very similar TRM directions and may have been emplaced contemporaneously. Ahuroa and Mamaku ignimbrites display TRM directions widely different to that expected from a dipole field, and were emplaced during polarity transitions in Earth's magnetic field. Geochemically, glasses and FeTi-oxides from the TVZ ignimbrites are homogeneous and typical of high-SiO v2 (>75 wt percent) rhyolites. They indicate little evidence of derivation from physically or compositionally zoned magma chambers, and allow individual eruptives to be fingerprinted. Variable compositions of whole pumice clasts from welded units, previously interpreted as evidence for chemical zonation can be explained by glass alteration and variable mineral components. Geochemical and chronological data suggest the Rocky Hill Ignimbrite and/or Unit E ignimbrite (ca. 1 Ma) may be correlatives of the Potaka tephra, found in sedimentary basins outside the TVZ. Rock magnetic fabric studies using anisotropy of magnetic susceptibility of ignimbrites allow paleoflow patterns to be determined. These patterns are generally consistent with source areas inferred from other data. The source for Mamaku Ignimbrite is consistent with an area on the western margin of Lake Rotorua. The Whakamaru group ignimbrites appear to have originated north of Lake Taupo, and in particular from an area near the Western Dome Belt. Glass shards from nonwelded bases of ignimbrites are well suited to dating by the isothermal plateau fission track (ITPFT) method. Any partial fading of the spontaneous tracks has been corrected by a single-step heat treatment of 150 degrees C for 30 days. The resulting ages and their uncertainties are comparable is caret 40Ar/caret 39Ar plagioclase determinations. The following new eruption ages were determined: Whakamaru group ignimbrites (0.34 Plus-minus 0.03 Ma), Matahina Ignimbrite (0.34 Plus-minus 0.02 Ma), Kaingaroa ignimbrite (0.33 Plus-minus 0.02 Ma), informally named unit Downer 8 (0.33 Plus-minus 0.02 Ma), and Mamaku Ignimbrite (0.23 Plus-minus 0.01 Ma). These data suggest a major phase of activity, with several different caldera forming events in the interval ca. 0.35-0.32 Ma. The age of Mamaku Ignimbrite constrains the paleomagnetic excursion recorded in the unit to ca. 0.23 Ma, similar to the age of the Pringle Falls geomagnetic episode recorded in the western USA.