Chronological and palaeomagnetic constraints on widespread welded ignimbrites of the Taupo volcanic zone, New Zealand

Abstract

Large volume (100-1000 km 3 ), widespread rhyolitic ignimbrites are the main products of the Taupo volcanic zone (TVZ) of New Zealand, one of the most active silicic volcanic regions on Earth. Several factors have made correlation and the eruptive history of the ignimbrites difficult to resolve, including limited exposure and chronological data, broadly similar lithologies and the lack of stratigraphic successions visible in the field. We have used the isothermal plateau fission track (ITPFT) method on glass shards from the nonwelded basal zones to obtain new eruption ages for the widespread units: Ongatiti (1.25B0.12 Ma), Whakamaru group (0.34B0.03 Ma), Matahina (0.34B0.02 Ma), Chimp (0.33B0.02 Ma), Kaingaroa (0.31B0.01 Ma) and Mamaku (0.23B0.01 Ma) ignimbrites. These glasses show little evidence of geochemical alteration and allow the units to be fingerprinted for correlation.The glass ages we have obtained for the late Quaternary units provide an independent check on chronological data obtained from phenocryst phases. The ITPFT method is a useful dating approach for sanidine-poor eruptives which limit the application of 40 Ar/ 39 Ar. Errors as limited as 10-30 ka can be obtained from the weighted mean of several age determinations. The thermoremanent magnetic (TRM) direction recorded in the units provides a means of correlation over a wide area of the TVZ, because each ignimbrite can be distinguished by its unique record of palaeosecular variation. These data indicate that the four separately mapped members of the Whakamaru group represent the same phase of activity, occurring within a period of 100 years. The TRM data indicate that the widespread Ahuroa ignimbrite erupted during an excursion in Earth's magnetic field, perhaps associated with the Cobb Mountain subchron (ca. 1.2 Ma). The youngest widespread welded unit, Mamaku ignimbrite (ca. 0.23 Ma), also erupted during an excursion and may represent a southern hemisphere record of the Pringle Falls geomagnetic episode found in the western United States. The palaeomagnetic and ITPFT data for the widespread late Quaternary ignimbrites suggest a major period of caldera formation at 0.34-0.30 Ma. This interval represents the eruption of multiple units from the Whakamaru caldera, followed by the formation of the Okataina and Reporoa calderas in rapid succession.

Stratigraphy, age and correlation of middle Pleistocene silicic tephras in the Auckland region, New Zealand: A prolific distal record of Taupo Volcanic Zone volcanism

“…A characteristic feature of AT-18 and -19 is that they display a wide range in glass composition similar to that of Unit D and Ahuroa exposed in the King Country to the west of the TVZ . The occurrence of two closely spaced but stratigraphically discrete tephra beds with identical glass chemistry supports the suggestion by Black et al (1996) that both Unit D and Ahuroa Ignimbrites were derived from the same magma and are part of the same eruptive phase. Correlation to Ahuroa/ Unit D is further supported by an ITPFT age of 1.16 ± 0.13 Ma obtained for AT-18.…”

Correlation of rhyolitic pyroclastic eruptive units from the Taupo volcanic zone by Fe-Ti oxide compositional data

“…At least eight major caldera centres Taupo and Okataina tephra names from Froggatt and Lowe (1990) and Wilson (1993). Ages and mineralogy from Froggatt and Lowe (1990) and Black et al (1996). T and fO 2 (this study) represent averages from number (N) of Fe-Ti oxide pairs and are calculated following Ghiorso and Sack (1991).…”

“…Dunbar et al 1989;Shane et al 1996;Black et al 1996). However, Briggs et al (1993) suggested that some large volume ignimbrites from the Mangakino centre are compositionally zoned.…”

Asymmetric, multiple-block collapse at Rotorua Caldera, Taupo Volcanic Zone, New Zealand

“…From the disparity between the two techniques Rogan (1982) suggested the presence of non-or reversely magnetised material at least 1 km thick. The Mamaku Ignimbrite has since been shown to have erupted during a magnetic excursion (Pringle Falls event) in which the North Pole was situated in an equatorial Atlantic position (Black et al 1996;Tanaka et al 1996). The intermediate magnetic direction of the Mamaku Ignimbrite probably masked the magnetic response of Rotorua Caldera measured by Rogan (1982), leading to the disparity of the basement depth calculations.…”

“…In places there is limited spatial overlap between calderas, adding to complications in developing simple caldera structures. Rotorua Caldera formed during a single eruption, that of the ~225 ka Mamaku Ignimbrite (Shane et al 1994;Houghton et al 1995;Black et al 1996).…”