CONSTRAINING THE MAGMATIC EVOLUTION OF RUAPEHU VOLCANO, NEW ZEALAND: NEW HIGH-PRECISION 40AR-39AR GEOCHRONOLOGY AND GEOCHEMISTRY

Models for the petrogenetic evolution of circum-Pacific arc magmas are dependent on volcanoes with precise temporal frameworks. Resolving the stratigraphy of composite volcanoes requires detailed interrelated physical volcanology and geochemical characterisation of eruptive products, combined with absolute dating. A new chronostratigraphic model is presented for Ruapehu volcano, an andesite-dacite composite volcano of the Taupo Volcanic Zone, New Zealand. The timing and duration of periods of effusive volcanic activity have been revised based on 32 new high-precision ⁴⁰Ar-³⁹Ar age determinations and high-resolution whole-rock major and trace element geochemistry of lava flows.

Effusive eruption of low-K basaltic andesite from > 200 – 170 ka and 160 – 120 ka constructed the oldest exposed parts of the Ruapehu edifice located on the northern and southern flanks, respectively. Following an apparent period of quiescence, several pulses of effusive activity prevailed from 45 – 5 ka, which built the modern cone. Dacite was erupted exclusively between 45 and 40 ka, whereas the composition of lavas erupted between 25 and 5 ka reverted to more mafic values (ca. 58 – 60 wt. % SiO₂). The presence of glacial ice on the volcano during this time period had a major effect on the distribution of lavas and, therefore, the morphology of the edifice. Widespread evidence for interaction with glacial ice during emplacement of lava flows erupted between 45 and 15 ka is reported for the first time. Lava erupted < 10 ka flowed into valleys where glacial ice had retreated. Eruption ages measured for these flows have associated uncertainties of < 1.2 kyr (1 s.d.) and represent the first absolute dates obtained for Holocene lavas on Ruapehu.

Our improved chronostratigraphy allows new and existing geochemical data to be (re-) interpreted in order to precisely constrain and sequence the magmatic evolution of Ruapehu volcano. Magma flux rates have been revised for Ruapehu, and are compared to other circum-Pacific arc volcanoes. This study is also contributing to the production of a 1:50,000 scale geological map for volcanoes of the Tongariro National Park, in a collaborative project between New Zealand and the US Geological Survey.