A high-resolution 40Ar/39Ar lava chronology and edifice construction history for Ruapehu volcano, New Zealand

Ruapehu is an active ~ 150 km3 andesite-dacite composite volcano located at the southern end of the Taupo Volcanic Zone, New Zealand. The growth of the present-day edifice has occurred throughout coeval eruptive and glacial histories since ~ 200 ka. We present high-precision 40Ar/39Ar eruption ages for 46 samples and whole-rock major element geochemical data for 238 samples from lava flows. These new and existing data are interpreted in the context of geomorphologic and geologic mapping, volcano-ice interaction processes and glacier reconstructions to present an improved chronostratigraphic framework and new edifice evolution history for Ruapehu. Sub-glacial to ice-marginal effusive eruption of medium-K basaltic-andesites and andesites constructed the northern portion of the exposed edifice between ~ 200 and 150 ka, and a wide southeast planèze as well as parts of the northern, eastern and western flanks between ~ 160 and 80 ka. None of the dated lava flows have ages in the range of 80-50 ka, which may reflect an eruptive hiatus. Alternatively the lack of ages may be the result of erosion and burial of lavas and syn-eruptive glacial conveyance of lava flows to the ring-plain during glacial advance at 70-60 ka. From ~ 50-15 ka edifice growth occurred via effusive eruptions onto the glaciated flanks of the volcano, resulting in construction of ice-bounded planèzes and ridges. The distribution of dated ice-marginal lavas indicates a general decrease in glacier thicknesses over this time, except for a short-lived period centred at ~ 31 ka when peak ice cover was reached. The compositional ranges of medium- to high-K andesite and dacite lava flows within 50-35 ka eruptive packages define broadly bimodal high- and low-MgO trends. Lavas erupted at ~ 35-22 ka have compositions that fill a transitional geochemical field between older dacites and younger andesites. Large-scale retreat of flank glaciers since ~ 15 ka has resulted in intra-valley lava flow emplacement at elevations below ~ 1500 m on the edifice. Between 15 and 10 ka unstable cones were constructed through effusive activity in the presence of remnant summit and upper flank glaciers and the emplacement of eruptive deposits on top of hydrothermally altered and fragmental sub-glacial and ice-marginal deposits. Debuttressing of two northern summit cones and a southern summit cone as ice underwent continued post-glacial retreat pre-conditioned the edifice for two major sector collapses. Ultimate triggering of the collapse events and deposition of debris avalanche deposits on the northern and south-eastern flanks may have been the result of tectonic or volcanic activity. The northern collapse scar is infilled by a new cone comprising < 10 ka lava flows that form the upper northern and eastern flanks of the modern edifice. Late Holocene-Recent eruptive activity has occurred through Crater Lake, which occupies the site of the collapsed southern cone. Deglaciation did not induce enhanced magma flux rates at Ruapehu as indicated by calculated eruptive volumes and their chronologies. However, volcano-ice interactions have had a fundamental influence on eruptive styles, sector collapse events, and the shape of the edifice by modifying the distribution, morphology and preservation of eruptive deposits for at least 200 kyr. Our approach to integrating field and geochronological datasets from Ruapehu is directly relevant to the evolution of glaciated Quaternary composite cones worldwide, particularly those along the circum-Pacific continental volcanic arcs.