New insights into the evolution of the magmatic system of a composite andesite volcano revealed by clasts from distal mass-flow deposits: Ruapehu volcano, New Zealand

Stratovolcanoes characteristically build large composite edifices over long periods with stacked lavas intercalated with pyroclastic deposits. In most cases, only the most recent volcanic products are exposed on the flanks of the volcano, and consequently the search for deposits recording an older eruptive and magmatic history is typically focussed far from the cone, within distal tephra deposits. Clasts within lahar and debris avalanche deposits may also provide unique insights into the earliest eruptive and magmatic history of long-lived volcanoes, especially when widespread fallout is absent. Careful sampling and subsequent petrological and geochemical analyses of lava and pumice clasts from six distal mass-flow deposit sequences (hyperconcentrated flow, debris flows and debris avalanche deposits) from Mt. Ruapehu (New Zealand), combined with detailed stratigraphic studies and radiometric age dating, give new perspectives on the pre-50 ka magmatic system of this complex volcano. A conglomerate emplaced between 340 and 310 ka contains evidence for the oldest episode of Mt. Ruapehu volcanism, and unusually for the composite cone, pumice clasts from this unit contain amphibole-bearing xenoliths. Chemical and petrological data for these oldest Ruapehu clasts indicate that a deep (similar to 40 km) crustal storage system had already developed under Mt. Ruapehu before similar to 340 ka. From the very earliest stages, evolution was largely controlled by magma mixing, along with decoupled assimilation and fractional crystallization within numerous isolated small-scale magma batches stored throughout the crust. From around 340 to 160 ka, there was a progressive shift towards more primitive compositions, suggesting that during this period large-scale replenishment events involving mantle-derived basaltic magmas occurred within the mid-to upper crustal storage system. Subsequent magmas became progressively more evolved due to decoupled fractional crystallization and assimilation processes accompanied by magma recharge events, which triggered major phases of eruptive activity.