Volcanic stratigraphy and phase chemistry of the 11 900 yr BP Waiohau eruptive episode, Tarawera Volcanic Complex, New Zealand

Abstract

The 11 900 yr BP Waiohau eruptive episode is a moderately sized rhyolitic event of the Tarawera Volcanic Complex. Subdivision of the pyroclastic fall deposits previously mapped as Waiohau Tephra has allowed the chronology of the eruption to be deciphered. At least 15 events (units A-O) with volumes in the range <0.01-1.7 km3 can be recognised in proximal to medial settings. The eruption commenced with a series of minor pyroclastic falls and surges, which rapidly changed to a major phase of pyroclastic surges (unit D) that reached up to 14 km from the vent with ash clouds recognisable up to 30 km. Eastern Dome is considered to have been extruded early in the episode. Several prominent plinian phases (unit E, L, M) followed, two of which are estimated to have had column heights of c. 30 km. One event was associated with the destruction of an obsidian dome and dispersal of obsidian lapilli (unit F). These plinian events were accompanied by thin pyroclastic flows punctuated by minor magmatic and phreatomagmatic fall eruptions. There is no evidence for prolonged quiescence during these eruptions. The climactic pyroclastic fall phase (unit M) was followed by a major failure of the northeast flanks of the Tarawera massif, either by a series of block and ash flows or debris avalanche. Effusive activity resulted in the emplacement of Waikakareao lava flows, and voluminous and widespread pyroclastic flows (unit N) to the north and northeast. The Waiohau episode concluded with effusive activity producing the Pokuhu flow and Kanakana Dome. A new volume estimate for the combined units A-O is 4.5 km3. This is a minimum because it does not properly incorporate distal ash (>50 km) poorly preserved in paleosols. Previous volume calculations are considered overestimates because they included tephric loess. Effusive activity represents 3.9 km3, and pyroclastic debris in the northern sector adds another 2.4 km3, but much of the latter is xenolithic. Deposits from the entire Waiohau eruptive episode were derived from a hypersthene (+ minor hornblende) rhyolite magma that lacked compositional (SiO2 = 78 ﾱ 0.2 wt%) and mineralogic gradients. Electron microprobe analyses reveal that phenocrystic phases lack compositional variation. The bulk of the magma also lacked physical gradients (T=780-790ﾰC; log fO2 = c. -15) as determined by Fe-Ti oxide geothermometry. Eastern Dome and the Pokohu lava display lower temperature (<740ﾰC) and log fO2 (c. -16). There is no evidence for mafic triggering of the Waiohau eruption, inlike other eruptive episodes of Tarawera, and the magma represents a separate batch, unrelated to earlier and later biotite-bearing magmas. The Waiohau eruptive products appear to have been derived from a rapidly convecting, low aspect ratio magma body that did not reside in the crust for a prolonged time.