Abstract:
New 40Ar/39Ar analysis results confirm that Hauhungatahi is approximately 800,000 years old based on analyses from lava samples taken near the summit. This establishes for the first time a valid geological age for the cone and makes it the oldest volcanic complex in the Tongariro Volcanic Centre (TgVC). The cone is therefore a unique opportunity to study eruptive deposits older than the Whakamaru ignimbrite (>34Ka) event that has buried most of the older Taupo Volcanic Zone (TVZ) that the TgVC is located in. Lava flows are present at the summit and southern foot of Hauhungatahi; between these flows are pyroclastic airfall deposits and lahars. The lavas contain porphyritic olivine and clinopyroxene. Plagioclase occurs only as groundmass microlites in most of the cone's lavas.
X-ray fluorescence (XRF) analyses of the cone's lavas indicate they are basaltic andesites ranging in silica from 52-56%. The deposits are low-K, calc-alkaline lavas with 8-9% MgO on average.
Multi-element diagrams show enrichments in Large Ion Lithophile Elements (LILE) with a corresponding depletion in High Field Strength Elements (HFSE).
Crystallization temperatures estimated for Hauhungatahi lavas from 2-pyroxene geothermometry are 1100-1170°C ± 70°C. These values are typically 100-200°C higher than Ruapehu lavas and typical orogenic andesites.
The multi-element pattern of LILE enrichment is typical for arc magmas where fluids from the subducting slab metasomatize the mantle wedge and initiate partial melting. The anomalously high crystallization temperatures and high MgO combined with the relative depletion to N-MORB in the High Field Strength Elements suggests that magmas originated from an already depleted upper mantle source and were later enriched by fluids from the subducting slab.
Nd/Sr isotopic data indicate Hauhungatahi lavas originate from MORB like magmas similar to the Kakuki basalt that is considered the most primitive basalt in the TVZ. Hauhungatahi may mark one of the first instances in the central North Island of decompressional melting, initiated by the extension of the underlying lithosphere, caused by the oblique relative plate motion of the Pacific-Australian plate boundary through New Zealand.
