Abstract:
A combined stratigraphic and geochemical examination of ~ 43 kyr of volcanic activity is presented for the Yenkahe Caldera, a mafic–intermediate volcanic system on the island of Tanna, in the Vanuatu Arc. Through this period two catastrophic ignimbrite-producing eruptions have occurred: the Siwi eruption and the older, Old Tanna Ignimbrite eruption. The latter was previously linked with a different edifice to the north-east, however re-examination has shown it was derived from the Yenkahe Caldera. Radiocarbon dating of this ignimbrite gives an age of ~ 43 kyr B.P. Both eruptions produced voluminous ignimbrite sheets, however differences in deposit sequences show that the eruptions followed distinct courses. Deposits from the more recent Siwi eruption display greater evidence for phreatomagmatic phases during eruption onset. Both ignimbrites are distributed asymmetrically about the caldera, indicating partial collapse in each case. The early stages of the Siwi eruption produced directed pyroclastic surges and spatter fountains. Between these two major eruptions, volcanic activity was maintained through the formation of small, discrete volcanic cones, such as Yasur, which is active today. Whole rock major and trace element data show that intra-caldera activity between cataclysmic eruptions produced magmas of uniform basaltic-trachy-andesitic composition (SiO2 ~ 56 wt.%). Minerals within these lavas appear to be in equilibrium with their host. The Siwi eruption produced the most evolved, trachy-andesitic magma (SiO2 > 58 wt.%), while the Old Tanna eruption is associated with less evolved, basaltic-andesite magma (SiO2 ~ 53 wt.%). Juvenile clasts from both ignimbrites display diverse mineral chemistry and mineral disequilibrium textures. From these variations in geochemistry and petrology we suggest that a crystal mush or resident magma remained following low-magnitude, intra-caldera activity. MELTS modelling suggest that this was stored at shallow depths, equivalent to pressures < 1 kbar. Injection and mingling/mixing with primitive magma rejuvenated this mush component to trigger the ignimbrite eruptions.
