Long-term changes in explosive and effusive behaviour at andesitic arc volcanoes: Chronostratigraphy of the Centre Hills Volcano, Montserrat

Volcanism on Montserrat (Lesser Antilles arc) has migrated southwards since the formation of the Silver Hills similar to 2.5 Ma, and has formed three successively active volcanic centres. The Centre Hills volcano was the focus of volcanism from similar to 1-0.4 Ma, before activity commenced at the currently active Soufriere Hills volcano. The history of activity at these two volcanoes provides an opportunity to investigate the pattern of volcano behaviour on an andesitic arc island over the lifetime of individual volcanoes. Here, we describe the pyroclastic stratigraphy of sub aerial exposures around central Montserrat; identifying 11 thick (>1 m) pumiceous units derived from sustained explosive eruptions of Centre Hills from similar to 0.8-0.4 Ma. Over 10 other, less well- exposed pumiceous units have also been identified. The pumice-rich units are interbedded with andesite lava breccias derived from effusive, dome-forming eruptions of Centre Hills. The stratigraphy indicates that large (up to magnitude 5) explosive eruptions occurred throughout the history of Centre Hills, alongside effusive activity. This behaviour at Centre Hills contrasts with Soufriere Hills, where deposits from sustained explosive eruptions are much less common and restricted to early stages of activity at the volcano, from similar to 175-130 ka. Subsequent eruptions at Soufriere Hills have been dominated by andesitic effusive eruptions. The bulk composition, petrography and mineral chemistry of volcanic rocks from Centre Hills and Soufriere Hills are similar throughout the history of both volcanoes, except for occasional, transient departures to different magma compositions, which mark shifts in vent location or dominant eruption style. For example, the final recorded eruption of Centre Hills, before the initiation of activity at Soufriere Hills, was more silicic than any other identified eruption on Montserrat; and the basaltic South Soufriere Hills episode marked the transition to the current stage of predominantly effusive Soufriere Hills activity. The compositional stability observed throughout the history of Centre Hills and Soufriere Hills suggests that a predominance towards effusive or explosive eruption styles is not driven by major compositional shifts of magma, but may reflect local changes in long-term magma storage conditions that characterise individual episodes (on 10(5) year timescales) of volcanism on Montserrat. (C) 2017 Elsevier B.V. All rights reserved.