Ignimbrite stratigraphy and chronology on Terceira Island, Azores

Ignimbrites are relatively rare on mid-oceanic volcanic islands, but they occur on at least three of the nine Azorean islands-São Miguel, Faial, and Terceira. Ignimbrites on Terceira, which are mostly comenditic trachyte in composition, constitute a significant portion of the island's volcanic stratigraphy exposed primarily in cliff sections along the southern and northern coastlines. The base of the most recent group of deposits, called the Upper Terceira Group, is marked by the Lajes-Angra Ignimbrite, an extensive ignimbrite sheet produced during the latest ignimbrite-forming episode ∼20-23 k.y. ago. The Lower Terceira Group consists of ignimbrites and interstratified pyroclastic fall deposits and lava flows predating the Lajes-Angra Ignimbrite. Including the Lajes-Angra Ignimbrite, the exposed volcanic succession on Terceira includes at least seven formations containing ignimbrites plus two isolated exposures of other pyroclastic density current deposits (a spatter flow and a block-and-ash-flow-like deposit). Most of these are from Pico Alto or, possibly, Guilherme Moniz, the middle pair of central volcanoes of the four that form Terceira. Radiocarbon ages complemented by 40 Ar/ 39 Ar age determinations on anorthoclase crystals from several ignimbrites suggest a narrow period of ignimbrite-forming volcanism on Terceira dating from ca. 86 ka to ca. 20-23 ka, which followed a period of predominantly effusive basaltic to trachytic/rhyolitic activity. However, a recurrence of ignimbrite volcanism on the island cannot be completely discounted. The results from this study also indicate that the 40 Ar/ 39 Ar method can be successfully applied to date anorthoclase crystals in volcanic rocks younger than 100 ka from Terceira, although it does not provide the same precision as radiocarbon dating at the younger age range of these volcanic rocks. © 2010 The Geological Society of America. All rights reserved.