Northeastern Section - 56th Annual Meeting - 2021

Paper No. 5-8
Presentation Time: 4:10 PM


VAREKAMP, Johan, Earth & Environmental Sciences, Wesleyan University, Middletown, CT 06459

In the late 1980s I travelled by VW microbus with Jelle de Boer and some students through Yugoslavia to Greece, to work on the Aegean arc. Jelle first visited Delphi, and we started our work on Nisyros. Located at the eastern end of the Aegean arc, Nisyros is a small active volcano with a 4 km wide summit caldera. It is also located on the margin of the submarine megacaldera related to the 161 ka Kos Plateau Tuff (KPT) eruption. The base of Nisyros consists of pillow lavas of andesite composition, a stratocone sequence of andesitic and dacitic lavas, scoria and pumices, capped by phreato-magmatic andesite scoria. These are overlain by two major pumice falls/flows and silicic lava flows. The most recent materials are the rhyolitic pumice and obsidian domes on the islet of Yali.

Two major silicic explosive eruptions (several km3 magma each), the Lower Caldera Pumice and the Upper Caldera Pumice, were each followed by caldera collapse, with resp. extrusion of the Nikia lava flow and intra-caldera domes. The lavas and domes bear evidence for extensive magma mixing, with mafic inclusions (MI) with quench rims, of basaltic andesite rich in hornblende. The Yali pumices are almost crystal-free rhyolites. Detailed chemical work on the host rocks and MI reveals a complex plumbing system over time, with coexistence of rhyolitic and basaltic magmas that seemed to have mixed just prior to eruption. The lower pumices are crystal-poor rhyo-dacites, whereas the upper dacitic pumices are crystal-rich, many derived through hybridization with the MI. The silicic magmas and MI are also isotopically distinct, with extremely primitive Sr-Nd isotope compositions for some MI in the Nikia flow and postcaldera domes (87Sr/86Sr = 0.7036, 143Nd/143Nd = 0.5128). The two caldera pumice layers are chemically distinct and have 87Sr/86Sr ~ 0.70455 and 143Nd/144Nd ~ 0.51263), suggesting that the MI and pumice magmas are not simply related through fractional crystallization. Presumably, a lower crustal ‘hot zone’ was formed by intrusions of largely mantle derived basaltic-andesitic magmas, which led to lower crustal melting, generating the silicic magmas. Comparisons of Nisyros with nearby Kos, Santorini and the other NE Aegean extinct volcanoes related to roll back of the subducting slab show a much more diverse array of chemical and isotopic compositions.