Miocene volcanism on Lesvos was particularly explosive giving rise to two extensive pyroclastic formations, the Sigri pyroclastics to the west and the Polychnitos ignimbrite to the east of the island. The Polychnitos ignimbrite at 17.2±0.5 Ma (Borsi et al.1972) is part of the shoshonitic succession on Lesbos which ranges in composition from basalt to rhyolite and is both underlain and overlain by calcalkaline volcanic rocks (Pe-Piper and Piper 1993) resting on a late Paleozoic metamorphic basement which has acted as an impediment to the free flow of the ignimbrite. The Polychnitos ignimbrite consists of eight lithological units, six of which are presumed to be facies of the same ignimbrite sheet ("PK", "PU", "MGF I, II, III", "Z"). Ignimbrite deposition at elevated temperatures is advocated by its columnar jointing, eutaxitic texture, gas escape structures and glassy zones of intense welding. The typical mineral assemblage of all Polychnitos ignimbrite units consists of plagioclase, Kfeldspar and biotite. It displays phenocryst microtextures indicative of magma mixing. Magma mixing is corroborated of glasses of two discrete compositions. Lithic clast measurements indicate a northeasterly trending fissure vent passing from the northeastern corner of the Kalloni Gulf.
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