Stratigraphy, structure and geology of Late Miocene Verkhneavachinskaya caldera with basaltic-andesitic ignimbrites at Eastern Kamchatka

We describe a new caldera-volcano in the volcanic front of Kamchatka named Verkhneavachinskaya caldera (VC). According to geological mapping, the VC is interpreted as an eroded shield volcano with a summit caldera exposing 1 km-thick lava-like ignimbrites. It is one of the largest (10 x 12 km diameter) and oldest (c. 5.78-5.58 Ma, Ar-Ar dating) morphologically preserved paleovolcano in Kamchatka. The welded ignimbrites with fiammes are andesites-basaltic andesites in composition, they are more mafic than most ignimbrites worldwide and than other post-Pliocene calderas in Kamchatka. The deposits of VC are interbedded layers of welded ignimbrites and volcanoclastic material, which we interpret as result of long-lived volcanic center activity with hot pyroclastic flows and subsequent accumulations of volcanoclastic suites (e.g. lahars). The most trace element ratios in VC rocks (e.g. La/Yb, Nb/La, BarTh) show similarity with post-accretion magmatism at Kamchatka and especially with Late Pleistocene-Holocene Bakening volcano located in the same area. The VC provides new insights into the early stages of Kamchatka frontal zone development after the Kronotsky Arc Terrane accretion. This potentially explains the origin of voluminous basaltic-andesitic ignimbrites formed on thin crust during the initial stage of the arc formation.