—We report the chemical and isotopic compositions of volcanic rocks of the Akhtang and Kostina mountain massifs in the Sredinny Range, Kamchatka. The analyzed rocks are similar in composition to the earlier studied volcanics of the eastern flank of the southern part of the Sredinny Range. Results of K–Ar isotope dating reveal three stages of volcanic activity in the two massifs. These stages are divided by long (1.4 and 2.4 Ma) periods of quiescence. In the Akhtang massif, the eruptive activity was at 4.9–4.0, 1.9–1.7, and 0.3–0.2 Ma, and in the Mt. Kostina massif, at ~8.0, 5.6–4.9, and ~3.5 Ma. Two early stages of both massifs are characterized by the eruption of island arc type rocks, and the late stage, by the eruption of rocks of hybrid geochemical type. The Mio-Pliocene (N1–N21) rocks of the Mt. Kostina massif are similar in geochemical features to the early Pliocene (N21) rocks of the Akhtang massif, and the late Pliocene (N22) lavas of the former massif are similar to the middle Quaternary (Q2) rocks of the superimposed monogenetic volcanism zone of the latter massif. For the Akhtang massif it has been first discovered that the volcanic reactivation after the long quiescence periods was accompanied by a change in the composition of rocks and in the type of eruptive activity (from the eruption of plateau-effusives rocks to the formation of stratovolcanoes and monogenetic volcanism zones). The obtained data on the age and composition of rocks as well as some morphological features of the studied massifs suggest that the plateau-effusive rocks of the Sredinny Range might be related to central-type eruptions.

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