The Pliocene–Quaternary Volcanism in the Interfluve of the Kozyrevka and Malaya Romanovka Rivers (The Sredinnyi Range of Kamchatka): The Ages and Compositions of the Rocks

The isotopic K–Ar ages of the volcanoes in the interfluve of the Kozyrevka and Malaya Romanovka rivers in the Sredinny Range of Kamchatka have been determined for the first time. We have identified six phases of activity: the end of the Miocene (5.8–5.5 Ma), the Early Pliocene (4.9–4.5 Ma), the Middle Eopleistocene (1.5–1.4 Ma), the Late Eopleistocene (1.1–1.0 Ma), and the Middle Pleistocene (two phases: 0.6 and 0.4–0.3 Ma). No later volcanism has been recorded in the region. It was found that the start of Quaternary volcanic activity was preceded by a repose period lasting approximately 3 Ma. It was found for the first time that the major fault striking northeast at the northern base of Romanovka Volcano could not have originated before 0.4–0.3 Ma B.P. An analysis of isotopic K–Ar dates suggests that the phases of Quaternary volcanic activity that occurred ∼1.5 and ∼0.3 Ma B.P. were regional in character. Regional activity was less pronounced for the phases ∼1.0 and ∼0.6 Ma B.P. The rocks discharged in the interfluve during the last six million years ranged between basalts and dacites. It was found that the extensive covers and major lava edifices are composed of intermediate and acid rocks, while basalts are confined to small monogenic centers. Overall, the volcanics studied here have chemical compositions that are similar to those of the Late Miocene–Quaternary rocks in the southern Sredinnyi Range. The results of our study alter significantly the previous age estimates for the volcanoes in the area of study at the Sredinnyi range. We suggest that no significant glaciation has occurred in the interfluve of the Kozyrevka and Malaya Romanovka at least during Quaternary time.