The Volcanism Occurring during the Initial Phase of Formation of the Northern Subduction Segment in the Pacific Plate (Kamchatka Peninsula, Kumroch Range)

The present paper reports a study of rocks in Mts. Baidara and Semkorok, which are situated in the northwestern flank of the Kumroch Range. The rocks are Amf-Px basaltic andesites and andesites, and are characterized by an arc type of trace element distribution. Some mineralogical and geochemical features in the composition of lavas sampled at Mt. Baidara (low concentrations of K 2 O, as well as of the entire range of REEs, of large-ion elements, thorium, and uranium) and at Mt. Semkorok (low concentrations of light REEs) are substantially different from the rocks of the adjacent Shiveluch Late Pleistocene–Holocene volcanic massif. The K-Ar isotopic lava ages are ~0.7 Ma (Baidara) and ~1.3 Ma (Semkorok), suggesting the inference that the eruptions might be related to the initial phase in the formation of the northern subduction segment in the Pacific plate.