New Data on the Age, Material Composition, and Geological Structure of the Central Kamchatka Depression (CKD). Part 2. The Mineralogical Composition of Volcanic Rocks and Mantle Xenoliths. Toward a Petrologic Model

Abstract

This study is concerned with the mineralogical composition of volcanic ejecta in the Central Kamchatka Depression: the plateau basalts of the Ozernaya Kamchatka, Kunch, and Nikolka Stratovolcano. These data were compared with rock compositions for some volcanoes in the Klyuchevskoi Group, the plateau basalts of Bakening Volcano, and data on xenoliths of two types, viz., the “island-arc” type (Klyuchevskoi, Kharchinsky, and Sheveluch volcanoes) and the “intraplate” type (the plateau of Bakening Volcano). We have identified complementary vulcanite–xenolith pairs for which we propose a concept of dynamic evolution that envisages the ascent of melts to the ground surface accompanied by nearly synchronous change in the mineralogical composition of rocks in both pairs, while we are merely able to detect some fragments of the process. Mineralogically, the CKD riftogenic vulcanites differ from the island-arc rocks of the Klyuchevskoi Group in (1) a greater iron content of the olivines and greater concentrations of MnO and CaO in these, (2) greater titanium concentrations (over 1 wt % in the former and below 1 wt % in the latter) in clinopyroxenes, their high sodium concentration (over 0.5 wt % in the former and below 0.5 wt % in the latter), and greater (occasionally reaching 9 wt %) alumina content, and (3) the appearance of high titanium chromous spinellids, anorthoclase, and sanidine. These differences are controlled by differences in alkalinities of the melts, in the depth, composition, and the degree of melting in the mantle substrate. The compositions of the CKD rocks that we have studied, considering that the depression is a riftogenic feature, could arise in connection with a low degree of melting in the mantle sources similar to those which gave rise to intraplate subalkaline basalts in the Bakening plateau, which we treated as the standard of riftogenic volcanic occurrence.