Amphibole Megacrysts in Pyroclastics from Secondary Eruptive Centers of Kronotsky Volcano

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The paper reports data on the morphology and composition of amphibole megacrysts found in pyroclastic material from two eruptive centers on the southern slopes of Kronotsky Volcano. The megacrysts ranging in size from 1 to 8 cm occur both within the matrix of volcanic bombs and as rounded lapilli in form of single crystals or their intergrowths. Compositionally, the megacrysts belong to the ferri-tschermakite–magnesio-hastingsite series with high Al 2 O 3 contents (14.8–16 wt %) and average magnesian index of Mg# = Mg/Mg + Fe 2+ = 0.67. The host volcanic bombs are low-K tholeiitic basalts (SiO 2 = 49.32–50.50 wt %, K 2 O = 0.24–0.27 wt %, FeO/MgO = 1.7–2.1). In addition, the pyroclastic material contains xenoliths of olivine pyroxenites, as well as coarse- and medium-grained amphibole–plagioclase intergrowths. Several hypotheses are proposed to explain the origin of this association, which is highly unusual for tholeiitic island arc basalts.