Keywords
amphibole
tholeiitic basalts
xenoliths
Kronotsky volcano
Kamchatka
Section
Abstract
Here we present 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 the form of single crystals or thier intergrowths. Compositionally, the megacrysts belong to the ferri-tschermakite–magnesio-hastingsite series with high Al2O3 contents (14.8–16 wt.%) and average magnesian index of Mg# = Mg/Mg+Fe2+ =0.67. The host volcanic bombs are low-K tholeiitic basalts (SiO2 = 49.32–50.50 wt.%, K2O = 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.
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