Types of Parental Melts of Pyroclastic Rocks of Various Structural–age Complexes of the Shiveluch Volcanic Massif, Kamchatka: Evidence from Inclusions in Minerals

The Shiveluch volcanic massif is one of the largest centers of andesite volcanism in Kamchatka. The paper reports pioneering data on inclusions in 24 samples of pyroclastic rocks characterizing principal evolutionary stages and episodes of the three structural–age complexes composing the massif. The rocks composing these complexes range from andesite to basalt. Data obtained by studying 145 inclusions of natural quenched glasses in minerals from the volcanic rocks of various age make it possible to distinguish three major melt types which gave rise to the whole spectrum of the pyroclastic rocks of the Shiveluch volcanic massif. These melts corresponded to picrobasalt and tephrobasanite (42–34 wt % SiO2, 7 wt % MgO, and up to 1.9 wt % K2O), dacite and trachydacite (59–66 wt % SiO2, 1.6–2.6 wt %, and up to 3 wt % K2O), and rhyolite (69–74 wt % SiO2, 0.2–0.5 wt % MgO, and up to 3.7 wt % K2O). The trace-element composition of the melts and their water contents are determined. All of the melt types had low Nb concentrations and were relatively rich in elements mobile in fluid (up to 50 ppm Rb, >700 ppm Ba, and up to 2–3 ppm Th) but were differently depleted in HREE (La/Yb ranges from 3 to 14) and enriched in HFSE. Along with the melts, the mineralogy and petrography of the rocks were examined. The rocks show evidence of hybridism, which should have played an important role in producing andesite of the volcano. The evolutionary history of the volcanic massif is reproduced based on tephrochronologic and petrologic data.