Chemical composition, volatile components, and trace elements in the melts of the Gorely volcanic center, southern Kamchatka: Evidence from inclusions in minerals

Melt inclusions in olivine and plagioclase phenocrysts from rocks (magnesian basalt, basaltic andesite, andesite, ignimbrite, and dacite) of various age from the Gorely volcanic center, southern Kamchatka, were studying by means of their homogenization and by analyzing the glasses in 100 melt inclusions on an electron microprobe and 24 inclusions on an ion probe. The SiO 2 concentrations of the melts vary within a broad range of 45–74 wt %, as also are the concentrations of other major components. According to their SiO 2 , Na 2 O, K 2 O, TiO 2 , and P 2 O 5 concentrations, the melts are classified into seven groups. The mafic melts (45–53 wt % SiO 2 ) comprise the following varieties: potassic (on average 4.2 wt % K 2 O, 1.7 wt % Na 2 O, 1.0 wt % TiO 2 , and 0.20 wt % P 2 O 5 ), sodic (3.2% Na 2 O, 1.1% K 2 O, 1.1% TiO 2 , and 0.40% P 2 O 5 ), and titaniferous with high P 2 O 5 concentrations (2.2% TiO 2 , 1.1% P 2 O 5 , 3.8% Na 2 O, and 3.0% K 2 O). The melts of intermediate composition (53–64% SiO 2 ) also include potassic (5.6% K 2 O, 3.4% Na 2 O, 1.0% TiO 2 , and 0.4% P 2 O 5 ) and sodic (4.3% Na 2 O, 2.8% K 2 O, 1.3% TiO 2 , and 0.4% P 2 O 5 ) varieties. The acid melts (64–74% SiO 2 ) are either potassic (4.5% K 2 O, 3.6% Na 2 O, 0.7% TiO 2 , and 0.15% P 2 O 5 ) or sodic (4.5% Na 2 O, 3.1% K 2 O, 0.7% TiO 2 , and 0.13% P 2 O 5 ). A distinctive feature of the Gorely volcanic center is the pervasive occurrence of K-rich compositions throughout the whole compositional range (silicity) of the melts. Melt inclusions of various types were sometimes found not only in a single sample but also in the same phenocrysts. The sodic and potassic types of the melts contain different Cl and F concentrations: the sodic melts are richer in Cl, whereas the potassic melts are enriched in F. We are the first to discover potassic melts with very high F concentrations (up to 2.7 wt %, 1.19 wt % on average, 17 analyses) in the Kuriles and Kamchatka. The average F concentration in the sodic melts is 0.16 wt % (37 analyses). The melts are distinguished for their richness in various groups of trace elements: LILE, REE (particularly HREE), and HFSE (except Nb). All of the melts share certain geochemical features. The concentrations of elements systematically increase from the mafic to acid melts (except only for the Sr and Eu concentrations, because of active plagioclase fractionation, and Ti, an element contained in ore minerals). The paper presents a review of literature data on volcanic rocks in the Kurile-Kamchatka area in which melt inclusions with high K 2 O concentrations (K 2 O/Na 2 O > 1) were found. K-rich melts are proved to be extremely widespread in the area and were found on such volcanoes as Avachinskii, Bezymyannyi, Bol’shoi Semyachek, Dikii Greben’, Karymskii, Kekuknaiskii, Kudryavyi, and Shiveluch and in the Valaginskii and Tumrok Ranges.