We developed a petrologic and geodynamic model for the magma generation of volcanic forma tions in the Tolbachik Dol area. Differences have been found in the evolution of volcanic ejecta and the min eral composition of rocks on Ostryi and Ploskii Tolbachik stratovolcanoes on the one hand and in that of the regional zone of cinder cones on the other. We conclude that the basaltic and the trachybasaltic magmas were generated independently of each other and that these magmas were mixed during the time when central vol canism was giving way to fissure volcanism. The leading process that was responsible for the generation of basaltic and trachybasaltic series was crystallization differentiation, while the compositional variation in the stratovolcano and cinder rocks can be explained by the supply of an alkaline component into the basaltic magma chamber and subsequent accumulation during magma crystallization.
Parageorgbokiite, β -Cu 5 O 2 (SeO 3 ) 2 Cl 2 , has been found at the second cinder cone of the Great Fissure Tolbachik Eruption, Kamchatka Peninsula, Russia. Ralstonite, tolbachite, melanothallite, chalcocyanite, euchlorine, Fe oxides, tenorite, native gold, sophiite, Na, Ca, and Mg sulfates, cotunnite, and some copper oxoselenites are associated minerals. The estimated temperature of the mineral formation is 400-625 ° C. The color is green, with a vitreous luster; the streak is light green. The mineral is brittle, with the Mohs hardness ranging from 3 to 4. Cleavage is not observed. The calculated density is 4.70 g/cm 3 . Parageorgbokiite is biaxial (+); α = 2.05(1), β = 2.05(1), and γ = 2.08(1); 2 V (meas.) is ~0 3 , and 2 V (calc.) = 0(5) ° . The optical orientation is X = a ; other details remain unclear. The mineral is pleochroic, from grass green on X and Y to yellowish green on Z . The empirical formula calculated on the basis of O + Cl = 10 is Cu 4.91 Pb 0.02 O 1.86 (ScO 3 ) 2 Cl 2.14 . The simplified formula is Cu 5 O 2 (ScO 3 ) 2 Cl 2 . Parageorgbokiite pertains to a new structural type of inorganic compounds. Its name points out its dimorphism with georgbokiite, which was named in honor of G.B. Bokii, the prominent Russian crystal chemist .
Abstract⎯This paper is concerned with the geological history and petrology of a major polygenic volcanic edifice dating back to Upper Pleistocene to Holocene time. This long-lived volcanic center is remarkable in that it combines basaltic and trachybasaltic magmas which are found in basaltic andesite and trachybasaltictrachyandesite series. The inference is that the coexisting parent magmas are genetically independent and are generated at different sources at depth in an upper mantle volume. The associated volcanic rocks have diverse compositions, stemming from a multi-stage spatio-temporal crystallization differentiation of the magmas and mixing of these in intermediate chambers. DOI: 10.1134/S0742046317040030INTRODUCTION The information on the Ploskie Sopki massif can be found to varying degrees of detail in (Sirin, 1968;Piip, 1956;Ermakov, 1977;Flerov and Ovsyannikov, 1991;Churikova, 1990Churikova, , 1993Churikova and Sokolov, 1993;Churikova et al., 2001Churikova et al., , 2012Churikova et al., , 2013. Our studies supplied considerable corrections to the interpretation of the volcanic history for the formation of the massif and the associated petrogenesis. The Ploskie Sopki massif (Fig. 1) is the largest polygenic (as to morphology and geology) volcanic edifice in the Klyuchevskoi Volcanic Cluster. It is made of the paired edifices of the Late Pleistocene Ushkovskii (3943 m) and Krestovskii (4108 m) stratovolcanoes. The summit part of Ushkovskii Volcano and part of Krestovskii are truncated by a 4.5-km × 5.5-km collapse caldera, which includes two cinder cones, Kherts and Gorshkov, and is filled with a glacier. Krestovskii Volcano is a fragment of the volcanic cone that has been spared by the collapse, and which makes up the N-NW-W steep wall of the collapse cirque. The slopes of the massif abound in cinder and cinder-lava cones due to parasitic eruptions at different hypsometric levels. Some of these are along the periphery of Ushkovskii Volcano, while most of the cones are confined to a narrow zone striking SSW-NNE-NE, which traverses the top of the massif and its slopes, and can be followed for a distance of 60 km, with the width varying between 6 and 8 km. One peculiar feature of the massif consists in the presence of normal and high alkalinity volcanic rocks within a single edifice, which
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