The paper presents new geological and isotope-geochemical data on rock composition from the head of the Kikhchik River, the outer NW part of Miocene-Pliocene volcanic belt of Central Kamchatka. The authors obtained age characteristics (40K−40Ar dates): 9.8 and 9.1 Ma for Kecheva massif and 8.3 and 7.8 Ma for Aopchi cones. New data on ages and analytical materials allow substantial updating our understanding of the rock composition from this volcanic belt as well as the beginning of volcanic activity. The authors revealed complex rocks, both completely identical to closely located volcanic manifestations (Kecheva) and abnormally enriched with Rb, Pb, Ba, Zr, Hf, Nb, U, Th and radiogenic Sr Aopchi, Kabanikha, littoral cones. The origin of these anomalous volcanites was likely caused by the partial melting of the mantle source close to the Sp- peridotite metasomatically reworked by the processes that may relate both to dehydratation of the subducting oceanic plate, and by the effect from the asthenospheric diapir.

The paper presents the data on age, mineralogy, geochemistry, and isotope composition of rocks from the
Bolshoi Payalpan Volcano (Sredinny Range, Kamchatka). We compared these data with those on the Nosichan
and Belogolovsky volcanoes, located within the Belogolovsky volcanic center. The basalts of the neck and the
upper lava complex of Bolshoi Payalpan are compositionally similar to the intraplate-type trachybasalts of the
Belogolovsky Volcano, and the basaltic andesites of the lower lava and the cone complex are similar to the
island arc rocks of the Nosichan Volcano. Analysis of the data obtained evidences that spatial and temporal
manifestations of intraplate and island-arc volcanism at the Bolshoi Payalpan Volcano are not accidental, but may
be a consequence of a change in the degree and depth of melting of the same deep source with the involvement
of a mantle diapir. The Belogolovsky volcanic center formed in a setting of the Late Miocene-Early Pliocene
rifting. Its evolution, right up to its extinction, proceeded in the same geodynamic setting with an increase in
depth of the mantle source and a decrease in the degree of its melting. Rock compositions of the Lower-Middle
Pliocene Nosichan Volcano remain of the island-arc type under conditions of rifting, since they are associated
with the mantle reservoir located at a shallower depth, which has experienced a higher degree of melting. There
is good reason for considering large volcanic centers as spontaneously-developing geological entities. As the
endogenous activity dies down, the degree of melting decreases and the depth of melting increases with the
replacement of island-arc volcanism by intraplate volcanism. The volcanic center becomes extinct.

Дается описание своеобразных пород субщелочного основного и среднего состава, распространенных в Срединном хребте Камчатки. Эти породы характеризуют вулканические, интрузивные и метасоматические образования, объединяемые авторами в единую вулкано-плутоническую формацию трахиандезит-габбро-сиенитового состава. Исходной магмой авторы считают базальтовую повышенной щелочности. Результаты петрографических работ, проведенных за последние годы в пределах Срединного хребта Камчатки, позволяют выделить в пределах его зону распространения своеобразных пород субщелочного состава.

Авторами настоящей статьи кратко описываются своеобразные пироксен-ортоклазовые метасоматические породы, приуроченные к области экзоконтакта порфировидных кварцевых сиенитов, прорывающих интрузив ультраосновных пород.

The evolution of the Quaternary Kekuknai volcanic massif (the western flank of the Sredinnyi Range in Kamchatka) has been subdivided into five stages: (I) the pre-caldera trachybasalt- basaltic andes- ite, (2) the extrusive trachyandesite-trachydacite, (3) the early trachybasalt, (4) the middle hawaiite- mugearite (with occasional occurrences of basaltic andesites), and (5) the late trachybasalt-hawaiite- mugearite (with occasional andesites) of areal volcanism. On the basis of petrologic data we identified the island arc and the intraplate geochemical types of rocks in the massif. The leading part in petrogenesis was played by dynamics of the fluid phase with a subordinated role of fractional crystallization and hybridism. Successive saturation of rocks with the fluid phase in the course of melt evolution stopped at the time of caldera generation when most fluid mobile elements and silica had been extracted. The geological and petrologic data attest to the formation of the massif in the environment of a backarc volcanic basin during the beginning of rifting with active participation of mantle plume components.

The Kekuknai massif was formed in the course of tectono-magmatic activity that involved the origin of a shield volcano and a caldera depression with associated emplacement of extrusions that terminated in intense post-caldera areal volcanism. The mineralogical compositions of the massifs rocks have been considered in detail. The use of previously known and newly developed indicator properties of rock-forming minerals allowed the reconstruction of the general picture of the magmatic melt evolution and conditions of rock crystallization (various fluid and water saturation levels, as well as the oxidation state of the system). Essentially island-arc or intraplate characteristics of the massif s rock compositions are found at different stages of development of a single fluid-magmatic system. Decompression evolution of the parent deep-seated basanitic magma occurred via occurrence in intermediate magma chambers of daughter magmas of trachybasalt (pre-caldera stage) or hawaiite (areal volcanism) composition. Subsequent emanate-magmatic differentiation of these melts, combined with crystallization differentiation under changing P-T-f0l conditions, resulted in the formation of the entire diversity of the Kekuknai rocks