The geology and petrology of the metavolcanic rocks of the Kvakhona Formation of the Sredinnyi Range crystalline massif in Kamchatka

The metavolcanic rocks of the Kvakhona Formation exposed on the western slopes of the Sredinnyi Range metamorphic massif are represented by two sequences. The lower sequence occupying the most part of the exposed formation is dominated by porphyric and aphyric clinopyroxene-plagioclase metabasalts and their tuffs with subordinate metapicritic basalts, metaandesites and their tuffs, and metadacites. The latter form isolated bodies in the northern part of the Kvakhona exposures. The upper sequence is composed of metaandesites, metabasalts, and their tuffs intercalated with terrigenous rocks (siltstones, sandstones, and carbonate graywacke) and metadacite bodies. The rocks were subjected to intense metamorphism under the greenschist facies conditions (t = 250–420°C, P _s around 1 kbar) with the replacement of clinopyroxene phenocrysts (or their clasts in the tuff varieties) by actinolite, chlorite, and epidote, while plagioclase phenocrysts are replaced by albite, muscovite, chlorite, and epidote. In the metabasalts enriched in ore minerals, clinopyroxene is replaced by very thin veinlets and the finest grains (about 20–30 μm) of Ca-Na and Na amphiboles (winchite, ferrowinchite, glaucophane, and ferroglaucophane). The groundmass of the rocks (or tuff cement) consists of variable combinations of titanite, magnetite, chlorite, epidote, silpnomelane, and albite. The metabasalts of the formation belong to the high-Fe and often high-Ti rocks, which makes them similar to the tholeiitic basalts of mid-ocean ridges or rifting zones. The metaandesites and metadacites also preserved an elevated Fe content and belong to calc-alkaline series rocks typical of island arcs and active continental margins. It is suggested that the primary rocks of the Kvakhona Formation were formed within volcanic centers on the floor of a vast Cretaceous epicontinental marginal basin, which accumulated thick sequences of terrigenous rocks. The detrital material for these rocks was supplied from the northeastern Asian continent. The geological and geochemical data testify to the similarity of the Kvakhona metavolcanic rocks and the greenstone altered volcanic rocks of the Pensantain Range of Western Kamchatka, which are dated by the U-Pb SHRIMP method at 90–100 Ma. The protolith of the metavolcanic rocks of the Kvakhona Formation was presumably formed within the same age interval.