Adakite Volcanism at the Continental Margin and Associated Problems. Part I. Adakites from the Upper Reaches of the Valovayam River: New Age, Mineral, and Chemical Data and Petrological Modeling

Resumo

The paper presents new age and petrological–geochemical data on the rocks of the Valovayam River area (Kamchatka Isthmus) and some areas of the Sredinny Range of Kamchatka (Orlov Volcano, Polovinnaya, Stolbovaya, and Matera mountains). Analysis of the obtained data shows that the spatial and temporal association of the intraplate and island-arc volcanism in the Valovayam River area is not accidental, but resulted from melting of a common source related to the mantle diapir. Additional data on the composition of the mantle xenoliths and the traces of their melting in the form of residual glasses make it possible to relate the formation of different rock series into a single process. A two-stage petrogenetic model is proposed: (1) formation of “black pyroxenites” as a result of melting of mantle lherzolites and (2) formation of the whole rock assemblage through melting of the same lherzolites and newly formed pyroxenites. The concurrent study of the rocks of similar age from the Valovayam River area and the Sredinny Range allows us to distinguish two types of adakites: high-Si adakites (HSA) in the former area and low-Si adakites (LSA) in the latter. The formation of the first type is related to the fractionation, melting, and breakdown of amphibole, while the formation of the second type was controlled by the fractionation and melting of pyroxene.