“Calc-alkaline” magmatism of the Omgon Range: Evidence for Early Paleogene extension in the western Kamchatka segment of the Eurasian continental margin

The hypabyssal rocks of the Omgon Range, western Kamchatka, that intrude Upper Albian-Lower Campanian deposits of the Eurasian continental margin belong to three coeval (62.5–63.0 Ma) associations: (1) ilmenite gabbro-dolerites, (2) titanomagnetite gabbro-dolerites and quartz microdiorites, and (3) porphyritic biotite granites and granite-aplites. The Early Paleocene age of the ilmenite gabbro-dolerites and biotite granites was confirmed by zircon and apatite fission-track dating. The ilmenite and titanomagnetite gabbro-dolerites were produced by the multilevel fractional crystallization of basaltic melts with, respectively, moderate and high Fe-Ti contents and the contamination of these melts with rhyolitic melts of different compositions. The moderate-and high-Fe-Ti basaltic melts were derived from mantle spinel peridotite variably depleted and metasomatized by slab-derived fluid prior to melting. The melts were generated at variable depths and different degrees of melting. The biotite granites and granite aplites were produced by the combined fractional crystallization of a crustal rhyolitic melt and its contamination with terrigenous rocks of the Omgon Group. The rhyolitic melts were likely derived from metabasaltic rocks of suprasubduction nature. The Early Paleocene hypabyssal rocks of the Omgon Range were demonstrated to have been formed in an extensional environment, which dominated in the margin of the Eurasian continent from the Late Cretaceous throughout the Early Paleocene. Extension in the Western Kamchatka segment preceded the origin of the Western Koryakian-Kamchatka (Kinkil’) continental-margin volcanic belt in Eocene time. This research was conducted based on original geological, mineralogical, geochemical, and isotopic (Rb-Sr) data obtained by the authors for the rocks.