Massifs of the heterogeneous mafic-ultramafic complex of the Olyutorsky zone, Koryak Mountains: Petrology and geodynamic aspects

The paper presents new data on the geochemistry and age of ultrabasic-basic and basic-granite massifs from the Olyutorsky zone of the Koryak Mountains , which are distinguished as a single mafic-ultramafic complex. Our study d emonstrated a complex structure of these massifs consisting of rocks of var ying age, geochemical characteristics, and genesis. The associations of lay ered ultrabasic-basic massifs include (a) layered dunite-clinopyroxenite-ga bbro series and (b) amphibole and phlogopite (alkaline) gabbroids. The rock s of the former association are most similar to those of the layered dunite -wehrlite-clinopyroxenite series and low-pressure clinopyroxene gabbro ophi olitic complexes. Gabbroids of Klamath- and Peninsular-type massifs, i.e., of island-are complexes, are analogues of the rocks of the second associati on. The associations of zoned basic-ultrabasic massifs (Alaskan-type) inclu de (a) dunites in the cores, (b) clinopyroxenite-gabbro composing the outer portion of the massifs, (c) fine-grained and doleritic gabbro, (d) gabbrod iorite of discordant bodies, and (e) felsic dike rocks. The massifs of basi c rocks and granitoids (closest analogues of Klamath- and Peninsular-type m assifs) are composed of the associations of(a) pyroxenite-gabbro and (b) fe lsic rocks of dikes and small intrusions. The distinguished associations of igneous rocks are derivatives of chemically diverse melts: tholeiitic, cal c-alkaline, potassium-rich calc-alkaline, subalkaline, and tonalite-trondhj emite. Changes in the composition of igneous rock associations reflect chan ges in the geodynamic conditions. The ages of rocks of various associations show a strong correlation with main evolution stages of the Bering Sea reg ion. The associations of layered dunite-clinopyroxenite-gabbro series, whic h are products of crystal fractionation of a basic melt close to tholeiitic , and the association of tholeiitic gabbro-dolerite and fine-grained gabbro were probably formed in pre-Maestrichtian time and related to the evolutio n of igneous chambers within spreading zones. The calc-alkaline, high-potas sium calc-alkaline, and subalkaline rocks (mainly gabbroids in composition) were generated in the Campanian-Maestrichtian and recorded various stages of the igneous evolution of an island-are system. The results of fission-tr ack dating of apatite demonstrated that, in the Late Maestrichtian-Paleocen e-Eocene, the rock associations of Cretaceous age were involved into accret ion processes and the association of silicic rocks of the tonalite-trondhje mite series was formed almost simultaneously with the collision of an islan d are and a continental margin, which occurred in the Eocene-Oligocene and could be related to the evolution of magmatism in an active continental mar gin or an ensialic island are.