New Data on the Age and Composition of Cenozoic Andesibasalts and Andesites of the Bolshaya Garmanda River (North Okhotsk Region)

Abstract

The work presents the K–Ar isotope age (54–65 Ma) and isotope–geochemical composition of the post-subduction volcanics which compose the basalt nappes in the Bolshaya Garmanda River valley (North Okhotsk Region). It is established that the age of the Garmanda volcanics differs from that of the rocks of the Kytyima Formation, in the composition of which these volcanics were earlier studied.
The isotope contents of Sr and Nd and low concentrations of heavy rare earth elements (HREE) relative to the MORB composition indicate the moderately depleted composition of the mantle source. The enrichment of the volcanics with large-ion lithophilic elements (LILE) can be explained by the role of fluids captured by the melts in the course of melting of the continental crust transformed as a result of the subduction metasomatism.