First data on REE-bearing silicates from volcanic rocks of Kamchatka

Abstract

The first finds of the REE-bearing silicates in the Kamchatka island-arc system were reported from K-Na alkaline trachytes from a large volcanic body of Bolshoi volcano in the back-arc zone of the Sredinny Ridge volcanic belt (56°24′10″ N and 157°56′45″ E) of Kamchatka. Chevkinite microlites are found in a trachytic groundmass in association with microlites of ferropigeonite, ferroaugite, anorthoclase, Na-sanidine, richterite, apatite, Nb-bearing ilmenite, zirkelite, baddeleyite, and zircon. The chevkinites from the Bolshoi volcano trachytes have higher Nb and Zr and similar REE contents compared to the chevkinite-group minerals from igneous rocks formed in different geodynamic settings. The crystallization conditions for chevkinites from Bolshoi volcano were determined by comparing the compositions of trachytes and their mineral assemblages with those of trachytes from Belogolovskii volcano of Sredinny Ridge, Kamchatka. The results show that chevkinites were formed during prolonged crystallization of a trachytic magma under reducing conditions at $\Delta \log f_{O_2 }$ ranging from −1.36 to −1.55 NNO, relatively low temperatures of 794–741°C, and moderate HF concentrations of magmatic fluids (0.003–0.0029 mol/dm³). These results, combined with the other data, are of crucial interest for studies of the REE budget during the evolution of high-alkaline magmas and can be used as indicators for determining the nature and role of different magma sources in the active continental margin setting of Kamchatka.