Petrological model for formation of felsic volcanic rocks of the Pecherkinskaya Formation (northwestern part of the Salair Ridge)

Abstract

The geochemical and isotopic (Nd, Sr) characteristics of felsic volcanic rocks of the Pecherkinskaya Formation in the northwestern part of the Salair Ridge were studied. These rocks are closely associated with industrially significant deposits and manifestations of gold- and silver-bearing pyrite-polymetallic formations with potentially gold-bearing weathering crusts on them. The study of the material composition of unaltered volcanic rocks of the Pecherkinskaya Formation is necessary for the reconstruction of the conditions of their formation, which contributes to estimation of the mineragenic potential of this formation and the conditions during the accumulation of ore matter. Felsic volcanic rocks of the Pecherkinskaya Formation are an extreme member of the basalt-plagioryolite series, have an extremely high-sodium type of alkalinity with a low potassium content, are calcareous and characterized by high silica contents, low iron content, have low REE concentrations, are enriched in Pb and U and depleted in Ta and Nb, are characterized by high values of εNd (t) (+6.36) and primary ratio 87Sr / 86Sr (0.706977). The REE distribution spectra show a low degree of fractionation with a pronounced negative "europium anomaly". The geochemical composition of the Pecherkinskaya Formation volcanic rocks indicates that primary magmas were formed by partial melting of rocks of the lower continental crust at a pressure of ~ 8 kbar in equilibrium with Hbl + Cpx + Pl ± Opx restite and a temperature of ~ 750-790 ° C. The isotope-geochemical characteristics indicating the participation of depleted mantle in the source are probably inherited from the juvenile crust, the rocks of which could have been formed as a result of differentiation of mantle magmas. Geological and geochemical features of felsic volcanic rocks of the Pecherkinskaya Formation are characteristic of igneous rocks formed under conditions of oceanic arcs.