Valaginskay Seria

This volcanic series, formed at the beginning of a geosynclinal cycle, consists of ultramafic volcanics, volcanic breccias, basaltic lavas, trachbasalts and trachyandesites, which have a total thickness of 7.5 km and form dykes and sills as well as flows. Picrite lavas (SiO2 39.44%, Na2O 0.07%, K2O 0.08%), tuffs and breccias cover an area of about 7 km2. Within the picritic tuffs are carbonatite bombs and blocks up to 1.5 m across. The carbonatites are light grey to green-grey rocks which are porphyritic with a fine-grained groundmass. The phenocrysts are diopside-augite and garnet which are set in a matrix consisting of calcite microlites and brown volcanic glass. Accessories are apatite, baryte, strontianite, sulphides and pseudomorphs after olivine. The carbonatite is enriched in Ba, Sr and Zr but the concentrations of P and Nb are very low. Determinations of 87Sr/86Sr in the carbonatite gave 0.70365_+0.00010, 0.70342_+0.00010 and 0.70369_+0.00010.