Age and Melt Sources of Ultramafic Dykes and Rocks of the Bolshetagninskii Alkaline Carbonatite Massif (Urik-Iya Graben, SW Margin of the Siberian Craton)

Abstract

The age of rocks of the Bolshetagninskii ijolite–syenite–carbonatite massif and ultramafic dykes within the Urik-Iya Graben in the southwestern part of the Siberian Craton was studied. An isochrone with an age of 640 ± 11 Ma was obtained by the 147Sm–143Nd method for rocks of the massif. 40Ar/39Ar dating of phlogopites from rocks of the dyke series provided two plateaus with ages of 644.1 ± 8.6 and 646.1 ± 8.6 Ma. The ranges of εNd(Т) values corrected for 640 Ma are from +4.2 to +5.0 for rocks of the massif and from +2.9 to +4.5 for dykes and characterize a mantle source close that of OIB. Ijolite and carbonatite of the massif have εNd(Т) values from +4.6 to +5.0 and εSr(Т) values from –7 to –10, which indicates a common silicate–carbonate parental melt for them. Variations in the initial (87Sr/86Sr)t ratio from 0.7025 to 0.7059 in dykes most likely reflect both the heterogeneity in the isotopic composition of the mantle source and the different degrees of contamination of mantle melts by the material of the upper continental crust.