ENRICHED ND-SR-PB ISOTOPIC SIGNATURES IN THE DOVYREN LAYERED INTRUSION ( EASTERN SIBERIA, RUSSIA): EVIDENCE FOR SOURCE CONTAMINATION BY ANCIENT UPPER-CRUSTAL MATERIAL

Abstract

Major- and trace-element concentrations and Nd-, Sr- and Pb-isotopic ratios are reported for the Dovyren layered mafic-ultramafic intrusion in the northern Baikal region, eastern Siberia. SmNd internal isochrons for an olivine gabbro from the layered series and a gabbronorite from a sill at the bottom of the Dovyren intrusion yield ages of 673 ± 22 and 707 ± 40 Ma, respectively. Initial isotopic ratios: (673) from 0.7101 to 0.7135, ϵNd(673 Ma) from −16.3 to −14.1 from 16.80 to 17.14, from 15.477 to 15.501 and from 37.17 to 37.59, are similar to those of late Archean-early Proterozoic upper continental crust, but do not appear to be a result of wallrock assimilation in the magma chamber. These isotopic features, as well as high K, Rb and LREE and low Ti concentrations in the calculated composition of the Dovyren parental magma, may be explained by subduction of sediments derived from upper continental crust into depleted mantle and subsequent melting of the metasomatized peridotite.