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

Major- and trace-element concentrations and Nd-, Sr- and Pb-isotopic r atios are reported for the Dovyren layered mafic-ultramafic intrusion in the northern Baikal region, eastern Siberia. Sm-Nd internal isochro ns for an olivine gabbro from the layered series and a gabbronorite fr om a sill at the bottom of the Dovyren intrusion yield ages of 673 +/- 22 and 707 +/- 40 Ma, respectively. Initial isotopic ratios: Sr-87/Sr -86 (673) from 0.7101 to 0.7135, epsilon(Nd)(673 Ma) from -16.3 to -14 .1, Pb-206/Pb-204 from 16.80 to 17.14, Pb-207/Pb-204 from 15.477 to 15 .501 and Pb-208/Pb-204 from 37.17 to 37.59, are similar to those of la te Archean-early Proterozoic upper continental crust, but do not appea r 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 concentra tions in the calculated composition of the Dovyren parental magma, may be explained by subduction of sediments derived from upper continenta l crust into depleted mantle and subsequent melting of the metasomatiz ed peridotite.