Abstract
Data on the composition and the He-Ar and Sm-Nd isotopic systems of ultramafic xenoliths in Cretaceous-Paleogene basalt in the North Tien Shan provide insight into a lateral heterogeneity of the upper mantle.The mantle beneath the northern and southern portions of the North Tien Shan has a lherzolite composition, whereas pyroxenites found east of Lake Issyk Kul were produced by the partial melting of and metasomatism in the primary lherzolite mantle under the effect of a mantle plume at 55 Ma.This lateral heterogeneity of the upper mantle is clearly reflected in variations in the velocities of seismic waves and gravity anomalies when the influence of the crust is subtracted, as was determined for the portion of the Tien Shan east of the Talas-Fergana Fault.