Sources of intraplate magmatism of western Transbaikalia in the late Mesozoic-Cenozoic: Trace-element and isotope data

At least fourteen stages of basaltic volcanism occurred in the western Transbaikalia volcanic area from the Late Mesozoic to the Cenozoic, being rather uniformly distributed over an interval of the past 170 m. y. The composition of the volcanic rocks evolved with time. The basalts of the Late Jurassic-Early Cretaceous stage have the highest SiO2 and Al2O3 and the lowest MgO, FeO*, and CaO contents. They are enriched in lithophile elements (particularly in Ba, Pb, Sr, and Li) and depleted in Ta and Nb and, to a lesser degree, in U and Th as compared to average GIB. The rocks are significantly enriched in radiogenic Sr (epsilon(Nd) > +6) at epsilon(Nd) ranging from 0 to -2. In contrast to these rocks, the Cenozoic volcanics are less siliceous and more magnesian. They are similar to OIB in trace-element concentrations and correspond to the slightly depleted mantle in isotope composition (epsilon(Sr) < 0, epsilon(Nd) > +2) The Late Cretaceous basalts have geochemical features transitional between the Late Jurassic-Early Cretaceous and Cenozoic basalts. It is shown that the variations of rocks compositions with time are related to the compositional evolution of their mantle sources in the sequence OIB EM-TI (Late Jurassic-Early Cretaceous) --> OIB DM (Late Cretaceous-Early Cenozoic) --> OIB EM-I (Late Cenozoic). The location of these sources in the structure of the upper and lower mantle of the area is analyzed.