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, b eing rather uniformly distributed over an interval of the past 170 m. y. The composition of the volcanic rocks evolved with time. The basalt s of the Late Jurassic-Early Cretaceous stage have the highest SiO2 an d Al2O3 and the lowest MgO, FeO, and CaO contents. They are enriched in lithophile elements (particularly in Ba, Pb, Sr, and Li) and deplet ed in Ta and Nb and, to a lesser degree, in U and Th as compared to av erage GIB. The rocks are significantly enriched in radiogenic Sr (epsi lon(Nd) > +6) at epsilon(Nd) ranging from 0 to -2. In contrast to thes e rocks, the Cenozoic volcanics are less siliceous and more magnesian. They are similar to OIB in trace-element concentrations and correspon d to the slightly depleted mantle in isotope composition (epsilon(Sr) < 0, epsilon(Nd) > +2) The Late Cretaceous basalts have geochemical fe atures transitional between the Late Jurassic-Early Cretaceous and Cen ozoic basalts. It is shown that the variations of rocks compositions w ith time are related to the compositional evolution of their mantle so urces in the sequence OIB EM-TI (Late Jurassic-Early Cretaceous) --> O IB DM (Late Cretaceous-Early Cenozoic) --> OIB EM-I (Late Cenozoic). T he location of these sources in the structure of the upper and lower m antle of the area is analyzed.