EVOLUTION OF MAGMATISM IN THE NORTHEASTERN BAIKAL RIFT SYSTEM

The K-Ar dating and study of the composition evolution of lavas from t he Udokan volcanic field (ICP-MS trace element concentrations; Ph, Sr, and Nd variations) revealed that recent magmatism developed in a cont inental lithosphere whose Ba/Rb, K/Nb, and K/Ta ratios were higher and Y/Ho values lower than those in the sources of oceanic basalt magmas. The localization of volcanism was governed by the recent activation o f the Kodar-Udokan weak zone of the Aldan Shield. The rift activation within the zone commenced at approximately 14 Ma simultaneously with e ruptions of alkaline ultramafic magma generated in a garnet-facies man tle. It is surmised that during the magma rise, some magma stagnated a t the crustal base and emitted enough heat and fluids to melt the rock s of the crust-mantle interface. The alkaline olivine basalt erupted o n the surface was the product of mantle and crustal magma mixing. The erupted basanite was derived from the underlying depleted mantle. Duri ng Pliocene-Quaternary time, the volume of the erupted mantle magma de clined, while that of the crustal magma increased. The Late Cenozoic a lkaline olivine basalts in and around the reactivated Early Proterozoi c weak zone of the Aldan Shield show great lateral variations of their trace element compositions. The rocks of the weak zone involved in me lting comprised the components of the Archean crust, ''island arc,'' a nd depleted mantle; the melts in the adjacent Aldan Shield included th e rocks of more enriched mantle and the Archean crust.