Composition and age of teshenite magmatism of Northern Transbaikalia

We present original precision data on the chemical and trace-element (Li, R b, Cs, Ba, Sr, Pb, Zn, Sn, Co, Ni, Cr, V, Cu, Sc, Mo, W, Nb, Zr, Hf, Th, U, REE, and Y) compositions of igneous rocks of the Tundak sill (Northern Tra nsbaikalia) - picroteshenites, meso- and leucoteshenites, and analcime syen ites. These rocks are shown to have formed through intrachamber crystalliza tion differentiation of the same teshenite magma within the sill. The K-Ar age of the teshenites is 118.83+/-3.56 Ma. The Sr isotope characteristics of the sill rocks and their hosted minerals disagree with the isochron model. We have established that the dispersion o f Sr isotope compositions is caused mainly by postmagmatic alterations of t hese rocks rather than by the isotopic heterogeneity of the initial melt. T he (Sr-87/Sr-86)(0) value of this melt (0.7053-0.7056) does not rule out th e presence of admixed assimilated crustal matter in it. Geological and geochemical analyses showed a great similarity of the North Transbaikalian and Late Cretaceous West Transbaikalian teshenites. We have also established that the study teshenites are close in age and composition to trachybasalts that effused in both regions at the earlier stages of the Late Mesozoic taphrogenic volcanism. We propose a genetic model for the fo rmation of basaltoid melts of both types (teshenite and trachybasaltic) fro m the same mantle source. The model implies that the compositional differen ces between these types are related to the long evolution of parental magma s and admits mixing of deeply differentiated and primitive melts in interme diate magma chambers.