post-Ordovician magmatism occurred within the Ordovician accreted elements and d 18 O, and simultaneous increase in high field strength terrane of Transbaikalia, during its 400 my of cratonization. Basic elements and K. We speculate on tectonic implications of the magmas may be critical in the generation of K-rich silicic magma, established chemical trends as reflecting progressive incompatible yet only subordinate volumes of coeval mafic rocks in the silicic element depletion and devolatilization of a mantle source and plutons and synchronous volcanics are present. Most of the mafic increasing prevalence of alkali basalt from the sublithospheric mantle rocks contain plagioclase megacrysts (1‐5 mm), and these were in the course of cratonization. used to reconstruct the primary basic magma chemistry and its evolution with time. Optical and scanning electron microscopy studies, and electron microprobe profiling through plagioclase megacrysts of diVerent ages revealed unzoned, Ca-rich cores in a number of crystals KEY WORDS: anorogenic; cratonization; ion microprobe; stable isotopes; in each sample. Several crystals within each rock in a number of trace elements rocks within each age group were studied. Several ion microprobe analyses inside each of these cores were made for concentrations of Li, Be, B, F, Mg, P, Cl, K, Ti, Fe, Co, Rb, Sr, Y, Zr, Nb, Cs, INTRODUCTION Ba, La, Ce, Pr, Nd, Sm, Eu, and Pb. In addition, partition coeYcients for the same trace elements and the relevant compositional range of plagioclase were used to convert trace element concentrations Secondary ion mass spectrometry studies in Transbaikalian plagioclase to parental magmatic values. Wholeof liquidus phases