400 my of basic magmatism in a single lithospheric block during cratonization: Ion microprobe study of plagioclase megacrysts in mafic rocks from Transbaikalia, Russia

Following accretion of southern Siberian microcontinents to the Siberian cr aton in the Early Paleozoic, five cycles of K-rich silicic magmatism, progr essively decaying in volume, occurred on batholithic scales throughout the Paleozoic and Early Mesozoic, followed by rift-related alkali volcanism of Jurassic to Recent age, Most of the post-Ordovician magmatism occurred with in the Ordovician accreted terrane of Transbaikalia, during its 400 my of c ratonization. Basic magmas may be critical in the generation of K-rich sili cic magma, yet only subordinate volumes of coeval mafic rocks in the silici c plutons and synchronous volcanics are present. Most of the mafic rocks co ntain plagioclase megacrysts (1-5 mm), and these were used to reconstruct t he primary basic magma chemistry and its evolution with time. Optical and s canning electron microscopy studies, and electron microprobe profiling thro ugh plagioclase megacrysts of different ages revealed unzoned, Ca-rich cal es in a number of crystals in each sample. Several crystals within each roc k in a number of rocks within each age group were studied. Several ion micr oprobe 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, Ba, La, Ce, Pr, Nd, Sm, Eu, and Pb. In addition, partition coefficients for the same tr ace elements and the relevant compositional range of plagioclase were used to convert trace element concentrations in Transbaikalian plagioclase to pa rental magmatic values. Whole-rock and whole-plagioclase analyses for oxyge n isotopes and trace elements were also made to constrain the amount of con tamination of basic magma and study its temporal trends. Plagioclase core c ompositions reveal up to one order of magnitude variation of some trace ele ments and ratios between suites, and show a progressive change in trace ele ment concentration with decreasing age. Plagioclase megacrysts and the reco nstructed basic magmas exhibit depletion in large ion lithophile elements. volatile elements, light rare earth elements and delta(18)O, and simultaneo us increase in high field strength elements and K. We speculate on tectonic implications of the established chemical trends as reflecting progressive incompatible element depletion and devolatilization of a mantle source and increasing prevalence of alkali basalt from the sublithospheric mantle in t he course of cratonization.