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 compositions reveal up to one order of magnitude variation of some trace elements and ratios between suites, and show a progressive Siberian craton in the Early Paleozoic, five cycles of K-rich silicic magmatism, progressively decaying in volume, occurred on batholithic change in trace element concentration with decreasing age. Plagioclase scales throughout the Paleozoic and Early Mesozoic, followed by megacrysts and the reconstructed basic magmas exhibit depletion in rift-related alkali volcanism of Jurassic to Recent age. Most of the large ion lithophile elements, volatile elements, light rare earth post-Ordovician magmatism occurred within the Ordovician accreted elements and dO, 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 different ages revealed unzoned, Ca-rich cores in a number of crystals