400 MY OF BASIC MAGMATISM IN A SINGLE LITHOSPHERIC BLOCK DURING CRATONIZATION: ION MICROPROBE STUDY OF PLAGIOCLASE MEGACRYSTS IN MAFIC ROCKS FROM TRANSBAIKALIA, RUSSIA

Abstract

Following accretion of southern Siberian microcontinents to the Siberian craton in the Early Paleozoic, five cycles of K-rich silicic magmatism, progressively 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 within the Ordovician accreted elements and d18O, 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. Whole-rock and whole-plagioclase analyses for oxygen isotopes and trace elements were also made to constrain the amount of contamination of basic magma and study its temporal trends. Plagioclase core compositions reveal up to one order of magnitude variation of some trace elements and ratios between suites, and show a progressive change in trace element concentration with decreasing age. Plagioclase megacrysts and the reconstructed basic magmas exhibit depletion in large ion lithophile elements, volatile elements, light rare earth elements and δ18O, and simultaneous 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 the course of cratonization.