Sources of the Late Mesozoic carbonatites of western Transbaikalia: Trace-element and Sr-Nd isotopic data

Carbonatites of the western Transbaikalia rift zone were formed in the midd le of the Early Cretaceous (130-120 Ma) and are represented by lavas, tuffs , dikes, and veins. They are associated with other highly alkaline magmatic rocks (tephrites, phonolites, shonkinites, and nepheline syenites). Alkali ne silicate rocks and carbonatites of these carbonatite associations have c ommon trace-element features. The silicate rocks are richer in Sr, Ba, Pb, REE (particularly, LREE) and much poorer in Rb, Nb, Th, Zr, and Hf than the oceanic island basalts (OIB). The anomalies of these elements are much str onger in the carbonatites, which contain a few percent of Sr, Ba, and REE a nd include specific mineralization with baritocelestine, barite, strontiani te, bastnaesite, and parisite. Sr and Nd isotopic compositions of the studi ed rocks vary from epsilon (Nd) = -1.6 and epsilon (Sr) = +11.8 in associat ions dominated by silicate rocks to epsilon (Nd) = -6.4 and epsilon (Sr) = +22 in associations almost solely composed of carbonatites. These variation s are probably related to the contribution to the magma compositions from t wo isotopically distinct sources, which we call as "tephritic" and "carbona titic." The proposed genetic model for the carbonatite associations suggest s the melting of a "tephritic" mantle, which was metasomatized by a carbona te fluid with isotopic characteristics of the "carbonatitic" source. These metasomatic events caused the formation of carbonates in the mantle source. The melting of this source produced carbonatite melts.