Carbonatites in collisional settings and pseudo-carbonatites of the Early Paleozoic Ol’khon collisional system

Carbonatites associated with syenites and subalkalic mafic rocks (lamprophyres) occur in the Himalayan continental collision zone, and this suggests their possible existence in other Phanerozoic collisional settings. The Early Paleozoic Ol’khon collisional system in Western Cisbaikalia is considered one of the possible occurrences. Subalkalic gabbroids as well as peculiar carbonate (brucite marbles) and calc-silicate rocks were found here, within the Tazheran massif of alkalic and nepheline syenites. Alkalic syenites, nepheline syenites, and calciphyres were dated at 471 Ma, 451–464 Ma, and 466 Ma, respectively, and their ages correspond to the main collisional events in the system. A geochemical description of igneous and carbonate rocks in the massif is provided. Close mapping showed unusual syenite and brucite marble combinations and the frequent vein or pipe-like form of carbonates and calciphyres corresponding to their magmatic intrusion. But carbonatite nature of the marble mentioned above does not fit their typical crustal geochemical features. Not ruling out the possibility of total change of geochemical signatures of mantle carbonatite in the collisional medium, we offer two other possible explanations for these facts: (1) melting of carbonate masses by syenite and mafic magmas, followed by carbonate melt intrusion into the upper crust, and (2) protrusion of carbonates into syenites and gabbroids at the late stages of the contact action of a silicate magma. In this case the above-mentioned carbonate structural features result from late recrystallization, whose mechanism is yet to be explained.