Assimilation of Carbonates by Mafic Magma: Fassaite Gabbro of the Olkhon Terrane (Western Baikal Region)

Specific gabbro with the main rock-forming mineral of fassaite—alumina-rich (up to 12 wt % Al₂O₃) calcium pyroxene, typical of high-temperature metasomatic rocks, are characterized. In terms of the geochemical characteristics, the fassaite gabbros are close to the subalkaline monzogabbros of the Ust’-Krestovsky complex, which are widely distributed within the Krestovsky subterrane of the Olkhon terrane (Western Baikal region). At the same time, they differ sharply from the latter in having a higher content of CaO and MgO and a lower content of SiO₂ and Al₂O₃. Fassaite gabbro composes several small bodies framed the Ust’-Krestovsky monzogabbro massif, without contacting the latter. A model of the formation of fassaite gabbro due to the assimilation of carbonate (crustal carbonate melts) by subalkaline mafic magma is proposed.