Assimilation of carbonates by mafic magma: fassaite gabbro of the olkhon terrane (western baikal region)

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Abstract

The paper characterizes specific gabbro, the main rock-forming mineral of which is fassaite – alumina-rich (up to 12% Al2O3) calcium pyroxene, typical for high-temperature metasomatic rocks. In terms of geochemical characteristics, the fassaite gabbro are close to the subalkaline monzogabbro 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 terms of higher content of CaO and MgO and lower content of SiO2 and Al2O3. Fassaite gabbro form several small massifs framed by the Ust-Krestovsky monzogabbro massif, without contacting the latter. A model of the formation of fassaite gabbros due to the assimilation of carbonate material (crustal carbonate melting) by subalkaline mafic magma is proposed.

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About the authors

E. V. Sklyarov

Institute of the Earth’s Crust, Siberian Branch of the Russian Academy of Sciences

Author for correspondence.
Email: skl@crust.irk.ru

Corresponding member of the RAS

Russian Federation, Irkutsk

A. V. Lavrenchuk

V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences; Novosibirsk State University

Email: alavr@igm.nsc.ru
Russian Federation, Novosibirsk; Novosibirsk

D. V. Semenova

V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences

Email: skl@crust.irk.ru
Russian Federation, Novosibirsk

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3. Fig. 1. Fragment of the geological map of the Olkhon terrane (after [10] with modifications and additions). 1 - porphyrites of the Birkhin volcano-plutonic association and amphibolites after them; 2 - high-temperature hornfelses after porphyrites; 3 - calcite marbles; 4 - dolomite and calcite-dolomite marbles, diopside-calcite rocks, diopsides, quartz-diopside rocks, quartzites; 5-7 - Ust-Krestovsky complex of gabbroids: fassaite gabbros (5), monzogabbro and quartz syenites (6), mingling zone of monzogabbro with granites, syenites and leucogabbro (7); 8 - granites; 9 - injection marbles and calciphyres; 10 - Quaternary deposits; 11 - viscous-plastic detachments; 12 – Geological boundaries. The inset shows the location of the map.

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4. Fig. 2. Calcite in clusters with plagioclase and pargasite (A) and in inclusions in pargasite (B). Photomicrographs in transmitted light, calcite is stained with alizarin. Cal – calcite, Fas – fassaite, Pl – plagioclase, Prg – pargasite, Spl – spinel.

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5. Fig. 3. Variational petrochemical diagrams and REE spectra of the Ust-Krestovsky massif rocks and fassaite gabbro. Arrows show the change in the average composition of the Ust-Krestovsky massif monzogabbro with assimilation of up to 70% calcite (Cal) and dolomite (Dol).

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6. Fig. 4. Concordia diagrams and cathodoluminescence images of zircons from fassaite gabbros (sample 4608) and subalkaline gabbroids from the mingling zone (sample 4609). The circles indicate the points where U‒Pb isotope studies were carried out with extracted age values ​​(Ma) for 206Pb/238U with an error of 2σ.

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