Crustal versus mantle events of 2.44–2.22 and 1.63–1.31 Ga at the junction between Khamardaban terrane, Tuva-Mongolian microcontinent, and Siberian paleocontinent: Petrogenetic consequences

The Tuva-Mongolian microcontinent and Khamardaban terrane are known as major tectonic units accreted to the Siberian paleocontinent. We report ²⁰⁷Pb/²⁰⁶Pb ages of 2.44–2.22 Ga for sources of Late Cenozoic volcanic rocks from the Tunka volcanic zone and of 1.63–1.31 Ga for those from the Khamardaban zone. The new ages are consistent with Precambrian geological events that are characteristic of the area and contradict the existing opinion about the Early Paleozoic collisional connection between these tectonic units inferred from dating of syn-collisional granites.

On the one hand, we constrain ore-forming processes in the Gargan block of the Tuva-Mongolian microcontinent and in the south of the Siberian paleocontinent between 2.45 and 1.4 Ga and between 1.3 and 0.25 Ga, respectively [Rasskazov et al., 2010]. The latest Pb-separating event in the Gargan block was followed by the generation of restite ultrabasic Ilchir belt that bounds the block from the south [Kiseleva et al., 2020]. So, we trace the boundary between the Gargan block and Ilchir belt to magma sources of the Tunka and Khamardaban zones that reasonably denote the root part of the Khamardaban terrane, accreted to the Tuva-Mongolian microcontinent and Siberian paleocontinent 1.63–1.31 Ga ago (Figure). On the other hand, we emphasize the importance of ore-forming events in the Gargan block, launched about 2.45 Ga, simultaneously with source generation in the Tunka zone. Basalts of this zone include xenoliths of fassaitic clinopyroxenites that show wide variations in the oxidation–reduction state. We suggest that fassaite (diopside) mineralization was due to interaction between orthopyroxene and calcite: (Mg, Fe)₂Si₂O₆ + CaCO₃ → (Mg, Ca)₂Si₂O₆ + CO₂ + FeO. Orthopyroxene of high-Mg spinel harzburgite xenoliths from Khobok River lavas (Tunka basin) shows SiO₂ content as high as 58.7 wt. %, while fassaite from pyroxenite xenoliths has SiO₂ content as low as 49 wt. %. Fassaitization of orthopyroxenites and harzburgites, obviously, releases both iron and silica. These components are found as amorphous Fe–Si phases in metasomatite xenoliths with low Mg/Si and Al/Si ratios [Ailow et al., 2021]. From data obtained, we speculate that fassaitization was an effective crust-mantle process of 2.4–2.2 Ga that could provide both the deep-seated Fe–Si mineralization and the generation of ferruginous quartzites displayed in the Great Oxidation Event.

Ailow Y. et al. // Lithosphere. 2021. V. 21, No. 4. P. 517–545.

Kiseleva O.N. et al. // Minerals. 2020. V. 10. P. 1077.

Rasskazov S.V. Brandt S.B., Brandt I.S. Radiogenic isotopes in geologic processes. Springer, 2010. 306 p.