The ages for zircons from xenoliths of Cenozoic volcanics from TransBaikalia and Mongolia

The ages for zircons from xenoliths of Cenozoic volcanics from TransBaikalia and Mongolia were determined by the same (LA-ICP-MS) aniCAP Q mass spectrometer (Thermo Scientific) and a NWR 213 in IGM SB RAS

The isotopic Pb/U and Pb/Pb ages  of the 6 zircon grains from the tuffs of the Bartoy volcano. They are plotting practically on  the207Pb/235U and 206Pb/238U concordia line (Figure 9). For the calculations ages the polynomial equations were used, obtained from the works of Khubanov et al., (2016-2024).

They may be divided into three groups. The ages of the zircons from tuffs 1160-1250 Ma mainly correspond to the meta-terrigenous rocks of the Shubutuiskaya Formation in the Khamar Daban zone (Gordienko et al., 2006). The Th/U ratio of 0.8-0.6 of these zircons corresponds to the common magmatic rocks, mainly of the basic type (Hawkesworth et al., 1997).

The ages near 800-860 Ma are determined for the suit from the metamorphic in Central Khamar-Daban (Shkol’nik et al., 2016). The collision events at the boundaries of the Paleoasian ocean occurred earlier in the Vend-Cambrian (Byzov, Sankov, 2024; Donskaya et al., 2013). Though some plutons with the model ages 1100-800 Ma were suggested by some authors to be referred to as collision. And elevated Th/U ratios of two zircons, 1.6-1.1, commonly correspond to the granites with the admixture of material of island arc environment with 3-5 Th/U ratios.

The age of granitic zircon, 300 Ma, just corresponds to the beginning of the Angaro-Vitim Batholith (AVP) formation (Tsygankov et al., 2010-2025). It may be connected to the movement of the superplume-formed kimberlites in Yakutia at the interval 420-340 Ma and later created the Biryusa and Tumanshet lamproites (Kostrovitsky et al., 2025) and later the Ingashi lamproites in Eastern Sayan ~306-309 Ma (Gladkochub et al., 2013). Further movement through Khamar-Daban and interaction with the lower crust and granulites brings to the creation of alkaline granitoids of AVP. But the Th/U ratio is rather low, 0.07, which commonly corresponds to the metamorphic type [90]; thus, they should be from granulites possibly remelted by a plume.

The ages of the granulite xenoliths from the Vitim picrobasalts (Ashchepkov et al., 2011) correspond to the initial stage of AVB.  And the next one, 873 may be correlated with the basic magmatism in Baikal uplift (Gladkochub et al., 2010).

In Shavaryn-Tsaram volcano two ages of zircons corresponds to Carboniferous stage 322 Ma of rifting in Mongolia (Kozlovsky et al., 2005) or close to last stage  AVP. The next one refers to the initial stage of Miocene plume magmatism (Ashchepkov et al., 2026).

The trace elements for two zircons determined in the granulites differ significantly. The inclined enriched in HREE pattern looks similar to carbonatitic zircons (Hardman, et al., 2025).  The next acid sample with La/Ybn <2 with  Eu minimum and without Ce anomaly. In MSD it shows the same peaks but without Y, Ta anomalies.

Work was done on state assignments of IGM SB RAS FWZN-2026-0007 and IG SB RAS. Russian Science Foundation Grant (23-17-00030).