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The Barrovian type metamorphism affecting the peri-Siberian tract of the Central Asian Orogenic Belt is mostly dated indirectly on zircon from (syn-tectonic) magmatic rocks as Late Proterozoic - Early Cambrian. However, in-situ monazite geochronology in micaschists and migmatite gneisses at the northern part of the Precambrian Baidrag block, central Mongolia, revealed that the Baikalian Late Proterozoic - Early Cambrian cycle overprints an earlier Tonian phase of metamorphism. The apparent Barrovian-type zoning ranging from garnet, staurolite, kyanite to kyanite/sillimanite migmatitic gneisses is thus false and points to hidden metamorphic discontinuities and mixed metamorphic histories from different times. Therefore, to decipher and interpret the record of different tectono-metamorphic events it is necessary to unreveal complete P-T-t paths from individual samples. Two localities with Tonian-age monazite show anticlockwise P-T paths: 1) Grt−Sil−Ky gneiss records burial to the sillimanite stability (~720°C, 6.0 kbar) followed by burial to the kyanite stability (~750°C, 9 kbar) and, 2) The Grt−St schist records burial to the staurolite stability field (~620°C, 6 kbar), further followed by almost isothermal burial (~590°C, 8.5 kbar). Based on monazite textural positon, internal zoning, and REE patterns, the time of prograde burial under a thermal gradient of 27-32°C/km is estimated at c. 890−853 Ma and further burial under a geothermal gradient of 18-22°C/km is dated at c. 835−815 Ma. On the other hand three localities with Late Proterozoic to Cambrian monazite ages show clockwise metamorphic paths at variable P-T gradients: 3) P-T conditions of the Grt schist reaches ~5 kbar and 500 °C and 4) the Grt−St−Ky schist reaches conditions of 9 kbar and 670 °C, indicating burial under a geothermal gradient of 20-26 °C/km. 5) Grt-Sil gneiss shows peak of 6-7 kbar and 700-750 °C, indicating melting conditions at 30-32 °C/km gradient. Monazite included in porphyroblasts and in the matrix indicate that these P-T conditions reached under variable geothermal gradient were semi-contemporaneous and occurred between 570 and 520 Ma. By correlation with published zircon ages of 600-530 Ma from granitoid magmatic rocks we suggest that the areas with higher geothermal gradient may be explained by closer vicinity of magmatic intrusions. These P−T and geochronology data from a continuous Barrovian metamorphic section suggest that anticlockwise P−T evolution from c. 930 to 750 Ma can be interpreted as a result of thickening of peri-Rodinian supra-subduction extensional and hot edifice. This metamorphic event was followed by a clockwise P−T evolution from c. 570 to 520 Ma possibly related to the collision of the Baidrag continental active margin with peri-Siberian continental mass further north.

The tectonometamorphic evolution of the peri-Siberian tract of the Central Asian Orogenic Belt is mainly characterized by Baikalian Late Proterozoic - Early Cambrian cycle related to amalgamation of Proterozoic oceanic and continent fragments to Siberain landmass. Here we present in-situ monazite geochronology linked to P−T modelling of micashischsts and migmatite gneisses at the northern part of the Precambrian Baydrag block (central Mongolia) previously considered as a part of Baikalian metamorphic belt. Garnet-sillimanite-kyanite gneiss records first burial to the sillimanite stability at ~725 °C and 6.5 kbar, followed by burial to the kyanite stability at ~650 °C and ~8 kbar. The garnet-staurolite schist records burial to the staurolite-stability at ~620 °C and 6 kbar, followed by a nearly isothermal burial to ~580 °C and 9 kbar. The monazite data yield a continuum of 207Pb-corrected 238U/206Pb dates of c. 926−768 Ma in the Grt−Sil−Ky gneiss, and c. 937−754 Ma in the Grt-St schist. Based on monazite textural positon and internal zoning, the time of prograde burial and peak under a thermal gradient of 28-32 °C/km is estimated at c. 870−890 Ma. It is not clear whether such high grade conditions prevailed until a phase of further burial under a geothermal gradient of 18-22 °C/km and dated at 800−820 Ma. Additionally, monazite with dates of c. 568−515 Ma occurs as whole grains or as rims with sharp boundaries on Grenvillean monazite in Grt-St schist testifying for minor Baikalian overprint. Metamorphic zircon rims with Th/U ratio ~0.01-0.06 in Grt−Sil−Ky gneiss with 877 ± 7 Ma age, together with lower intercepts of zircon discordia lines in both Grt-Sil-Ky gneiss and Grt-St schist further support the Tonian age of high grade metamorphism. The P−T and geochronology data show anticlockwise P−T evolution from c. 930 to 750 Ma which is interpreted as a result of thickening of suprasubduction extensional and hot edifice - probably of back arc or arc type. This kind of prograde metamorphism was so far described only on the northern part of the Tarim block and interpreted as a result of initiation of peri-Rodinian subduction of Mirovoi Ocean. Here, we further discuss geodynamic consequences of a unique discovery of Tonian metamorphism in term of tectonic switch related to initiation of peri-Rodinian oceanic subduction during supercontinent assembly followed by strong mechanical coupling potentially related to onset of Rodinia splitting.

Long-lived accretionary evolution of the Mongolian part of the Central Asian Orogenic Belt was intensively studied during the last decades, however, it is not completely known yet. Types of geodynamic processes forming this orogenic system and their timing are addressed through petrological, geochemical and/or sedimentological study combined with U-Pb zircon geochronology. The publications included in this thesis present research of two magmatic and one sedimentary system in the western Mongolia. Both the ancient magmatic complexes and sedimentary successions provide opportunity to study geotectonic evolution in a different way. Therefore, mutually combined results of these two geological disciplines allow more complex insight into the geodynamics of the region. The geochemical, isotopic and geochronological investigations of the high-grade basement complexes in the eastern Hovd Zone reveal an existence of the Early-Mid-Ordovician intra- oceanic arc and associated fore-arc volcano-sedimentary complex. The superimposed Mid- Silurian intra-plate magmatism is interpreted as a result of lithospheric thinning related to the same but already retreating subduction. Striking similarities of the studied basement with neighbouring Lake Zone margin suggest that the both units originally belonged to a single...

Supplementary material A1: Analytical methods

Supplementary material A2: Detailed samples description

Supplementary material A5: SiO2–K2O (in wt%) classification diagram for metaigneous rocks of the KU of Peccerillo & Taylor (1976).