Geological structure of Transbaikalia

The territory of Transbaikalia and the adjacent regions of Mongolia , amounting to more than 800 thousand km² ^[1] , is unique in the scale of distribution and variety of granitoids , which occupy more than 70% of the area, and the formation of acidic magmas occurred from Archean to Early Cretaceous. They are confined to the Mongol-Okhotsk (Mongol-Transbaikal) mobile belt , which has a complex and long history. This belt was formed as a result of the development of the Paleo-Asian Ocean . According to other researchers, this structure was formed as a result of subduction of the Andean type on the outskirts of the East Siberian craton , joining of the island arc , collision and rifting in the Triassic and Cenozoic . At most of these stages, granitoids formed, which determined their great diversity.

In the long geological history of Transbaikalia, six major stages of magmatism are distinguished:

1. Wend - early Cambrian - the founding of the Caledonian eugeosyncline, basic volcanism, ultrabasic intrusions.
2. Cambrian - Silur - the formation of external carbonate troughs in areas adjacent to the eugeosyncline, mass formation of granites.
3. Devon - orogenous activation, local development of acidic and mixed volcanism. Intrusions of alkaline-earth syenites, granites and alaskite granites.
4. Carbon - Perm - tectonomagmatic activation, intrusive series of gabbro-monzonite-syenite, alkaline-syenite and alkaline-granite composition.
5. Triassic is a chalk series of tectonomagmatic activations with the occurrence of volcanotectonic structures, superimposed depressions with intrusions of normal and alkaline-earth granodiorite - granite-leucogranite series and outflows of basaltoids.
6. The Quaternary Period - rift formation and outflow of alkaline basaltoids.

The geological history of Transbaikalia in the Paleozoic and Mesozoic is closely connected with the evolution and closure of the oceanic space that once separated the Siberian and Mongol-Chinese (North China) paleocontinents. The history of the paleo-oceans is deciphered primarily by ophiolite complexes that carry direct information about the oceanic stages of development of modern structures. However, the ophiolites of the Mongol-Okhotsk belt underwent intensive tectonic and metamorphic processing, which greatly complicates their interpretation. At the same time, the Mongol-Okhotsk belt is characterized by the broadest development of different age granitoid magmatism, which for many of its segments is the only indicator of the paleogeodynamic conditions of its various links located on both sides of the modern Mongol-Okhotsk seam - active and passive margins of continents, island arcs and collision zones . History of Magmatism of the Mongolo-Okhotsk Belt From the Cambrian to the Triassic, several stages of granitoid magmatism are distinguished in the work, which are tied to the main stages of the development of the Mongolo-Okhotsk mobile belt.

1. Stage of the active continental margin and the preceding island-arc stage (Cambrian): tonalite-granodiorite-granite series
2. Continental collision stage (Ordovician-Silurian): quartz monzonite-granite series (Angara-Vitim batholith (Barguzinsky) and smaller plutons).
3. Stage of postorogenic stretching (presumably Devonian or Carboniferous): quartz syenite granite series.
4. The stage of intracontinental riftogenesis (Perm - the beginning of the Triassic), the various stages of which are recorded by the formation of bimodal and co-igneous volcanic series: Monzonite syenite granite (initial stage), syenite granite (middle stage) and alkaline syenite granite (final stage).