Duration and geodynamic nature of giant Central Asian batholiths: geological and geochronological studies of the Khangai batholith

In the Late Paleozoic and Early Mesozoic, during about 100 m.y., the world’s three largest batholiths (Angara-Vitim, Khangai, and Khentei, each up to 1 000 000 km3 in volume) had formed within the limits of the Central Asian orogenic belt. Considering the case of the Khangai batholith, the problem of how, when, and why such an extensive granite formation took place is analyzed. The geochronological data for granitoids of the batholith by U–Pb (ID-TIMS) and 40Ar/39Ar dating methods are systematized to distinguish three age groups of rocks. These rock groups are correlated to the geological events occurred in the region. The earliest group includes granitoids formed in the interval of 302–283 Ma. They tend to the western and southern framings of the batholith and correspond to the fragments of two igneous belts that crossed the region, where the batholith formed later, and reached the areas far beyond. The youngest group of igneous rocks (230–200 Ma) is developed in the eastern periphery of the batholith and corresponds to the marginal part of the large Early Mesozoic Mongol-Transbaikalian igneous zone, with the main part being located far away to the east of there. Igneous complexes that formed in the interval of 273–238 Ma correspond to the batholith proper. They are concentrated within the zone of 350 × 400 km in size and are represented by rocks of two associations: granite-granodiorite (Khangai complex) and granite-leucogranite (Sharaus Gol complex). The coeval analogs of these rocks are reported only in the framing of the batholith. The comparison between the Khangai batholith and two other giant ones (Angara-Vitim and Khentei) revealed their similarity in terms of structure and evolution. They are all composed of similar rock associations and are of comparable sizes and age intervals of formation. For example, the Angara-Vitim and Khentei batholiths formed mainly in the intervals of 305–275 and 229–195 Ma, respectively. The obtained estimates of formation time of ~30 m.y. should seemingly be considered as the time necessary for chambers of anatectic magmas, which to certain degree formed giant (~1 000 000 km3 in volume) batholiths, to cool down in the Earth’s interior. The formation of giant batholiths is attributed to the effect of mantle plumes on the lithosphere of a young fold zone that appeared as a result of accretionary-collisional events in the marginal part of the Siberian paleocontinent.