Multi-stage emplacement of alkaline and peralkaline syenite-granite suites in the Mongolian-Transbaikalian Belt, Russia: Evidence from U-Pb geochronology and whole rock geochemistry

The Mongolian-Transbaikalian Belt (MTB) is the largest and most representative component of the Central-Asian network of Late Paleozoic and Mesozoic granites and syenites and associated volcanic rocks. The MTB incorporates over 350 individual plutons and numerous volcanic fields, predominantly of alkaline and peralkaline composition. We present new U-Pb zircon and titanite geochronological data from five plutonic complexes located in the central part of the MTB. Four of them (Bryansky, Khorinsk, Kharitonovo and Malo-Kunalei) are peralkaline and alkali-feldspar granitoids, and one plutonic complex (Ust-Khilok) represents a shoshonitic series (K-rich gabbro, monzonite and syenite). The new data confirm the presence of three distinct stages of highly alkaline magmatic activity within the MTB: Early Permian (280-275. Ma), Late Triassic (230-219. Ma) and Late Jurassic (~. 152. Ma). The dated shoshonitic plutonic complex formed between 280 and 278. Ma, overlapping with the formation of the Early Permian alkali-feldspar-peralkaline granitoid rocks. The ages of the alkali-feldspar and peralkaline magmatic suites are indistinguishable within a single magmatic complex. This suggests that several episodes of syenite and granite magma emplacement occurred within <2myr intervals. The close association in space and time of geochemically distinct igneous rocks points to co-existence of discrete magma chambers comprising magma from different sources. Early Permian magmatic activity in the MTB completed a continuous post-collisional cycle of magmatism that started in the Carboniferous (at ~. 330. Ma) and lasted for about 50myr. The Late Triassic stage marks the beginning of anorogenic tectonic setting in the MTB evolution. Late Jurassic peralkaline magmatism in the territory of the MTB is related to the formation of Mesozoic metamorphic core complexes, which are abundant in northern Mongolia and Transbaikalia and formed in an anorogenic within-plate environment.