Alkaline granitic magmatism of theWestern Transbaikalia: Petrogenetic and geodynamic implications from U-Pb isotopic-geochronological data

TheMongolian-Transbaikalian volcanoplutonic belt extends for almost 2000 kmthrough northern Mongolia and Transbaikalia and hosts several hundreds of plutons of alkali feldspar syenites (AFS) and peralkalinegranites (PA) that formed at 280-270 and 230-210 Ma. The giant (similar to 220 km(2) in area) Sherbakhta pluton in the Vitim plateau (western Transbaikalia) is located and composed of metaluminous (A/CNK <= 1), high ferroan AFS syenites (10.7-12.3 wt% (NaO)-O-2 + (KO)-O-2; NK/A < 1) and A-type peralkaline granites (8.7-9.5 wt% Na2O + K2O; NK/A 0.95-1.05). The U-Pb (SHRIMP-RG, LA-ICP-MS) age of the pluton is 260 Ma. The isotope and geochemical characteristics of the studied pluton (epsilon Nd(T) -2.4 to -2.9; T-Nd(DM-2) 1.25-1.27 Ga; epsilon Hf(T) 1.1-0.6, T-Hf(DM-2) 1.02-1.05 Ga, delta O-18 of quartz 7.3-8.22%) indicate that the parental trachytic-trachyandesitic melt is compositionally similar to high-Sr syenites of the Sherbakhta pluton and derived by the melting of the mafic, moderately potassium-rich lower crustal source. The magmatic evolution of the parental melt toward a residual ferroan peralkaline graniticmelt in the intermediate chamberwas controlled by fractional crystallization of sodic plagioclase and K-feldspar, followed by amphibole and aegirine under lowoxygen fugacity. The A-type granitoids in the Transbaikalia region formed in the intracontinental geodynamic setting, probably as a result of impact of the mantle plume on heated plastic crust of a young orogen, which formed during the Late Hercynian fold thrust deformations. (c) 2021 Elsevier B.V. All rights reserved.

Automated summary

The Mongolian-Transbaikalian volcanoplutonic belt in northern Mongolia and Transbaikalia hosts hundreds of plutons formed between 280-270 and 230-210 Ma, including the giant Sherbakhta pluton consisting of AFS syenites and peralkaline granites. The isotopic and geochemical characteristics suggest that the parental melt underwent fractional crystallization to form residual ferroan peralkaline graniticmelt in an intermediate chamber. This A-type granitoids in the Transbaikalia region formed in an intracontinental geodynamic setting due to the impact of a mantle plume on the heated plastic crust of a young orogen.