The Mongolo-Transbaikalian alkali-granite province stretches for about 2000 km and includes 350 plutons. The largest, Bryansky, pluton covers more than 1600 km2 and is made up of two almost coeval syenite-granite series: alkali-feldspar (AFS) (287 Ma) and peralkaline (PAS) (285 Ma). In the period between the AFS and PAS formation, a bimodal trachybasalt-comendite series erupted. Comendite is regarded as a volcanic rock comagmatic to peralkaline granites. Data on mineralogy and geochemistry of the granitoids suggest that the rocks in each series were produced by fractional crystallization in deep magma chambers. Although AFS and PAS are closely associated in space and time, there are no direct genetic links between them. Studies of melt inclusions (MI) in quartz and c1inopyroxene from peralkaline syenite, granite, and comendite show that the crystallization of syenitic magma commenced at T > 930–940 °C, which is much higher than the determined Iiquidus temperature of granitic melts (760–790 °C). Both the syenitic and highly silicic magmas were enriched in Cl (about 0.2–03 wt.%). As the fractional crystallization proceeded, the residual silicic melts became enriched in F, up to 1.5–1.7 wt.%. Based on a study of MI in comendite, the temperature of generation of peralkaline silicic magma has been estimated at > 1000 °C. Similar high temperatures have been established for other comendites of the province. This suggests that the silicic magma was generated at depths significantly greater than the normal crust thickness.

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First page of GENESIS OF TWO SYENITE-GRANITE SERIES: BRYANSKY PLUTON (<italic>Transbaikalia</italic>)
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