Petrogenesis of two syenite-granite series: Bryansky pluton. (Transbaikalia)

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 nearly coeval syenite-granite suites: alkali-feldspar (AFS) (287 Ma) and peralkaline (PAS) (285 Ma). In the period between the AFS and PAS formation, a bimodal trachybasalt-comendite series was 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. Though AFS and PAS are closely associated in space and time, there is no direct genetic relation between them. Studies of melt inclusions (MI) in quartz and clinopyroxene from peralkaline syenite, granite, and comendite show that the crystallization of syenitic magmas commerced at T = >930-940°C, which is much higher than the determined liquidus temperature of granitic melts (760-790°C). Both the syenitic and highly silicic magmas were enriched in Cl (about 0.2-0.3 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 is 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.