Biotite stability in peraluminous granitic melts: Compositional dependence and application to the generation of two-mica granites in the South Bohemian batholith (Bohemian Massif, Czech Republic)

Abstract

Crystallization experiments have been conducted in the system Na 2O–K 2O–MgO–FeO–Al 2O 3–SiO 2–H 2O (with 4% normative corundum) in order to constrain the stability of biotite as a function of water activity and the Mg# of biotite [Mg/(Mg +Fe total)] in equilibrium with peraluminous granitic melts. The temperature at which biotite breakdown starts is strongly dependent on the Mg# of biotite. At 500 MPa, the temperature of biotite breakdown to form orthopyroxene increases from 750 °C to 830 °C, as the Mg# of biotite increases from 0.4 to 0.5. Considering that the system investigated is relevant for Ca-poor peraluminous biotite-bearing rocks (metapelites), the biotite dehydration curves obtained are used to discuss the melting reactions and the temperatures that lead to the formation of two distinct types of two-mica granites found in the South Bohemian batholith (specifically the Eisgarn and Deštná granites). The phase relationships were determined experimentally for the composition of these two granites in order to constrain the composition of the biotite in equilibrium with the melt in the protoliths. We demonstrate that Eisgarn granitic melts may have been generated at temperatures in the range 830–850 °C from melting reactions involving biotite with a Mg# up to 0.5 as a reactant. In contrast, Deštná granitic melts cannot have been generated from dehydration melting reactions involving biotite.