Phase relations in albite granite-H2O-HF system and their petrogenetic applications.

Phase relations were experimentally determined in albite granite-H2O-HF system with 2, 4 and 6 wt.% fluorine at 100 MPa pressure. With the increase of fluorine content from 2 to 6 wt.% in the system, liquidus temperature increased from 768 to 790°C and the solidus temperature decreased from 540 to 490°C. Maximum temperature of quartz and topaz stability field increased with F content, whereas that of alkali-feldspar decreased and its stability field disappeared at 6 wt.% fluorine. The mineral assemblage of topaz granite (quartz + albite + K-feldspar + mica + topaz) was stable above solidus at F ≤ 4 wt.%, and that of topaz greisen (quartz + topaz + mica) was stable above solidus at F = 6 wt.%. The experiments showed that the residual melt with high F content can be formed by fractional crystallization of a leucogranitic melt with relatively low F content. These experimental results provide reliable evidences for the formation of topaz granite and topaz greisen at low temperature magmatic conditions.