Abstract
The vapor-absent partial melting of biotite-bearing quartzofeldspathic assemblages may play an important role in the petrogenesis of high-temperature granitoid rocks. F-bearing biotite has been reported in high-grade terranes; therefore, melting equilibria involving fluorhydroxy micas may be better models for lower-crustal anatexis than those involving the hydroxy end-member. In order to make an initial assessment of the effect of F on vapor-absent melting, the reaction fluorhydroxy phlogopite + quartz = enstatite + liquid has been investigated experimentally near 8 and 15 kbar, using a synthetic phlogopite of F/(F + OH) = 0.58 (±0.12). Near 15 kbar, melting occurs between 1020 and 1050 °C; at 8 kbar, melting occurs between 950 and 1000 °C. The equilibrium was reversed at l5 kbar by a two-stage experiment between 1100 and 1000 °C, and it was reversed at 8 kbar by a two-stage experiment between 1050 and 950 °C. Reversal criteria included textural evidence for mica regrowth from melt. Results of this study indicate that ~60 mol% substitution of F for OH in phologopite stabilizes the phlogopile + qtrarlz assemblage by as much as 175 °C relative to enstatite and liquid. This significant stabilizing effect indicates that vapor-absent partial melting of rocks containing high fluorine micas may be restricted to temperatures in excess of 950 °C, leaving a residual unmelted micaceous assemblage of higher F content. The stabilizing effect of F in this system is broadly consistent with current models for the petrogenesis of A-type granites. Quantitative analyses of quenched liquid generated from the vapor-absent fluorhydroxy phlogopite + quartz assemblage, however, indicate melts are lamproitic in composition, not granitic.
