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.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.