Role of K-feldspar During Partial Melting in the Continental Crust

Jagmohan Singh Mineral Physics Lab., G.N.D.University, Amritsar 143005, India

cse @

W. Johannes Institut für Mineralogie, Universität Hannover, 30167 Hannover, Germany)

K-feldspar is an important breakdown product of partial melting of biotite bearing assemblages. During experiments on dehydration melting of tonalites (phlogopite/biotite-plagioclase-quartz assemblages) within temerature range of 700-950°C and pressure range of 2-15 kbar, K-feldspar was observed to form at the initial breakdown of biotite. The other phases observed include orthopyroxene, clinopyroxene, amphiboles and melt of granitic to granodioritic composition.

The dehydration melting in the phlogopite-plagioclase (An45)-quartz assemblage begins at 760°C at pressures below 8 kbar, whereas at higher pressures the melting begins at temperatures as low as 700šC. The K-feldspar persists for a temperature interval of nearly 50°C below 8 kbar, but this temperature interval increases upto 200°C at 12 kbar, whereas phlogopite is still present. In case of biotite (Ann50)-plagioclase (An45)-quartz starting composition, the dehydration melting solidus lies at 710°C at pressures ¾ 5 kbar, and at 690°C or even lower at higher pressures. As the melting progressed in this composition, the K-feldspar persists upto approximately 825°C at P ¾ 8 kbar and the K-feldspar out curve extends upto 900°C at higher
pressures. The biotite-out curve in this assemblage lies 10-40°C lower than the K-feldspar ouut curve.

The H2O content of the melt decreases with temperature, whereas the K2O content increases. The K2O content of the melt is highest when the biotite breaks down. The recrystallized biotite shows a consequent decrease in K2O. This study shows that the formation of K-feldspar as a breakdown product of the biotite bearing assemblages is controlled by composition of the starting biotite, pressure temperature conditions and the composition of the melt and coexisting biotite.