DEHYDRATION MELTING OF TONALITES .2. COMPOSITION OF MELTS AND SOLIDS

Dehydration melting of tonalitic compositions (phlogopite or biotite-p lagioclase-quartz assemblages) is investigated within a temperature ra nge of 700-1000 degrees C and pressure range of 2-15 kbar. The solid r eaction products in the case of the phlogopite-plagioclase(An(45))-qua rtz starting material are enstatite, clinopyroxene and potassium felds par, with amphiboles occurring occasionally. At 12 kbar, zoisite is ob served below 800 degrees C, and garnet at 900 degrees C. The reaction products of dehydration melting of the biotite (Ann(50))-plagioclase ( An(45))-quartz assemblage are melt, orthopyroxene, clinopyroxene, amph ibole and potassium feldspar. At pressures > s kbar and temperatures b elow 800 degrees C, epidote is also formed. Almandine-rich garnet appe ars above 10 kbar at temperatures greater than or equal to 750 degrees C. The composition of melts is granitic to granodioritic, hence showi ng the importance of dehydration melting of tonalites for the formatio n of granitic melts and granulitic restites at pressure-temperature co nditions within the continental crust. The melt compositions plot clos e to the cotectic line dividing the liquidus surfaces between quartz a nd potassium feldspar in the haplogranite system at 5 kbar and a(H2O) = 1. The composition of the melts changes with the composition of the starting material, temperature and pressure. With increasing temperatu re, the melt becomes enriched in Al2O3 and FeO+MgO. Potash in the melt is highest just when biotite disappears. The amount of CaO decreases up to 900 degrees C at 5 kbar whereas at higher temperatures it increa ses as amphibole, clinopyroxene and more An-component dissolve in the melt. The Na2O content of the melt increases slightly with increase in temperature. The composition of the melt at temperatures > 900 degree s C approaches that of the starting assemblage. The melt fraction vari es with composition and proportion of hydrous phases in the starting c omposition as well as temperature and pressure. With increasing modal biotite from 20 to 30 wt%, the melt proportion increases from 19.8 to 22.3 vol.% (850 degrees C and 5 kbar). With increasing temperature fro m 800 to 950 degrees C (at 5 kbar), the increase in melt fraction is f rom 11 to 25.8 vol.%. The effect of pressure on the melt fraction is o bserved to be relatively small and the melt proportion in the same ass emblage decreases at 850 degrees C from 19.8 vol.% at 5 kbar to 15.3 v ol.% at 15 kbar. Selected experiments were reversed at 2 and 5 kbar to demonstrate that near equilibrium compositions were obtained in runs of longer duration.