DEHYDRATION MELTING OF TONALITES .1. BEGINNING OF MELTING

The beginning of dehydration melting in the tonalite system (biotite-p lagioclase-quartz) is investigated in the pressure range of 2-12 kbar. A special method consisting of surrounding a crystal of natural plagi oclase (An(45)) with a biotite-quartz mixture, and observing reactions at the plagioclase margin was employed for precise determination of t he solidus for dehydration melting. The beginning of dehydration melti ng was worked out at 5 kbar for a range of compositions of biotite var ying from iron-free phlogopite to iron-rich Ann(70), with and without titanium, fluorine and extra aluminium in the biotite. The dehydration melting of phlogopite + plagioclase (An(45)) + quartz begins between 750 and 770 degrees C at pressures of 2 and 5 kbar, at approximately 7 40 degrees C at 8 kbar and between 700 and 730 degrees C at 10 kbar. A t 12 kbar, the first melts are observed at temperatures as low as 700 degrees C. The data indicate an almost vertical dehydration melting so lidus curve at low pressures which bends backward to lower temperature s at higher pressures (> 5 kbar). The new phases observed at pressures less than or equal to 10 kbar are melt + enstatite + clinopyroxene potassium feldspar +/- amphibole. In addition to these, zoisite was al so observed at 12 kbar. With increasing temperature, phlogopite become s enriched in aluminium and deficient in potassium. Substitution of oc tahedral magnesium by aluminium and titanium in the phlogopite, as wel l as substitution of hydroxyl by fluorine, have little effect on the b eginning of dehydration melting temperatures in this system. The dehyd ration melting of biotite (Ann(50)) + plagioclase (An(45)) + quartz be gins 50 degrees C below that of phlogopite bearing starting compositio n. Solid reaction products are orthopyroxene + clinopyroxene + potassi um feldspar +/- amphibole. Epidote was also observed above 8 kbar, and garnet at 12 kbar (750 degrees C). The experiments on the iron-bearin g system performed at less than or equal to 5 kbar were buffered with NiNiO. The f(O2) in high pressure runs lies close to CoCoO. With the s ubstitution of octahedral magnesium and iron by aluminium and titanium , and replacement of hydroxyl by fluorine in biotite, the beginning of dehydration melting temperatures in this system increase up to 780 de grees C at 5 kbar, which is 70 degrees C above the beginning of dehydr ation melting of the assemblage containing biotite (Ann(50)) of ideal composition. Thr dehydration melting at 5 kbar in the more iron-rich A nn(70)-bearing starting composition begins at 730 degrees C, and in th e Ann(25)-bearing assemblage at 710 degrees C. This indicates that qua rtz-biotite-plagioclase assemblages with intermediate compositions of biotite (Ann(25) and Ann(50)) melt at lower temperatures as compared t o those containing Fe-richer or Mg-richer biotites. This study shows t hat the dehydration melting of tonalites may begin at considerably low er temperatures than previously thought, especially at high pressures (>5 kbar).