FLUID ABSENT MELTING OF A LAYERED CRUSTAL PROTOLITH - IMPLICATIONS FOR THE GENERATION OF ANATECTIC GRANITES

We report the result of H2O-undersaturated melting experiments on char ges consisting of a layer of powdered sillimanite-bearing metapelite ( HQ36) and a layer of powdered tonalitic gneiss (AGC150). Experiments w ere conducted at 10 kbar at 900-degrees, 925-degrees and 950-degrees-C . When run alone, the pelite yielded approximately 40 vol% strongly pe raluminous granitic melt at 900-degrees-C while the tonalite produced only approximately 5 vol% weakly peraluminous granitic melt. At 950-de grees-C, the pelite and the tonalite yielded approximately 50 vol% and approximately 7 vol% granitic melt, respectively. When run side by si de, the abundance of melt in the tonalite was approximately 10 times h igher at all temperatures than when it was run alone. In the pelite, t he melt abundance increased by approximately 25 vol%. When run alone, biotite dehydration-melting in the tonalite yielded orthopyroxene and garnet in addition to granitic melt. When run side by side only garnet was produced in addition to granitic melt. Experiments of relatively short duration, however, also contained Al-rich orthopyroxene. We sugg est that the large increase in melt fraction in the tonalite is mainly a result of increased activity of Al2O3 in the melt, which lowers the temperature of the biotite dehydration-melting reaction. In the pelit e, the increase in the abundance of melt is caused by transport of pla gioclase component in the melt from the tonalite-layer to the pelite-l ayer. This has the effect of changing the bulk composition of this lay er in the direction of ''minimum-temperature'' granitic liquids. Our r esults show that rocks which are poor melt-producers on their own can become very fertile if they occur in contact with rocks that contain c omponents that destabilize the hydrous phase(s) and facilitate dehydra tion-melting. Because of this effect, the continental crust may have a n even greater potential for granitoid melt production than previously thought. Our results also suggest that many anatectic granites most l ikely contain contributions from two or more different source rocks, w hich will be reflected in their isotopic and geochemical compositions.