GENESIS OF PERALUMINOUS GRANITES .1. EXPERIMENTAL INVESTIGATION OF MELT COMPOSITIONS AT 3 AND 5 KB AND VARIOUS H2O ACTIVITIES

Melting experiments have been performed on a peraluminous quartzo-feldspathic gneiss from Northern Portugal. This gneiss occurs as xenoliths in the Tourem anatectic complex and is the most probable source rock for the surrounding anatectic granites. On the basis of field, petrological, and geochemical data, it can be shown that anatexis took place in the stability field of cordierite + biotite and that the evolution of magmas is the result of processes involving segregation of partial melt and separation of restite minerals. Experiments were performed at 700, 750, and 800-degrees-C, 3 and 5 kb, and various H2O activities (aH2O) to clarify the influence of aH2O and melt fraction on the composition of the generated melts. Biotite and cordierite are the two main ferromagnesian phases observed in the run products. Cordierite is formed by incongruent melting of biotite. For relatively low melt fractions (< 30-35 wt.%), the partial melts coexisting with quartz, alkali feldspar, and plagioclase have a minimum or near-minimum melt composition. The melts become richer in potassium with decreasing aH2O. This result using a natural rock as starting material is in good agreement with results achieved in the synthetic Qz-Ab-Or system. In the stability field of biotite and cordierite, the influence of aH2O on melt composition is at least as important as the effect of changing P and/or T. For higher melt fractions the composition of the melt is strongly controlled by the disappearance of alkali feldspar and the melts become richer in An and poorer in Or with increasing degree of melting. The wide range of compositions (especially for K2O, which varies from 3.5 to 5.4%) observed in the experimental peraluminous melts demonstrates that a wide variety of granitoid magmas may be produced from the same source rocks. The application of the experimental results to the Tourem anatectic complex shows that melting occurred at H2O-undersaturated conditions (4-5 wt.% H2O in the melts, corresponding to aH2O of approximately 0.5 at 5 kb). Experimental melts similar in composition to the most leucocratic granite of the Tourem anatectic complex (considered to approximate the composition of the generated melt) were obtained around 800-degrees-C, suggesting that this temperature was attained during the peak of anatexis.