Experimentelle Gesteinsmetamorphose—IV: Bildung anatektischer Schmelzen aus metamorphisierten Grauwacken

Abstract

Experimental high-grade metamorphism and anatexis of different greywackes has been studied under 2000 atm H 2O-pressure. The high-grade metamorphic mineral assemblage is that of biotitecordierite-plagioclase-alkalifeldspar-quartz-gneisses, with in some cases sillimanite. Quantitatively, the mineral content differs (Table 3) due to the different chemical compositions of the metamorphosed greywackes. If the positions of the points in the ACF-diagram are corrected for the amount of brotite formed, then they give the right ratios of the quantities of the other observed minerals. For most of the investigated rocks an anatectic melt begins to form at 685°C, for others this temperature is higher, up to 725°C. As with metamorphosed clays, the first melt to be formed is an aplitic alkalifeldspar-quartz melt. But with small rise in temperature plagioclase is fractionally dissolved in the melt, together with a certain amount of biotite, cordierite and sillimanite. The composition of the melt changes quickly with rise in temperature from aplitic over granitic to granodioritic and in most cases to tonalitic. A tonalitic or sometimes a granodioritic composition is a final stage in the anatectic process of previous greywackes, because further rise in temperature scarcely changes the composition of the melts. The cotectic formation of tonalitic anateotic melts have been observed which have dissolved more quartz than would be supposed. The amount of melt increases quickly with rising temperature, but at about 780°C a maximum is reached. In most cases over 70 up to 96 per cent of the metamorphosed greywackes have been liquified; if the metamorphite contained especially high quantities of quartz and/or An-rich plagioclase then the amount of melt is smaller, but nevertheless it still comprises two-thirds of the anatectic rock complex. If separated from the crystalline remainder it can intrude and form tonalites or granodiorites, respectively. If not separated far from the place of formation it may represent the “granitoid” portion of migmatites. Results are given for the solidus-liquidus relations of plagioclases in complex anatectic systems; these are compared with those in the system Ab-An-H 2O.