GRANITE FORMATION CONTROLLED BY PHASE EQUILIBRIA IN THE CRUSTAL COLUMN. EXPERIMENTAL PERSPECTIVES FROM EQUILIBRIUM AND FRACTIONAL CRYSTALLISATION (Invited Presentation)

Recent studies on granitoids and thermal migration experiments questioned whether the granite solidus is well defined, yet voluminous granitoids and their effusive counterparts make an essential contribution to the continental crust. We present experimental results designed to better understand how the diversity of granitoid magmas formed and tested fundamental fractionation processes in the deep crust. We discuss results from nominally dry and moderately hydrous experiments designed to understand fractionation processes, with an emphasis on the roots of magmatic arcs by simulating temperature decrease in a stepwise manner. An evaluation of the major element composition indicates that the cumulate line of descent (CLD) of hydrous systems is fundamentally different from dry systems. Cumulates derived from hydrous experiments display elevated CaO and Al₂O₃ contents at low SiO₂, producing voluminous andesitic to rhyolitic liquids, while dry systems follow plagioclase dominated, different fractionation paths, producing much less evolved liquids. The mineralogical and chemical composition of cumulates converges for very different hydrous primary magmas, indicating that fundamental phase equilibria under the conditions prevailing in the roots of magmatic arcs exert a strong control on the compositions of derivative andesitic to rhyolitic liquids. We argue that even if the solidus of granite systems is lower than commonly assumed, the trivial volume produced at temperatures below 650°C is insignificant for the formation of continental crust.