Melting of vapour-absent tonalite at 10 kbar to simulate dehydration–melting in the deep crust

Abstract

Granitoids may be formed by differentiation of mantle-derived basalts, by partial fusion of crustal rocks, or by interaction of melts from these two sources. Except in special circumstances, the amount of pore fluid in the deep continental crust is likely to be so small that partial melting of rocks will take place under vapour-absent conditions, with water becoming available through dehydration–melting reactions. Here we present the results of experiments in which garnet tonalite is melted at 10 kbar in the absence of vapour. Between 825 and 900 °C the biotite melting reaction produces 20% melt; by 1,000 °C hornblende has melted, yielding 35% melt. As a melt fraction of 30–50% is required before a granodioritic melt can be segregated from its source rock²¹ our results imply that, whereas migmatites may be generated at typical high-grade metamorphic temperatures of 825 °C, the segregation of large-volume granitoid magmas from orthogneisses requires temperatures greater than 950 °C. In the absence of large amounts of externally-derived aqueous fluid, underplating or emplacement of hot basalt into the crust is required to raise the temperature sufficiently for magma segregation.