Melting of Crustal Rocks During Continental Collision and Subduction

Abstract

The continental crust can partially melt to generate silicic magmas. Compositional diversity among these magmas is determined by the pressure of melting, by the availability of free aqueous fluids, and by the composition of the protolith. Because crustal rocks are subject to extreme pressure and temperature conditions during continental collisions, collisional orogens potentially are environments for magma generation. This chapter discusses the nature of the melts and solid residues likely to be formed in response to continental collision.

In the absence of H₂O-rich fluids, melting of metamorphic rocks is triggered by the breakdown of hydrous minerals. These incongruent dehydration-melting reactions give rise to H₂O-bearing melts and to anhydrous (or less hydrous) solid residues. With the exception of muscovite-rich metapelitic rocks at pressures of less than ~1 GPa, dehydration melting of common crustal rocks requires temperatures in excess of 850°C. Heating of crustal rocks to such temperatures in collisional orogens takes place only if the lithospheric mantle becomes detached from the crust, if the crust is invaded by mantle-derived magmas, or if subduction transports rocks to the upper mantle. Melts generated by dehydration melting of a wide range of quartzofeldspathic rocks at temperatures of 850–1100°C are granitic, and become less ferromagnesian and richer in total alkalis and alumina with increasing pressure. The solid residues are more variable and depend on source composition, but are generally granulitic at P ≤ 1 GPa and eclogitic at P ≥ 2 GPa, with a transitional interval of garnet granulite. In thickened continental crust underlain by a lid of lithospheric mantle, and in slices of continental crust that are buried by subduction and exhumed rapidly, temperatures are unlikely to exceed 800–850°C. Under these conditions only muscovite-rich metapelitic schists can undergo dehydration melting, yielding peraluminous leucogranites. The rather flat dP/dT slope of the muscovite dehydration-melting reaction means that melting most likely takes place at relatively shallow depth (<0.8 GPa), during decompression caused by tectonic exhumation of deep-seated rocks. Deeper melting of any quartzofeldspathic rock in these relatively “cold” environments requires influx of H₂O-rich fluids. Melts formed in this manner are more sodic than the hotter melts formed by dehydration melting, and the residues are rich in micas.