What do experiments tell us about the relative contributions of crust and mantle to the origin of granitic magmas?

TLDR

Granite formation is studied experimentally as melting of metamorphic rocks and as reactions between basaltic magmas and supracrustal rocks, with peraluminous leucogranites representing pure crustal melts and all other types being hybrid magmas. Peraluminous leucogranites arise from dehydration‑melting of muscovite‑rich metasediments during rapid adiabatic decompression, while hybrid magmas and associated mafic cumulates form through simultaneous mantle‑crust interactions. The variation in granite types reflects differences in source material and pressure of mantle‑crust interaction, correlates with tectonic setting, and is accompanied by cumulate assemblages that vary with pressure and contribute to lower continental crust, indicating that granitic magma generation is largely linked to crustal growth rather than recycling.

Abstract

Abstract The origin of different kinds of granitic rocks is examined within the framework of experimental studies of melting of metamorphic rocks, and of reaction between basaltic magmas and metamorphic rocks. Among the types of granitic rocks considered in this chapter, only peraluminous leucogranites represent pure crustal melts. They form by dehydration-melting of muscovite-rich metasediments, most likely during the fast adiabatic decompression that results from tectonic collapse of thickened intracontinental orogenic belts. All other granitic rocks discussed here represent hybrid magmas, formed by reaction of basaltic melts with metamorphic rocks of supracrustal origin. These hybrid rocks include Cordilleran granites, formed at or near convergent continental margins, strongly peraluminous ‘S-type’ granites, alumina-deficient ‘A-type’ granites, and rhyolites associated with continental flood basalts. The differences among these types of granites reflect differences both in their source materials and in the pressures at which mantle-crust interactions take place. In turn, these variables are correlated with the tectonic settings in which the magmas form. Hybrid mafic cumulates are also produced by mantle-crust interactions, simultaneously with the granitic melts. These cumulates range from orthopyroxene + plagioclase-rich assemblages at low pressure to clinopyroxene + garnet-rich assemblages at high pressure, and are known to be important constituents of the lower continental crust. With the exception of peraluminous leucogranites, generation of granitic magmas is almost always associated in space and time with growth, rather than just recycling, of the continental crust.