Experimental interaction of granitic melt and peridotite at 1.5 GPa: Implications for the origin of post-collisional K-rich magmatism in continental subduction zones

Abstract

To decipher the genesis of post-collisional magmatism in continental subduction zones, we conducted a series of experiments on the interaction of granitic melt derived from continental crust with harzburgite at 1.5 GPa and temperatures ranging from 950 °C to 1250 °C. The two end-members were homogeneously mixed with different crustal melt/peridotite ratios and under water-saturated or water-undersaturated conditions. Interaction takes place in two stages: the initial reaction, followed by re-melting of the metasomatized product. In the first stage, crustal melt reacts with the harzburgite, consuming olivine and forming orthopyroxene and phlogopite. In the second stage, re-melting of the metasomatized product is dominated by the incongruent melting of both orthopyroxene and phlogopite, leading to increasing peritectic olivine production as melting progresses. Upon reaction with the peridotite and melting of the metasomatized product, initially granitic melts evolve in composition to trachytic and trachyandesitic. The crustal melt/peridotite ratio does not appear to affect the SiO2 content of melt that equilibrates with olivine and orthopyroxene, whereas the SiO2 content of the melt is higher when olivine is absent from the residue. K2O in the melt is buffered by coexisting phlogopite: this buffering effect of phlogopite weakens at higher crustal melt/peridotite ratio and in water-undersaturated conditions, leading to higher K2O in the melt. Trace element compositions of the hybrid melt generally inherit the characteristics of the crustal melt: most LILEs and transition elements are buffered by phlogopite, with Rb and V having the highest partition coefficients. Our experimental melts show compositional parallels with mantle-derived post-collisional magmas in the Variscan orogen, verifying the involvement of granitic continental crust melt in their source.