Mutual Hybridization and Complex Zircon Populations in Magmatic Arcs Caused by Magma Exchange between Two Mushes

Abstract Magma hybridization, a common process in magmatic arcs, is difficult to distinguish from other magmatic processes (e.g. fractional crystallization) because of the geochemical similarity between their products. This study uncovers mutual hybridization between two mushes in the Gangdese arc, southern Tibet: a crystallizing hornblendite dyke developing a cumulate texture, and a migmatitic tonalite host rock undergoing melting. The contact between these two mushes is diffuse and accompanied by leucosomes that cross the contact suggesting exchange of magmas. The hornblendite dyke is part of a large pluton emplaced in the lower crust (~12 kbar). Mineral composition indicates that the hornblende crystalized from a parental magma of granitic composition (SiO2 ~ 70 wt %) with high H2O content (~10 wt %). Mass balance calculation indicates that a substantial proportion of evolved melt (~50–70 wt %) was extracted from the hornblendite. Zircons from the hornblendite and its leucosomes define a prolonged growth history (96–83 Ma) and the rock captured zircon xenocrysts (177–164 Ma) from the older migmatitic tonalite host rock. This migmatitic host rock contains garnet and hornblende, interpreted to be peritectic phases in and around leucosomes, with older zircon ages at 178–152 Ma recording its original crystallization, and younger ages of 98–85 Ma recording remelting at the time of hornblendite crystallization. These features suggest that the tonalite underwent H2O-fluxed melting induced by the intrusion of the pluton and its dykes. The ca. 90 Ma hornblendite zircons have trace element signatures indicative of the influence of hornblende-zircon-apatite-titanite fractionation. However, zircons in its leucosome have trace elements indicative of the increasing influence of garnet, a mineral that is present only in the tonalite host rock. The same trace element patterns are also found in the contemporaneous anatectic zircons in the tonalite and its leucosomes. Considering: (1) the loss of residual melt in the hornblendite cumulate, (2) the seamless leucosome network crossing the contact between both rocks, (3) the transfer of ca. 170 Ma zircon xenocrysts from tonalite to hornblendite dyke, and (4) the shared age and chemistry of zircons in the leucosome network connecting both rocks, we argue that this was an open system where the two mushes developed an interconnected leucosome network, exchanging interstitial melt and zircons. The loss of residual melt and gain of exotic magma, shifted the composition of the hornblendite away from the fractional crystallization trend of arcs magmas. This two-way exchange between mushes could explain large-scale hybridization that influences the diversity of arc magmas and contributes to the formation of complex zircon populations in magmatic rocks.