Syntectonic generation and segregation of tonalitic melts from amphibolite dikes in the lower crust, Striding‐Athabasca mylonite zone, northern Saskatchewan

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Abstract
Vapor‐absent melting in a swarm of amphibolite dikes in the lower crust has produced segregations of tonalitic and trondhjemitic composition in a variety of textures and structures that dramatically illustrate the mechanisms of melt segregation during deformation. The 3.2 Ga Chipman dikes intrude the mylonitized Chipman tonalite within the Striding‐Athabasca mylonite zone of northern Saskatchewan. Dike emplacement spans a major sinistral transpressive ductile deformation and granulite facies metamorphic event. Older dikes are intensely folded and sheared; younger dikes are undeformed. Major and trace element analyses indicate that the dikes are tholeiitic basalts, similar in composition to midocean ridge basalt. Thermobarometry finds conditions of 750–850°C, 1.0 GPa. The youngest, most pristine dikes contain hornblende and plagioclase with minor clinopyroxene and garnet. Older, migmatitic dikes have tonalitic to trondhjemitic segregations spatially associated with garnet crystals. Where small, the segregations occupy tails or strain shadows on every garnet crystal. Where garnets and segregations are large, leucosomes form an interconnected network that extends into the host tonalite. Tonalitic pools, probably of dike origin, collect in boudin necks and fold hinges. The Chipman dikes are interpreted to have been emplaced, solidified, and partially melted during ductile shearing in the lowermost crust, perhaps near the base of an Archean island arc. They appear to offer an exceptional view of magma genesis where underplated mantle‐derived mafic magmas provide not only heat but also a component of material to new felsic magmas. Deformation is essential to the process, allowing access of mafic magma into the lower crust and facilitating the segregation of the new felsic melts from their source rocks.