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 uncleformed. Major and trace element analyses indicate that the dikes are tholeiitic basalts, similar in composition to midocean ridge basalt. Thermobarometry finds conditions of 750-850øC, 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. 15,717 15,718 WILLIAMS ET AL.: TONALITE GENERATION FROM AMPHIBOLITE is to describe the structural and tectonic setting of the migmatized dikes and to illustrate, on a variety of scales, the textures and fabrics which resulted from melting and melt segregation. Geologic Background The Striding-Athabasca mylonite zone [Hanmer and Kopf, 1993; Hanmer et al., 1994; Hanmer, 1995] is a 400-km-long segment of the Snowbird tectonic zone, a regional northeasttrending linear anomaly in the horizontal gravity gradient map of the Canadian Shield [Goodacre et al., 1987]. The Snowbird zone has been interpreted as the boundary, possibly a suture, between the Rae and Hearne Archean crustal provinces [Hoffman, 1988]. The Striding-Athabasca mylonite zone is particularly well exposed in the East Athabasca Mylonite Triangle (EAMT), at the eastern end of Lake Athabasca, Saskatchewan. There, a 40 km-wide zone of anastomosing granulite facies mylonites is separated from the Rae (northwest) and Hearne (southeast) province wall rocks by narrow greenschist facies shear zones (Figure 1) [Hanmer, 1995; Hanmer et al., 1992, 1994]. The EAMT can be divided into three structural domains (Figure 1). All three have shallowly plunging, northeast trending ...

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The geophysically defined Snowbird tectonic zone is manifested in northernmost Saskatchewan as a deep-crustal, multistage mylonitic structure, the East Athabasca mylonite triangle. The triangle, located at the northeastern apex of a stiff, crustal-scale "lozenge," is composed of mid-Archean annealed mylonites and late Archean ribbon mylonites, formed during two granulite facies events (850–1000 °C, 1.0 GPa). The flow pattern in the mylonites is geometrically and kinematically complex, and corresponds to that expected adjacent to the apex of a stiff elliptical volume subjected to subhorizontal regional extension parallel to its principal axis. The late Archean mylonites are divided into an upper structural deck, entirely occupied by a dip-slip shear zone, and an underlying lower deck. The latter is divided into two upright conjugate strike-slip shear zones, separated by a low-strain septum, which deformed by progressive coaxial flow. The flow pattern in the mid-Archean mylonites is compatible with that of the late Archean mylonites, and suggests that the crustal-scale lozenge influenced deformation since the mid-Archean. In the interval ca. 2.62–2.60 Ga, deformation in the upper and lower decks evolved from a granulite facies pervasive regime to a more localized amphibolite facies regime. With further cooling, deformation was localized within very narrow greenschist mylonitic faults at the lateral limits of the lower deck. By the late Archean, the East Athabasca mylonite triangle was part of a deep-crustal, intracontinental shear zone. This segment of the Snowbird tectonic zone was not the site of an Early Proterozoic suture or orogen.

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