The origin of retrograde shear zones in the Napier Complex: implications for the tectonic evolution of Enderby Land, Antarctica

Abstract

Retrograde shear zones (RSZs) in granulites at Fyfe Hills in the Napier Complex, East Antarctica, formed during a Late Proterozoic or Early Palaeozoic reactivation event. These RSZs preserve a structural and metamorphic record of excavation from depth equivalents of 6–8 kbar (20–26 km) to depth equivalents of 3–5 kbar (10–17 km). Excavation was essentially isothermal suggesting that the RSZs formed in an allochthonous terrain during crustal thickening by low-angle thrusting. Rehydration of the RSZs was accompanied, in part, by large-ion lithophile element enrichment and reduction in K/Rb, features which are attributed to: (1) the retrograde fluid having originated in continental crust that had not previously undergone granulite-facies metamorphism; and (2) the fluid having been introduced into the RSZs by the intrusion of granitic dykes and pegmatites. The saturation of these RSZ pegmatites in components characteristically depleted in deep-crustal and mantle sources (such as boron and beryllium) supports a low-grade source for the retrograde fluid. Furthermore, a rehydration mechanism involving intrusion of pegmatites suggests that the Fyfe Hills granulites are underlain by a significantly lower-grade, hydrous terrain.