Quantifying rates of dome-and-keel formation in the Barberton granitoid-greenstone belt, South Africa.

Abstract

The Barberton granitoid-greenstone belt is a classic dome-and-keel province, characterized by kilometerscale gneiss domes and elongate keels of largely folded supracrustal rocks. Combined U–Pb SHRIMP data and structural mapping demonstrate that the geometry of the Barberton belt reflects events that occurred over ∼30 million year interval, from ca. 3230 and 3203 Ma. Early deformation with NW–SE shortening in the upper crust was accompanied by emplacement of tonalite-trondhjemite-granodiorite TTG magmas at 3234±12 and 3226±9 Ma. Much of the structural grain of the greenstone belt relates to a long episode of post-orogenic extension, with NE-directed extension in the lower crust leading to exhumation of high-grade gneisses in the southern Barberton terrane. Advective heat transfer during emplacement of kilometer-scale (TTG) plutons around the margins of the greenstone belt facilitated the infolding of the relatively denser and colder greenstone sequence. The end of this tectonic cycle is punctuated by the emplacement of the undeformed, 3203±7Ma Dalmein pluton, which sharply truncates not only anticlines and synclines in the greenstone belt but also the dominant fabric in the high-grade gneisses.