Sheared Peridotite and Megacryst Formation Beneath the Kaapvaal Craton: a Snapshot of Tectonomagmatic Processes across the Lithosphere–Asthenosphere Transition

Abstract

AbstractThe cratonic lithosphere–asthenosphere boundary is commonly invoked as the site of sheared peridotite and megacryst formation, a well-recognized petrological assemblage whose genetic relationships—if any—remain poorly understood. We have undertaken a comprehensive petrology and Sr–Nd–Hf–Ca isotope study of sheared peridotite xenoliths and clinopyroxene megacrysts from the c. 1150 Ma Premier kimberlite pipe on the central Kaapvaal craton in South Africa. New textural and mineral trace element evidence suggests that strong tectonic and magmatic overprinting affected the lower cratonic mantle over a vertical distance of ≥50 km from the lithosphere–asthenosphere boundary located at ∼200–225 km depth. Although modification of the central Kaapvaal cratonic mantle is commonly linked to the c. 2056 Ma Bushveld large igneous event, our thermobarometry, mantle redox, and Sr–Nd–Hf–Ca isotope data support a model in which volatile-rich low-volume melts and associated high-density fluids refertilized the lithosphere base shortly before or during asthenosphere-derived kimberlite and carbonatite magmatism at around 1150 Ma. This episode of lithospheric mantle enrichment was facilitated by exceptionally strong shear movements, as are recorded in the plastically deformed peridotites. We argue that stress-driven segregation of percolating carbonated melts contributed to megacryst formation along, or in close proximity to, shear zones within the cratonic mantle lithosphere. Integration of our results from the Kaapvaal craton and modern petrological concepts allows for the identification of a lithosphere–asthenosphere transition zone between ∼150 and 225 km depth. This horizon is defined by intersections of the ∼40–42 mW m–2 Premier paleogeotherm with (1) CO2–H2O-present solidus curves for peridotite (upper bound), and (2) typical mantle adiabats with potential temperatures between 1315 and 1420 °C (lower bound). At Premier, the most strongly deformed sheared peridotites occur mainly between ∼160 and 185 km depth, firmly within the lithosphere–asthenosphere transition zone. Contrary to many previous models, we suggest that sheared peridotite formation occurs in localized deformation zones spaced out across the entire width of the lithosphere–asthenosphere transition zone, rather than being restricted to a single thin layer at the craton base where mantle flow causes viscous drag. Hence, plate-tectonic stresses acting on the lower cratonic lithosphere may be accommodated by extensive networks of shear zones, which provide transient pathways and sinks for percolating volatile-rich melts, linking the formation of megacrysts and sheared peridotites.