Ultrahigh-temperature metamorphism in the lower crust during Neoarchean Ventersdorp rifting and magmatism, Kaapvaal Craton, southern Africa

Abstract

A diverse suite of ultrahigh-temperature (UHT) granulite xenoliths from kimberlites of the central Kaapvaal craton (South Africa) record the deep crustal manifestation of Neoarchean Ventersdorp extensional tectonics and magmatism. U-Pb geochronological data for texturally characterized metamorphic zircon and monazite from a number of metasedimentary (garnet + quartz ′ sapphirine ′ sillimanite ′ anti-perthite) UHT granulites are combined with published geothermabarometry to elucidate the pressure-temperature-time (P-T-t) path of metamorphism. Prograde metamorphic growth of zircon and monazite indicates ramping of lower crustal temperatures 225 Ma prior to peak UHT metamorphic conditions at ca. 2720-2715 Ma. The subsequent cooling path is established by dating of 2690 Ma zircon in retrograde kyanite-rich leucosomes, indicating initial cooling rates of ≥10 °C/Ma. Preservation of peak and prograde monazite dates through the UHT conditions support a very high volume diffusion closure temperature for this mineral, similar to that of zircon (>1000 °C). The 5-10 Ma duration of UHT metamorphism in the lower crust is directly correlative with the rapid eruption of the Ventersdorp flood basalts through the central Kaapvaal craton at 2714 ′ 8 Ma, and associated crustal melting, plutonism, and widespread extensional tectonics. UHT metamorphism is postulated to have occurred in response to nonuniform intracratonic lithospheric thinning and superimposed magmatic heat advection. The links between upper and lower crustal manifestations of 2.7 Ga magmatism and extension support the hypothesis that the present-day flat seismic Moho of the central and western Kaapvaal craton is a lower crustal rheological response to Ventersdorp tectonomagmatism. The lithospheric thermal gradient necessitated by the UHT lower crustal granulites demands a transient removal of the thermal lithosphere localized along the north-northeast-trending axis of the Ventersdorp rift at 2.72 Ga. However, contemporaneous maintenance of a coherent mantle keel with thermal gradients 200 km distal to the rift is required in order to preserve ancient (3.2-2.9 Ga) diamondiferous lithosphere. The contrast between the craton-wide geography of crustal extension and the more restricted localization of thermal effects in the lithospheric mantle embodies a profound crust-mantle decoupling in the Kaapvaal craton during intracratonic rifting, and bears upon our understanding of the generation and preservation of cratonic lithospheric mantle keels.