Proterozoic Stability of the Kaapvaal Craton from Titanite (U-Th)/He Thermochronology and Strong Influence of Radiation Damage on this Underutilized Thermochronometer
Abstract:The Kaapvaal craton of southern Africa is an archetypal Archean craton that formed and initially stabilized between 3.7 and 2.7 Ga. Geochronology and isotopic studies have constrained periods of lithospheric growth and stabilization, and low temperature thermochronology has yielded information about the Mesozoic-Cenozoic history of burial and erosion across the craton. However, there is a substantial thermal history gap between these end-member events, because few thermochronometers provide access to temperatures of ~300-120°C. Such data are critical for evaluating Kaapvaal’s response to Proterozoic lateral accretion and intracontinental magmatism. Our study assesses cratonic stability by applying a little-utilized but promising mid-temperature thermochronologic technique, titanite (U-Th)/He dating, to decipher cooling through 230-190°C (based on early diffusion studies).
We obtained titanite (U-Th)/He data for Archean basement samples across an ~200,000 km2 area of the northern Kaapvaal craton. Multiple samples with titanite eU values < 70 ppm yield He dates as old as 1200-800 Ma. In contrast, titanites with eU of 70-700 ppm yield younger dates (350-20 Ma) that display a dramatic correlation between date and eU. This pattern clearly manifests the influence of radiation damage on titanite He retentivity that has been observed in other He thermochronometers, but never previously documented for titanite. There is strong future potential to exploit this effect to decipher more detailed thermal histories, as has been done for apatite and zircon He thermochronometry. In our dataset, the oldest titanite results postdate extensive ~1.4-1.2 Ga carbonatite and kimberlite magmatism across the Kaapvaal craton, and overlap with ~1.2-1.1 Ga Namaqua-Natal arc accretion and ~1.1 Ga Umkondo intraplate large igneous province activity. The volcanic character of many of the northern Kaapvaal alkaline and carbonatite complexes indicates that the basement was exhumed to the surface by the time of their emplacement. This constraint, combined with the titanite He dates that record cooling through ~200 °C, implies 7-10 km of regional unroofing at ca. 1.0 Ga. Additional work is underway to determine the spatial extent of this substantial, but previously unidentified, erosional event to better establish its cause.