Abstract
Apatite (U-Th)/He (AHe) dating of kimberlite pipes provides a unique way to assess the timing and cause of unroofing and elevation change in cratonic interiors. Kimberlites are small volume, volatile-rich magmas emplaced rapidly from mantle depths. By dating kimberlites of differing emplacement ages using AHe thermochronometry the potential exists to more tightly bracket the timing of denudation than by dating Precambrian basement characterized by a more protracted thermal history. Moreover, the chemistry of the kimberlites themselves and the mantle xenolith suites that they contain provide a window into the state of the lithosphere below at the time of kimberlite emplacement. Thus, coupling AHe thermochronometry with mantle xenolith data from kimberlites may allow direct documentation of the surface response to deep-seated changes in the continental lithosphere and the mantle beneath. The southern African plateau currently sits at approximately 1000 m average elevation, but marine sedimentary rocks preserved on the plateau surface show that the continent was at sea level at approximately 255 Ma. The timing, rate, and cause(s) of this surface uplift remain enigmatic. The region was pierced by numerous kimberlites in post-Paleozoic time, including several areas of repeated intrusion. In a pilot study, four kimberlites with emplacement ages ranging from 143 Ma to 74 Ma from a small geographic area straddling the southwestern margin of the Kaapvaal Craton were dated using AHe. The oldest of the pipes, a group II kimberlite, shows a significantly younger AHe date than emplacement age, suggesting significant post-emplacement unroofing. The youngest of the pipes has an AHe date similar to its emplacement age suggesting only limited unroofing since kimberlite intrusion. Thermobarometry on mantle nodules contained within these pipes shows warming of the mantle in this region over the same period, and chemical studies indicate metasomatism and suggest lithospheric thinning. Our results provide evidence for a direct temporal link between the thermochronologically constrained unroofing history, surface uplift, and a change in the southern Africa lithospheric mantle at depth which could be related to other dynamic mantle processes. Together this suggests a deep mantle origin for the surface uplift and the erosional response documented by the thermochronometry data.