Apatite fission track ages associated with the altered igneous intrusive in Beacon sandstone near the base of CRP-3, Victoria Land Basin, Antarctica

The Cape Roberts drillhole 3 (CRP-3) on the western edge of the Victoria Land Basin, Antarctica cored through a highly altered igneous intrusion intruded into Paleozoic sedimentary basement. This intrusion was regarded as highly enigmatic as its origin could represent volcanism associated with early rifting of the Victoria Land Basin, later renewed rifting within the Terror rift, or Jurassic tholeiitic magmatism. Direct methods to date the intrusion by U-Pb dating of zircon or fission track analysis failed due to insufficient quantities of these minerals. Apatite fission track (AFT) analysis on the adjacent sedimentary basement, the Devonian Beacon Supergroup sandstone, yielded an age of 101 ±6 Ma and a mean track length of 12.3 μm with a 1.9 μm standard deviation. The fission tracks were not annealed in the Cenozoic and thus the intrusion must be older than this. This observation, plus trace element chemistry of the intrusion suggest it is most likely the same age and original composition as the middle Jurassic sills and dykes of the Transantarctic Mountains. The AFT age is similar to the onshore regional AFT stratigraphy and reflects complete thermal overprinting in the Jurassic, residence in an apatite partial annealing zone, followed by exhumation in the early Cenozoic and down-faulting at least 3 km to its present position. However, the sample of Beacon sandstone has an AFT age "too young" and a confined track length distribution "too short" relative to the results a sample from an onland equivalent stratigraphic position should yield in the simplest scenario. This is possibly due to the position of the CRP-3 basement on the western edge of the West Antarctic rift system, where it underwent periods of rifting and elevated thermal gradients in the Jurassic, Cretaceous, Eocene and Oligocene causing annealing of the sample. Alternatively, this sample reflects a more complex thermal history involving Cretaceous as well as Cenozoic denudation, prior to being down faulted to its present position.