Feldspar Pb isotopes as a tool for unveiling East Antarctica’s cratonic framework

Bulk terrestrial Pb isotopic compositions can be modelled to understand the evolution of crust and mantle differentiation episodes through Earth’s history and to the formation of the Earth itself. Deviations from these models are attributed to distinct crustal evolution pathways and are often unique between different tectonic terranes. Initial studies utilised whole-rock media for Pb isotopic analysis, however whole rocks are subject to time-integrated radiogenic ingrowth of Pb from U- and Th-bearing minerals the rock contains. K-feldspar readily incorporates Pb into its crystal lattice but not U or Th, hence the Pb isotopic compositions measured in K-feldspar are time invariant and preserve initial equilibrated values during crystallisation. K-feldspar Pb isotopes are thus a powerful tool for delineating units with fundamentally different geological evolution histories. When mapped, feldspar Pb data can define otherwise obscure terranes and their sutures.

Here we present the first regional feldspar Pb data from the Palaeoproterozoic mountain block of the Shackleton Range in East Antarctica which permits a robust test of pre-determined tectonic suture zones. In particular, the data support a complex tectonic arrangement for the Shackleton Range that is consistent with the stacking of several thin-skin nappes during the Ross Orogeny. Moreover, comparisons of these data with Pb isotope datasets from elsewhere in East Antarctica and contiguous terranes from continents modelled to be previously juxtaposed can assist in testing the role in which the Shackleton Range played during the Nuna-Rodinia supercontinent cycle.

These and ongoing analyses by these authors aim to build a feldspar Pb isotopic map for Antarctica that will help evaluate competing tectonic models for unexposed East Antarctica, and provide a reference against which measurements from glacial and ice rafted transported feldspar grains may be compared and thus help with evaluations on past ice sheet stability.