Mapping ancient geochemical terranes using feldspar Pb isotopes

Combined with geophysical, geochronological and structural data, isotope geochemical proxies (including Pb, Nd, Hf and Sr) are commonly used to correlate basement terranes and hence to test and refine supercontinent configurations. In the case of Pb, many such studies rely on whole-rock isotopic data, which being time-dependent, can be difficult to interpret.

By contrast, feldspars, by virtue of their low U/Pb and Th/Pb ratios, have time invariant Pb isotope compositions. They are also modally abundant rock-forming minerals and are amenable to in situ analysis and textural discrimination. This study evaluates the utility of Pb isotopes in feldspar as a mechanism to identify terrane boundaries and evaluate large-scale continental reconstruction models.

We are systematically collating feldspar Pb isotope data from Archaean and Paleoproterozoic basement rocks in the North Atlantic region (Laurentia-Baltica and the intervening continental plateaux) from literature sources as well as new measurements from outcrop and offshore wellbores from Scotland, Norway, Greenland and Russia.

A particular focus is on the Lewisian Complex of Scotland because these rocks are argued to link with terranes in both Greenland and Baltica. Our Pb isotopic data identify four Pb isotopic domains in Scotland. The NAC, Nagssugtoqidian and Rinkian terranes are recognised onshore and offshore west of Shetland and correlate strongly with Greenland. Moreover the boundaries between them are consistent with large-scale sutures proposed on other grounds although they do not support the traditional nine-terrane model for the Lewisian Complex [1]. Feldspar Pb isotopic data from the Kola Peninsula and northern Fennoscandia are consistent with the suggestion that the sub-Moine inliers (previously considered to have Archaean protoliths, equivalent to the Lewisian Complex) have an affinity with Baltica [2].

References:
[1] Kinny et al. (2005) Journal of the Geological Society, London 162, 175-186.
[2] Strachan et al. (2020) Geology 48, 1094-1098.