Feldspar Pb isotopes differentiate Lewisian Complex tectonic models and support an exotic origin for sub-Moinian “Lewisianoid” inliers

Characterisation of terrane boundaries is a key component of palaeogeographic and hence tectonic reconstruction of ancient continental lithosphere and is achieved through various combinations of geophysical, geochronological, structural and isotope geochemical datasets.

In this study feldspar Pb isotopic data have been obtained from the Lewisian Complex (NW Scotland) to test if proposed terrane boundaries can be imaged. The Lewisian Complex gneisses were originally regarded as a contiguous Archaean block that has been variably reworked (deformed and metamorphosed) during the Palaeoproterozoic Laxfordian orogeny. Some authors have subdivided the Lewisian into multiple terranes[1] based on a combination of geochronology and structural constraints. A simpler “two-plate” model has also been proposed[2] , based primarily on structural and metamorphic features and correlation of the Scottish Lewisian with gneiss terranes in Greenland. In the two-plate scenario most of the Lewisian is equivalent to the Rae Craton (lower plate) with remnants of the North Atlantic Craton (upper plate) structurally on top of it. Distinguishing between these alternatives has implications for the thermal evolution of the lithosphere, and the locations of major ore bodies.

The results identify four distinct Pb isotopic domains in the Lewisian Complex of mainland Scotland, Outer Hebrides and offshore west of Shetland and Orkney. Three of these Pb domains correspond to the expected subdivisions of the two-plate model[2] for the Laxfordian orogen, i.e., a pristine Archaean footwall mainly located in the subsurface west of Shetland, extensively reworked Archaean on the Scottish mainland and Outer Hebrides and the variably but generally unreworked high grade Archaean hanging wall. These correspond to the larger-scale Rae Craton, Nagssugtoqidian and North Atlantic Craton terranes of Greenland. Although the multiple-terrane model[1] is not supported, in several cases the mapped Pb isotopic boundaries effectively image known structural breaks and shear zones.

The final feldspar Pb isotopic domain comes from sub-Moinian “Lewisianoid” inliers in the hanging wall of the Moine Thrust (previously considered equivalent to the Lewisian Complex) are consistent with the suggestion [3] that they have an affinity with Baltica.

[1] Kinny, P.D. et al. 2005. J. Geol. Soc. Lond.; [2] Park, R.G. 2022. Scot. J. Geol.; [3] Strachan, R.A. et al. 2020. Geology