Establishing terrain boundaries within Archaean lithospheric units is challenging because, like those that form the Lewisian Complex in Scotland, they commonly comprise rather uniform-looking gneisses. Identifying terrane boundaries has, therefore, leant heavily on various isotopic dating techniques and geochemical tracer methods. 

The geological evolution of the Lewisian Complex of northwest Scotland is much debated; there are conflicting models for its formation and the number, character, and location of terrane boundaries within it. Originally regarded as a continuous unit, it was subsequently subdivided into multiple terranes based on in situ U–Pb zircon geochronology (Friend and Kinny 1995; Kinny et al. 2005). A radically different two-plate model has also been suggested (Park 2005, 2022), primarily based on correlating structural and metamorphic features with major tectonic units in Greenland. 

In an attempt to reconcile the conflicting models for the Lewisian Complex, we have paired field observations with feldspar Pb isotopic compositions in order to build a geochemical map for the complex. Feldspars, both K-feldspar and plagioclase, are abundant rock-forming minerals with low U/Pb and Th/Pb ratios and, hence, have time-invariant Pb isotope compositions. We systematically collected nearly 160 onshore samples to cover all the major outcropping regions. Also, we included 15 drilled cores from offshore sites of the Hebridean Shelf to test previously established terrain models. Our results are most consistent with a two-plate model, where the Assynt, Gruinard, and East Barra regions likely correlate with the North Atlantic Craton of Greenland. The rest of the onshore Lewisian Complex feldspar Pb isotopic data, together with the southernmost offshore borehole data, strongly overlap with compositions obtained from the Nagssugtoqidian terrane in southeast Greenland. The remaining offshore samples, situated north of the Lewisian outcrop, have a very different and distinct Pb isotopic signature and may correlate to a third terrane, likely the Rae Craton of Greenland.

​BIOGRAPHY Eszter Badenszki is a postdoc researcher in the iCRAG geochemistry platform. Eszter obtained her M.Sc at Eötvös Loránd University (Budapest, Hungary) and her Ph.D. from UCD, working with Prof. Stephen Daly. Eszter’s PhD research focused on the age and development of the deep crust of the Scottish Midland Valley. As an SFI-funded postdoc at UCD, she has been working on lower crustal xenoliths from central Ireland, especially their possible involvement in base metal mineralization. Eszter joined iCRAG in June 2016 where she is responsible for laser-ablation techniques and developing new methods for isotopic analysis.