T26A-04 – Feldspar Pb isotope compositions shed light on Archaean terrane dimensions during the Palaeoproterozoic assembly of the Lewisian Complex, Scotland

The identification of lithospheric boundaries within Precambrian gneiss terranes has drawn heavily on a variety of dating and geochemical isotopic tracer methods. For example, the Neoarchaean to Palaeoproterozoic Lewisian Complex of NW Scotland, long regarded as a contiguous Archaean block, was subdivided on geochronological grounds into multiple separate terranes. A simpler “two-plate” model has also been proposed and is based primarily on structural and metamorphic features placed in the wider regional context, i.e. the Scottish Lewisian gneisses are correlated with the major terrane units in Greenland. In this scenario most of the Lewisian gneisses are equivalent to the Rae craton (lower plate) with remnants of the North Atlantic craton (upper plate) structurally lying on top of it. Distinguishing between these alternatives has implication for the thermal evolution of the lithosphere and the dimensions of microplates over time.

We have addressed this issue from a geochemical point of view using the Pb isotope compositions of feldspars to map regional geochemical variations. Feldspars are ideal for this purpose being abundant rock-forming minerals with low U/Pb and Th/Pb ratios and hence with time invariant Pb isotope compositions. The feldspar Pb data (measured by laser ablation MC-ICPMS) distinguish two geochemical terranes supportive of the “two-plate” terrane model. Importantly, assigning sampling sites to a Pb isotopic terrane is generally independent of crosscutting relationships identified in the field as well as the intensity of subsequent tectonism and isotopic re-equilibration.

To further refine the position of the terrane boundaries, the feldspar Pb study is being extended to include Archaean-Palaeoproterozoic gneiss terranes across the North Atlantic region. Results are aimed to identify the key lithospheric-scale structures that were active during and since amalgamation of the supercontinent of Columbia, and to evaluate their coincidence with large mineral deposits.