A reappraisal of the Lewisian Gneiss Complex: geochronological evidence for its tectonic assembly from disparate terranes in the Proterozoic

New U-Pb single-zircon geochronology undertaken on tonalitic gneisses, gran ite sheets, migmatites and metasediments from the Lewisian Gneiss Complex o n the mainland and the northern part of the Outer Hebrides, NW Scotland, ha ve been used to test the correlation of so-called Laxfordian events across the complex from the Outer Hebrides to the mainland, and the current model for the evolution of the complex as a whole. The study has revealed that th e granite sheets originated in two quite different melting events. Those on the mainland at Loch Laxford are ca. 1,855 Ma old whereas those on Harris and Lewis, with which they are presently correlated, are ca. 1,675 Ma old. Grey gneisses associated with granites on the south side of Loch Laxford ar e confirmed to belong to the 'northern region'. A migmatitic grey gneiss on Harris has given a protolith age of ca. 3,125 Ma, the currently oldest rec ognised in the complex. Detrital zircons in the Leverburgh and Langavat bel ts range in age from 2,780 to 1,880 Ma and unequivocally demonstrate deposi tion in the Palaeoproterozoic. The granulite facies metamorphism in this bl ock is dated from zircon overgrowths at ca. 1,880 Ma. The Laxford Shear Zon e which separates the northern and central regions is interpreted to have e volved post-1,860 Ma, during amphibolite facies metamorphism accompanying d eformation which took place at ca. 1,740 Ma in both regions. On Harris, the Langavat-Finsbay shear zone developed after 1,675 Ma when a ca. 1,880-Ma g ranulite facies Proterozoic arc was juxtaposed against amphibolite facies A rchaean rocks to the north. Therefore, the shear zones which bound tectonic blocks in the Lewisian Complex evolved at different times and can be inter preted as terrane boundaries. The new data confirm that the Lewisian Comple x was not constructed from one contiguous piece of Archaean crust reworked in the Proterozoic but was progressively assembled from several discrete te rranes during the Proterozoic. Accordingly, the former regional divisions o f the Lewisian Complex are here renamed as follows. On the mainland, the no rthern region is called the Rhiconich terrane, and the central region the A ssynt terrane. On the Outer Hebrides, the Archaean gneisses of Lewis and th e northern part or Harris comprise the Tarbert terrane, whereas the newly a ccreted Proterozoic blocks are called the Roineablial terrane in Harris and the Niss terrane in the north on Lewis. Wider correlations show that the g eology of the Outer Hebrides has more in common with East Greenland than ma inland Scotland on the eastern side of the Minch Fault.