The Lewisian terrane model: a review

Abstract

Synopsis The Lewisian terrane model of Friend and Kinny divides the Lewisian complex into nine separate terranes believed to have amalgamated during the Palaeoproterozoic. This paper analyses the rationale for the terrane model and suggests criteria to evaluate and refine it. Similarities and differences in Palaeoproterozoic structural, metamorphic and igneous features are used to divide the complex into 14 separate blocks of three types: type A (upper-plate) characterized by retrogressed high-pressure granulite-facies Archaean metamorphic rocks and localized Laxfordian deformation; type B (lower plate) characterized by amphibolite-facies Archaean metamorphic rocks originating at higher crustal levels, intense Laxfordian deformation, and swarms of Laxfordian granites and pegmatites; and third, Palaeoproterozoic complexes comprising oceanic and volcanic arc elements. The boundaries between the type A and type B blocks, where seen, are major shear zones believed to represent a deformed and disrupted collisional suture between two large continental plates, the central Greenland craton (CGC) to the NE and the North Atlantic craton (NAC) to the SW. The following sequence of events is proposed. The Palaeoproterozoic complexes were accreted to the upper plate (NAC) at c. 1.9 Ga. This was followed by collision with the CGC at c. 1.87 Ga, causing the intense early Laxfordian deformation and high-grade metamorphism. At c. 1.74 Ga a second collision took place with the 1.80 Ga ‘Malin’ volcanic arc to the south, resulting in reactivation and late Laxfordian deformation, producing NW-trending folds, dextral shear zones, granite sheets and pegmatites. These late Laxfordian movements are considered to have resulted in considerable disruption of the early Laxfordian terranes. They include substantial strike-slip movements that may have led to the creation of further displaced terranes, some of which may prove to be exotic.