The Lewisian complex: a typical Precambrian high-grade terrain?

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Abstract
Summary: The main structural, lithological and geochemical features of the Lewisian complex are summarized in relation to available geochronology and an attempt is made to integrate this information into an evolutionary model for the Lewisian from the Archaean through the Proterozoic. The phase of extensive crustal generation which formed the early Lewisian (Scourian) complex occurred at about 2900 Ma and was marked by the production of vast volumes of tonalite which, together with supracrustal components, were affected by strong horizontal thrusting and deformation. The deeper parts of the Lewisian crust were subject to (Badcallian) granulite facies metamorphism culminating at c. 2700 Ma. Major NW-SE shear zones developed shortly after this and resulted in the segmentation of the Lewisian crust and juxtaposition of different crustal levels. This marked a period of quite extensive retrogression of the high-grade gneisses, which continued (with further local shearing) during and after the emplacement of the Scourie dyke suite at c. 2400 Ma. Extension of the Lewisian crust occurred in the southern (Gairloch) area around 2000 Ma and extensive outpourings of mafic volcanics, with associated exhalative mineralization, took place accompanied by sedimentation in the developing basin to form the Loch Maree Group. Igneous activity related to the South Harris igneous complex may have occurred at this period on the Outer Hebrides. Laxfordian deformation and metamorphism began before 1900 Ma and resulted in major reactivation of the earlier shear zones between and within the juxtaposed crustal blocks. More pervasive deformation accompanied the closure of the Loch Maree volcano-sedimentary basin, with major overthrusting and isoclinal folding. Culmination of Laxfordian metamorphic activity occurred at c. 1900 Ma with migmatization and granite injection at Laxford and on Harris, and locally in the southern area. Shear deformation, increasingly more brittle and localized, continued to affect the Lewisian complex until 1400 Ma and probably until 1000 Ma. Interestingly, the main shear zones established at the end of the Archaean crust-forming episode at 2600 Ma continued to be the focus of tectonic, metamorphic and magmatic activity over the following 1500 million years.