The Lewisian Complex: insights into deep crustal evolution

Abstract

AbstractThe Lewisian Complex is an Archaean/Proterozoic craton fragment found in NW Scotland and throughout the Outer Hebrides. The 1907 memoir recognized, simply from field relationships and petrographic observation, key features of Lewisian evolution. The bulk of the Lewisian is an old, deformed complex consisting mainly of acid igneous rocks, with some basics, ultrabasics and metasediments. In the Central District of the mainland these are pyroxene bearing (now recognized as granulite facies). The Lewisian Complex was intruded by a suite of basic and ultrabasic dykes which show variable states of later deformation, the intensity of strain being correlated with the development of hornblende schist in the dykes and amphibolite facies assemblages in the country rocks. In the Northern and Southern Districts, this deformation is pervasive and the dykes become concordant hornblende schist sheets. The new foliation with transposed dykes and metasediment sheets is then folded around NW–SE axes. Today there is no single agreed model for the evolution of the complex but an outline is as follows. In the pre-dyke (Scourian) history, subduction led to melting of oceanic crust which provided vast volumes of tonalite-trondhjemite-granodiorite in the period 3100–2700 Ma. Ages show geographic variations but it is not proven whether that implies large displacements between pieces of crust or whether it represents intrusions into other intrusions. The subcontinental lithospheric mantle dates from c. 3000 Ma. K, U and other large ion lithophile elements are depleted in the Central District of the mainland; this is due to depletion in the downgoing oceanic slab which in turn is a result of dehydration prior to melting. Other areas are not depleted in such elements, so various tectonic settings were involved. Remnants of metabasic material in the Lewisian may be relics of oceanic crust. Granulite facies metamorphism with, in places, P>10 kb and T>1000 °C occurred a considerable time after intrusion so is not necessarily linked to igneous events. This ‘Badcallian’ episode affected mainly the Central District and a part of the southern Outer Hebrides; other areas show only amphibolite facies. Zircon dating indicates two high-grade events at 2500 and 2700 Ma. During the ‘Inverian’ episode a series of wide amphibolite-facies shear zones affected the granulite-facies Scourian gneiss prior to the intrusion of the Scourie dykes. The Scourie dykes were intruded from 2400–2000 Ma and are largely quartz tholeiites derived from enriched subcontinental lithospheric mantle; there are some picrites which yield the oldest ages but are also seen to crosscut basic dykes. The dykes trend NW–SE and are steep where not affected by later deformation except where they intrude along, and are controlled by, Inverian fabrics. Post-dyke (Laxfordian) history involves the development of calc-alkaline igneous rocks in the Outer Hebrides and mainland (c. 1900 Ma). Volcanics associated with sediments younger than 2000 Ma comprise an accretionary complex formed in a subduction setting; they are now intercalated between slabs of Archaean basement indicating that the complex was involved in collision with continental crust. Huge strains transposing dykes and country rocks affected almost all of the Outer Hebrides and the mainland except for the Central District. The NW–SE trending lineation indicates the collision direction; the metasediments on the mainland and the South Harris Igneous Complex may mark a folded suture between two continents. Metamorphism was amphibolite facies almost everywhere; in South Harris it was granulite facies at c. 1880 Ma. At 1750–1675 Ma, a distinct event, called late Laxfordian but much younger than earlier Laxfordian metamorphism and with a distinct tectonic setting, caused folding of the previous structures along NW–SE axes, migmatization and renewed amphibolite facies metamorphism.