Mafic dyke remnants in the Lewisian Complex of the Outer Hebrides, NW scotland: a geochemical record of continental break-up and re-assembly

Abstract

In the predominantly late Archaean Lewisian Complex of NW Scotland, the Palaeoproterozoic (2.4–2.0 Ga) Scourie dykes have been used to discriminate between late Archaean events, and ca.1.7 Ga Laxfordian reworking. On the Outer Hebrides metabasite dykes intrude the Lewisian Complex, and were previously assumed to be correlatives of the Scourie dykes of the Scottish mainland. New geochemical data allow the Outer Hebrides metabasites to be divided into two groups. Dykes intruded into Archaean Gneisses north and south of the ca. 1.9 Ga South Harris Complex (SHC) are identical, and have depleted geochemical signatures. They are linked with an early episode of rifting (ca. 2 Ga) related to the fragmentation of the Lewisian crust and eventual formation of ocean crust. Later subduction of this ocean crust generated the ca. 1.9 Ga arc related rocks of the SHC, which contain a second group of subduction-related metabasites that are different from the dykes in the adjacent Archaean gneisses. Closure of this ocean and subsequent Laxfordian continent-arc-continent collision at ca. 1.7 Ga, sutured the rifted Archaean fragments back together. The northeast boundary of the SHC has previously been interpreted as a terrane boundary. However, the new dyke data demonstrates that any terrane boundary actually lies within the SHC. The northeast segment of the SHC, the Langavat Belt, contains dykes chemically identical to those intruding the Archaean gneisses to the northeast. The southwest segment of the SHC, the Harris Granulite Belt, contains arc related plutons and metabasites with arc signatures, which are distinct from mafic dykes in the adjacent Archaean gneisses and Langavat Belt. Consequently, much of the Langavat Belt is unrelated to the arc complex. Data from the Outer Hebrides dykes casts doubt on their correlation with the Scourie dykes, and demonstrate that the Outer Hebrides dykes have a closer affinity with the Palaeoproterozoic Kangâmiut dykes of Greenland, than to the Scourie dykes.