Palaeoproterozoic evolution of the Lewisian complex of the Outer Hebrides, Northwest Scotland : the South Harris complex, a possible accreted island arc?

The Palaeoproterozoic evolution of the central Outer Hebrides can be explained by modern plate-tectonic processes and is consistent with the products of a Wilson cycle. Local continental crust comprising c. 2.8 Ga tonalitic orthogneiss underwent extension and rifting leading to the deposition of clastic sediments derived from a late Archaean source, and was followed by the emplacement of c. 2 Ga mafic dykes. The latter are probably not directly equivalent to the Scourie dykes of the Scottish mainland. Rupturing and the formation of an ocean basin followed, and this was subsequently subducted around 1.9 Ga leading to the development of a volcanic arc, which now forms much, but not all of the South Harris Complex. This arc was founded on continental crust formed in part by the c. 2.5 Ga South Harris anorthosite. During subsequent collision events the late Archaean tonalitic gneisses and enclosed dykes were sutured back together trapping arc and probable ophiolitic material in the process. The latter is concentrated in a major imbricate zone lying immediately NE of the arc rocks, and formed by the Langavat Belt. In addition to probable ophiolitic material, this zone includes Archaean-derived metasediments, and slivers of Archaean basement gneiss. This represents the most probable location of the major suture zone associated with the reassembly of the Archaean gneisses. Suturing was followed by the initiation of kilometre-scale post-collisional shear zones. One of these, the Langavat Shear Zone was initiated prior to -1740 Ma, and exploits the Langavat Belt imbricate zone. The evolution of the Lewisian Complex of the Outer Hebrides is closely comparable to the Nagssugtoqidian orogen of southern Greenland, with which it is correlated. In fact, the geology of the Outer Hebrides bears a closer affinity with the Nagssugtoqidian belt than it does to the Lewisian rocks of the Scottish mainland.