The South Barra shear zone: a composite Inverian-Laxfordian shear zone and possible Terrane boundary in the Lewisian gneiss complex of the Isle of Barra, NW Scotland

The Lewisian Gneiss Complex (LGC) of NW Scotland largely comprises Archaean tonalite–trondhjemite–granodiorite (TTG) gneisses variably reworked in the Palaeoproterozoic. One form of this heterogeneous reworking is the formation of major shear zones in the earliest Palaeoproterozoic which are then reactivated in the late Palaeoproterozoic. Recent remapping of the LGC of SE Barra, in the southern Outer Hebrides, has led to the interpretation of such a composite shear zone, the Southeast Barra Shear Zone (SBSZ). The SBSZ separates a block of heterogeneously overprinted pyroxene-bearing TTG gneisses from a dioritic meta-igneous complex to the west. Despite a lack of geochronological data, the juxtaposition of different protolith rocks along the SBSZ raises the possibility it may be a previously unrecognized terrane boundary.