U-Pb geochronology of Lewisian orthogneisses in the Outer Hebrides, Scotland: implications for the tectonic setting and correlation of the South Harris Complex

摘要

The South Harris Complex of the Scottish Outer Hebrides forms a distinctive component of the Lewisian Complex and contains three supposedly co-magmatic meta-igneous bodies, a diorite, a ‘norite’ and an anorthosite, believed to be part of a Palaeoproterozoic arc. New U–Pb zircon ages obtained from these bodies of 1888 ± 2 Ma, 1890 +2/–1 Ma, and 2491 +31/–27 Ma, respectively, demonstrate that the anorthosite cannot be related to the younger calc-alkaline diorite and ‘norite’. The diorite and ‘norite’, however, probably are related products of arc magmatism. The spatial association of the diorite and ‘norite’ with the substantially older anorthosite suggests that the younger intrusions were emplaced into continental crust. Any suture associated with the South Harris Complex remains cryptic, and is not marked by an obvious change in protolith ages in the adjacent tonalitic gneisses.