The geochemistry and geochronology of the gneisses and pegmatites of the Tollie antiform in the Lewisian complex of northwestern Scotland

Abstract

The Tollie antiform occupies a tectonic break between an area to the north, where remnants of Archaean structures are preserved, and an area to the south, characterized by substantial Proterozoic reworking. The antiform produced in this tectonic boundary is a multistage structure containing three components Creag Mhor Thollaidh in the north, faulted against a central zone, which passes southwards into the highly sheared and attenuated Buanichean gneisses of the southwestern limb. The compositions of gneisses in the antiform assign them unambiguously to the Archaean gneisses of the Gruinard and Lochinver areas to the north. Gneisses of this extensive area were brought together within the narrow confines of the Tollie antiform. As the antiform evolved, the Buanichean gneisses acted as an anatectic source to a set of shear-refined potash-rich pegmatites, which penetrated north into the central zone of the developing antiformal structure. In this central zone, some of the pegmatites were deformed into open folds, whereas others were unmodified. The pegmatites thus form a point of reference within the evolution of the Tollie antiform. Rb–Sr isotopic data on the pegmatites give an age for their intrusion and crystallization of 1660 Ma. Downward shear of the southwestern limb of the antiform continued until about 1500 Ma. By the time this episode of structural restyling in the Lewisian had been completed, a deep-rooted wedge of Proterozoic cover sediments had become encased in an Archaean basement.