The Lewisian complex or Lewisian gneiss is a suite of Precambrian metamorphic rocks that outcrop in the northwestern part of Scotland, forming part of the Hebridean Terrane and the North Atlantic Craton. The oldest protoliths of the Lewisian Gneiss Complex may span the Paleo-, Meso- and Neo-Archean and as such, provide evidence for geological processes operating during early Earth history, e.g., plate tectonics.
By 3,100 million years ago, it is thought that the processes of plate tectonics had begun to operate, and so continents moved slowly across the surface of the globe, colliding with each other to form mountain ranges or splitting apart to produce new oceans. The surface of the Earth would have looked very different to today; the only life-forms were simple clusters of cells floating in the oceans, and there was little or no oxygen in the atmosphere. The Lewisian Gneiss Complex is composed of Archean gneisses which have been variably reworked during the Proterozoic and has proven to be a key region in the study of crustal growth and continental tectonics.
Although the Lewisian Gneiss Complex has been the subject of geological research for over a century, beginning with Peach et al. (1907), many aspects of its geological evolution remain controversial. Indeed, some of these outstanding controversies are likely a legacy of the long history of research in this area in that many isotope studies were under-taken when techniques were still in their infancy and have since either been improved upon or disregarded as obsolete (e.g.Giletti et al., 1961;Pidgeon and Aftalion, 1972). Among the key points still disputed are: (1) protolith ages of the gneisses, (2) age of granulite metamorphismin the Assynt block and (3) relative affinities of different fault bounded blocks.
The refractory nature of zircon means it often records and preserves relic age signatures, even when the original zircon has been subjected to burial deep in the Earth’s crust and has experienced extreme metamorphic conditions. Although such information is useful for re-constructing geological histories of complex terranes, age determinations on zircon displaying multi-age domains are not without their analytical challenges. Zircon from the Lewisian Gneiss Complex often display extremely complex zonation in CL images (e.g. Corfu et al., 1998; Goodenough et al., 2013) and it is difficult to further our understanding of evolution of the Lewisian Gneiss Complex, and some aspects of global Archean and Proterozoic tectonic processes, using conventional TIMS or micro-beam dating methods. A technique integrating some form of spatial resolution with the high analytical precision of TIMS may resolve outstanding issues for this enigmatic and classic area of geology.