The crustal evolution of Central Scotland and the nature of the lower crust: Pb, Nd and Sr isotope evidence from Caledonian granites

Abstract

The identification of the wide range of isotope characteristics and chemical heterogeneities within the sub-continental mantle has demanded a re-appraisal of the crustal evolution of many terrains. A recent publication has suggested that such heterogeneities may also exist beneath Scotland [1]. This, together with inadequate knowledge of the age and geochemical nature of the lower crust has fuelled the controversy of the relative involvement of mantle and crust in the formation of the Caledonian granites [2–8]. This paper reports 31 Pb isotope analyses, on feldspars from six granite complexes, and four new Nd and four new Sr whole-rock analyses. These data demonstrate that a major source of the granites is the lower crust which is characterised by relatively unradiogenic Sr, Pb and Nd. It is, by inference, old, depleted in U and Th, and has a low |Th/U ratio. Isotopic constraints suggest the sub-Grampian (Central Scottish) lower crust is dominated by Grenville (ca. 1200-1000 Ma) age rocks in addition to an older Scourian (ca. 2700 Ma) component. The major episodes of crustal growth in Central Scotland occurred during the Scourian and Grenville orogenies. The Caledonian orogeny produced only limited crustal addition.