Evidence for early structures in xenoliths in the South Harris anorthosite, Outer Hebrides

Abstract

Since the early work of Jehu and Craig (1927) the metasedimentary gneisses of the Leverburgh belt and the intrusive plutonic rocks of the South Harris igneous complex have been the subject of many geological studies (for example, Dearnley 1963; Coward et al. 1969; Wood 1975; Graham 1980). The establishment of the age of the South Harris igneous complex anorthosite component at 2180 ± 60 Ma, and an age of 1870 ± 30 Ma for a subsequent granulite facies event (Cliff et al. 1983) has helped to resolve two of the debates over the geological evolution of the area. First, the correlation with the Scourie dykes of basic bodies cutting the complex (Dearnley 1963; Witty 1975; Dickinson and Watson 1976; Heyes 1978; Horsley 1978). Second, the debate over the timing and number of granulite facies events. Dearnley (1963) suggests that evidence for an early granulite facies event (Scourian) was preserved in the pyroxene granulite (norite) bordering the Sound of Harris (Fig. 1). However, Palmer (1971) and Graham (1980) consider that the norite forms a part of the South Harris igneous complex, in which case the granulite facies assemblages would thus be Laxfordian. An important component of the Leverburgh belt are units comprising highly aluminous quartz-rich metasedimentary rocks with variably retrogressed high-pressure granulite-facies mineral assemblages (Dearnley 1963; Coward et al. 1969; Wood 1975; Dickinson and Watson 1976). These lithologies in particular demonstrate the severe effects of the Laxfordian remobilisation and deformation. Relict structures, which might indicate the pre-Laxfordian history of these rocks, . . .