New evidence for protolith ages of Lewisian granulites, Northwest Scotland

The granulite facies Scourian rocks in the central region of the Lewisian complex of northwest Scotland play an important role in the understanding of the development of mid-Archean high-grade gneiss complexes. Whereas the later, largely Proterozoic history of the Scourian has been clarified in a detailed U-Pb study, unraveling the Archean ages of the protoliths using conventional isotope techniques has proved impossible. In order to advance our understanding of the deep crustal processes responsible for these granulites, it is essential to establish the age(s) of accretion of the protoliths and their subsequent tectonothermal history. A combined cathodoluminescence (CL) and sensitive high-mass resolution ion microprobe (SHRIMP) single-zircon study has revealed a hitherto unrecognized morphological complexity in zircons from the type localities of the granulites. Use of CL allowed identification of relict oscillatory igneous zoning, metamorphic overgrowths, and irregular areas of recrystallization. From the SHRIMP data, an age of ca. 2960 Ma is inferred for the gneiss protoliths, which were altered considerably during an important metamorphic event ca. 2490 Ma.