NEW EVIDENCE FOR PROTOLITH AGES OF LEWISIAN GRANULITES, NORTHWEST SCOTLAND

The granulite facies Scourian rocks in the central region of the Lewis ian complex of northwest Scotland play an important role in the unders tanding of the development of mid-Archean high-grade gneiss complexes. Whereas the later, largely Proterozoic history of the Scourian has be en clarified in a detailed U-Pb study, unraveling the Archean ages of the protoliths using conventional isotope techniques has proved imposs ible. In order to advance our understanding of the deep crustal proces ses responsible for these granulites, it is essential to establish the age(s) of accretion of the protoliths and their subsequent tectonothe rmal history. A combined cathodoluminescence (CL) and sensitive high-m ass resolution ion microprobe (SHRIMP) single-zircon study has reveale d a hitherto unrecognized morphological complexity in zircons from the type localities of the granulites. Use of CL allowed identification o f relict oscillatory igneous zoning, metamorphic overgrowths, and irre gular areas of recrystallization. From the SHRIMP data, an age of ca. 2960 Ma is inferred for the gneiss protoliths, which were altered cons iderably during an important metamorphic event ca. 2490 Ma;