CHRONOLOGY AND MECHANISM OF DEPLETION IN LEWISIAN GRANULITES

The pattern of Th, U and Rb depletion has been investigated in a suite of contrasting lithologies from the classic Lewisian granulite terrain at Scourie, N.W. Scotland. The study is based on lithologies ranging from ultramafic to tonalitic gneisses, together with calcsilicate and pelitic metasediments, sampled on the metre scale. Because of their close proximity, all of these lithologies have experienced identical P-T-t histories since depletion; however, their expected dehydration and melting characteristics are markedly different. Pb isotope analyses of the samples define the latest time of U/Pb fractionation as 2.665 +/- 0.026 Ga, and indicate substantial U and Th depletion relative to Pb in tonalitic and mafic lithologies, and in some metasediments. In contrast, ultramafic samples show no U and Th depletion. Sm-Nd isotope analyses of the mafic and ultramafic samples define an age of 2.707 +/- 0.052 Ga, which is interpreted as the time of igneous differentiation of the mafic-ultramafic bodies. This time is indistinguishable from the latest time of Th-Pb and U-Pb fractionation. Overall there is a straightforward relationship between the magnitude of depletion, lithology and the stability of mineral phases. Tonalitic and mafic gneisses are depleted in Th and U relative to Pb, whereas ultramafic gneisses which retain amphibole show no evidence for depletion. The observations are consistent with loss of U, Th and Rb in fluids produced by metamorphic dehydration and melting. A role for externally derived CO2 in the depletion process is not excluded by the data but there is no evidence for it having existed. Limits are placed on the length scale of transport for several elements. U, Th and Rb are depleted regionally from mafic and tonalitic lithologies; transport distances must have been on the kilometre scale. In contrast, movements of Sm and Nd have been more restricted.