Reconstruction of Crustal Genesis in a High-Grade Gneiss Terrane Using Sm/Nd-Whole Rock Systematics and Pb-Isotopes of Leached Feldspar

Abstract

Sm/Nd whole rock and Pb-feldspar data each provide important information on the history of old continental crust. However, the combination of both data sets allows new insights that prove most valuable in high-grade and polymetamorphic terranes, where a lot of information is lost during metamorphism. The data set from Tanzania comprises granitoid rocks from the Archaean craton and granulite and eclogite facies rocks from the polymetamorphic Mozambique Belt (MB) in Eastern Tanzania. The evolution of the Mozambique Belt in eastern Africa (Holmes 1951) has been a matter of discussion for a long time, with essentially two contrasting models: the older one proposing an emialic orogeny with reworking of older crustal material (Watson 1976; Krrner 1980) while the second, new model is based on plate tectonics involving continent-continent collision (Shackleton 1976; Muhongo 1989) with generation of new ernst. However, the amount of new crust added during metamorphic events to pre-existing Archaean material and its distribution within the metamorphic belt was hitherto unknown. Some U-Pb zircon data on the granulites showed either evidence for Proterozoic (Maboko et al., 1985) or Archaean (Coolen 1982) precursors. Several zircon samples just yielded upper intercepts around 700Ma that were interpreted as the age of Panafrican metamorphism. Although Nd model ages are not unequivocal in their interpretation due to mixing effects in the ernst and possible fractionation of the Sm/Nd ratio during high-grade metamorphism, they provide first order constraints on age provinces within metamorphic belts. Whereas the Sm/Nd ratio suffers most severe fractionation during melt extraction from the mantle i.e. the forming of new crust, changes in Th/Pb and U/Pb ratios occur afterwards during crustal processes such as metamorphism, sedimentation and weathering. Pb isotope systematies of leached feldspars from the same samples, therefore, give complementary results to Nd systematics because they reveal the time integrated effects of the crustal history and preserve information on the time of present parent-daughter fractionation.