ND AND SR ISOTOPIC MAPPING OF THE ARCHEAN-PROTEROZOIC BOUNDARY IN SOUTHEASTERN TANZANIA USING GRANITES AS PROBES FOR CRUSTAL GROWTH

Granitoids of two different generations occur across the Archaean-Prot erozoic boundary in southeastern Tanzania. The first generation, which is confined to the Archaean Tanzania Craton, yields Rb-Sr whole-rock isochron ages of 2600 Ma and low, mantle-like, initial Sr-87/Sr-86 rat ios (0.702-0.704). These coupled with positive or near-zero epsilon(Nd ) values (-0.2 to 2.0) suggest that these granitoids represent juvenil e mantle material which was added to the continental crust at about 26 00 Ma. The second generation of granitoids intrudes the Palaeoproteroz oic Usagaran Belt and is characterised by Rb-Sr whole-rock isochron ag es of about 1900 Ma. These granitoids show Sm-Nd crustal formation age s which are 200 to 600 Ma older than their emplacement ages and negati ve epsilon(Nd) values (-2.2 to -6.2). Mixing calculations suggest that the Proterozoic granitoids may have formed by partial melting of 2000 Ma (Usagaran) mantle-derived material which had incorporated between 15 and 45% of an Archaean component from the adjacent Tanzania Craton.