New U-Pb-Hf zircon isotope data for the Paleoproterozoic Ubendian belt in the Chimala area, SW Tanzania

We present new U-Pb and Hf zircon isotope data on a suite of granitoids from a hitherto unstudied area of the Palaeoproterozoic Lupa terrane of the Ubendian belt in SW Tanzania. The major part of the area comprises a sequence of upper amphibolite grade paragneisses and migmatites. Subsequently, the field relations show a complex history of granitoid emplacement from early pre-tectonic strongly foliated diorite-tonalite-granodiorite orthogneiss to late tectonic, weakly deformed ellipsoidal granitoid plutons. These were followed by emplacement of undeformed K-feldspar rich porphyritic alkali granite. U-Pb zircon data on the granitoids show their emplacement ages span the period ∼1925 Ma to 1890 Ma and constrain the culmination of the Ubendian orogeny in the Lupa terrane to about 1900 Ma. The undeformed K-feldspar rich Chimala granite, previously considered to be unconformably overlain by the Buanji Group volcano-sedimentary rocks (recently shown to be ∼1675 Ma, not Neoproterozoic as originally thought) was dated at ∼1407 Ma showing that it must intrude the sedimentary rocks, although the age relationships are not exposed in the mapping area. This is the first record of early Irumide/Kibaran-aged post-tectonic granites in this part of East Africa. Hf isotope data from the zircons show that the granitoids have a long crustal pre-history stretching back to as far as ∼3.9 Ga, and showing that the Lupa terrane may form part of the “metacratonic” margin of an originally much more extensive Tanzania Craton.