Precambrian Research 113 (2002) 227 – 242 Isotopic dating of Neoproterozoic crustal growth in the Usambara Mountains of Northeastern Tanzania: evidence for coeval crust formation in the Mozambique Belt and the Arabian – Nubian Shield M.A.H. Maboko a, *, E. Nakamura b a Department of Geology, Uniersity of Dar Es Salaam, PO Box 35052, Dar Es Salaam, Tanzania b The Pheasant Memorial Laboratory for Geochemistry and Cosmochemistry Institute for the Study of the Earth’s Interior, Okayama Uniersity, Misasa, Tottori -ken, 682 -01 Japan Received 29 February 2000; accepted 27 July 2001 Abstract Granulite-facies orthogneisses of andesitic to dacitic composition in the Usambara Mountains of north eastern Tanzania yield a Sm–Nd whole rock isochron age of 815 58 Ma and an initial  (Nd) value of 4.1. This age is interpreted as dating Sm – Nd fractionation during extraction from the mantle and immediate subsequent crystallisa- tion of the granulite protolith during an event of regional calc-alkaline magmatism in the area. Isotopic and geochemical characteristics of the rocks are consistent with a convergent margin setting for the magmatism with minimal contamination by older continental crust. The isotopic data from the Usambara Mountains demonstrate that Neoproterozoic crust formation in the Arabian – Nubian Shield and parts of the Mozambique Belt was broadly contemporaneous. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Mozambique Belt; Tanzania; Sm – Nd geochronology; Crust formation; Granulites www.elsevier.com/locate/precamres The Neoproterozoic East African orogeny (Stern, 1994) played the most important role in shaping the geology of a large part of the eastern seaboard of the African continent, stretching from Mozambique in the south northwards into the Arabian Peninsula (Fig. 1). Whereas models for the formation of the northern part of the orogen (the Arabian – Nubian Shield) in terms of plate tectonic processes, involving the assembly of sev- eral arc and back-arc basin systems, are widely accepted (Stoeser and Camp, 1985; Vail, 1985; Kroner et al., 1987; Stern, 1994), the origin of the southern part, the Mozambique Belt of Holmes (1951) is still open to debate. In particular, the question of whether Neoproterozoic tectonism in the Mozambique Belt was preceded by significant juvenile crust formation, as was the case in the Arabian – Nubian Shield or only involved rework- * Corresponding author. Fax: +255-51-410258. E-mail address: mmaboko@hotmail.com (M.A.H. Maboko). 0301-9268/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S0301-9268(01)00213-3