Journal of Petrology, Volume 39, Issue 4: April 1 1998.

Crustal Age Domains and the Evolution of the Continental Crust in the Mozambique Belt of Tanzania: Combined Sm-Nd, Rb-Sr, and Pb-Pb Isotopic Evidence

ANDREAS MÖLLER^1,2, KLAUS MEZGER² AND VOLKER SCHENK¹

¹MINERALOGISCH-PETROGRAPHISCHES INSTITUT, UNIVERSITÄT KIEL, 24098 KIEL, GERMANY ²MAX-PLANCK-INSTITUT FÜR CHEMIE, POSTFACH 3060, 55020 MAINZ, GERMANY

Combination of Nd and Sr model ages and Pb isotopes on leached feldspars reveals distinct model age provinces not recognized previously within the Mozambique Belt of Tanzania. Most boundaries of these age domains are overprinted by Neoproterozoic (Pan-African) tectonism and metamorphism. Granitoids from the Archean craton show Nd model ages of 2·7-3·1 Ga and very primitive Pb isotope systematics in feldspars. Amphibolite-facies migmatites and granulites from the Mozambique Belt have similar characteristics, yet their high ²⁰⁸Pb/²⁰⁴Pb values point to U loss in the Archean, possibly during high-grade metamorphism, and subsequent Pan-African reworking. Eclogite-facies metapelites of the Early Proterozoic Usagaran Belt likewise exhibit Archean Nd model ages, but higher Pb isotopic ratios are consistent with last recrystallization of feldspar at 2 Ga. Granulites with Nd model ages from 1 to 1·5 Ga only occur in NE Tanzania; because of their restricted range in Pb isotopic composition they are interpreted as juvenile additions during late Proterozoic time. Granulites of the W Uluguru Mts have Nd model ages between 2·1 and 2·6 Ga, and highly variable feldspar Pb isotope composition indicating possible derivation from cratonic and/or Usagaran material, reworked and mixed with a small proportion of younger Proterozoic material during the Pan-African orogeny. This could indicate the suture zone between a western Archean-Proterozoic continental mass and juvenile arc-terranes docking on from the east during subduction of the Mozambique Ocean. The combined isotope data provide strong evidence that parts of the East African crust grew by lateral accretion of Early and Mid Proterozoic segments onto an Archean nucleus. However, the Neoproterozoic (Pan-African) orogeny not only led to addition of new crust in the NE of Tanzania, but also reworked older crustal material in most other parts of the Mozambique Belt. This juxtaposition of ancient with juvenile crustal segments is consistent with an active continental margin setting before or during orogenesis. Correlation with adjacent terranes indicates similar processes of mixing and limited juvenile addition prevailing throughout central Gondwana during the Pan-African orogeny.

Keywords: age province boundaries;crustal evolution; model ages; Mozambique Belt; Sr-Nd-Pb isotopes

Pages 749-783

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