Isotope chemostratigraphy of marbles in northeastern Mozambique: Apparent depositional ages and tectonostratigraphic implications

Marbles are minor but characteristic components of metasedimentary units within nappes in the Pan-African Mozambique Belt in NE Mozambique. Metasedimentary units remain largely undated, and carbon and strontium isotope stratigraphy of marbles has been used for indirect dating of the depositional history in this part of the Mozambique Belt. Sixty-nine samples from nine occurrences of dolomite, calcite and magnesite marbles in the Montepuez, Xixano, Lalamo, Ocua and Nampula metamorphic complexes were analysed for major and trace elements, and a subset of 39 samples for C, O and Sr isotopes. The least altered δ13C values range from −3.5 to +7.1‰ (V-PDB) and 87Sr/86Sr ratios from 0.70504 to 0.70671. These values are considered as the best proxy to seawater composition at the time of deposition. The apparent deposition ages, derived from available seawater evolution curves, range from c. 1250 to c. 660 Ma. An age of 1250–910 Ma is obtained from a tripartite marble unit in the Montepuez Complex which is exposed in the Montepuez quarries. Five other age-groups are represented by marble units with apparent depositional ages of 800–750 Ma (Xixano North), 800–660 Ma (Montepuez West), c. 750 Ma (Nampula), c. 740 Ma (Xixano South and Lalamo), and 740–670 Ma (Montepuez East). The data suggest that: (i) Pan-African nappes in NE Mozambique include Neoproterozoic and probable Mesoproterozoic sediments; (ii) Neoproterozoic rocks of the Xixano and Nampula complexes might have different ancestry and were tectonically juxtaposed during the Neoproterozoic Pan-African orogeny.