Precambrian Research 148 (2006) 19–31 Depositional environments and an apparent age for the Geci meta-limestones: Constraints on the geological history of northern Mozambique V.A. Melezhik a,∗ , A.B. Kuznetsov b , A.F. Fallick c , R.A. Smith d , I.M. Gorokhov b , D. Jamal e , F. Catuane f a Geological Survey of Norway, 7491 Trondheim, Norway b Institute of Precambrian Geology and Geochronology, nab. Makarova, 2, 199034 St. Petersburg, Russia c Scottish Universities Environmental Research Centre, G75 0QF East Kilbride, Glasgow, Scotland d British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, Scotland e Eduardo Mondlane University, PO Box 257, Maputo, Mozambique f National Directorate for Geology, Box 217, Maputo, Mozambique Received 26 August 2005; received in revised form 22 February 2006; accepted 7 March 2006 Abstract Strongly contrasting rocks were juxtaposed during the long tectonometamorphic history of the Mozambique Orogenic Belt in northern Mozambique. The latest depositional event was expressed by accumulation of marine sediments, which were subsequently mildly metamorphosed and tectonically juxtaposed with granulite facies complexes. The low-grade metasedimentary rocks comprise the Geci group which was mapped, and studied petrographically, geochemically and isotopically in order to provide constraints on the depositional environments, age and latest history of the Mozambique Orogenic Belt. The group occurs as several large, tectonically dissected, intensively sheared, folded and mylonitised, SW–NE trending lenses within Unango Complex granulite rocks. In places, primary depositional features are well preserved. The dominant rocks are calcarenites, dolarenites, calcite matrix- supported and dolostone clast-supported carbonate breccias forming beds with erosional bases, normal and reverse graded bedding, and well-developed Bouma sequences. Dolomicritic, microbial and oolitic dolostone clasts were apparently derived from the margin of a shallow-water carbonate platform and redeposited by turbidity currents on a continental slope with calcareous sedimentation. The Geci meta-carbonate rocks have low SiO 2 and Al 2 O 3 contents. MgO/CaO ratios fluctuate between 0.05 and 0.70 averaging 0.15 ± 0.18 (1σ, n = 111). Acid-soluble constituents have moderate concentrations of Fe (777 ± 310 ppm), Mn (131 ± 85 ppm) and Sr (566 ± 145). Mn/Sr ratios are relatively low (0.26 ± 0.19), δ 13 C and δ 18 O values are invariably high, with only moderate scatter: +4.0 ± 0.6% V-PDB and 25.8 ± 0.5% V-SMOW, respectively. The least altered 87 Sr/ 86 Sr ratio of calcite is 0.70708, whereas dolomite is enriched in 87 Sr with the lowest ratio of 0.70730. The least-altered 87 Sr/ 86 Sr and δ 13 C ratios suggest an apparent depositional age of either 595–585 or 630–625 Ma. This provides a lower age limit for juxtaposition of the low-grade Geci group rocks and granulite facies rocks of the Unango Complex. © 2006 Elsevier B.V. All rights reserved. Keywords: Mozambique; Orogenic belt; Limestone; Dolostone; Carbon; Strontium; Isotopes; Age; Glaciation ∗ Corresponding author. Tel.: +47 73 90 42 41; fax: +47 73 92 16 20. E-mail address: victor.melezhik@ngu.no (V.A. Melezhik). 0301-9268/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2006.03.003