DEPOSITIONAL ENVIRONMENTS AND AN APPARENT AGE FOR THE GECI META-LIMESTONES: CONSTRAINTS ON THE GEOLOGICAL HISTORY OF NORTHERN MOZAMBIQUE

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 SiO2 and Al2O3 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), δ13C and δ18O 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 87Sr/86Sr ratio of calcite is 0.70708, whereas dolomite is enriched in 87Sr with the lowest ratio of 0.70730. The least-altered 87Sr/86Sr and δ13C 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.