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Shear-zone patterns and eclogite-facies metamorphism in the Mozambique belt of northern Malawi, east-central Africa: Implications for the assembly of Gondwana

Abstract:

We report on the first occurrence of Pan-African eclogite from the Mozambique belt of northern Malawi, east-central Africa. We describe aspects of (1) the pattern of Pan-African transcurrent and subhorizontal shear zones and how these shear zones relate to eclogite-facies metamorphism and (2) the P-T-t evolution of the eclogite. Finally, we discuss the significance of eclogite-facies metamorphism and shear-zone deformation for the assembly of Gondwana. The first major deformation event involved sinistral and dextral transcurrent and top-ESE shearing and occurred at ∼ 580-550 Ma. Transcurrent and top-ESE shearing resulted from ESE-WNW horizontal crustal shortening and commenced under upper amphibolite-facies metamorphism at pressures < 10 kbar. This first major deformation event affected the grey-gneiss basement and pre-dated eclogite formation by >40 My. Pan-African eclogite occurs as slices in the upper parts and also on top of the grey-gneiss basement. Estimated eclogite-facies metamorphic conditions are 17-18 kbar and 660-780 °C. The pressure-temperature evolution of the eclogite is characterised by near-isothermal decompression. The eclogite was penetratively deformed at its margins during greenschist-facies metamorphism when it was juxtaposed with the grey gneiss. Shrimp ages of magmatic zircons yielded an age of ∼ 1000 Ma for the emplacement of the eclogite precursor. 207Pb/206Pb and U-Pb zircon ages of small, partly rounded zircons suggests that high-pressure metamorphism occurred between ∼ 530 and 500 Ma. An 40Ar/39Ar age on hornblende yielded an Ordovician age, which might be close to the time of juxtaposition of the eclogite and the grey-gneiss basement. The eclogite and the grey-gneiss basement were both deformed by subhorizontal top-NE shearing and folding during greenschist- and sub-greenschist-facies conditions. Both sets of transcurrent shear zones remained active during top-NE tectonic transport implying continued or renewed ESE-WNW crustal shortening. The top-NE structures facilitated the final juxtaposition of the eclogite and the grey-gneiss basement. Overall, the data suggest that the eclogite and the underlying grey-gneiss basement did not share the same orogenic history. Late Neoproterozoic ESE-WNW crustal shortening during amphibolite-facies metamorphism at ∼ 580-550 Ma did not cause overthickened crust and is, therefore, not related to continent collision. We propose that considerable crustal thickening and eclogite-facies metamorphism occurred during the subduction/collision transition when East and West Gondwana finally collided in the Cambrian. © 2002 Elsevier Science B.V. All rights reserved.