THE TRIANGLE SHEARZONE, ZIMBABWE, REVISITED - NEW DATA DOCUMENT AN IMPORTANT EVENT AT 2.0-GA IN THE LIMPOPO BELT

The Limpopo Belt in Southern Africa has been used to demonstrate that modern-style continent-continent collision operated during the Late Archaean (2.6-2.7 Ga). We have studied the age and PT conditions of strike-slip tectonism along the important right-lateral Triangle Shearzone. Our results substantiate existing Proterozoic metamorphic mineral age data of prior uncertain significance. Using the Pb-Pb and Sm-Nd garnet chronometers and the Ar-Ar step heating technique for amphibole, we have dated pre- and syn-tectonic metamorphic minerals at 2.2 and 2.0 Ga. Thus the Triangle Shearzone can now be regarded as an important Proterozoic suture. Examination of corresponding high-grade PT conditions, reaching similar to 800 degrees C at 9 kbar, indicates a clockwise metamorphic evolution with pronounced isothermal uplift. Although the evidence that thrusting of the Marginal Zones of the Limpopo Belt over the adjoining cratons occurred during the Late Archaean clearly remains, it is now very uncertain to which event the various PT paths obtained in the Limpopo Belt may be assigned. Therefore the question of whether the 2.6-2.7 Ga tectonism fits on its own a modern-style continental collision model remains open and has to be reassessed.