The significance of Rb-Sr ages of biotite and phlogopite for the thermal history of the Central and Southern Marginal Zones of the Limpopo Belt of southern Africa and the adjacent portions of the Kaapvaal Craton

Generally, Rb-Sr biotite and phlogopite ages range from ∼ 2050 Ma to ∼ 1950 Ma between the Murchison-Thabazimbi and Triangle-Tuli-Sabi shear zones of southern Africa in the rocks comprising the Central and Southern Marginal Zones of the Limpopo Belt and adjacent portions of the Kaapvaal Craton. In some shear zones, however, similar ages ocurr as young as ∼ 800 Ma. In contrast, within the Kaapvaal Craton south of the Murchison-Thabazimbi Shear Zone, Rb-Sr biotite ages range between ∼ 3150 Ma and ∼ 2100 Ma and show no regular distribution. In all cases, the ages are interpreted to indicate the time elapsed since the biotite last cooled below its closure temperature, approximately 320°C. The regular pattern of 2050 Ma to 1950 Ma Rb-Sr biotite ages north of the Murchison-Thabazimbi Shear Zone suggests that the present erosional surface approximates a paleo-isothermal surface. This surface was exposed by uplift and erosion which was completed by ∼ 1900 Ma to 1800 Ma ago when the sedimentary and volcanic rocks of the Soutpansberg Group were deposited. The regional uplift was accommodated by motion on the Murchison-Thabazimbi and Triangle-Tuli-Sabi Shear Zones and probably was related to tectonism associated with the emplacement of the anorogenic Bushveld and Phalaborwa complexes. Other anorogenic granitic magmatism accompanied uplift. The depth represented by the paleo-isothermal surface and the magnitude of the uplift that exhumed it cannot be constrained precisely. It is suggested, however, that the majority of the 10 to 15 km by which crust in this area is thinner than that of the adjacent cratons is the manifestation of this uplift.