Lithospheric structure, evolution and diamond prospectivity of the Rehoboth Terrane and western Kaapvaal Craton, southern Africa: Constraints from broadband magnetotellurics

摘要

A1400 km-long, 2-D magnetotelluric (MT) profile, consisting of 69 sites at 20 km intervals, across the western part of the Archaean Kaapvaal Craton, the Proterozoic Rehoboth Terrane and the Late Proterozoic/Early Phaiie-rozoic Ghanzi-Chobe/Damara Belt reveals significant lateral heterogeneity in the electrical resistivity structure of the southern African lithosphere. The lithospheric structures of the Rehoboth Terrane and Ghanzi-Chobe/ Damara Belt have not been imaged previously by geophysical methods. Temperature is the primary control on the resistivity of mantle minerals, and the MT derived lithospheric thicknesses therefore provide a very reasonable proxy for the "thermal" thickness of the lithosphere (i.e., the thickness defined by the intersection of a conductive geotherm with the mantle adiabat), allowing approximate present-day geotherms to be calculated. The work indicates the following present-day average lithospheric thicknesses, to a precision of about ± 20 km, for each of the terranes traversed (inferred geotherms in brackets): Eastern Kimberley Block of the Kaapvaal Craton 220 km (41 mW m~(-2)), Western Kimberley Block 190 km (44 mW m~(-2)), Rehoboth Terrane 180 km (45 mW m~(-2)) and Ghanzi-Chobe/Damara Belt 160 km (48 mW m~(-2)). A clear relationship between the electrical resistivity structure of the lithosphere and the tectonic stabilisation-age of the terrane is evident Good agreement between the inferred present-day lithospheric geotherms and surface heat flow measurements indicate the latter are strongly controlled by variations in lithospheric thickness. A significant difference in lithospheric thickness is observed between the Eastern and Western Kimberley blocks, and is consistent with previous seismic tomography images of the Kaapvaal Craton. The present-day lithospheric thickness, and reduced depth extent into the diamond stability field, accounts for the absence of diamondiferous kimberlites in the Gibeon and Gordonia kimberlite fields in the Rehoboth Terrane. Previously published mantle xenolith P-T arrays from the Gibeon, Gordonia and Kimberley fields, however, suggest that the Rehoboth Terrane had equilibrated to a cooler conductive palaeo-geotherm (40-42 mW m~(-2) ), very similar to that of Eastern Kim-