INTRACRUSTAL RECYCLING AND UPPER-CRUSTAL EVOLUTION - A CASE-STUDY FROM THE PAN-AFRICAN DAMARA MOBILE BELT, CENTRAL NAMIBIA

Clastic sediments from the Pan-African Damara mobile belt show a wide range in depositional ε{lunate}Nd (ε{lunate}Nd(0) = -18 to -1) and several key trace-element ratios, notably Th/Nd and La/Sm, increase (from 0.2 to 0.7 and 4.0 to 8.0, respectively) with decreasing ε{lunate}Nd(0). Such trace-element ratio-ε{lunate}Nd(0) data arrays are attributed to binary mixing within the sedimentary mass and the data array defined by the Damara sediments is consistent with data from other regions. The low-ε{lunate}Nd(0) end-member is characterised by high Th/Nd, Th/Y, La/Sm and Hf/Zr ratios and reflects highly differentiated upper crust dominated by granitoids. Pb-isotope data suggest that the low-ε{lunate}Nd(0) end-member also had relatively high time-integrated Th/U (∼4). By contrast, the high-ε{lunate}Nd(0) end-member reflects juvenile upper crust, but this end-member has La/Sm, Th/Nd and Hf/Zr ratios (∼4.0, ∼0.2 and ∼0.02, respectively), slightly lower than those of Taylor and McLennan's preferred model for bulk crust, and significantly lower than those of the "andesite model". Intracrustal differentiation is discussed in terms of bulk crust, present-day average upper crust, post-Archaean shale and a highly differentiated low-ε{lunate}Nd(0) end-member. Trace-element ratios which are not significantly fractionated during erosion and sedimentation, e.g. Th/Nd, Th/Y and La/Sm, show relatively smooth increases between these reservoirs, whereas other trace-element ratios which are fractionated both by sedimentary and magmatic processes, e.g. Rb/Sr and Rb/Zr, increase dramatically in the more differentiated crustal reservoirs. © 1990.