Granite petrogenesis and crystal evolution studies in the Damara Pan-African orogenic belt, Namibia

Abstract

Abstract provided by author:

Damara granitoids define a hyperbola on an [symbol like e] Sr vs. e Nd diagram but cannot be modelled as simple binary mixtures between old continental crust and depleted mantle end-members. An episodic intracrustal remobilisation model is proposed to explain the hyperbola defined by granitoid data on an e Sr vs. e Nd diagram

Within the 10 km thick Damara metasedimentary pile 87Sr/36Sr ratios increase systematically with depth. This trend is accompanied by a decrease in 143Nd/144Nd ratios. Model Nd ages (CHUR) for the stratigraphically oldest Damara metasediments (Nosib Group) are about 2. 0 Ga whereas the younger metasediments (Kuiseb Formation and Nama Group) have model Nd ages (CHUR) about 1. 0 Ga reflecting sediment input from younger source terrains. The ratio of model ages (TNdCHUR/TSrBE) is used as an index of intracrustal reworking since it provides a measure of Rb/Sr fractionation (increases) relative to Sm/Nd fractionation. The older Damara metasediments (Nosib Group) have the highest model age ratios suggesting that their source terrains have suffered the largest amount of intracrustal reworking. However, the rate of intracrustal reworking was greatest in source terrains sampled by the younger Damara metasediments (Kuiseb Formation and Nama Group). This suggests that the rate of intracrustal reworking increased through time in the interval (2. 0 - 1. 0 Ga) in this segment of continental crust

All Damara granitoids have model Nd ages (DM) older than about 1. 0 Ga suggesting that intracrustal reworking was the dominant process and crustal growth was negligible