Granite petrogenesis and crustal evolution studies in the Damara Pan-African orogenic belt, Namibia
Major and trace element data for Oamara granitoids distinguish three
geochemically distinct granitoid groups, - crustal-melt granitoids, calc-alkaline granitoids and
within-plate granitoids. The overwhelming majority of the Oamara granitoids are peraluminous
crustal-melt leucogranites which have elevated 87 Sr/86Sr ratios (> 0.710) and old model Nd
ages (- 2.0 Ga).
Calc-alkaline diorites are metaluminous and have the lowest 87Sr/86Sr ratios
(0.704 - 0.707) and model Nd ages (OM) in the range 1.1 -1.7 Ga. Within-plate granitoids are
characterised by elevated high-field strength (HFS) abundances and have model Nd ages (OM)
in the range 1.1 - 1.6 Ga.
All Oamara granitoids have model Nd ages (OM) older than about 1.0 Ga
suggesting that intracrustal reworking was the dominant process and crustal growth was
Oamara granitoids define a hyperbola on an 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 Oamara metasedimentary pile 87 Sr/86Sr ratios increase
systematically with depth. This trend is accompanied by a decrease in ' 43Ndl 144Nd ratios.
Model Nd ages (CHUR) for the stratigraphically oldest Oamara 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 (TNd CHUR/TSr BE) is used as an index of intracrustal reworking since it
provides a measure of Rb/Sr fractionation (increases) relative to SmlNd fractionation. The older
Oamara metasediments (Nosib Group) have the highest model age ratios suggesting that their
source terrains have suffered the largest amount of intracrustal reworking. However, the !ala of
intracrustal reworking was greatest in source terrains sampled by the younger Oamara
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