1. The Geochemistry andGeochronology of the Barby Formation, Southern
Namibia
Malobela, T.R.1, Mapani, B.S.1 Cornell, D.H.2
1University of Namibia,Geology Department
2University of Goteborg, Department of Earth Sciences
Volcanicandsedimentaryrocksof the SinclairSupergroup occur in the Konkiep Terrain of Southern
Namibia. Three conformable volcano-sedimentary cycles are recognised. In this work we describe
and date volcanic rocks of the Barby Formation, a key unit in the Sinclair area. The rock units in the
Barby are basaltic trachyandesites, trachydacites and rhyolites. Based on geochemistry of the rock
samples, the bulk of rock types are calc-alkaline. The plate tectonic setting of the regional arc is
constrained in the Barby Formation which has high calc-alkaline rocks that suggest that the
emplacementwasinan active continental marginsetting.The Barbyrockscontainsilicabetween51-
78 wt. % SiO2 and abundant K2O. This work presents results of U-Pb dating and Lu-Hf isotopic
analyses of zircon grains from four samples. Most of the analysed zircons have two domains and
faint oscillatory zonation distinguished using cathodoluminescent images. The Barby Formation
samplesrecorda U-Pbage of 1213.7 ± 4.8 Ma, the SpesBonasyenite isslightly older than the Barby
formationhavingaU-Pb age of 1217 ± 3 Ma. The coeval SpesBonasyenite and the Barby units have
a Lu-Hf crustal residence age between1682 and 1873 Ma suggestingthe twoformationsare formed
froma mixture of younger and older material. Though they have two different U-Pb ages, they are
fromthe same geological environment.The BarbyFormationreflectsasubductioneventwhich took
place during the assemblage of the Rodinia supercontinent. Rocks in the area of the Kairab
Formationhave been dated at 1370 Ma, showing the initiation of the collision event, and ended at
about 1100 Ma as evidenced by late granites of the Rooiberg Formation.