Controls on polymetamorphism in the Southern Pan-African Kaoko belt (Namibia) by petrochronology with EPMA Th-U-Pb monazite dating and geothermobarometry in Garnet metapelites

Petrochronology with garnet and monazite was performed on refurbished metapelite samples from the Central Kaoko Zone (CKZ), the Western Kaoko Zone (WKZ) and the Orogenic Core Zone (OCZ). P-T path segments of garnet crystallisation were reconstructed by geothermobarometry based on continuous cation exchange and net transfer reactions. The oldest metamorphic monazite populations of 1490 ± 15 Ma and 635 ± 7 Ma occur enclosed in garnet porphyroblasts and signal Meso-Proterozoic and early Pan-African high-grade metamorphic events. According to the sample position next to a ~ 550 Ma granitoid gneiss, a monazite population at 552 ± 4 Ma may be assigned to a local contact metamorphism. In the WKZ and OCZ the further monazite age isochrones range from 546 ± 12 Ma to 532 ± 5 Ma, and in the CKZ from 528 ± 4 Ma to 502 ± 5 Ma. When interpreted in terms of metamorphic P-T paths and the stability field of monazite, the formation of these late Pan-African monazites are pre-, syn-, and postcrystalline to late Pan-African garnet blastesis. P-T conditions for Mg-rich garnet rims increase from E to W within the CKZ from 550 to 650 °C at 9 kbar. Maximal P-T conditions in the OCZ were 705 °C/7.9 kbar. Garnets in the WKZ display retrograde zonation trends with Mg-rich cores crystallising at 700 °C/4–6 kbar. In the westernmost WKZ sample the garnet core crystallised at 780 °C/7.4 kbar and could be of early Pan-African age. Monazite ages and geothermobarometry confirm results from isotope age dating methods.