The P-T-t paths of high-grade gneisses, Kaoko Belt, Namibia: Constraints from mineral data, U-Pb allanite and monazite and Sm-Nd/Lu-Hf garnet ages and garnet ion probe data

The Damara Orogeny is a late Neoproterozoic to Cambrian (ca. 570-480Ma) intracratonic event that affected the Kaoko Belt, the inland branch of the Damara orogen and the Gariep Belt in Namibia and South Africa. This study focuses on the Pan-African evolution of part of the Kaoko Belt between the Puros shear zone and the Village mylonite zone which consists of Mesoproterozoic migmatitic para- and orthogneisses with minor granulite and amphibolite. Pseudosection modeling combined with thermobarometric calculations indicate that the para- and orthogneisses equilibrated at about 670-800°C and ca. 0.6-0.8GPa. Some garnets display a pronounced bell-shaped Ca, HREE, Y and Sr zoning, flat zoning profiles of Mn and Fe and concave upward concentration profiles of Sm and Nd. Pressure-temperature estimates obtained on these garnets reveal similar temperatures of 700-750°C but slightly higher pressures of ca. 0.9GPa. The preservation of distinct major and trace element zoning in garnet and the existence of broadly similar (near prograde) Sm-Nd and Lu-Hf garnet-whole rock ages of ca. 525Ma obtained on the same sample indicate an extremely fast cooling path. Retrograde conditions persisted until ca. 490Ma indicating a slow, late stage near isobaric cooling path. The resulting clockwise P-T-t path is consistent with crustal thickening through continent-continent collision followed by post-collisional extension and suggests that the upper amphibolite to granulite facies terrain of the central Kaoko Belt formed initially in a metamorphic field gradient of ca. 25-35°Ckm^-1 at moderately high pressures.