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

Abstract

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.