Trace element composition of rutile and the application of Zr-in-rutile thermometry to UHT metamorphism (Epupa Complex, NW Namibia)

The trace element composition of rutile and Zr-in-rutile temperatures were determined for ultra-high temperature (UHT) metamorphic rocks from the Epembe Unit of the Epupa Complex in NW-Namibia. Ortho- and paragneisses record Mesoproterozoic peak metamorphic UHT conditions of 970 ± 40 °C at 9.5 ± 2 kbar, as estimated from conventional thermobarometry and constraints from pseudosection modeling. Rutile exhibits superchondritic concentrations of V, Zr, Nb, Hf, Ta, Th and U while rare earth elements (REE) are far less enriched. Zr and Hf correlate positively with two distinct trends. Nb and Ta as well as Cr and V show a positive correlation although with less clear trends. Only Hf correlates with Zr, suggesting a decoupling of Zr and Hf from the other high field-strength elements (HFSE) probably during retrogression. In general, the non-homogeneous HFSE distribution in rutile indicates that equilibrium trace element distribution achieved during UHT peak metamorphic conditions was either almost completely erased or had never been achieved as a common feature of all rutile grains. The retrograde metamorphic evolution of the UHT rocks is interpreted to be responsible for trace element redistribution under equilibrium conditions restricted to small domains. This has affected the trace element composition of the rutile grains investigated here thereby disturbing their UHT signature, which may cause problems for provenance studies involving such ‘disturbed’ grains. A systematic comparison of all available Zr-in-rutile thermometer calibrations shows that beside of one, all give similar temperature estimates for the studied samples. No systematic differences regarding the Zr content were observed between rutile grains in different textural positions (i.e. matrix grains, those shielded by host minerals or post-peak grains). However, calculations revealed a broad range of temperatures between  1000 °C. The large spread of calculated temperatures is interpreted to result from intergrain diffusion and trace element exchange by fluid-mediated recrystallization during the retrograde metamorphic evolution. This interpretation is supported by the presence of extensively formed retrograde reaction textures involving hydrous phases such as cordierite and biotite in the studied samples. In addition, tiny (≤ 5 μm) Zr-silica-rich phase separations which occur either homogeneously or heterogeneously distributed in single rutile grains may cause intergrain Zr variations.