Title:
Trace element composition of rutile and the application of Zr-in-rutile thermometry to UHT metamorphism (Epupa Complex, NW Namibia)
Publication Year:
2011
Abstract:
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. Keywords: Rutile trace element composition, LA-ICP-MS, Epupa Complex, UHT granulites, Zr-in-rutile thermometry.
Publication Title:
Lithos
Volume:
126
Issue:
3-4
Pages:
388-401
Item Type:
Journal Article
Language:
en

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