PROBLEMS WITH MODELING DEFORMATION IN THE MENDERES MASSIF (WESTERN TURKEY) USING GARNET-BASED THERMOBAROMETRY

The Menderes Massif covers an area of >40,000 km² in western Turkey, and is a prime example of an area that experienced a complex, multi-stage geologic history. The Massif is considered as a metamorphic core complex, the result of extreme extensional deformation in continental regions. To identify if Menderes Massif garnets record a polymetamorphic history as suggested by recent in situ monazite ages, an electron microprobe was used to X-ray element map garnets and obtain peak P-T conditions. Garnet-based P-T conditions are frequently used to evaluate and develop models for the tectonic evolution of the Menderes Massif. If the garnets are detrital, polymetamorphic, or developed during a previous metamorphic event, using their compositions in combination with matrix minerals generates misleading conditions and erroneously constrained tectonic models. X-ray element garnet maps and compositional transects across garnets from samples collected from the same unit, even the same outcrop, in the central Menderes Massif show very different zoning profiles. For example, garnets from samples MM03-22 and MM03-23 from the same outcrop in the central Menderes Massif have similar Mn and Ca zoning, but very different Fe and Mg zoning, suggesting the rocks experienced similar baric conditions, but different thermal conditions. The P conditions of these two rocks are within uncertainty (11.4 ±1.6 kbar and 9.0±2.4 kbar, respectfully), but yield different temperature results of 650±40°C and 505±55°C. Note that the P-T conditions are consistent with their mineral assemblages and zoning profiles. Multiple samples from the same outcrop in the Menderes Massif should be analyzed for consistency of the P-T results. For example, we have confidence in the P-T conditions of samples MM03-33 and MM03-38, as they were collected in close proximity of each other and yield similar peak conditions and garnet zoning (680±45°C; 10.3±3.1kbar and 640±40°C and 8.9±2.3 kbar).