Polymetamorphic Garnets in the Menderes Massif (Western Turkey): Insights into the Metamorphic History of a Complexly-Deformed Region

The Menderes Massif in the Aegean region is a prime example of an area that experienced a complex, multi-stage metamorphic history. Post-collisional extension exhumed the Menderes Massif, which covers an area of >40,000 km². The Massif experienced polyphase deformation, but distinguishing Pan-African events from Alpine metamorphism, and discriminating Eocene-Oligocene compression from the post-collisional extension are controversial. To identify if Menderes Massif garnets record this polymetamorphic history, we used an electron microprobe to X-ray element map and identify zoning types. One southern Menderes Massif sample has garnet + biotite + muscovite + chlorite + plagioclase + ilmenite + zircon + allanite + monazite + quartz. X-ray element maps of the garnets show a discontinuous Mn increase not found near minerals characteristic of retrogression, defining a ∼30-μm plateau. The Mn increase appears at the same location as a sharp Ca decrease and Mg increase. The profile leaves open the possibility the garnets grew during two stages and/or experienced a change in bulk composition during rim growth. One garnet from the Kuzey Detachment, which bounds the northern edge of the Central Menderes metamorphic core complex, shows no zoning in Mn, Fe, Mg, or Ca, and has a 483+/-5 Ma monazite inclusion. This result indicates the sample experienced significant diffusional relaxation since its time of formation. 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.