Insights into the deformation history of the Menderes Massif (Western Turkey)

The Menderes Massif (western Turkey) has a complex internal structure recording a multiphase deformation history and repeated metamorphism since the Late Proterozoic. Pressure-temperature-time (P-T-t) conditions are key for ascertaining the mechanisms by which extension created the massif. Geochronologic data show evidence of multiple tectonic events, from the Cambro-Ordovician to the early-late Cenozoic. Here we report Th- Pb ion microprobe monazite ages and garnet-based P-T conditions samples collected along 3 transects across the northern (Alasehir) detachment of the central Menderes Massif. Samples collected along A-A' are part of the Bayindir nappe. Garnets in sample MM03-28 contain mm-sized inclusions of staurolite, but the rock lacks biotite and aluminosilicates. Garnets in these rocks may be being consumed via the reaction biotite + aluminum silicate + garnet + H2O to staurolite + muscovite + quartz. The reaction is the reverse of the staurolite-out isograd and the sample may have experienced a decrease in P and/or T. Biotite and aluminosilicates were completely removed from the system. Garnet was present in excess, and was left as relics and evidence for the reaction. Although alternatives exist for this interpretation, eight monazite grains dated in the matrix and in garnets in this rock yield a weighted mean age consistent with a single population at 31.0±0.7 Ma (±1σ, MSWD=0.7). This age is consistent with the timing of initial extension in the Menderes Massif during the Oligocene reported elsewhere. Samples collected ~5 km further west along B-B' are thought to be part of the Bozdag nappe. Two samples from this transect record peak P-T conditions within uncertainty of 680±45°C and 10.3±3.1 kbar (sample MM03-33) and 640±40°C and 8.9±2.3 kbar (sample MM03-38). Five monazite grains dated in the matrix and in garnets in sample MM03-33 yield a weighted mean age consistent with a single population at 25.4±1.5 Ma (±1σ, MSWD=0.9). Taking into account the uncertainty, the age is 3-8 m.y. younger than the monazites in sample MM03-28. We suggest the sample records monazite growth during continued movement along the Alasehir detachment. Rocks were also collected ~20 km west of B-B' from the Bozdag nappe along transect C-C'. Sample MM03-48 contains garnets of varying size, with larger subhedral garnets in mica- rich layers and smaller euhedral garnets in coarser-grained quartz veins. Matrix monazite grains from this rock range from 31.5±2.7 Ma to 22.8±2.4 Ma. Results from these samples are consistent with the hypothesis that extensional exhumation in the Menderes Massif began in Oligocene and continued during the early to mid Miocene. The mid Miocene constraint arises from monazite ages of 15±2 Ma for synextensional granites along the Alasehir detachment that are also reported in this session.