Deciphering the P-T-t conditions of garnet-bearing metamorphic rocks in the Southern Menderes Massif, SW Turkey

Ataktürk, Katelyn Rahşan
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The Aegean region contains numerous metamorphic core complexes that reflect post-collision extensional tectonics. The largest of these is the Menderes Massif of western Turkey, which covers an area of ~40,000 km². The Selimiye Shear Zone bounds its southern border and is a key location for studying the metamorphic history of the massif. Models of the tectonic evolution of the region requires an understanding of the peak pressure-temperature (P-T) conditions recorded by rocks in the massif, and the time (t) at which they achieved those conditions. However, limited P-T-t data exists in the Southern Menderes Massif. Here, P-T-t data was obtained from garnet-bearing rocks collected perpendicular to strike along seven transects spaced about 35 km across the Selimiye Shear Zone. Garnets in nine samples from four transects were analyzed using high-resolution back-scattered electron (BSE) imagery, X-ray element (Fe, Mg, Mn, Ca and Y) maps, and quantitative compositional analyses. Both zircon and monazite grains were dated in rock thin section using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Secondary Ion Mass Spectrometry (SIMS) to ascertain the timing of events recorded by the rocks. Some garnet X-ray element maps show zoning consistent with multiple stages of growth, diffusion, and retrogression. Distinct zones in each sample are visible on high contrast BSE images and can be related to Y, Fe, and Mn contents. The conventional garnet + biotite geothermometer and garnet + plagioclase + muscovite + biotite geobarometer were used to estimate peak metamorphic P-T conditions. These range from 556±10°C to 671±27°C, and 15.3±0.2 kbar to 22.4±0.5 kbar. The temperatures are similar to previous estimates, but the pressures appear about ~10 kbar higher the previous estimates. U-Pb zircon ages range from 2022±30 Ma (13.4% disc.) to 254±5 Ma (13.4% disc.). Based on Th/U contents, the oldest ages are likely related to inherited grains from magmatic sources. The youngest zircon age is the first reported Triassic grain from the Southern Menderes Massif and may relate to the closure event of the Paleo-Tethyan Ocean. A younger history is recorded by U-Pb and Th-Pb monazite ages, which range Cretaceous to Miocene. Monazite geochronology is hindered by the contamination of high amounts of common Pb, but U-Pb and Th-Pb age calculations show Miocene to Jurassic ages. The textures of monazite (i.e. drusy, filling cracks of garnet and in reaction with allanite) imply that ages could record crystallization and/or fluid dissolution/reprecipitation mediated events in the Oligocene. Data reported here support the observation that polymetamorphic events are recorded in the Southern Menderes Massif rocks in close proximity to the Selimiye Shear Zone. Two options are possible environments of continental exhumation of rocks along the zone: (1) a polymetamorphic history that records relict high-pressures from previous metamorphic events or (2) a single-stage exhumation of high-pressure rocks.