TIMING EXTENSIONAL EVENTS USING MONAZITE: EXAMPLES FROM THE MENDERES MASSIF (WESTERN TURKEY)

Monazite, a rare earth-, radiogenic-bearing phosphate mineral, is typically applied towards understanding the timing of prograde events. The mineral forms at the garnet isograd from the breakdown of accessory minerals (e.g., allanite) which are stable at lower grades. Some ideal characteristics of monazite as a geochronometer are the incorporation of high amounts of Th or U while excluding common Pb and retaining radiogenic Pb to high temperatures. In the Central Menderes Massif in western Turkey, possibly the planet’s largest metamorphic core complex, we find monazite forms under retrograde conditions in garnet-bearing metamorphic assemblages from the breakdown of allanite. The pre-extensional history of the Central Menderes core complex is largely comprised of a series of nappes placed during Eocene Alpine compression. The nappes are exposed to the surface today due to Cenozoic extension. Granites that are deformed along a large-scale detachment surface constrain the timing of extension to the Miocene (9.6+/2.6 to 21.7+/-4.5 Ma). These granites that contain monazite as the only rare-earth bearing phase have CaO + Al₂O₃ contents restricted to <16 wt%. We identify retrograde monazite in the massif’s metamorphic assemblages as having ages ranging from 17 to 5 Ma and attribute them to Cenozoic extensional tectonics. Extensionally-created monazite shows no correlation with the rock’s major element chemistry and typically forms as overgrowths on allanite likely through fluid-mediated reactions. These monazite contain higher amounts of common Pb than are typical and can yield meaningless ages due to this problem. They also show mottling and fibrous textures and cathodoluminescence of the regions surrounding them document the presence of fluids. Due to the continuous nature of the retrograde conditions experienced by the rock, attributing their ages to timing the initiation of extension can be problematic. However, monazite geochronology may be another tool to time episodes of extension in metamorphic core complexes and lend insight into the dynamics of these regions. We find garnet-bearing rocks of the Menderes Massif contain allanite as the dominant rare-earth-bearing accessory mineral, thus when monazite is identified, it signals a change in conditions that potentially can be timed.