APPLYING GARNET-MONAZITE THERMOMETRY TO SCANDIAN THRUST SHEETS IN NORTHWEST SCOTLAND

Northwest Scotland features a series of east dipping ductile thrust faults cutting the metasedimentary Moine Supergroup. From the structurally lowest Moine thrust in the foreland to the most hinterland-positioned Naver thrust, there is a continuous increase in metamorphic grade from greenschist facies to sillimanite-bearing amphibolite facies. To further understand the tectonic history of this area, it is essential to establish temperature-time constraints for individual thrust sheets. Electron microprobe and laser ablation ICP-MS are used to analyze trace elements and U-Pb isotopes in samples throughout the study area. Some of our samples show clear REE zoning in garnet and monazite. In two samples from the leading edge of Naver thrust sheet, monazite crystals typically have low Y domains, which are likely to be in equilibrium with garnet crystals that display relatively flat REE zoning. Temperatures based on the garnet-monazite thermometer of Pyle et al. (2001), are estimated at 540-560 ˚C, in good agreement with other petrological constraints. Concordia ages of low Y monazite from these two samples are 426±2 and 425±4 Ma. Another sample from the leading edge of the Naver thrust sheet contains two distinct groups of garnet, each with distinct textures and REE zoning patterns. Which is challenging to interpret, but the full range of garnet compositions are consistent with temperatures of 620-680 ˚C, if garnet and monazite are in equilibrium. The concordia age of monazite in this sample is 420±2 Ma. One sample from the underlying Moine thrust sheet has distinct and consistent high Y garnet rims, which we interpret to be in equilibrium with monazite. Garnet-monazite temperature estimates for this sample are 580-590 °C, also consistent with other petrological constraints. The concordia age of monazite in this sample is 416±2 Ma. Importantly, previous geochronology (Goodenough et al., 2001) has suggested that transitional ductile-brittle deformation (300-400 °C) in the structurally underlying Moine thrust zone occurred at c. 430 Ma. In contrast, our results imply that the rocks in the ductilely-deformed hanging wall to Moine thrust were buried much deeper than those in the footwall until at least 420-415 Ma. Therefore, final movement on the Moine thrust likely occurred long after 430 Ma and perhaps even later than 415 Ma.