DECOMPRESSIONAL GARNET GROWTH DURING PROGRADE METAMORPHISM IN CALEDONIAN THRUST SHEETS, NW SCOTLAND

Recent quantitative thermometric analyses (quartz c-axis fabric opening angle and Fe-Mg garnet-biotite exchange) of samples collected in foreland to hinterland transects across the Moine, Ben Hope, and Naver-Sgurr Beag thrust sheets in northwest Scotland provide a comprehensive characterization of the Scandian orogenic retro-wedge thermal architecture. However, due to a lack of petrologically important metamorphic mineral phases (e.g. Al-silicate, staurolite, etc, insight into synorogenic P-T paths and particle trajectories has previously been limited, and this in turn has hindered our ability to examine the nature of orogen-scale kinematic and thermal coupling.

New isochemical pseudosection models in the system MnNCKFMASHT were produced for multiple garnet metapelite samples from the top of the Moine thrust sheet (footwall of Ben Hope thrust) and into the base of the Ben Hope thrust sheet. The presence of syn- to post-kinematic staurolite in samples from the top of the Moine thrust sheet constrains peak metamorphism to c. 620 °C at c. 6 kbar, similar to temperatures calculated using garnet-biotite (600 ± 50 °C) and quartz c-axis (590 ± 50 °C) thermometry. However, modeling indicates that garnet growth initiated at 545 °C and c. 8 kbar, suggesting growth and heating coeval with decompression. This is similar to samples from the base of the Ben Hope thrust, where peak pressures of c. 8.5 kbar occurred at c. 560 °C. Garnet growth during decompression is also consistent with petrographic observation of rutile and ilmenite inclusions in the core and rim of garnets, respectively. Some overstepping of the garnet-in reaction is attributed to initial emplacement of each successive thrust sheet, resulting in sharp pressure perturbations across this reaction. Orogen-scale surface-directed isotherm perturbations due to progressive thrusting and subsequent erosion explains this inferred temperature increase during the early stages of exhumation. Lower P-T conditions towards the foreland in the west, which is indicated by the abundance of chlorite and lack of staurolite/kyanite in Al-saturated rocks, results from decreased thrust-related burial and a reduced thermal gradient driven by subduction of pro-wedge lithosphere beneath the retro-wedge.