Elastic Thermobarometry on Metapelites Across the Crustal Section of the Famatina Arc, Argentina

The Famatina orogen in northwestern Argentina preserves a near full crustal section of the Ordovician Famatina magmatic arc. This Paleozoic orogenic system provides an excellent natural laboratory to study subduction dynamics and whole-arc processes. Here, we performed elastic, Raman thermobarometry on metasedimentary rocks along transects through the crustal section at Sierra Valle Fertil - La Huerta. We targeted garnet-bearing migmatites from different paleo-depths for quartz-in-garnet barometry and zircon-in-garnet thermometry. We prepared grain mounts using ~60 handpicked garnets, and then identified ~15 quartz and zircon inclusions hosted in those garnets. We measured the inclusion pressure of quartz and zircon inclusions via Raman spectroscopy, calculating the spectral shift of the inclusions relative to free reference quartz and zircon crystals. Entrapment conditions were calculated using elastic models based on hydrostatic pressure calibrations and residual strain components. Across the suite of samples, elastic thermobarometry recovers entrapment conditions of 350 to 1200 MPa and 760 to 980 °C. The conditions are broadly consistent with anticipated temperatures and pressures along the crustal section, derived using conventional, thermodynamic geothermobarometers appropriate for metapelites (Tibaldi et al. 2013). For a given sample, however, elastic thermobarometry results are ~200 MPa and ~100 °C higher than conventional geothermobarometry. The discrepancy may result from uncertainties in elastic thermobarometry. Quartz-in-garnet inclusion pressures plot along isomekes near the α-β quartz phase transition, which introduces large uncertainties. Temperature estimates derived from zircon-in-garnet rely upon accurate assessment of the degree of metamictization in zircon inclusions, a variable that can be challenging to constrain. The discrepancy between elastic and conventional thermobarometry may instead indicate the equilibrium-based approach captures closure temperatures after regional cooling from peak conditions. If true, then elastic thermobarometry may better estimate peak metamorphic conditions. Our work highlights the importance of combining elastic and thermodynamic thermobarometry to better examine the full P-T evolution of an arc.