How continuous and precise is the record of P-T paths? Insights from combined thermobarometry and thermodynamic modelling into subduction dynamics (Schistes Lustre's, W. Alps)
Abstract
Pressure-temperature (P-T) paths as complete as possible and with a precision on the km-scale or less are needed to further improve the knowledge of deformation, re-equilibration processes and element ⁄ fluid transfer, in particular along subduction zones. This contribution attempts to (i) critically evaluate the precision and continuity with which metamorphic P-T histories are retrieved today and (ii) discuss implications for regional-scale accretionary processes in subduction zones, through application to the Schistes Lustre's complex (Haute Maurienne, W. Alps). P-T estimates are compared and combined using several independent approaches: (i) from minerals assumed to be in textural equilibrium; (ii) from electron microprobe compositional maps; and (iii) from pseudosection modelling predictions. Multi-equilibrium calculations were performed with TWEEQU and THERMOCALC, and pseudosections were built with PERPLE_X and THERIAK ⁄DOMINO. These P-T estimates were also compared with maximum temperatures (Tmax) deduced from the Raman spectroscopy of carbonaceous matter. The different methods used here yield the peak of pressure for the lower structural unit of the Schistes Lustre' s at 480 C and 23 kbar and document the retrograde path for both the Median and Lower Units. The results show that P-T conditions are recorded almost continuously and can be determined with a precision of ±1 kbar and ±30 C at best. This study underlines the complementarity of the various thermobarometric methods and demonstrates that precision could be increased by improving solid solution models for chlorite. Observed tectonic patterns, major lithological boundaries, pressure-temperature and Tmax data suggest that underplating processes and early structural development played a key role in the Schistes Lustre's accretionary complex.