Nanoscale chemical zoning of chlorite and implications for low-temperature thermometry: Application to the Glarus Alps (Switzerland)

A combination of adapted chlorite thermometry and high-spatial-resolution analytical techniques (TEM-EDX/FIB) shows that the low-grade metamorphic chlorites of the Glarus Alps, Central Alps (Switzerland), do not record the peak metamorphic conditions as commonly assumed in previous studies of this reference area for low-grade metamorphism. Chlorites have rather recorded several stages of the retrograde path, through an intracrystalline nanometric-scale compositional zoning. The consistency of the nanoscale zoning patterns observed both within sample and along the transect suggests that local equilibrium was achieved at this spatial scale and maintained during growth or re-equilibration. Applying recent thermobarometers, we highlight that chlorites recorded a distinct behaviour between the northern and southern part of the studied transect: the south of the Glarus area displays a regular P-T exhumation path, from 3.0 ± 0.2 kbar and 310 ± 20 °C for maximum P-T (estimated with conventional thermometers), to ~0.8 kbar and ~220 °C according to chlorite crystal-core analysis, and to ~0.3 kbar and ~190 °C according to chlorite crystal-rim analysis. On the contrary, the north of the Glarus area shows an apparent break in the exhumation, with P estimates from chlorite crystal rims (~1.3 kbar) higher than from chlorite crystal cores (~0.8 kbar). Even if the absolute pressure values are fraught with large uncertainty, their contrasting core/rim pattern is not model dependent. We try to correlate these new thermobarometric results with independent data to refine the exhumation scenario of the North-Helvetic flyschs, confirming differential uplift along the transect. This reappraisal of Glarus chlorite thermometry demonstrates that an adapted thermometry/analysis protocol opens new prospects for investigating the evolution of low-grade metamorphic terranes.