How continuous and precise is the record of P–T paths? Insights from combined thermobarometry and thermodynamic modelling into subduction dynamics (Schistes Lustrés, 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 (T max ) 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 T max data suggest that underplating processes and early structural development played a key role in the Schistes Lustre´s accretionary complex.

Zagros blueschists: Episodic underplating and long-lived cooling of a subduction zone

“…The nature of the juxtaposition of the Seghin slice below the Ashin unit is kinematically challenging to understand because it involves underthrusting of higher pressure material underneath lower pressure units ( Fig. 8; see also similar observations in the Franciscan suture: Platt, 1986; in the W. Alps: Plunder et al, 2012;Angiboust et al, 2014;in Turkey: Plunder et al, 2015). This accretion process could result from large-scale geodynamic perturbations of the subduction system (e.g., late Cretaceous plate reorganization marked by acceleration of closure and obduction events; Agard et al, 2007;Monié and Agard, 2009) and/or by the entrance and the underthrusting of more buoyant material in the subduction channel promoting accretion and exhumation along the interface (e.g.…”

“…Natural remnants allowing to understand the structure and the nature of deeper (e.g. 30-50 km) accretionary processes are relatively rare (Kimura and Mukai, 1991;Plunder et al, 2012;Angiboust et al, 2013) and/or have not been recognized on the field so far. Yet, the question of subduction accretion processes is fundamental as it results in the progressive abandonment of the main subduction thrust fault and leads to the downwards migration of the seismically active thrust beneath the accreted material.…”

Subduction zone metamorphic pathway for deep carbon cycling: I. Evidence from HP/UHP metasedimentary rocks, Italian Alps

“…Protoliths of these rocks could have been sediment sections such as those subducting into the modern-day subduction zone at the Timor/Banda forearc, sampled at ODP Sites 262 and 765 (Ernst, 2005;Trümpy, 2003; description of this section in Plank, 2014). In addition to the thorough geothermobarometry and geochronological studies (Agard et al, 2001a(Agard et al, , 2002Beyssac et al, 2002;Takeshita et al, 2004;Gabalda et al, 2008;Plunder et al, 2012), some stable isotope work has previously been done on the Cottian Alps traverse (Henry et al, 1996;Busigny et al, 2003;Bebout et al, 2013).…”

“…In this study, we assessed the extent of retention of C in sediments subducted to depths approaching those beneath volcanic fronts through study of the concentrations and stable isotope compositions of carbonate and reduced/organic C in HP/UHP metasedimentary rocks in the Italian Alps. Our focus was on a well-studied traverse of the Schistes Lustrés in the Cottian Alps, exposures of sediments associated with the Zermatt-Saas ophiolite in the Val d'Aosta region, and UHP metapelitic and metacarbonate rocks exposed at Lago di Cignana, Valtournenche (see Agard et al, 2001a;Angiboust et al, 2009;Plunder et al, 2012;Bebout et al, 2013;see Fig. 2).…”

Thermodynamic forward modeling of retrogressive hydration reactions induced by geofluid infiltration

“…The limitations of the forward models are due to some important differences between the conditions and behavior of prograde dehydration and retrograde rehydration reactions, even though rehydration is simply a reverse dehydration reaction. Rehydration reactions commonly proceed heterogeneously in a non-equilibrium state (e.g., Brodie and Rutter 1985;Plunder et al 2012), whereas dehydration reactions occur under relatively homogeneous conditions in a perfect equilibrium state. As described in detail in the next section, rehydration reactions are generally fluid-induced and occur via replacement of relict minerals by equilibrated minerals (e.g., Andersen et al 1991;Imon et al 2002;Ota et al 2004;Okamoto and Toriumi 2005;Putnis 2009;Putnis and Austrheim 2010;Centrella et al 2015).…”