Francesco Nosenzo, Paola Manzotti and Martin Robyr, 2023. H2O budget and metamorphic re-equilibration in polycyclic rocks as recorded by garnet textures and chemistry. Lithos: 452–453, 107230.

https://doi.org/10.1016/j.lithos.2023.107230

Abstract

Massive dehydration is expected to occur during oceanic subduction. The situation is quite different during continental subduction, where often a large amount of crustal material has experienced a first orogenic cycle before the burial during the second subduction/collision cycle (rocks are therefore polycyclic). The amount and timing of dehydration and/or hydration episodes in polycyclic rocks strongly controls the extent of metamorphic re-equilibration during the second orogenic cycle. This study aims at estimating the fluid budget in polycyclic metapelites from the Muret Unit (Dora-Maira Massif, Western Alps). The excellent preservation, in a kilometre-scale low-strain domain, of pre-Alpine minerals and structures allows the comparison with the textures and structures observed in the pervasively foliated adjacent rocks.

In the low-strain domain, the main foliation is pre-Alpine and defined by high-temperature minerals whereas the Alpine high-pressure overprint is static. Pre-Alpine garnet porphyroblasts were fractured and partially dissolved before the growth of the Alpine garnet over pre-Alpine garnet fragments. The preservation of the overall shape of the original pre-Alpine porphyroblasts suggests that pre-Alpine garnet crystals were pseudomorphically replaced by chlorite during the late Variscan retrogression. This process was likely triggered by an episode of fluid-rock interaction and moderate hydration (⁓ 1–2 wt% H2O) which is also responsible of the growth of metamorphic zircon at ∼304 Ma.

In the high-strain domain, the dominant fabric is Alpine and developed at high-pressure conditions. Despite the difference in the strain intensity, metapelites from both low- and high-strain domains developed the same peak Alpine assemblage garnet-chloritoid-glaucophane-muscovite-rutile in the presence of a free H2O phase at 21–22 kbar and 530–560 °C. Thermodynamic modelling indicates that after the peak pre-Alpine metamorphism, a minimum re-hydration of at least 1 wt% was needed in order to reach H2O-saturation during the Alpine cycle.

Alpine garnet mainly sealed fractures and formed thin discontinuous overgrowth on the partially dissolved pre-Alpine garnet. Its growth occurred during the Alpine prograde to peak evolution, involving progressive consumption of chlorite and lawsonite and resulting in a prograde growth zoning. The preservation of the Alpine garnet growth zoning and the absence of complex compositional modifications suggest that the rock did not record massive pulses of fluid infiltration during subduction. Instead, the main fluid-rock interaction episode was limited and occurred before the Alpine re-equilibration during the late Variscan evolution.

Fig. 1. (a) Geological map of the South-Western Alps. The location of the Muret area is indicated with a red star (modified from Ballèvre et al., 2020). (b) Detail geological map of the Muret area (darker colour = outcropping rocks; lighter tone = interpretation under Quaternary deposits). Sampling sites in low-strain and high-strain Alpine domains are indicated with red circles. Original field data were projected on a digital topographic map (DTM; www.geoportale.piemonte.it). The southern slope of the Punta Muret is characterised by continuous outcrops along the ridge and large-scale landslides along the slopes. The landforms are therefore dominated by trenches and depressions that can be related to slope movements. For this reason, interpretative lithological contacts outside the low-strain domain have not been traced. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)