Garnet growth across the quartz–coesite transition in metapelites: equilibrium vs. kinetics

Paola Manzotti, Federica Schiavi, Michel Ballèvre, and Francesco Nosenzo.  https://doi.org/10.5194/ejm-37-455-2025

Abstract
This study explores the growth of garnet across the quartz–coesite transition in metapelite. Garnet chemistry and texture were investigated in several metapelite samples (garnet-chloritoid micaschist) collected across different units in the northern Dora-Maira Massif (Western Alps), and thermodynamic modelling was used to constrain the P–T (pressure and temperature) conditions of garnet growth. Two groups of garnet-chloritoid micaschist were identified.

The first one displays evidence for a single garnet generation (Alpine in age) and occurs both in the basement (Muret Unit and Chasteiran Unit) and cover (Serre Unit) of the northern Dora-Maira Massif. In these rocks, garnet crystals display similar texture, chemistry, and P–T conditions of nucleation and growth. Coesite is found as tiny inclusions in the garnet outer cores. Thermodynamic modelling indicates that garnet cores (alm₇₆prp₆grs₅₋₆) nucleated in the quartz stability field or at the quartz–coesite transition at 2.5–2.7 GPa and 470–530 °C. Its growth (alm₇₈₋₈₀prp₈₋₁₀grs₅₋₆) culminated at 2.9–3.0 GPa and 530–550 °C in the coesite–chloritoid stability field. Peak burial conditions are very similar from sample to sample, irrespective of the unit where they occur, and samples were modelled in P–T conditions of 0.1 GPa and 20 °C. These P–T values are considered to be within the uncertainties accepted for thermodynamic modelling, suggesting that overstepping did not play a major role during garnet formation. A second stage of garnet growth (garnet rim), characterized by a sharp increase in grossular (up to 26 mol %) and decrease in pyrope (up to 3 mol %) is commonly observed: it developed during decompression in the quartz stability field (from 2.4–1.5 GPa) and slight cooling (540–500 °C).

The second group of metapelite was identified only in the basement (Muret Unit). In these samples, coesite is absent and polycyclic garnet crystals are present. A narrow Alpine garnet rim overgrew pre-Alpine garnet relicts at much lower P (∼ 2.1–2.2 GPa) than the quartz–coesite equilibrium. The absence of garnet growth at ultrahigh pressure (UHP) conditions in the second group of metapelite may be related to the details of the reaction mechanisms, especially the timing and amount of fluid access inside the system, as well as the consequent changing scale of the effective bulk compositions during garnet growth. In the polycyclic rocks, kinetics plays a major role.

We performed a review of garnet compositions in metapelites from a variety of terrains and P–T conditions, and we modelled the bulk-rock composition of an average metapelite. This shows (i) systematic variations of garnet composition with P–T conditions (decreasing grossular content with increasing P and increasing pyrope content with increasing T) and (ii) that the garnet compositions reported from the studied area are consistent with those described in other terrains equilibrated at the quartz–coesite transition.

Figure 1. Structural setting of the Dora-Maira Massif (modified from Manzotti et al., 2025). The black rectangle indicates the location of the geological map of Fig. 2.