Effects of bulk-rock chemistry on paragonite stability in high-pressure epidote-amphibolites from the Yuli metamorphic belt, Taiwan

Paragonite is an important high-pressure (HP) indicator in epidote-amphibolites; however, its occurrence in these rocks is notably rare. Paragonite-bearing and paragonite-free epidote-amphibolites from Taiwan offer a great opportunity to understand the influence of bulk-rock chemistry on paragonite stability by phase equilibrium modelling. The investigated epidote-amphibolites have basaltic compositions with elevated Al₂O₃ content (16.2–19.7 wt%). The paragonite-bearing (PEA) type is characterised by pargasitic hornblende + epidote + paragonite + rutile + quartz ± garnet assemblages, while the paragonite-free (EA) type contains pargasitic hornblende + epidote + rutile + quartz ± chlorite assemblages. The mineralogical difference between the two types is attributed to a variation in bulk-rock Mg# (PEA: 28–54; EA: 63–67). Both types experienced similar peak pressure–temperature (P–T) conditions at 1.2–1.6 GPa and 575–625 °C (M1 stage). Although the P–T estimates are comparable to that of some eclogites, these rocks do not exhibit eclogite facies mineral assemblages. The corresponding paleo-geothermal gradients of 12–16 °C km^− 1 indicate a warm subduction environment, likely reflecting the thermal structure of a young intra-oceanic subduction zone. Although these rocks show little or no retrogression, we still identified two post-peak metamorphic stages, including a blueschist facies overprint at 0.7–0.8 GPa and 440–475 °C (M2 stage), and a greenschist facies overprint at 0.4–0.6 GPa and 400–475 °C (M3 stage). These P–T estimates suggest a cooling and decompression from M1 to M2, followed by a near isothermal decompression from M2 to M3. P–T–X modelling in the MnNCFMASHTO (K-free) systems shows that paragonite preferentially stabilises in high-Al, high-Na/(Na + Ca), high-Fe³⁺/ΣFe, or low-Mg# metabasic compositions. H₂O saturation, or near H₂O saturation, is also essential for paragonite stability. Using the median worldwide metabasite composition in Forshaw et al. (2024) as a global reference, the P–X modelling in the NCFMASHTO (K-free) system predicts that paragonite is stable when any one compositional parameter meets the following approximate thresholds: Al₂O₃ > ~17.0 wt%, Na/(Na + Ca) > ~ 0.45, or Fe³⁺/ΣFe > ~ 0.50. However, these values should be considered as a first-order approximation, rather than strict criteria. Our results emphasise the pivotal role of bulk-rock chemistry in controlling the occurrence of paragonite in metabasites.