The cover image is based on the article An Inverse Method for Quantifying Petrological Parameters and Uncertainty in Phase Equilibrium Modelling by Ian Cawood and T. Mackay-Champion, https://doi.org/10.1111/jmg.70016.
The cover image is based on the article An Inverse Method for Quantifying Petrological Parameters and Uncertainty in Phase Equilibrium Modelling by Ian Cawood and T. Mackay-Champion, https://doi.org/10.1111/jmg.70016.
The metamorphism of amphibolite facies Archean supracrustal rocks has variably been interpreted to relate to subduction–collision or drip/sagduction tectonics. This paper presents a systematic metamorphic study of supracrustal amphibolites from the Hongshanzi area in the Yinshan Block, North China Craton, to decipher their metamorphic evolution and tectonic implications. The amphibolites occur as rafts within host TTGs (trondhjemite–tonalite–granodiorite) and are metamorphosed under lower to upper amphibolite facies conditions. Two samples of lower-amphibolite facies rocks record one episode of metamorphism that exhibits clockwise P–T paths involving pre-Pmax burial heating and post-Pmax decompressional heating and subsequent cooling. The Pmax conditions are constrained to ~595°C, 8.5–9 kbar and ~650°C, 8 kbar, using isopleths of the maximum Ti content in amphibole and minimum XAn (anorthite) in plagioclase in P–T pseudosections. This approach is quite effective for the P–T estimation especially of subsolidus amphibolites. The mAl (Al in M2 site) and tAl (total Al) in amphibole have high potentials to be geobarometers, but they are commonly modified during decompression and tend to underestimate pressures for amphibolites. One sample of upper-amphibolite facies records two episodes of metamorphism. The early episode is characterized by a decompression and cooling evolution from the peak at 790°C–830°C, 7–8 kbar, defined using Ti and tAl in amphibole in P–T and T–M (TiO2) pseudosections because the rock is unsaturated with respect to TiO2. The later metamorphic event shows a clockwise P–T path and peak condition of 560°C–600°C, 6–7.5 kbar. Zircon dating yields a crystallization age of 2.53–2.52 Ga and three metamorphic ages at 2.52–2.51 Ga, ~2.46 Ga and 1.86–1.82 Ga that are interpreted to date amphibolite facies metamorphism, modification and the overprinting event, respectively. The Neoarchean metamorphism is characterized by variable apparent geothermal gradients from 18°C/km to 29°C/km that occurred coeval with the regional TTG magmatism and that was more likely correlated with a drip or sagduction regime. In addition, the Yinshan Block was involved in the Paleoproterozoic orogeny at 1.86–1.82 Ga.
Phase equilibrium modelling offers a powerful quantitative framework for understanding petrological processes. Yet, many studies still rely on qualitative comparisons between natural datasets and these forward modelled predictions to constrain model parameters, commonly pressure–temperature (P–T) conditions. Compounding this, uncertainties from the observed data or within the modelled predictions are rarely quantified, limiting confidence in the estimated P–T conditions and resulting petrological interpretations. We introduce LinaForma, an inverse modelling workflow that determines best-fit P–T conditions (or other petrological parameters) and their associated uncertainties for a given rock system by minimizing the misfit between observed data (e.g., mineral compositions or modal proportions) and their forward modelled predictions. Uncertainty is quantified by resampling of the observed data with replacement. Diagnostic metrics identify poorly performing variables and assess the sensitivity of the inversion result to variable uncertainty. Applied to an amphibolite-facies pelite and metabasite from the Greater Himalayan Sequence (Zanskar Himalaya, NW India), the approach proves effective across contrasting model systems that require different sets of solution models and variables, and it produces P–T estimates consistent with classical thermobarometry. The workflow offers several advantages: compatibility with outputs from any forward modelling software; flexible variable selection; systematic grid-search inversion in multidimensional space; a robust L1-norm misfit function resistant to outliers; and sensitivity and uncertainty analysis via bootstrap resampling. Limitations include the increasing computational demands for high-dimensional grids (N > 2) and the absence of explicit quantification of uncertainties inherited from the thermodynamic dataset and solution models. Alongside other emerging quantitative methods, LinaForma enables petrologists to make more informed interpretations of complex metamorphic systems and target improvements to thermodynamic datasets.
Rutile provides a wealth of petrochronological information in metamorphic geology and due to its high stability during processes of the sedimentary cycle, rutile takes a special position in sedimentary provenance analysis. Besides being one of the classical minerals datable using the U–Pb system, rutile incorporates a broad range of trace elements, many of those being incompatible in most of the common metamorphic minerals. Although numerous multivariate statistical or machine-learning tools are available, current rutile discrimination schemes suffer from focusing on uni- and bivariate approaches leading to large compositional overlaps. Here we compiled and enlarged a dataset of 2335 rutile trace-element analyses (1646 new analyses) from 110 metamorphic rock samples of 48 localities covering a wide range of pressure–temperature conditions. After showing that the subsampling and testing strategy of the classical random forest algorithm is inappropriate for such hierarchical data structures, we introduce a modified version (random forests for hierarchically structured data) which provides realistic and generalized error estimates, improving hyperparameter tuning and performance. By applying this concept, we present a novel and multivariate rutile discrimination scheme, using the concentrations of 16 elements. The model correctly predicts the source rock composition (felsic versus mafic) in ~89% of the cases and the metamorphic gradient (≤ 350 °C/GPa versus > 350 °C/GPa) in ~84%. Combined with U–Pb dating, this will enable time-resolved insights into the geodynamic evolution of the hinterland, taking rutile provenance analysis to the next level. This approach furthermore highlights specific elements and ratios — previously understudied — that reflect bulk-rock and partitioning effects, making them valuable for future investigation of metamorphic processes. For the ease of application, a user-friendly “rutileRF-HSD” web application is provided.
The breakdown of hydrous minerals during subduction results in a densification of the rocks, producing volume changes that can trigger fluid pressure fluctuations and hydrofracturing. These volume changes are controlled by the Clapeyron slope of the reaction and are quantified here using a macroscopic framework that integrates a zero-dimensional mechanical model and a one-dimensional petrological model. A Mohr–Coulomb–based mechanical model is coupled with phase equilibrium calculations (Theriak-Domino), oxygen isotope fractionation and trace element partitioning, using the compressed compensated Redlich–Kwong (CORK) equation of state to describe fluid properties. This model can predict fluid pressure fluctuations and brittle-failure events from successive Gibbs free energy minimisations while accounting for tensile strength and differential stress in the mechanical model. The code is released as an open-source Python library ThorPT. For intermediate subduction geotherms, results indicate up to 5 vol.% fluid-filled porosity without brittle failure, consistent with findings from several geophysical studies. In basalts and serpentinites, the calculated time-averaged permeability values show intermittent episodes of permeability exceeding 10−19 m2. In serpentinite, the permeability increase can be attributed to olivine formation. In mafic rocks, episodes of increased permeability are associated with the breakdown of chlorite, amphibole and lawsonite during the transition from blueschist to eclogite. In the case of warm subduction geotherms, the model results show a strong correlation between dehydration-driven brittle failure, fluid migration and seismicity, particularly in the mafic crust. The blueschist–eclogite transition is associated with intense brittle failure, matching geothermal and geophysical models linking this depth range to seismic double layers. Our results highlight the coupled mechanism of dehydration reaction and brittle failure as a driver of episodic fluid extraction, with broad implications for fluid–rock interaction and mass transfer in subduction zones.
Microanalysis of trace elements in garnet can yield new and important insights into the kinetics of garnet growth, metamorphic reactions and fluid–rock interaction in subduction zones; however, differentiating the effects of these processes on a garnet-by-garnet basis can be challenging. In this study, we couple microanalyses of trace elements and stable oxygen isotopes in garnet to isolate the causes of trace element variation in mineralogically banded blueschists and eclogites from the Franciscan Complex. Trace element variations in garnet are not apparently driven by interaction with externally derived fluids as recorded by rimward decreases in δ18O and can be best explained by local mineral reactions. Intrasample heterogeneity in garnet trace element zoning patterns can be attributed to localized differences in mineral assemblages, which impact the availability of trace elements to growing garnets. These mineralogical heterogeneities also control the extent of reaction with externally derived fluid and resulting δ18O zoning patterns through differences in reaction-induced porosity. Our study highlights the importance of linking petrography to advanced microanalysis and has important implications for understanding the chemical composition and physical pathways of fluids in the eclogitized downgoing slab.
Elevated heat flow associated with mafic magmatism in accretionary orogens has often been proposed as a driving mechanism for (ultra)high-temperature (UHT) metamorphism and anatexis. The Sancheong–Hadong complex, located in the southern Yeongnam Massif, Korea, consists of a ca. 1.87–1.86 Ga anorthosite–mangerite–charnockite–granite (AMCG) suite hosted by granulite-facies gneisses. We present new evidence for UHT metamorphism and near-isobaric cooling in garnet-orthopyroxene granulite xenoliths entrained in the Sancheong anorthosite. These xenoliths record partial melting, typified by leucosomes containing peritectic garnet and orthopyroxene. The development of garnet coronas or garnet-quartz symplectites at the orthopyroxene-plagioclase interface in the granulite reflects near-isobaric cooling under a low fH2O condition. P–T pseudosection modelling of a garnet-orthopyroxene granulite reveals peak metamorphic conditions of 900°C–930°C and ~8.0 kbar, followed by near-isobaric cooling and melt crystallization at 800°C–830°C and ~7.4 kbar. Such a UHT condition is further supported by zircon crystallization temperatures of ~900 ± 30°C, estimated from Ti-in-zircon thermometry. Zircons in the granulite and leucosome are distinctly zoned into three domains (cores, inner rims and outer rims) that reflect successive recrystallization under high-temperature conditions. U(–Th)–Pb dating of zircon and monazite from two garnet-orthopyroxene granulites and one leucosome yields weighted mean ages of 1861 ± 5 Ma (n = 9) and 1863 ± 12 Ma (n = 3), respectively, coeval with the emplacement of anorthosite at 1861 ± 2 Ma. This result suggests an almost complete resetting of the U–Pb systematics during the transient UHT event at ~1860 Ma. The δ18O values of zircon in the granulite are uniform at 8.15 ± 0.11‰ (n = 83). Together with the zircon ɛHf(t) values ranging from −7.0 to −0.3, such an elevated δ18O value indicates a derivation from crustal sources. Zircon rims in the leucosome yield slightly lower δ18O values (7.67 ± 0.10‰; n = 17) which are consistent with 7.52 ± 0.14‰ (n = 20) estimated from monazites of the granulite. In contrast, monazite from the leucosome has an even lower δ18O value of 6.57 ± 0.43‰. These results suggest that δ18O values decrease during cooling and melt crystallization, most likely due to the influx of lower δ18O juvenile fluids and/or melts during AMCG emplacement. Taken together, the lower crustal xenoliths of the southern Yeongnam Massif underwent UHT metamorphism and anatexis at ~1860 Ma, accompanied by anorthosite emplacement, within the context of a prolonged melt-bearing system during the late stage of the Paleoproterozoic hot orogenesis in the North China Craton.
Evidence of metamorphism at ultrahigh-pressure (UHP) conditions is documented by the presence of coesite, diamond and/or majoritic garnet. However, the growth of UHP-stable phases such as majoritic garnet is often volumetrically low, and overprinting during exhumation can obscure evidence of UHP growth, making it difficult to positively identify UHP rocks. In this study, we selected garnet-kyanite schists from three microdiamond-bearing localities within the Rhodope Metamorphic Complex, located in eastern Greece. Samples from Xanthi, Sidironero, and Kimi have similar bulk rock compositions, but the pressure–temperature (P–T) paths differ. Because the major phases record vanishingly little evidence of metamorphism at UHP conditions, we analyzed zircon grains with complex textures to evaluate if zircon preserves a record of UHP metamorphism. Zircon grains from all localities have cores and rims separated by a characteristic interface domain, as revealed by cathodoluminescence (CL) imaging. The detrital igneous cores range in age from c. 2.5 Ga to 220 Ma and exhibit a negative Eu* anomaly, a Yb/Gd of 10–100, and variable Th/U (0–1.2). Rims yield dates of 150–125 Ma with Yb/Gd of 0.1–10 and Th/U of 0–0.2. Interface domains yield dates 165–145 Ma with Yb/Gd ranging between 0–1000 and Th/U < 0.2. We interpret the distinctive CL textures and Yb/Gd of the interface domains as evidence of zircon that reacted at UHP. The interface domain in zircon from all petrographic contexts yields variable Yb/Gd ratios that are significantly higher than both cores and rims. We therefore interpret that zircon recrystallized via interface-coupled dissolution–reprecipitation reaction; this process preferentially partitioned heavy rare earth elements within the interface domain, which explains the higher Yb/Gd ratios. The rim domains equilibrated with the matrix, producing a relatively homogeneous and low Yb/Gd ratio in these domains.
The spatial extent and degree of preservation of interface domains are interpreted as a function of the P–T path and minor variations in bulk composition. Interface domains are best preserved in rocks from Xanthi and Sidironero; in these samples, thin, homogeneous, garnet-stable rims only partially overprint and crosscut the interface domain. In contrast, rocks from Kimi followed a higher-temperature trajectory and the zircon grains grew large rim domains that overprinted much of the interface domain and the detrital core. Zircon grains from plagioclase-rich versus quartz-rich domains within samples from Sidironero show differences in texture, which indicates that local bulk composition can affect what evidence of UHP metamorphism is preserved. Collectively, these samples provide a new, durable marker of metamorphism in UHP rocks and yield new insight about which factors affect the preservation of UHP textures.
Metaophiolites and metasediments of the Piedmont Zone are exposed in the Soana Valley, a poorly studied area of the Western Alps, located between the Europe-related Gran Paradiso massif and the Adria-related Sesia–Lanzo zone. Previous investigations reported the occurrence of only greenschist-facies metamorphic assemblages in the metasediments. We present new petrographic analyses and phase equilibria modelling of garnet-chloritoid- and garnet-lawsonite-choritoid-bearing mica schists from the Soana Valley. These mica schists preserve relicts of Alpine high-pressure mineral assemblages, testifying both a prograde and a decompressional evolution. Four main metamorphic stages are recognized from compositional zoning of garnet in a garnet-lawsonite-chloritoid-bearing mica schist: a subduction-related prograde stage at 17.0–18.5 kbar and 420°C–440°C (high-pressure blueschist-facies), a pressure peak at 19.0–21.5 kbar and 440°C–460°C (blueschist–eclogite-facies transition) and two decompressional stages at 17.5–18.5 kbar and 470°C–490°C (high-pressure blueschist-facies) and 14–15 kbar and 490°C–510°C (peak temperature). The peak pressure conditions of the Soana Valley metasediments are comparable (∆P ≤ 2–3 kbar) to those reported for some units of the external Western Alps located at structurally lower levels. Our results lead us to interpret the Piedmont Zone as a stack of tectonic slices, each one with its own metamorphic history, accreted together during subduction, rather than an assemblage of two juxtaposed belts characterized by distinct metamorphic histories, as traditionally reported.
Dehydration of rocks and minerals during high-pressure metamorphism is controlled by the thermal structure of the subduction zone and can be traced by the stability fields of hydrous phases. Consistent with the results of experimental works, the content of silicate components in fluids is assumed to increase from the fields of subcritical to supercritical fluid at higher pressure and to hydrous melt at higher temperature. As there is no direct information about the composition of these fluids along the subduction zone, the multiphase (polyphase) inclusions in anhydrous minerals in eclogite facies rocks are frequently interpreted as products of such fluids or melt, which became entrapped and subsequently crystallised. Although some of these multiphase inclusions are formed by quartz and feldspars and have a composition of granitic melt, most of them contain phases that do not support their formation from granitic or other types of melt. The assumption of their formation from supercritical fluids or melt has stimulated laboratory research to reproduce the original fluids or melts from which these multiphase inclusions could have originated. Therefore, petrological data useful to verify or falsify this assumption is essential for further direction in multidisciplinary research on fluid and melt generation in the subduction zone. This paper presents textural and compositional relations among multiphase inclusions in garnet from various lithologies subjected to high- to ultrahigh-pressure metamorphism. The aim is to substantiate the in situ formation of multiphase inclusions through recrystallisation of pre-existing (hydrous) solid phases enclosed within host minerals. In addition to the presence of original hydrous solid phases, evidence is provided for their transformation into high-pressure phases during prograde metamorphism as well as their subsequent modification and decomposition during retrogression or heating associated with decompression. Furthermore, it is discussed whether the crystallisation of garnet with inclusions is in agreement with bulk-rock fractionation and thermodynamic modelling and whether the compositions of multiphase inclusions correspond to the pressure–temperature fields of hydrous melts or supercritical fluids from experimental data. The results of the study on the textural and compositional relationships of the multiphase inclusions, along with the pressure stability fields of the host minerals, support the interpretation that these inclusions formed by recrystallisation of originally enclosed aqueous and anhydrous phases, rather than by entrapment from a melt.
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