Journal of Metamorphic Geology最新文献

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.

The cover image is based on the Original Article Thermodynamic and Kinetic Controls on the Growth of Metapelitic Garnet in the Danba Dome (SW China): Insights From Microstructure, Element Mapping and Thermodynamic Modelling by Zhen M. G. Li et al., https://doi.org/10.1111/jmg.70005.

Orogenic belts that sustain elevated temperatures at intermediate crustal depths for tens of millions of years are known as hot orogens. The evolution of these hot orogens is largely influenced by thermal maturation, primarily driven by the distribution of heat-producing elements (HPEs), such as K, Th and U in the overthickened crust. This process involves widespread anatexis, granulite facies metamorphism, extensive transfer of fluids and magma and large-scale crustal flow. Although most of the thermal evolution of hot orogens is controlled by HPEs, episodic heat transfer may also contribute to their geodynamic development. The Araçuaí orogen, located in southeastern Brazil, represents a Neoproterozoic hot orogen that exposes in its internal domain deeper levels of an overthickened crust, resembling the Tibetan plateau. Its evolution involves more than 120 million years of magmatism and metamorphism, including ultra-high-temperature metamorphism. Different geodynamic models have been suggested to explain the thermal evolution of this orogen, ranging from episodic heat sources within subduction-collisional orogeny to continuous long-lived heat sources in collisional or intracontinental settings. In this paper, we integrate detailed thermodynamic modelling, Lu–Hf and Sm–Nd garnet dating, and U–Pb and REE zircon data from an outcrop that includes granulite facies metasedimentary rocks (Nova Venecia complex) and an intrusive gabbroic stock (São Gabriel pluton). This key outcrop allows us to investigate the history of high-grade regional metamorphism within the internal domain of the Araçuaí orogen, as well as the thermal impact of the late-stage high-temperature magmatism on the later evolution of this hot orogen. Our data indicate that the studied rocks reached near-peak conditions (6.5–10 kbar and ~800°C) at 535–530 Ma and followed a near-isothermal decompression path to 5–6 kbar at ~530–520 Ma. Only local effects of contact metamorphism (~5 kbar and 900°C–1000°C) were observed along the contact between the gabbroic stock and its host rock. Based on this newly integrated dataset and the compilation of existing information, we argue that the Araçuaí orogen evolved from a single, continuous, long-lived heat source controlled by radiogenic decay from HPEs, rather than episodic advective heating from subduction/collision-related processes.

Understanding how Paleoproterozoic orogenic processes shaped the assembly of Laurentia remains a critical puzzle in deciphering Earth's ancient tectonic history. To address this challenge, the Montana metasedimentary terrane in the northwest Wyoming Province, which preserves a complex record of multiple metamorphic episodes, provides a unique opportunity. In this study, we integrate garnet composition with coupled Lu-Hf and Sm-Nd geochronology to unravel the polymetamorphic history of this terrane and constrain the timing and mechanisms of orogenic processes. Our new garnet Lu-Hf and Sm-Nd ages reveal three distinct age groups: 2.42, 2.19–2.06 and 1.83–1.70 Ga. Most analysed garnet samples demonstrate typical prograde zoning patterns with enriched Mn and Lu contents in their core, suggesting that the Lu-Hf ages from these samples reflect the timing of prograde metamorphism. The ~30 Myr younger Sm-Nd ages for most samples document the cooling process that followed peak metamorphic conditions during the orogenic cycle. One amphibolite sample shows reverse Lu zoning that likely resulted from mineral breakdown reactions, suggesting complex trace-element incorporation during high-grade metamorphic processes. By integrating our age data with the estimated peak metamorphic conditions reported by previous studies, we identify a spatial and temporal trend of decreasing metamorphic grade that progressed southeastward from the northwestern boundary of the terrane between 1.8 and 1.7 Ga. This pattern reveals a propagation of orogenic activity during the Big Sky orogeny, providing insights into the thermal evolution and incorporation of terranes during Laurentia assembly.

Understanding whether deformation occurred in the presence of aqueous fluid or silicate melt is crucial for interpreting ductile shear zones, impacting their thermal and geochemical evolution, and having rheological consequences. To identify the syn-deformational fluid type, we investigate contrasting shear zones active during the Alice Springs Orogeny in central Australia, focusing on their effects on dry granulite facies gneisses transformed into greenschist–amphibolite facies schists. Shear zones in the north-western part of the orogen (Reynolds–Anmatjira Ranges) exhibit greenschist–lower amphibolite facies muscovite–chlorite assemblages, quartz veins and microstructures indicative of solid-state deformation. These features collectively suggest deformation in the presence of aqueous fluid. In contrast, shear zones in the south-eastern part (Strangways Range) display upper amphibolite facies garnet–biotite–sillimanite assemblages, along with granitic dykes and lenses retaining igneous textures. Microstructures, such as ‘string of bead’ textures and felsic minerals forming films along grain boundaries or exhibiting low apparent dihedral angles, indicate the former presence of melt in high strain rocks. This suggests that hydration in the south-eastern shear zones was driven by externally sourced silicate melt and melt–rock reactions. Differentiating between the two types of shear zones using whole rock major and trace element data is challenging. However, rare earth element (REE) analyses show potential. Limited REE metasomatism is observed where aqueous fluids are inferred, with three samples in a transect displaying consistent patterns. In contrast, where silicate melt is interpreted as the metasomatic agent, REE metasomatism is more variable, exhibiting atypical REE patterns relative to common rock types and considerable variability between samples in a transect. This contrast is attributed to greater mobility of REEs in silicate melt compared to aqueous fluid.

Geothermobarometry provides crucial constraints on the physical conditions of metamorphism, offering insights into petrogenetic processes and providing key information on thermal regimes and metamorphic depths to other geological disciplines. However, calibrating a thermobarometer from the natural record is challenging because independent pressure (P) and temperature (T) estimates are required, and the compositional variation of minerals—governed by multiple metamorphic reactions—must be captured in a complex function. This work calibrates a machine learning thermobarometer for biotite using relative P–T estimates based on mineral assemblage sequences. A neural network is used as a flexible model to fit a high-dimensional thermobarometric regression curve. To address the challenge of sparse training data, a transfer learning strategy is employed, where the model is primarily trained on a large dataset generated with phase equilibrium modelling before refinement with natural data. A general framework for calibrating machine learning thermobarometers is outlined using a neural network thermobarometer for biotite as an example. Selection of the best-performing model is guided by k-fold cross-validation alongside complementary accuracy checks using metamorphic sequences and precision assessments via Monte Carlo error propagation. Evaluation on an independent test dataset, compiled from the literature, indicates that the model is a potential biotite single-crystal thermometer with a root mean square error of ± 45°C, consistent with the estimated uncertainty of Ti-in-Bt thermometry applied to the same data. A potential barometer is affected by systematic underestimation of pressures above 0.6 GPa due to regression to the mean of the natural database, which is biased towards low-pressure metamorphism. This limits its applicability in higher-pressure regimes. This study highlights the potential of using neural networks with transfer learning in petrological applications since they are often constrained by limited natural data.