ABSTRACT Reconstructing the pressure–temperature–time ( P–T–t ) evolution of (ultra)high‐pressure rocks is essential for constraining subduction and collisional dynamics. Subduction zones are often investigated using basic eclogites, whereas nonbasic high‐pressure rocks remain comparatively understudied, potentially hindering a comprehensive understanding of complex subduction systems. The Bughea Complex is a modern‐style mélange within the pre‐Alpine basement of the South Carpathians (Romania) that hosts a wide variety of eclogitic rocks, providing an opportunity to investigate their response to subduction processes. Here, we integrate phase equilibrium modelling, rutile thermometry, zircon and apatite U–Pb dating coupled with trace element analysis and garnet Lu‐Hf dating to reconstruct the P–T–t evolution of three different eclogitic rocks from the Bughea Complex: a kyanite eclogite, a basic eclogite and an eclogitic schist. The kyanite eclogite is interpreted to derive from a continental sedimentary protolith based on its continental crust‐like trace‐element patterns and detrital‐like zircon U–Pb age spectrum (ranging from Archean to Cambrian). It records the highest peak pressure conditions (~3.2–3.6 GPa at 700°C–820°C), consistent with ultrahigh‐pressure metamorphism, followed by decompression and cooling to < 1.6 GPa and < 675°C. In contrast, the basic eclogite, interpreted to derive from altered oceanic crust based on its MORB‐like trace‐element patterns, and the eclogitic schist, which exhibits trace‐element signatures similar to trench sediments in volcanic‐arc settings, record lower peak pressures (~2.0–2.3 GPa at 580°C –610°C and ~1.9–2.4 GPa at 530°C–600°C, respectively). The former records near‐isothermal decompression (~1.3 GPa at 585°C–615°C), whereas the latter records decompression and minimal heating (~1.0–1.2 GPa at 600°C–615°C). Distinct garnet Lu–Hf ages (469 ± 96 to 345 ± 21 Ma) suggest diachronous metamorphism during a prolonged Variscan subduction, whereas apatite U–Pb age (ca. 340 Ma) likely records late‐stage cooling and exhumation. Differences in P–T–t records may reflect mélange rheology and variations in subduction dynamics, whereas limited heating during retrogression and compositional buoyancy appear critical for preserving peak conditions, particularly in nonbasic rocks. Overall, these findings demonstrate that continental‐derived metasedimentary rocks can preserve evidence of extreme pressure conditions not always recorded by associated basic eclogites, highlighting the importance of integrating nonbasic high‐pressure rocks into P – T – t reconstructions of subduction‐related mélanges to achieve a more complete understanding of complex collisional systems.
ABSTRACT Fluid‐assisted Mg metasomatism profoundly alters crustal rocks, producing phyllosilicate‐rich assemblages, with major implications for rheology as well as petrophysical properties such as permeability. However, the effects such processes remain poorly constrained due to limitations in our ability to model the sequence of mineral reactions and bulk composition changes during fluid–rock interactions. Here we address this challenge through a phase equilibrium analysis of Mg–Ca/Na–Si metasomatism of a granodiorite infiltrated by an ultramafic‐derived fluid, using chemical potential phase projections and sections of mobile elements. We investigated a shear zone, in a granodiorite located in the Zillertal massif (Neves area, Tauern window in Eastern Alps, Italy), where Alpine amphibolite‐facies metasomatism and deformation (550°C—0.55 GPa) transformed the late‐Variscan granodiorite into amphibole‐bearing schists (garbenschiefer) and ultimately into a chlorite–muscovite schist. Mass balance calculations show strong MgO enrichment (up to 370%) and H 2 O gain, coupled with substantial CaO, Na 2 O and SiO 2 loss (−92%, −86% and −13%, respectively). Chemical potential phase diagrams of these most mobile elements computed at constant pressure and temperature demonstrate that amphibole is a transient phase stabilized by MgO and CaO gain at the expense of quartz, before it breaks down into a CaO‐free chlorite–muscovite assemblage. Although absent in the studied area, our modelling indicates that greater MgO gain and SiO 2 loss could stabilize that talc‐bearing assemblage: the so‐called whiteschist in the Alps. We infer that the metasomatizing fluid is externally derived from the dehydration of nearby ultramafic bodies, consistent with processes described in the Greiner shear zone, located to the north of the studied area. Finally, we demonstrate that our modelling approach can be applied to model metasomatic zones at contacts between chemically contrasting lithologies, such as ultramafic and sediments, like in subduction mélange zones.
ABSTRACT The rheological behaviour of solid‐solution minerals may play an important role in determining how metamorphic crust accommodates deformation. Whether or not a mineral's deformation is diffusion or dislocation‐mediated depends on a range of extrinsic variables such as stress, strain rate and temperature, as well as intrinsic variables such as diffusivity and water activity. However, it remains unknown if the local gradients in chemical potential that develop between metamorphic phases with evolving pressures and temperatures can also impact the deformation mechanics of metamorphic rocks. Here, we pair electron‐backscatter diffraction (EBSD) data with phase equilibria modelling of high‐variance, low‐variance and compositionally layered blueschist mineral assemblages to demonstrate that the local chemical driving force may directly influence the relative prevalence of dislocation creep and diffusion creep in amphibole. In high‐variance assemblages with glaucophanite domains ( F = 7), glaucophane displays subgrain boundaries with a consistent misorientation axes distribution indicative of dislocation creep. In domains with rare (meta stable) omphacite, fewer subgrains are observed in glaucophane and the distribution of subgrain boundary misorientation axes is more diffuse. Whereas in neighbouring rocks with low‐variance assemblages ( F = 1–3), glaucophane subgrain boundaries are rare and the distribution of low‐angle misorientation axes is random. The amphibole microstructures in the low‐variance assemblages are consistent with diffusion creep having been the dominant deformation mechanism. The increased reactivity associated with low‐variance assemblages enhances the chemical driving force for intracrystalline diffusion, thereby accelerating mass transfer and favouring diffusion‐mediated deformation. In contrast, higher variance assemblages exhibit a reduced contribution from chemically driven diffusion and accommodate strain primarily by dislocation creep.
ABSTRACT Recent studies have suggested various mechanisms to generate melt during exhumation of deeply subducted crust, including dehydroxylation of omphacite, fluid‐absent omphacite‐ and/or phengite‐breakdown melting and fluid‐present melting. However, the space–time relationships among these mechanisms remain uncertain. Here, we report microstructural evidence of the reactions responsible for, and compositions of, low‐volume melts frozen in situ (as leucosome pockets) within weakly deformed granitic veins that cross‐cut foliated UHP eclogite boudins in gneisses from the central Sulu belt, China. Phengite in the granitic veins records crystallization pressures of 3.4–2.7 GPa, and the granitic veins have whole‐rock Sr–Nd isotope compositions and trace element patterns consistent with derivation from the eclogite. The granitic veins likely crystallized from a solute‐rich supercritical fluid or hydrous melt generated by dehydroxylation of nominally anhydrous minerals during the early stage of exhumation (decompression). Subsequently, leucosome pockets and grain‐boundary films of melt formed in the granitic veins by the successive breakdown of omphacite and phengite. Based on the mineral modes and chemical compositions of 55 leucosome pockets (cf. microgranitoids), we distinguish those with (1) high Na/K ratios mainly composed of plagioclase and euhedral amphibole with skeletal omphacite and (2) low Na/K ratios predominantly composed of K‐feldspar and plagioclase and which contain phengite with corroded margins and fine‐grained biotite. High Na/K leucosome pockets (HLPs) are consistent with a melting reaction involving mostly omphacite‐breakdown, whereas low Na/K leucosome pockets (LLPs) are inferred to have formed by a reaction consuming variable proportions of phengite and omphacite. We argue that the reactions to form HHP then LLP initiated at approximately 1.5 and 1.1 GPa, respectively, and document two different mechanisms to generate melt during exhumation of deeply subducted crust in which breakdown of omphacite occurs before (deeper than) phengite. At higher temperatures, these melt‐producing reactions would generate a larger volume of melt that potentially could facilitate exhumation and increase crust–mantle interactions, thereby increasing the compositional heterogeneity of orogenic mantle.
In the middle to lower continental crust, mineral reactions can play a key role in controlling rheological behaviour by generating fine-grained, mechanically weak domains. In granulite-facies rocks from Kross & oslash;y (Bergen Arcs, Norway), we document how garnet breakdown produced interconnected networks of orthopyroxene + plagioclase +/- spinel that formed along grain boundaries and reaction fronts. We interpret the reaction as being initiated during early Caledonian decompression and cooling, which changed from Grenvillian high-pressure granulite- to lower pressure granulite-facies. The networks are characterized by fine-grained Ca-enriched plagioclase (relative to the surrounding coarse-grained plagioclase) and orthopyroxene symplectite (10-50 mu m) surrounded by larger feldspar grains (1-2 mm). Phase-equilibrium modelling indicates that the formation of fine-grained Ca-enriched plagioclase networks occurred under lower pressure granulite-facies conditions and predates the main amphibolite-facies overprint. These reaction products form mm- to cm-scale bands that align under stress and are reactivated during amphibolite-facies conditions. Our observations show that reaction-driven grain size reduction, even when limited to 5%-10% of the rock volume, produces mechanically weak plagioclase networks that are potential sites for later strain localization. This suggests that the early formation of fine-grained reaction products can locally weaken the crust, forming distinct rheological domains and controlling where deformation and later metamorphic evolution are concentrated.
Understanding the pressure of emplacement of granitic intrusions is crucial to understanding the exhumation history of plutons and constraining the tectonic setting of magma emplacement. However, P-T and geochronological constraints from exhumed plutons are often characterized by large uncertainties, especially in shallow crustal settings with p < 2-3 kbar, where minimal changes in estimated pressure can drastically change the exhumation model. We show an example from a typical upper crustal intrusion, the Porto Azzurro Pluton on the Island of Elba (Italy). This pluton emplaced at similar to 6.5 Ma at less than 7-8-km depth and is believed to have been exhumed under a regional low-angle normal fault, known as the Zuccale Fault. Phase equilibrium modelling of garnet +/- cordierite +/- andalusite-bearing parageneses in hornfels shows that the metamorphic pressure registered by the contact aureole rocks is lower than previously thought, constraining the Porto Azzurro Pluton to similar to 3.8-4.6-km depth (p = 1.0-1.2 kbar). Our P-T data constrain, for the first time, the minimum and maximum metamorphic pressure of the Zuccale Fault footwall and hanging wall blocks. P estimates overlap in the footwall (p = 0.3-1.8 kbar) and hanging wall (p = 0.4-1.2 kbar) blocks of the Zuccale Fault. Moreover, both fault blocks show a similar thermal evolution, which indicates that the fault contributed little, if at all, to the exhumation of the intrusion.
Shear zones act as preferential fluid pathways during prograde and retrograde stages of metamorphism. Nonetheless, we still have limited knowledge of the drainage and permeability of natural settings. The preserved signature of fluid in exhumed rocks provides insights into fluid flow during burial and exhumation. This study investigates fluid flow processes recorded by garnet in quartz-schists from the As Sheik shear zone (Saih Hatat window, NE Oman). We observed garnet as equant, oblate and honeycomb (i.e., skeletal) shapes that document distinct fluid-related growth stages from peak-pressure to early exhumation, both occurring at eclogite facies conditions. Garnet nucleated at 2.0-2.2 GPa and 500 degrees C-550 degrees C after the chloritoid-out dehydration reaction. Subsequent decompression and heating (1.3-1.5 GPa, 600 degrees C-650 degrees C) promoted further fluid release and honeycomb garnet growth. We infer that dissolution, transport and precipitation rates primarily influenced whether garnets grew as oblate grains (i.e., as pseudomorphs on peak-pressure chloritoid grains) or newly nucleated equant grains. Honeycomb garnet was interpreted to show the permeability network that records migration and drainage of fluid. This internally produced fluid escaped from the shear zone using pre-existing grain boundaries and reaction-formed pathways. Assuming 1 km of shear zone thickness, we computed a time-integrated fluid flux of similar to 34 m(3) m(-2) for the entire duration of garnet growth. This study highlights garnet morphology as a tracer of transient fluid pathways during a burial-exhumation cycle of an eclogitic shear zone. The close connection between garnet morphology and fluids calls for a re-evaluation of similar microstructures in different tectonic settings.
The rubidium-strontium (Rb-Sr) system has a long pedigree as a geochronological tool that relies on the application of an isochron approach. In particular, biotite contains strongly radiogenic Rb-Sr isotopes, and thus can yield meaningful crystallisation or cooling ages via this approach. However, the response of the Rb-Sr system in biotite to multiple tectonothermal events, and interpretation of the resulting apparent ages, requires a nuanced understanding of the degree to which isotopic equilibration has been attained. Here, we investigate biotite in metapelitic rocks from the Fraser Zone, Albany-Fraser Orogen, Western Australia, a fault-bound block of Mesoproterozoic crust. Using a combination of phase equilibrium modelling, in situ laser ablation Rb-Sr analyses of biotite and U-Pb monazite dating, we show that these rocks reached peak metamorphic conditions of 830 degrees C-875 degrees C and 7.8-11.3 kbar at 1280 +/- 5 Ma, during late Stage I of the Albany Fraser Orogeny. Rb-Sr biotite model ages, ages calculated from single analyses and a set initial 87Sr/86Sri, reveal a direct relationship to their petrographic setting; grains associated with quartz-K-feldspar segregations have lower 87Sr content and younger model ages than grains within biotite- and sillimanite-rich bands. The model ages imply that biotite closed to Sr isotope exchange at different times depending on petrological context. Ages of c. 1200 Ma associated with biotite-sillimanite matrix bands likely record cooling through 300 degrees C-400 degrees C during Stage II of the Albany Fraser Orogeny, whereas younger, c. 1100 Ma Rb-Sr biotite ages are associated with leucosome segregations and were influenced by later Sr loss. The presence of variable apparent ages from the same fabric within single samples implies differential Sr equilibration controlled by those minerals in contact with biotite.
Garnet in eclogites is routinely analysed to reconstruct detailed P-T paths, as its chemical zoning records multiple metamorphic stages. In this aim, eclogites from the Serkout area (Aleksod terrane, Central Hoggar, Southern Algeria) have been petrologically investigated and thermodynamically modelled to trace their P-T history. Our study reveals that the samples have preserved the eclogite-facies paragenesis, characterised by garnet, omphacite, quartz, clinozoisite and amphibole, indicative of prograde to peak metamorphism. However, these minerals have been partially replaced during retrogression by clinopyroxene-plagioclase and amphibole-plagioclase symplectites. Coarse garnet crystals have highly heterogeneous, almandine-dominated compositions (X Alm ranging from 0.30 to 0.56), whereas omphacite has a maximum jadeite content (X Jd) of 0.31. Coarse amphibole, assumed to be of eclogite facies, is magnesio-hornblende with NaM4 = 0.31-0.47. Secondary plagioclase shows a wide range of compositions (X An = 0.20-0.99), from albite-rich in plagioclase-diopside symplectites after omphacite to anorthite-rich in plagioclase coronae around clinozoisite. The garnet grains in the Serkout eclogites display clear core-rim zoning, which, together with the evolution of mineral inclusions, allows the garnet growth to be correlated with different metamorphic events. Garnet cores formed under low-pressure, relatively high-temperature conditions (first M1 event), whereas garnet inner to outer rims developed during the prograde path from M1 to M2 stages. The late symplectites and coronas are related to retrogression (M3 stage). The combination of detailed mineralogical analysis, Ti-in-amphibole thermometry and thermodynamic modelling (P-T-M H2O pseudosections), using THERMOCALC-345, has revealed the existence of two distinct events: (a) an older high-temperature event (718 degrees C) most likely Paleoproterozoic in age, and (b) a younger eclogitic event characterised by (i) a prograde P-T path starting at 9 +/- 1 kbar and 610 +/- 50 degrees C under H2O-undersaturated conditions, progressing to peak conditions of 21.5 +/- 1 kbar and 720 +/- 50 degrees C under H2O-saturated conditions, and (ii) a retrograde path characterised by nearly isothermal decompression towards 5.5 kbar and 685 degrees C.
The absence of jadeite in regions where subduction of continental crust has been documented by other high-pressure (HP) or ultrahigh-pressure (UHP) minerals is striking. This study presents only the second discovery of jadeite in the crystalline rocks of the Erzgebirge, a region known for its occurrences of microdiamond, coesite and omphacite in eclogite, felsic granulite and other quartzofeldspathic rocks involved in continental subduction. The studied quartzofeldspathic gneiss is composed of quartz, garnet, plagioclase, K-feldspar, muscovite, biotite, rutile and kyanite. Garnet occurs mainly as large porphyroblastic grains of Grt1 overgrown by thin discontinuous rim of Grt2. Jadeite occurs as inclusions in kyanite and Grt1 garnet, the latter also containing coesite. Domains up to 3 mm in size of fine-grained plagioclase-muscovite symplectite, surrounded by plagioclase-muscovite mosaic and occasionally associated with small garnet grains of Grt2, are interpreted as pseudomorphic replacement of jadeite. The pressure-temperature (P - T) evolution of the samples was reconstructed by comparing the composition and proportion of the observed mineral assemblage with pseudosections constructed in THERMOCALC. Prograde metamorphism is constrained by the presence of garnet and jadeite inclusions in kyanite. Peak P - T conditions reaching > 28 kbar and similar to 600 degrees C-800 degrees C are constrained by the composition of porphyroblastic garnet Grt1 containing coesite. Early exhumation is constrained by the rim composition of Grt1 containing jadeite. Modelling results suggest that exhumation was accompanied by an influx of H2O, leading to a significant increase in the proportion of Ti-bearing muscovite. The subsequent exhumation and cooling are associated with a second generation of garnet Grt2, which partly preceded the breakdown of jadeite into plagioclase-muscovite symplectite and intergrowths. The late-stage P - T conditions, constrained by the composition of albitic plagioclase and small garnet Grt2 grains in the pseudomorphs, are estimated at similar to 9-13 kbar and similar to 420 degrees C-530 degrees C. The balanced reaction of jadeite breakdown fits the observed mineral compositions and the plagioclase-muscovite proportions in the pseudomorphs. Furthermore, the reaction balance reveals that while K and Na are highly mobile, the Al/Si ratio in the former jadeite remains unchanged in the resulting plagioclase-muscovite pseudomorphs. The jadeite-and coesite-bearing rocks reported in this study broaden the known extent of UHP metamorphism in the Erzgebirge tens of kilometres further to the south, calling for revision of existing geodynamic models of this area.
Regional metamorphism within the Central Qilian Block provides critical constraints on the tectonic evolution and geodynamic processes in the Qilian orogenic system. Combined phase equilibrium modelling and multiple isotope-dating methods reveal the P-T-t evolution of the Barrovian metamorphic zones preserved in the Huangyuan Group in the northern Central Qilian Block. A metamorphic zircon U-Pb age of 466 +/- 2 Ma from a paragneiss, a muscovite Ar-40/Ar-39 age of 434.8 +/- 3.9 Ma and a biotite Ar-40/Ar-39 age of 365.6 +/- 1.2 Ma from a micaschist and a biotite Ar-40/Ar-39 age of 372.4 +/- 0.9 Ma from an amphibolite in the group record Palaeozoic polyphase metamorphism. Phase equilibrium modelling indicates clockwise P-T paths with peak conditions of 6.1-6.8 kbar and 575 degrees C-625 degrees C in the garnet zone, 6.2-7.2 kbar and 610 degrees C-670 degrees C in the staurolite zone and 6.0-7.3 kbar and 750 degrees C-780 degrees C in the sillimanite zone. The Barrovian metamorphism is inferred to result from crustal thickening and the resulting elevated geothermal gradient in the northern Central Qilian Block. A Palaeozoic thickened continental arc resulting from southward subduction of the North Qilian Ocean at similar to 466 Ma is proposed for the northern Central Qilian Block. Southward subduction of the North Qilian Ocean beneath the Central Qilian Block lasted until 457-428 Ma. With the waning of arc-related magmatism, a decrease in the regional geothermal gradient led to greenschist-facies to amphibolite-facies retrograde metamorphism in the Huangyuan Group at similar to 435 Ma. After the closure of the South Qilian Ocean, the Quanji block collided with the Hualong and Central Qilian Blocks. Post-collisional extensional collapse at 400-350 Ma affected the Central Qilian Block, resulting in hydrothermal activity in the Huangyuan Group at 372-366 Ma.
The Quetico subprovince of the Superior Province is among the largest metasedimentary belts globally and provides evidence of tectonic processes at play during the final stages of Archean cratonization. Although previous studies have shown that deformation and metamorphism of the 2.7-Ga turbiditic protolith resulted in regional subvertical tectonic fabrics and symmetrical N-S field gradients with the highest conditions in the east-central part of the belt, constraints on the spatiotemporal evolution of metamorphism and deformation in the Quetico belt are underdeveloped. To obtain new insights into the tectonometamorphic history of the belt, 12 samples from garnet-in to orthopyroxene-in were subjected to garnet Lu-Hf chronology and trace-element mapping. Garnet Lu-Hf ages from the garnet, staurolite-andalusite, and cordierite zones in the northwestern and southwestern Quetico overlap between 2664 and 2657 Ma, demonstrating that subsolidus metamorphism was coeval in these parts of the belt. In the eastern Quetico, ages are younger in the staurolite-andalusite, sillimanite, melt, K-feldspar, and orthopyroxene zones (2651-2643 Ma). Subsolidus garnet features Cr, V, Ti, Y, and HREE zoning showing a ghost phyllosilicate fabric that is continuous and parallel, or slightly rotated relative to the external foliation, implying syn to post-tectonic garnet growth. Suprasolidus garnet features more homogenized Cr, V, Ti, Y, and HREE distributions and higher Zr and P concentrations indicative of prograde suprasolidus growth. Prominent HREE-rich annuli around embayed garnet boundaries in the orthopyroxene zone are attributed to garnet resorption near peak temperatures in the belt. These observations indicate a single, protracted tectonothermal event that extended across the Quetico, rather than a series of discrete events involving syn-tectonic medium-P/T metamorphism followed by low-P/T overprinting and plutonism. The data are interpreted to record melt-enhanced advective heat transport and exhumation within the upper crust during transpressive shortening, characteristic of ultra-hot orogens (UHO). East-west diachroneity in garnet growth and differences in peak metamorphic grade indicate two loci where the sequence of heating and extrusion occurred at different times. This model provides a testable hypothesis for the tectonic evolution of UHO belts in Archean terranes.
The metamorphic zoning and geothermobarometry of the Ryoke metamorphic complex in the Yanai area, southwest Japan, show that its thermobaric structure was buffered by the dehydration melting of biotite. The temperatures over most of the area covered by the three high-grade zones (8.5- to 19.0-km depth) are consistent with those of the dehydration melting reaction and indicate a geothermal gradient of 9.2 degrees C/km. The deepest area (> 19-km depth) of the highest grade zone has a domal structure and records temperatures that are > 50 degrees C higher than the melting reaction. In contrast, the low-grade zones (< 8.5-km depth) record a geothermal gradient of > 100 degrees C/km. The spacing of the isobars indicates that the crust was vertically thinned at all these depths. Thermal modelling suggests that the formation of this thermobaric structure requires thermal buffering by the dehydration melting reactions of muscovite and biotite and also advective heat transfer related to the upward migration of melt produced by these reactions. The melt migration through the thermally buffered middle crust enabled heat to advect efficiently to the shallower crust and modify the conductive geothermal gradient. The spatial-temporal scale of this thermal event in the study area is comparable to that of coeval and present-day volcanism, suggesting a close link between deep crustal and surface processes.
This study investigates garnet-clinopyroxene double-layered coronae in a metagabbronorite from the high-grade metamorphic Gf & ouml;hl Unit (Bohemian Massif). The coronae formed at the interfaces between relic magmatic orthopyroxene, still preserved in the cores of the coronae, and the plagioclase-rich rock matrix. They comprise an inner clinopyroxene layer and an outer garnet layer, both of which are polycrystalline. The coronae record a polyphase metamorphic evolution. The first metamorphic stage occurred under high-pressure-high-temperature (HP-HT) conditions and produced the garnet-clinopyroxene coronae. The garnet layer exhibits a pronounced overall compositional zoning, with inward-decreasing grossular and outward-decreasing pyrope contents, reflecting chemical potential gradients that drove the necessary chemical mass transfer during corona growth. Subsequent decompression led to a granulite-facies overprint, which is manifest from discontinuous layers of secondary orthopyroxene and plagioclase along the interface between the stage-I garnet and clinopyroxene layers. In addition, a secondary compositional zoning developed on the scale of individual garnet grains, which is characterised by a rimward decrease in the grossular content and a concomitant increase in the pyrope and almandine contents. The resulting complex compositional zoning of garnet allows distinguishing between the features generated during the early HP-HT evolution and those associated with decompression. Thermodynamic modelling yields pressure-temperature conditions indicating formation of the primary garnet-clinopyroxene double-layered coronae at conditions corresponding to a geothermal gradient of approximately 15 degrees C/km, followed by isothermal decompression and, finally, by rapid cooling. The inferred conditions for the HP-HT stage likely reflect a transitional geodynamic setting involving deceleration of a subducting slab and associated thermal relaxation.
Understanding metamorphic overprinting is crucial for identifying transitions in tectono-thermal regimes during orogenesis. Here, we report the first definitive example of polymetamorphism in metapelites of the Baoyintu Group, central Inner Mongolia. Our analyses included petrographic observations, phase equilibrium modelling, as well as U-Pb and Rb-Sr geochronology. Garnet-white mica schist exhibits two stages of garnet growth: an early-stage assemblage of chlorite + garnet (I) with epidote + phengite + quartz + rutile (I) inclusions and a late-stage assemblage of chlorite + garnet (II) + muscovite + quartz + ilmenite + staurolite + rutile (II) in the matrix. Phase equilibrium modelling yields pressures of 16-22 kbar and temperatures of 400 degrees C-500 degrees C for the early-stage assemblage corresponding to blueschist facies metamorphism, and 6-7 kbar and similar to 550 degrees C for the late-stage assemblage corresponding to amphibolite facies metamorphism. A garnet-two mica schist contains a mineral assemblage similar to the late-stage assemblage in garnet-white mica schist, yielding peak pressures of 4-5 kbar and temperatures of 570 degrees C-580 degrees C. Zircon and rutile U-Pb geochronology of the metapelite samples yielded two groups of metamorphic ages (c. 384 and c. 259-236 Ma). In situ mica Rb-Sr geochronology of the garnet-two mica schist yielded an age of 215 Ma interpreted to record cooling after the Permian-Triassic metamorphic peak temperature. These results reveal that medium- and low-dP/dT metamorphism occurred during the late Permian-early Triassic that overprinted Devonian high-dP/dT metamorphism. Based on previous studies of magmatism in the region, we relate the two stages of metamorphism to subduction of Paleo-Asian oceanic lithosphere (Devonian, c. 384 Ma) and back-arc extension (Permian-Triassic, 259-236 Ma). This contribution fills a critical gap in our understanding of the tectono-thermal transition history of subduction within the Paleo-Asian Ocean and provides a robust framework for interpreting polymetamorphism in accretionary orogens.
Metamorphic diamonds offer insights into Earth's evolution, modulating the global carbon cycle through subduction into the mantle and exhumation. However, the formation and internal structure of minute diamonds remain poorly understood. Here, we study the internal structures of diamond-bearing inclusions from the Eastern Alps using state-of-the-art analytical techniques. Atomically resolved images revealed intricate structures, including well-faceted monocrystalline diamonds, complex nanocrystalline clusters of intergrown diamond and graphite and amorphous matter. Associated fluids contain CO2, CH4 and H2O, indicating formation from C-O-H fluids. The proposed model of C-O-H fluid behaviour suggests monocrystalline diamond precipitation at the onset of retrograde metamorphism. Amorphous layers formed through interaction between the trapped fluid and the host garnet during the further exhumation. When the fluid reached the P-T conditions of the transition zone between the diamond and graphite stability fields, polycrystalline clusters formed. For the first time, we can see a detailed record of metamorphic fluids evolution, preserved in the internal structures of microsized diamond inclusions.
Eclogite lenses are exposed within the orthogneiss-dominated core of the Orlica-& Sacute;nie & zdot;nik Dome in the Sudetes, which forms the northeastern margin of the Bohemian Massif (Variscan Belt of Central Europe). The presence of coesite inclusions in garnet and omphacite confirms that these eclogites underwent ultrahigh-pressure metamorphism. This interpretation is further supported by phase equilibria modelling, which indicates peak-pressure metamorphic conditions of 2.9-3.2 GPa and 750 degrees C-830 degrees C. The results are consistent with estimates derived from conventional geothermobarometry and Zr-in-rutile thermometry applied to rutile inclusions in garnet. Based on quartz-in-garnet elastic barometry, a maximum entrapment pressure of approximately 2.0 GPa is obtained. We interpret this discrepancy as a result of viscous relaxation of garnet at high temperature. The first stage of re-equilibration during decompression occurred at a pressure of 2.0-2.2 GPa and a temperature of 680 degrees C-770 degrees C. The observed rock associations exhibit similarities to other UHP occurrences within the Saxo-Thuringian Zone, suggesting a comparable exhumation mechanism. This likely involved initial buoyancy-driven exhumation within a subduction channel, followed by crustal-scale folding. Furthermore, the maximum pressure recorded in the eclogites may partly reflect nonlithostatic components, such as transient pressure variations arising from rheological heterogeneity between the eclogites and their host rocks.
Incomplete retrograde reaction of sodic-calcic pyroxene occurred during the exhumation of Eocene eclogite and blueschist in NE New Caledonia. Sodic-calcic pyroxene is well-preserved in reaction relationships with paragonite and structurally late albite, in conflict with the prediction that it should transform efficiently during decompression to albite, paragonite, glaucophane and quartz. The microstructural distribution of albite accompanying crenulation cleavage development is consistent with albite growth having been restricted by space-filling requirements. In contrast, domains of high strain enable the accommodation of volume and fluid infiltration, increasing the size of the equilibration volume leading to the complete conversion of sodic-calcic clinopyroxene to albite. Despite decompression-driven dehydration, limited volume accommodation presents a plausible mechanism to metastably preserve high-P (up to 18 kbar) sodic-calcic clinopyroxene-bearing assemblages to lower-P conditions (similar to 10 kbar) that should stabilise albite. Efficient retrogression is facilitated by dynamic recrystallisation inducing dilation.
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.
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 similar to 89% of the cases and the metamorphic gradient (<= 350 degrees C/GPa versus > 350 degrees C/GPa) in similar to 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.
