ABSTRACT The Western Dharwar Craton (WDC) hosts some of the oldest preserved ultramafic sequences, offering key constraints on Mesoarchean mantle dynamics and crust–mantle interactions. This study integrates petrography, whole-rock geochemistry, mineral chemistry, and zircon U–Pb geochronology to investigate metapyroxenites from the Coorg Massif (CM), southern part of the WDC, and to evaluate their relationship with Sargur Group (SG) ultramafic rocks exposed farther north. The CM metapyroxenites occur as dykes and dismembered enclaves within charnockites and granite gneisses and record upper amphibolite–lower granulite facies metamorphism characterized by diopside, enstatite, pargasitic-hornblende, andesine, pleonaste, and Fe-Ti oxide-bearing assemblages. Despite high-grade metamorphic overprinting, coherent MgO-correlated major- and trace-element trends, immobile HFSE-REE systematics, and mantle-like Nb/Ta and Zr/Th ratios indicate that primary magmatic signatures are largely preserved, with negligible crustal contamination. Geochemically, the CM metapyroxenites and SG ultramafic rocks display Barberton-type komatiitic basalt affinity, defining a continuous compositional spectrum from peridotitic komatiites to evolved komatiitic basalts. Flat to weakly fractionated HREE patterns, variable Al systematics, and trace-element ratios indicate derivation from a heterogeneous depleted mantle source within the spinel–garnet lherzolite field, followed by olivine-dominated fractionation and non-equilibrium crystallization from initially Mg-rich parental melts. These features are best explained by the melting of an FeO-enriched pyrolitic mantle in a plume-related thermal regime, without requiring significant subduction-related metasomatism. HFSE-based tectonic discrimination diagrams yield mixed arc, back-arc, and oceanic plateau affinities, which, when integrated with field relations and metamorphic gradients, suggest emplacement of plume-derived magma into arc-modified lithosphere, most plausibly within a Mesoarchean back-arc setting. Zircon U–Pb ages of 3159 ± 11 Ma and 3147 ± 7 Ma constrain the crystallization of the CM metapyroxenites to the younger (~3.1 Ga) Mesoarchean mafic–ultramafic magmatic episode recognized across the WDC, coeval with SG ultramafic magmatism, tonalite-trondhjemite-granodiorite (TTG) emplacement, and early charnockite formation. Together, these results support a three-stage Mesoarchean evolution involving plume-driven mantle melting, plume–arc interaction with variable-depth ultramafic emplacement, and subsequent crustal thickening and stabilization. The CM metapyroxenites are thus interpreted as deeper-crustal, Type-II equivalents of SG ultramafic assemblages, recording a unified plume-influenced geodynamic system that shaped the lithospheric architecture of the southern part of WDC during the Mesoarchean.

Repeated high-grade metamorphic overprinting usually results in incomplete re-equilibrium. This blurs the pressure-temperature-time (P-T-t) information preserved in ancient terranes, complicating the reconstruction of orogenic evolution and tectonics. This study investigated metabasites from the Daqingshan area of the North China Craton, comparing samples distal and proximal to the deformation zone. An undeformed granulite and a plastically deformed amphibolite both record an earlier ultrahigh-temperature (UHT) metamorphism peaking at 950−1000 °C and 8−9 kbar with post-peak cooling ending at fluid-absent solidi, followed by a later granulite-facies overprinting to peak conditions of 8−9 kbar and 870−890 °C, likely driven by prograde hydrational P-T rises. These P-T conditions were reliably constrained by three key factors: tetrahedral Al in clinopyroxene, anorthite in plagioclase, and Ti in amphibole. The overprinting assemblages vary in extent between samples (e.g., local embryos versus widespread neoblasts), suggesting that more complete petrologic records of metamorphic overprinting benefit from deformation and fluid availability. Moreover, zircon and monazite dating show that the earlier UHT metamorphism cooled at 1.94−1.92 Ga, whereas the later overprinting occurred at 1.87−1.83 Ga. The UHT metamorphism is attributed to post-orogenic or backarc extension, preceded by an orogenic event that produced high-pressure granulite-facies metamorphism at ca. 1.96−1.94 Ga. The granulite-facies overprinting is attributed to a separate orogenic event involving typical crustal thickening after the UHT metamorphism, based on its P-T path and nearby geologic data.

ABSTRACT The role of the Yangtze Block in the Rodinia Supercontinent is still debated. The garnet-biotite metatexite in the Micangshan area, northwestern margin of the Yangtze Block, mainly consists of garnet, biotite, plagioclase, K-feldspar, and quartz with minor ilmenite and accessory minerals of zircon and monazite. The prograde-stage is inclusions inside the garnet, including quartz, biotite, plagioclase, and monazite. The peak-stage is characterized by equilibrium garnet, biotite, plagioclase, K-feldspar, quartz, and ilmenite. The retrograde-stage represents the anhedral small flaky biotite and the coexistence of minor anhedral plagioclase and K-feldspar. Combining the P-T estimation from the pseudosection by Perple_X, quartz-in-garnet (QuiG) Raman-barometer, and conventional geothermobarometers, the prograde-stage P-T condition is constrained at 477–555°C, 5.0–6.5 kbar, and the peak and retrograde stages were determined as 800–830°C, 7–8.8 kbar and 603–674°C, 4.5–5.5 kbar, respectively. A clockwise P-T path was constructed. Metamorphic zircons in this rock yield metamorphic ages of 808 ± 5 Ma and 728–746 Ma. Zircons formed at ca. 808 Ma revealed a crystallization temperature of 727 ± 12°C, which is lower than the peak-stage temperature. Thus, zircons were regarded as retrograde products. Inherited zircons in this rock yielded 206Pb/238U apparent dates of 828–939 Ma. These zircons show geochemical characteristics of orogenic or arc-related origins. The 206Pb/238U dates of the monazite span in the range of 788–822 Ma, which suggests two centres at 817 Ma and 803 Ma. As monazite hosted in the garnet, it could record the metamorphic history from the prograde. Thus, 817 Ma is believed to be the time of peak-stage metamorphism, and the 808–788 Ma is interpreted as retrograde time. Based on all of the above, the northwestern margin of the Yangtze Block is suggested to have experienced the collision process, resulting in granulite facies metamorphism at ca. 817 Ma. This study provides additional constraints to decode the role of the Yangtze Block in the Rodinia evolution.

The Castillon massif, in the northern Pyrenees, features a complex of decimetric to metric ultramafic and mafic layers emplaced within metasedimentary series (from the bottom to the top: garnet, sillimanite, and kyanite-bearing gneisses and sillimanite + cordierite-bearing gneisses). Ultramafic and mafic layers and metasediments have been deformed and metamorphosed under granulitic facies conditions during the Hercynian orogenesis. The mineralogical, petrologic, and geochemical characteristics of the studied samples allow us to define two distinct series: 1) a pyroxene-bearing magmatic series (UM-M1) consisting of ultramafic (UM: dunites, harzburgites, and orthopyroxenites) and mafic (M1: norites, gabbro–norites, and gabbros) rocks; and 2) a pyroxene-free and hornblende-bearing series (M2; mela-, meso-, and leucogabbros). The leucogabbros exhibit some characteristics of anorthosites, including the high Al 2 O 3 whole-rock content (31 wt%), high An content (An 84–96 ) in plagioclase, weak rare earth element enrichment, and very positive Eu anomalies. We propose that the rocks of the ultramafic and pyroxene-bearing rock-series (UM-M1 series) are all associated with the same magmatic event and that the M2 series rocks are associated with a distinct separate event. Isotopic data suggest that these formations are Ordovician. The M2 rocks have juvenile Nd isotopic signatures (εNd (460) from +4.59 to +8.11), suggesting that they are derived from superheated alumina-rich basaltic or basaltic–andesite melts extracted from a relatively depleted mantle source. In contrast, most of the M1 rocks derived from parental basaltic melts show partially crustal contamination, with only a few clearly juvenile samples (εNd (460) from +0.45 to +6.59). We propose a geodynamic evolution for the Castillon massif involving a two-stage genesis and the emplacement of the two series. First, the emplacement of olivine-saturated basaltic melts in a deep metasedimentary crust resulted in the M1 series. The second step involves the emplacement of alumina-rich basaltic or basaltic–andesite melts to produce the M2 hornblende-bearing series (mela-, meso-, and leucogabbros) devoid of pyroxenes. The leucogabbros show strong similarities with common anorthosites although they have not been previously observed in the Variscan Pyrenees.

Abstract Greywacke is a rock type pervasively present in orogens, and anatexis of greywackes is a pivotal process for crustal differentiation and granitoid formation. However, it remains challenging to characterize pristine compositions of anatectic melts derived from diverse lithologies. This study investigates nanogranitoids (i.e. crystallized melt inclusions) in metagreywackes from the Rauer Islands, eastern Prydz Bay, East Antarctica. This is the first study to recognize nanogranitoids in metagreywackes dominated by Bt dehydration melting. Results from phase equilibrium modeling and Zr-in-rutile thermometry indicate that the peak metamorphic P–T conditions were ~0.95 GPa and ~ 850°C, and that the melts were trapped under near-peak conditions. Zircon and monazite dating suggests that the granulite facies metamorphism and anatexis occurred during the Pan-African orogeny. Nanogranitoids (2 to 40 μm in size) occur in garnet and are composed of cryptocrystalline daughter phases including phlogopite, muscovite, cristobalite, quartz, plagioclase, kokchetavite, and kumdykolite. They were experimentally re-homogenized under 850°C, with the glass showing silicic (SiO₂ = 74.2 ± 3.1 wt %), peraluminous (aluminum saturation index = 1.1–1.6), K-enriched (K/Na = 1.1–2.6), and low-maficity (FeO + MgO = 1.7–4.1 wt %) compositions. Micro-Raman spectroscopy suggests that H2O contents in the melts vary from 0.44 to 1.08 wt %, with an average value of 0.75 ± 0.2 wt %. Compared with bulk rock compositions, the investigated melts show enrichment in Rb, Cs, and U, depletion in Ce and Sr, and elevated Rb/Sr ratios (3.8–13.7). From these geochemical characteristics of nanogranitoids, we conclude that (1) K2O and Rb enrichment in melts points to incongruent melting involving dehydration of biotite; (2) depletion of Sr reflects moderate melting degrees (i.e. plagioclase is still abundant at peak stage); and (3) zircon saturation temperatures (775–837°C at 1.0 GPa) imply disequilibrium melting of accessory minerals during high-temperature anatexis. Physical properties of the melt (2.43 ± 0.06 g/cm3 density, and ~ 3.5 μW/m3 heat production) suggest that anatexis of metagreywackes would facilitate efficient melt extraction and redistribution of some large ion lithophile elements (Rb and Cs) and heat-producing elements (K and U). A detailed comparison between published data suggests that biotite dehydration melting usually generates granitic melts with high heat production values. Considering the wide distribution and efficient melt production, anatexis of metagreywackes may have a remarkable contribution to chemical differentiation across crustal sections, prompting stabilization of the crust.

The Cheng-gang crystalline graphite deposit is a recently discovered medium-to-large-sized deposit within the Tan-Lu Fault Zone (TLFZ), East China. However, the knowledge on this deposit remains limited, resulting in a poor understanding of its genesis. In this study, this deposit is chosen to elucidate the degree of graphite mineralization, the nature and depositional environments of the protoliths, and the carbon source of graphite through geochemical and stable isotope investigations, and mineralogical analysis. The fixed carbon contents in the graphite-ore-bearing layers range from 2% to 3%. X-ray diffraction analyses reveal a high degree of graphitization. Analyses of elemental ratios indicate that the protoliths of metamorphic rocks predominantly consist of felsic rocks derived from the upper crust and deposited in brackish-water and reducing environments (anoxic to dysoxic). Stable carbon isotope analyses show that CH4 with lighter carbon isotopes released from the decomposition of pristine organic matter was trapped into adjacent inorganic reservoirs and the residual fraction with heavy carbon isotopes evolved to become graphite under metamorphism. Assuming the existence of isotope exchange between carbonate minerals and graphite, the temperature of peak metamorphism is estimated to be 580–860 °C, corresponding to amphibolite–granulite facies during regional metamorphism. The direct mixing of organic fluids and adjacent inorganic reservoirs may have contributed to graphite ore formation and needs to be further explored in future studies. The findings shed light on the genesis of the TLFZ graphite deposits, providing practical implications for local mineral exploration.

Abstract The carbonate-dominated supracrustal sequences of the western Grenville Province formed between ∼1200 and ∼1300 Ma and are well-known for high-grade metamorphosed SEDEX and VMS deposits. However, there is limited information on base metal mineralization hosted in the clastic-dominated supracrustal sequences in the Allochthonous Medium to Low Pressure Belt of the central Grenville Province. The Moucou zinc occurrence (>1.3 wt.% Zn), located 50 km NW of Lac Saint-Jean, Quebec, provides an example of such mineralization. This zinc occurrence is hosted in metavolcanic rocks interbedded with quartzite layers of the Barrois Complex, a clastic-dominated supracrustal sequence, and metamorphosed under granulite facies. We present geochemical data, titanite geochronology, and petrographic observations for the Moncou zinc occurrence. The metavolcanic rocks have a transitional signature, Nb and Ta negative anomalies, and Ba and Th enrichments suggesting a back-arc setting comparable to the well-mineralized carbonate-dominated supracrustal sequences of the western Grenville Province. Zinc mineralization consists of (i) a first sphalerite generation (Sp1) systematically surrounded by a thin layer of plagioclase+quartz (Pl+Qz); (ii) deformed Zn-rich biotite crosscut by the titanite grains dated at ∼1000 Ma, indicating the minimum age for the first zinc remobilization; and (iii) Zn-rich chlorite spatially associated with a second sphalerite generation (Sp2). These observations suggest that Zn mineralization occurred following this sequence: a pre-metamorphic event crystallizing Sp1 as it is surrounded by anatectic melt (Pl+Qz) and shows typical high-temperature crystallization textures, then a syn- to post-metamorphic event characterized by the Zn-rich biotite, and finally a low-temperature hydrothermal event crystallizing the Zn-rich chlorite and the second sphalerite generation. This study reveals the potential for Zn mineralization of the clastic-dominated supracrustal sequences of the Medium to Low Pressure Belt such as the Barrois Complex.

Research subject. The material composition, formation conditions, and age of amphibolites of the Central zone of metamorphism (CZM) of the Rai-Iz massif. Materials and methods. Microprobe studies of minerals were conducted using a Cameca-SX100 microanalyzer; the content of petrogenic elements was determined by an X-ray multichannel spectrometer CPM-35; the REE content was determined by a mass spectrometer with inductively coupled plasma NexION 300S in the Collective Use Center “Geoanalitik”, Institute of Geology and Geochemistry of the Ural Branch of the Russian Academy of Sciences. 40 Ar/ 39 Ar dating was carried out at the Institute of Geology and Mineralogy of the Siberian Branch of the Russian Academy of Sciences according to A.V. Travin’s method. Results. The petrography and geochemistry of the amphibolites of the CZM Rai-Iz massif were studied; the parameters of their metamorphism were established, and the age of 40 Ar/ 39 Ar was determined. Two types of amphibolites were identified: garnet amphibolites, epidote-garnet amphibolites, and clinopyroxene-amphibole and amphibole-phlogopite-garnet rocks. The main minerals were found to be amphibole and garnet. Amphibole corresponds to edenite, pargasite, and ferropargasite. In some amphibole samples, chemical zonation was established, manifested in the depletion of the marginal parts of Al 2 O 3 and FeO grains relative to the central ones, while the MgO content increased from the center to the edge. Garnets from garnet and epidote-garnet amphibolites exhibited an almandine-grossular composition with pyrope (3–16 %) and spessartine (3–13 %) components; in amphibole-phlogopite-garnet rocks, garnet demonstrated an almandine–pyrope composition. In the pomegranate, the following chemical zonation was established: the FeO content increases from the center to the edge of the grains, while the MnO content decreases. The age of amphibolites determined by the 40 Ar/ 39 Ar method (405.2 ± 5.4 Ma) corresponds to the Lower Devonian. The averaged formation parameters of amphibole-bearing rocks correspond to P = 6–13 kbar, T = 430–860° C, and to the boundary of amphibolite and greenschist facies and the boundary of amphibolite and granulite facies. REE spectra in the studied rocks, normalized relative to chondrite, showed the predominance of heavy lanthanides over light ones. The character of REE distribution is close to N-MORB basalts. Conclusion. The studied rocks formed at the boundary of the early and middle Devonian. Their formation was associated with regional metamorphism in the conditions of the onset of collision.

ABSTRACT This study reports a sequence of high- and medium-grade metamorphic rocks beneath structurally overlying mantle peridotites, referred to as a ‘metamorphic sole’ in the Nagaland–Manipur Ophiolite Belt, NE India. Here, we characterize the petrography and mineral chemistry, constrain the pressure-temperature conditions, and show the sole records an inverted metamorphic gradient. Structurally downwards beneath the deformed mantle peridotite, the sole sequence consists of mafic granulites (peak P–T at ~1.35–1.48 GPa, ~870–950°C) at the top, through garnet-clinopyroxene bearing amphibolite (~1.25–1.29 GPa, ~785°C) and garnetiferous and non-garnetiferous amphibolite (~1.17–1.27 GPa, ~700–720°C) to epidote amphibolite (~1.2–1.33 GPa, ~625–680°C). The studied sole sequence is distinct as it records high-pressure, ultrahigh-temperature granulite facies metamorphism at the tip of the subducted oceanic crust during subduction initiation. The granulite facies metamorphic sole rocks exhibit a progressive decrease in peak metamorphic temperatures from ~950°C to ~870°C at ~1.4 GPa, structurally down the section. The metamorphic sole sequence defines a metamorphic field gradient with either a near-isobaric temperature gradient at mantle depths or a gradient with a gentle, positive slope in the P–T space. These findings, when collated with the results of thermo-mechanical modelling and theoretical considerations, suggest that subduction in the southeastern arm of the Neo-Tethys was most likely initiated through a subduction initiation mechanism, after which the different slices of sole rocks were progressively accreted at either uniform depths under mantle condition or at shallower depths (between 1.5 and 1.2 GPa), as the subduction interface cooled.

ABSTRACT The Proterozoic Eastern Ghats Mobile Belt (EGMB) in the eastern Indian shield is not only characterised by its structural and metamorphic complexity but is also known for its rich deposits of graphite and manganese. In Odisha, graphite is reported in six geographical belts in the northern part of the EGMB. A new greenfield prospect, the Daspalla Graphite Belt (DGB) in the Nayagarh District of Odisha, is reported in the present investigation. An attempt has been made to study the genesis, control and mode of occurrence of graphite in the DGB. Laser Raman Spectroscopy (LRS) was conducted to characterise and decipher the molecular structure of graphite, and carbon isotope studies were conducted to determine δ13C values and total organic carbon content. In EGMB, graphite is present in high-grade granulite facies metasediments, including its migmatised variants. Graphite occurrence is primarily governed within selective lithology, while its mineralisation is controlled by regional structural fabrics. In DGB, graphite, along with the host rock, forms a pair of westerly plunging antiform and synform structures. The northern limb of the antiform is truncated by the WNW-ESE-trending Mahanadi Shear Zone (MSZ), whereas the southern limb of the synform is truncated against the NNW-SSE-trending Rushikulya Shear Zone, resulting in an S-shape to the belt. Axial trace of the antiform and consecutive synform trends is almost E-W. LRS study indicates the presence of a single major ~1580 cm−1 (G) band. Some minor bands at ~1350– 1360 cm−1 and ~2720 cm−1 are also observed. In the carbon isotope study, the δ13C of samples ranges from −23.4‰ to −27.6‰ with total organic carbon content from 2.3 to 11.5%. The δ13C value less than −20‰ suggests that the graphite of DGB was formed by conversion of organic or carbonaceous matter during granulite facies metamorphism of the sediments.

Porphyroblastic garnet grains are found in the mafic granulite enclave in the eastern region of the Chotanagpur Gneissic Complex (CGC). Grain composition varies from the core to the rim of this porphyroblastic garnet. According to the petrography and geothermobarometry investigation, there is a breakdown symplectite texture and temperature change from the garnet grain's centre to its rim. These occurrences demonstrated that this rock would undergo decompression once it reaches its peak metamorphic state. Garnet porphyroblasts in a retrogressed mafic granulite enclave create Cpx-Plag-Ilm intergrowth as a result of decompression and cooling. The chemical composition of the core, rim, and symplectite section clearly differs, according to Electron Micro Probe Analysis (EPMA). Garnet from symplectite (Cpx-Plag-Ilm) and high-grade granulite facies rock breakup is an obvious sign of isothermal decompression or rock upliftment.

Anatexis is a key process linking deep crustal metamorphism, tectonic deformation, and magmatic activity in orogenic systems. Understanding continental arc crustal metamorphism and anatexis is crucial for comprehending crustal differentiation and reworking. The North Wulan metamorphic complex, located along the northern margin of the Qinghai-Tibet Plateau, northern Tibet, contains a rock sequence that outcrops from deep to shallow crustal levels of a continental arc. In this paper, we present systematic studies on different types of migmatite in the North Wulan metamorphic complex to constrain the pressure-temperature-time conditions of metamorphism and partial melting within the deep crust of continental magmatic arcs. The biotite-amphibole gneiss formed through the remelting of preexisting Cambrian arc rocks, whereas the felsic gneiss originated from the partial melting of the Paleoproterozoic basement within the arc crust. Zircon U-Pb geochronology reveals that the igneous protoliths of the biotite-amphibole gneiss crystallized at 503−500 Ma. U-Pb data and Hf isotopic data from zircons indicate that these Cambrian arc rocks and the Paleoproterozoic basement underwent contemporaneous metamorphism and anatexis at 465−458 Ma. Based on both petrographic and geochemical evidence, the leucosomes in the migmatites formed from water-fluxed melting. Petrographic analysis shows diffuse boundaries between the leucosome and gneiss, along with an absence of anhydrous peritectic minerals in the leucosomes. Geochemical analysis supports this conclusion, with data showing specific correlations in element ratios (Rb/Sr versus Sr, Rb/Sr versus Ba, Ta versus Nb, and U versus Th). Phase equilibrium modeling indicates that partial melting of Cambrian arc rocks and felsic gneiss occurred under water-saturated conditions (with 1.48 wt% and 1.74 wt% H2O, respectively). The zircon Eu/Eu* data reveal that the switch from compression to extension occurred at ca. 480 Ma. As previous studies have concluded, we suggest that asthenosphere upwelling through thinned lithospheric mantle introduced high heat flow into the lower crust due to the rollback of the subducted oceanic plate. This caused water-fluxed melting in low-pressure/high-temperature granulite facies and the reworking of the continental arc crust during the subduction of the Qaidam oceanic slab in the early Paleozoic.

The Anápolis-Itauçu Complex is part of the Neoproterozoic Brasília Fold Belt and constitutes its metamorphic core, or internal zone. It is constituted of granulites and layered mafic and granite bodies. Diagnostic ultra-high temperature mineral assemblages are recognized among the granulites, being spinel + quartz the most common. Ultra-high temperature conditions are recovered from several rocks using conventional thermobarometry, based on the orthopyroxene and garnet pair, and on the Zr-in-rutile thermometer, with values above 900 o C. In some outcrops, an intense retrometamorphism affected the rocks, and the granulite facies mineral assemblage was replaced by typical greenschist facies ones such as kyanite + chloritoid + muscovite + chlorite. In this sample, the high-grade aspect in outcrop is preserved, as attested by the presence of leucosome and residue. Thermodynamic modeling indicates that intense water influx occurred, and temperatures around 500 o C were necessary to produce the observed mineral assemblage, at pressures lower than 5 kbar. With the present composition, thermodynamic modeling points to a possible generation of spinel + quartz, should the rock be submitted to ultra-high temperature conditions and some Fe 3+ were present.

Abstract Holsnøy, Norway, offers a world-class natural laboratory for studying the impact of fluid on subducting lower crust. Holsnøy is composed of dry, metastable lower crustal granulite that was infiltrated by fluids along shear zones and seismic fractures during subduction. The infiltration facilitated the localized growth of eclogite facies mineral assemblages along the fluid flow pathways. The duration of the eclogite facies metamorphism, however, remains uncertain. Previous garnet diffusion chronometry studies have estimated timescales ranging from hundreds of years to millions of years based on diffusional relaxation between metastable granulite facies garnet cores and eclogite facies garnet rims and fractures. The shorter timescales are inferred from extremely sharp Ca gradients across chemical contacts present in some garnets whereas the longer timescales are from wider Mg and Fe profiles present in all garnets. The different timescale estimates have led to divergent models for the region’s tectonometamorphic evolution. Here we show that the sharp Ca contacts can be explained by diffusion-induced compositional stress. As Ca is significantly larger than Mg and Fe, its movement strains the crystal lattice and generates stress that limits the relaxation of sharp chemical contacts. When compositional stress is accounted for, the sharp contacts yield timescales that are consistent with the wider Mg and Fe diffusion profiles. We determine that eclogite facies conditions (670–700 °C, 1.5–2.2 GPa) lasted a maximum of c. 300 kyr. The relatively short duration of eclogite facies conditions requires that multiple transient heating events were superimposed on a longer (>106 yr) overall timescale of metamorphism. Granulite facies garnet cores are surrounded by multiple generations of eclogite facies rims formed by interface-coupled dissolution–reprecipitation (ICDR) reactions. The garnet rims indicate two rapid, regional-scale fluid pulses and additional smaller, more localized pulses. The fluid pulses may be linked to episodes of seismic moment release as well as transient heating via exothermic hydration reactions and/or shear deformation. Our model results predict up to 400 MPa of differential stress at the garnet core–rim contacts, consistent with observed eclogite facies microfractures that extend into relic granulite facies garnet cores. The microfractures indicate that ICDR was aided by compositional stress: diffusion ahead of the reaction front generated stress and fracturing that created porosity for further ICDR. Thus, compositional stress can markedly impact both diffusion systematics and intracrystalline deformation. Together, these results show that despite their brevity, transient thermal, fluid flux, and/or baric episodes may exert the primary controls on the mineralogical and rheological development of subducted lithologies, in contrast to the long, slow burial and exhumation typically envisioned for regional metamorphism.

ABSTRACT 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.

The study area is inserted in the context of the interference zone between the Brasília and Ribeira orogens. Based in microscopic evidence, four metamorphic stages were identified in the São João Marcos Iron Formation (SJ MIF) and associated metamafic and metaultramafic rocks. The first (E1), characterized by pyroxene relicts, marks the metamorphid peak in granulite facies. The second (E2) registers transition from granulite to amphibolite facies conditions, with the development of amphiboles (mainly hornblende) and plagioclase. The final moments of this stage is marked by the development of garnet coronas between hornblende and plagioclase, in a IBC (isobaric cooling) path. The third (E3) stage is defined by symplectites developed in a ITD (isothermal decompression) path, possibly related to the orogenic collapse. Finally, cummingtonite-grunerite appears marking the fourth (E4) stage, suggesting higher temperatures in relation to the previous stage. Regarding the geochronology, the metamafic and metaultramafic rocks present mostly archean and paleoproterozoic provenance, with subordinate neoproterozoic grains. 641,5 ± 2,9 Ma was calculated as crystallization, as well as 3 groups of metamorphic ages: 622 – 613 Ma; 599 – 598 Ma and 587 – 586 Ma. The associated metassedimentary rocks present archean to neoproterozoic provenance ages, with maximun depositional age in 686 Ma and 3 groups of metamorfic ages: 647 – 640 Ma, 609 – 605 Ma and 599 Ma. The stage E1 is related to a metamorphism between 647 – 640 Ma, contemporary to the crystallization of the metamafic and metaultramafic rocks; E2 and E3 are contemporary to metamorfic events between 622 – 598 Ma, which are related to Southern Brasília belt evolution.

Abstract Understanding reactive melt flow is crucial for advancing our knowledge of crustal differentiation; however, the mechanisms governing melt migration remain debated, particularly in deep magmatic arc environments. A composite sample from the Central Qilian continental arc, NE Tibet, preserves the transition from hornblende gabbronorite to garnet granulite, offering a rare opportunity to study reactive melt flow in the arc root. Thermodynamic modeling showed that the hornblende gabbronorite was metastable under lower‐crustal conditions (6.2–8.2 kbar, 900–931°C). To equilibrate with the normal thermal regime of the middle to lower crust, it underwent near‐isobaric cooling to 816 ± 16°C, whereas its transformation into garnet granulite occurred under higher pressure and temperature conditions (10.2–12.2 kbar, 833–865°C). The sample records melt‐rock interactions during the transition from the magmatic stage to garnet granulite facies metamorphism. Reactive melts infiltrated grain boundaries, inducing mineral replacement via dissolution‐precipitation and metasomatism. Enriched rare earth elements (REEs) in blue‐green pargasite, reaction microstructures and hydrous products attest to melt‐rock interactions involving Mg‐Sr‐REE‐enriched silicate melts. Trace element mapping reveals a correlation between reaction microstructures and high‐Sr plagioclase bands, highlighting grain boundary pathways for melt migration. Replacement microstructures illustrate permeable reactive melt flow pathways within the lower arc crust. Reactive melt flow enhanced chemical disequilibrium and mineralogical reorganization, driving textural maturation through coupled dissolution‐reprecipitation. This pervasive melt‐rock interaction mechanism likely governs both crustal differentiation and the development of high Sr arc magmatic signatures.

ABSTRACTUnderstanding 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.

Petrogenesis and geochronology of the granulite facies gneissic suite hosting the Katanning Gold Deposit, Yilgarn Craton, Southwest Western Australia

In the absence of appropriate tools and a knowledge base for exploring high-grade metamorphic terrains, felsic gneiss complexes at granulite facies have long been considered barren and have remained undermapped and understudied. This was the case of the Bondy gneiss complex in the southwestern Grenville Province of Canada which consists of 1.39–1.35 Ga volcanic and plutonic rocks metamorphosed under granulite facies conditions at 1.19 Ga. Iron oxide–apatite and Cu-Ag-Au mineral occurrences occur among gneisses rich in biotite, cordierite, garnet, K-feldspar, orthopyroxene and/or sillimanite-rich gneisses, plagioclase-cordierite-orthopyroxene white gneisses, magnetite-garnet-rich gneisses, garnetites, hyperaluminous sillimanite-pyrite-quartz gneisses, phlogopite-sillimanite gneisses, and tourmalinites. Petrological and geochemical studies indicate that the precursors of these gneisses are altered volcanic and volcaniclastic rocks with attributes of pre-metamorphic Na, Ca-Fe, K-Fe, K, chloritic, argillic, phyllic, advanced argillic and skarn alteration. The nature of these hydrothermal rocks and the ore deposit model that best represents them are further investigated herein through lithogeochemistry. The lithofacies mineralized in Cu (±Au, Ag, Zn) are distinguished by the presence of garnet, magnetite and zircon, and exhibit pronounced enrichment in Fe, Mg, HREE and Zr relative to the least-altered rocks. In discrimination diagrams, the metamorphosed mineral system is demonstrated to exhibit the diagnostic attributes of, and is interpreted as, a metasomatic iron and alkali-calcic (MIAC) mineral system with iron oxide–apatite (IOA) and iron oxide copper–gold (IOCG) mineralization that evolves toward an epithermal cap. This contribution demonstrates that alteration facies diagnostic of MIAC systems and their IOCG and IOA mineralization remain diagnostic even after high-grade metamorphism. Exploration strategies can thus use the lithogeochemical footprint and the distribution and types of alteration facies observed as pathfinders for the facies-specific deposit types of MIAC systems.