Jadeitites are commonly found in serpentinite mélanges and form by fluid flow across the subduction interface. Petrological analysis of jadeitites from various localities (Myanmar, Guatemala, Cuba, Russia, and Iran) coupled with structural characterization enabled the identification of successive jadeite/omphacite generations with subordinate amphibole and mica. These parageneses reflect metasomatism coeval with brittle and/or ductile deformation and complex crosscutting relationships. The composition of fluid inclusions (salinity, gas, δ18O, δD) reveals a wide range of fluid species pointing to a diversity of jadeitite-forming metasomatism. In situ trace element analysis and isotopic measurements (δ11B, 87Sr/86Sr, δ18O) indicate a fluid source dominated by altered oceanic crust (AOC) with a minor sedimentary component. Despite marked changes in major element content during protracted metasomatism, trace elements evolve only moderately while isotopes are virtually homogeneous, recording only small variations of fluid composition over time. Jadeitite evolution is strongly related to the ongoing serpentinization of the mantle wedge, promoting a longer fluid time-residence at the interface associated with chemical exchange and pore-pressure build-up. This suggests that (i) First jadeite generations formed by percolation of highly channelized AOC-derived-fluids in a dry mantle wedge, while later generations record fluid interaction with sediments and the serpentinized mantle. (ii) Fluid pulses across the subduction interface and rheological behavior of the near interface mantle wedge are not controlled by drastic changes in the nature of the slab input, but rather by the cooling of the serpentinizing subduction environment. (iii) The re-use of the same fluid pathways above the slab promotes the re-equilibration of isotopic signatures. (iv) Overpressures may build up upon jadeitite formation and promote brittle deformation events. This may lead to switches in deformation style and variations in permeability, thus changing fluid flow mode along the base of the mantle wedge.

Lower Jurassic Abshour gabbro (178.3 f 2.3 Ma and 187.2 f 4.7 Ma) and Gowd-e-Howz granitoid (180.3 f 1.3 and 178.9 f 1.2 Ma) intrusions are exposed in the Esfandaghe area, Kerman province, SE Iran, as part of the southern Sannandaj-Sirjan metamorphic-magmatic zone (SSMMZ). These igneous bodies intruded into Upper Paleozoic-Triassic metamorphic rocks. The Gowd-e-Howz granitoid stock primarily consists of coarse to mediumgrained granodiorite, with subordinate diorite, quartz diorite, quartz monzonite, granite, and aplitic-pegmatitic alkali granitic veins, accompanied by a small amount of gabbro and cut by middle Jurassic quartz monzonitic dykes. Mafic microgranular/microgranitoid enclaves (MMEs) of various sizes are present in the stock. The granitoid rocks exhibit a low to medium-K calc-alkaline nature, with enrichment in large ionic lithophile elements (LILEs) such as Rb, Ba, K, and Ce, alongside a depletion in high field strength elements (HFSEs) like Ti, Y, Nb, and Zr. The REE patterns display relatively parallel, smooth LREE-enriched chondrite normalized trends, indicating the influence of fractional crystallization, along with contamination, assimilation, and magma mixing/ mingling processes in magma evolution. The type of lithologic association (gabbro, diorite, granodiorite, granite), the presence of MMEs, the prevalence of Fe-Mg silicate minerals (Cpx, Amph. Bt), and the plots of the geochemical data on various whole rock and mineral chemistry discriminant diagrams suggest that the Gowd-eHowz granitoid stock possesses characteristics of metaluminous to slightly peraluminous, calc-alkaline, magnetite series, I-type granitoids typical of continental volcanic arc settings formed from partial melting of a subducting oceanic slab and lower crust (CI-type granite) in an active continental margin subduction zone. In this model, the initiation of NE-dipping subduction of the Zagros Neotethyan oceanic lithosphere beneath the Central Iran continental lithosphere during the Late Triassic-Early Jurassic (approximate to 180 Ma) may have accounted for the generation of mafic and felsic magmas in the Andean-type arc magmatism of the SSMMZ, where gabbroic and granitoid plutons formed and intruded in metamorphic country rocks. Thermobarometric calculations indicate that magma storage, plumbing, and fractionation occurred at three levels (40-45, 14-16, and 5-7 km) in the base, middle, and upper continental crust, respectively.

The western-central European Variscan orogen developed during the Upper Devonian to Carboniferous continental collision between Gondwana and Laurussia, culminating in the assembly of Pangea. This orogen records a complex sequence of tectono-thermal events providing insights into continental crustal evolution and lithospheric deformation mechanisms. Late Devonian D1 contractional deformation is associated with crustal thickening, Mississippian D2 extensional deformation was probably caused by gravitational collapse of the mountain range, and Pennsylvanian D3 contractional deformation represents subsequent crustal shortening. The Iberian Massif, located at the core of the Variscan Orogen, offers exceptional conditions for detailed analysis of deep-to-shallow crustal Variscan tectono-thermal processes, preserving these three superimposed deformation events.In the Ossa-Morena Zone (SW Iberian Massif), the youngest Variscan orogenic activity is associated with the emplacement of syn- to late-D3 plutons. We present new geological mapping, whole-rock geochemistry, and zircon U-Pb geochronology from the Pennsylvanian Figueira e Barros-Ervedal and Fronteira plutons (west-central Ossa-Morena Zone). These shallow-crustal, calc-alkaline, peraluminous granodioritic to granitic intrusions are syn- to late-D3 because they crosscut D2-D3. SHRIMP U-Pb zircon dating indicates crystallisation ages of 307 ± 3 Ma and 308 ± 2 Ma for the Figueira e Barros-Ervedal and Fronteira plutons, respectively.Their host metamorphic succession consists of Silurian-Devonian siliciclastic flysch, containing olistostromes and olistoliths, overlying a bimodal volcanic-sedimentary complex assigned to the Cambrian-Ordovician (?). Both stratigraphic units underwent post-kinematic contact metamorphism associated with the emplacement of these Pennsylvanian plutons, producing pelitic hornfels, dominated by spotted mica schists with post-kinematic porphyroblasts. Prior to this contact metamorphism, regional M2 Buchan-type metamorphism produced pre- to syn-kinematic garnet porphyroblasts and syn-kinematic andalusite and staurolite porphyroblasts. These mineral assemblages are associated with the development of a flat-lying pervasive S2 foliation and mineral lineation, defined by biotite and muscovite (after sillimanite?), which is comparable to that observed in the hanging-wall blocks of Mississippian gneiss domes in the Iberian Massif, including in nearby sectors of the Ossa-Morena Zone. It should also be noted that locally, pre-early-kinematic garnets preserved as cores or as isolated minerals, together with possible high-pressure/low-temperature mineral assemblages in the kyanite zone, were also identified, pointing to a pre-D2 process of regional pressurisation (Barrovian metamorphism), which possibly represents D1-M1(?). About 20 km northwest of the Pennsylvanian Figueira e Barros-Ervedal and Fronteira plutons, the Mississippian Ponte-de-Sôr gneiss dome exhibits a pervasive S2 foliation and top-to-the-SE tectonic transport synchronous with M2 Buchan-type metamorphism. We propose that a comparable, though cryptic, D2 gneiss dome developed in the study area prior to the emplacement of the syn- to late-D3 Figueira e Barros-Ervedal and Fronteira plutons.Work supported by FCT, I.P./MCTES through national funds (PIDDAC): LA/P/0068/2020- https://doi.org/10.54499/LA/P/0068/2020, UID/50019/2025, https://doi.org/10.54499/UID/PRR/50019/2025, UID/PRR2/50019/2025, and by the Spanish Ministerio de Ciencia e Innovación, Fondos Feder, PID2023-149105NA-I00. L.S.H. benefits from the FCT PhD scholarship UI/BD/154616/2023, I.D.S from the FCT research contract DL57/2016/CP1479/CT0030 (https://doi.org/10.54499/DL57/2016/CP1479/CT0030), J.C.D. from FCT contract CEECINST/00032/2018/CP1523/CT0002 (https://doi.org/10.54499/CEECINST/00032/2018/CP1523/CT0002), and M.F.P. from grant Nº. FCT/UIDB/06107-Center for Sci-Tech Research in Earth System and Energy-CREATE.

Zircon studies in mafic and ultramafic rocks are particularly valuable because they can reveal deep-mantle petrogenetic and geodynamic processes. These studies, however, are hindered by the scarcity and uneven distribution of zircon in the mantle and mantle-derived rocks. Consequently, finding zircon in these rocks is inherently difficult and strongly influenced by the zircon abundance, grain size, volume, and the number of samples searched. Here, we describe a method for in-situ zircon identification and extraction for further analytical studies. To this end, we cut four representative 2 × 3 cm slabs, one from the granite and three from the gabbro, and carefully polished one side to enable automated large-area elemental mapping with SEM, EPMA, and µ-XRF using WDS and or EDS. Of all these, µ-XRF offers the best balance among acquisition time, surface coverage, spatial resolution and result quality. To overcome spectral interferences affecting the Zr signal, the method uses multidimensional analysis based on suitable ratios of the X-ray lines produced by the sample's major and, eventually, minor elements. Zircon-bearing pixels are then discriminated utilising a combination of global, local, and probabilistic statistical classifiers, such that zircon identification is accepted only where consistent statistical behaviour is reproduced across independent methods. Overall, the protocol enables reproducible detection and extraction of zircon down to 50 µm in grain size while minimising both false positives and false negatives. The same methodology can be used to locate other Zr-bearing (e.g., baddeleyite, zirconolite, srilankite) and Zr-lacking (e.g., apatite, xenotime monazite, rutile, chromite) accessory minerals that may be present in the rock sample.

Subsolidus muscovite dehydroxylation represents a significant internal fluid source in felsic rocks. To investigate its effects, non-equilibrium differential-heating experiments were conducted at 1 bar and 750–1000 °C, together with piston–cylinder experiments at 800 °C and 500 MPa, using a two-mica orthogneiss and a biotite orthogneiss. The two lithologies show markedly different behaviour. In the biotite orthogneiss, only limited transformations occur. In contrast, muscovite dehydroxylation in the two-mica orthogneiss generates internally derived H2O fluids capable of reactivating mineral-replacement reactions without external fluid addition. Structurally bound OH released during dehydroxylation destabilises adjacent biotite, producing fine-grained orthopyroxene + spinel aggregates accompanied by the loss of H2O, K, Ti and F. The generated fluids also promote titanite growth and pervasive alkali metasomatism. Plagioclase is replaced by transitional Or–Ab feldspar, porous albite, and newly formed K-feldspar, producing textures comparable to those in natural hydrothermal and lower-crustal metasomatic systems. These microstructures indicate efficient fluid-mediated mass transfer even at extremely low fluid abundances (<1 wt%).The contrasting responses of the two orthogneisses highlight the role of muscovite in controlling fluid availability, alkali mobility, and subsolidus reaction pathways. A major consequence is the transfer of the K-feldspar component hosted in biotite into newly formed K-feldspar, shifting the bulk composition towards granite and potentially enhancing fertility during subsequent water-fluxed melting. These experiments demonstrate that mica-derived fluids can act as effective metasomatic agents during thermal shock events, linking dehydration reactions, feldspar re-equilibration, and the preconditioning of felsic rocks for anatexis.

The disturbance of the U-Pb isotopic system in zircon (ZrSiO4) is an important factor for U-Th-Pb geochronology, therefore understanding zircon nanostructure is crucial for reliable age determination. To understand the process of Pb mobilization in zircon, heating experiments on zircon grains from two samples of the Central Iberian Zone (CIZ), a Variscan tonalite and a Cambro-Ordovician orthogneiss, were performed. Samples were heated at 1400 degrees C for 30, 90 and 180 days in a horizontal furnace in N2 at 1 atm. Following the experiment, nano- and microstructural analyses were performed using Transmission Electron Microscopy (TEM). Mobilization of Pb was documented only in zircons from orthogneiss - these grains contain Pb nanospheres. Zircon grains from orthogneiss showed a significant influence of heating during the experiment on the microstructure. We observed Pb nanospheres (Pb0) and Pb nanoinclusions containing Pb in different oxidation states, namely Pb2+ and Pb4+. This is the first paper documenting nanospheres of metallic Pb existing together with Pb oxides (PbO, Pb2O3, Pb3O4 and PbO2) in one sample. As a result of heating, Pb was oxidized and after 6 months of heating, the spheres of metallic Pb were no longer found. The formation of metallic Pb nanospheres is explained by an annealing process at elevated temperature, that caused the concentration of Pb in noncrystalline, metamict domains of zircon. This study indicates that more than one mechanism can be responsible for the formation of nanospheres. In this case, chemical elements present in zircon as inclusions react with Pb causing its oxidation.

Biotite plays an important role in the geochemical cycle of Li, Rb, Cs, and Ba in the upper continental crust, as it is a significant carrier of Li and large-ion lithophile elements in felsic igneous rocks and high-grade detrital metasedimentary rocks. During its interaction with meteoric and hydrothermal fluids, biotite can be transformed into various types of clay minerals (mostly, interlayer-deficient biotite, vermiculites and smectites). These transformations can cause fractionation of the alkaline trace-element ratios Rb/Li, Cs/Li and Rb/Cs between biotite and its replacement products. This study examines the mineral transformations that occur when biotite interacts with aqueous and saline fluids and the poorly understood geochemical behaviour of the resulting phyllosilicates. For this purpose, we performed batch hydrothermal experiments of the interaction of biotite + quartz + graphite with ultrapure H2O, and 2 M NaCl, 2 M CaCl2 and 1 M NaF brine fluids at 170 degrees C and 10 bar using Teflon bombs, and at 550 degrees C and 800 to 1400 bar using autoclave apparatus. At lower-T conditions, biotite was replaced by 2:1 trioctahedral clay minerals (interlayer-deficient biotite, smectite, vermiculite, and other phyllosilicate species with higher interlayer charge) and Fe oxy-hydroxide minerals by coupled dissolution- precipitation mechanisms. At higher-T conditions, these mechanisms caused the transformation of biotite into the mineral assemblages (quartz + graphite): diopside + anorthite + titanite (CaCl2 brine experiments), albite + ilmenite + clay minerals (NaCl brine experiments), and cryolite + alkali feldspar with albite rimmed by Kfeldspar + Fe-oxides (NaF brine experiments). Therefore, a significant reduction of the clay mineral stability in the presence of NaF and CaCl2 brine fluids is inferred. The biotite replacements by phyllosilicates were mostly controlled by the ion exchange of K+ by H+ (or its hydrate state H3O+), hydrated Na+ and Ca2+, and NaF in the interlayer site. Conservation of the total mass and the Si, Al and Mg abundances occurred in most experimental phyllosilicates. However, in the products of the low-T NaF brine + graphite experiments, the total mass may have a gain of 5.3-11 % assuming Mg conservation. Sc, V, Nb and Ta abundances were also conserved, but a significant fractionation of the Rb/Li, Cs/Li, and Ba/Li ratios occurred in the experimental phyllosilicates. The experiments predict the generation of highly fractionated Rb/Li and Cs/Li phyllosilicates by replacement of biotite during interaction with aqueous fluids and, mostly, NaCl and NaF brine fluids at high-T and low-T conditions, respectively. This demonstrates a key role of biotite in the fractionation of Rb/Li, Cs/Li and Rb/Cs during the hydrothermal alteration of felsic igneous rocks. Conversely, a reversal in the mobility of Li with respect to Rb and Cs occurred in the phyllosilicate products when biotite interacted with NaCl or CaCl2 brine fluids at relatively low-T conditions. These experimental results highlight the key role of biotite-fluid interaction processes in controlling the budget of alkaline trace elements in the continental crust.

Jadeitites are commonly found in serpentinite melanges and form by fluid flow across the subduction interface. Petrological analysis of jadeitites from various localities (Myanmar, Guatemala, Cuba, Russia, and Iran) coupled with structural characterization enabled the identification of successive jadeite/omphacite generations with subordinate amphibole and mica. These parageneses reflect metasomatism coeval with brittle and/or ductile deformation and complex crosscutting relationships. The composition of fluid inclusions (salinity, gas, 518O, 5D) reveals a wide range of fluid species pointing to a diversity of jadeitite-forming metasomatism. In situ trace element analysis and isotopic measurements (511B, 87Sr/86Sr, 518O) indicate a fluid source dominated by altered oceanic crust (AOC) with a minor sedimentary component. Despite marked changes in major element content during protracted metasomatism, trace elements evolve only moderately while isotopes are virtually homogeneous, recording only small variations of fluid composition over time. Jadeitite evolution is strongly related to the ongoing serpentinization of the mantle wedge, promoting a longer fluid time-residence at the interface associated with chemical exchange and pore-pressure build-up. This suggests that (i) First jadeite generations formed by percolation of highly channelized AOC-derived-fluids in a dry mantle wedge, while later generations record fluid interaction with sediments and the serpentinized mantle. (ii) Fluid pulses across the subduction interface and rheological behavior of the near interface mantle wedge are not controlled by drastic changes in the nature of the slab input, but rather by the cooling of the serpentinizing subduction environment. (iii) The re-use of the same fluid pathways above the slab promotes the re-equilibration of isotopic signatures. (iv) Overpressures may build up upon jadeitite formation and promote brittle deformation events. This may lead to switches in deformation style and variations in permeability, thus changing fluid flow mode along the base of the mantle wedge.

Orogenesis involves a continuum of complex natural phenomena within the context of the Wilson Cycle, the backbone of modern plate tectonics. While topographic effects of present-day orogenic cycles are readily visible, eroded old orogens represent windows that expose the crust's interior and facilitate the study of complex lithospheric processes.The object of this research lies within the Devonian-Carboniferous Variscan collisional orogen that extends from Southern Europe to Northern Africa. This orogenic belt resulted from the convergence and collision between the passive margin of north Gondwana and the active margin of southern Laurussia, forming the Pangea Supercontinent. The Iberian Massif, located in the core of Pangaea, is one of the best exposures of the Variscan orogen in Europe, and a unique natural laboratory to study deep-to-surface geodynamic phenomena. Studying this sector of the Pangea supercontinent raises new important questions about how modern collisional orogens evolve and how their crustal architecture develops.Field and analytical data compiled in the last 20-30 years in Iberia has revealed a complex basin-cover architecture derived from the deformation and metamorphism of the Ediacaran to Carboniferous stratigraphy. Ongoing research in a critical and representative region of the SW Iberian Massif (i.e. Ossa-Morena Zone), reveals a close relationship between the deformation, metamorphism, magmatism and sedimentary processes involved in deep to shallow lithospheric dynamics, during both orogenic thickening and gravitational collapse. The systematic study of key outcrops was performed, to define first-order geological contacts between major tectono-metamorphic, stratigraphic and magmatic units. This information made it possible to define the architecture of the crust along a transverse across the central region of the Ossa-Morena Zone (Estremoz, Portugal). With the structural relationships well defined, the main units were sampled to control the ages of the orogenic events and to correlate the tectono-metamorphic fabrics found in the Variscan basement regionally. This research focused on SW Iberian Massif will give important constraints to develop state-of-the-art conceptual and numerical models of the tectonic evolution of the Variscan Orogen during the assembly of the Pangaea supercontinent. The combination of field and petrography data with numerical modelling can be very useful for better understanding the role of different lithospheric processes in orogenic building and gravitational collapse as Supercontinents are formed. This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020 (https://doi.org/10.54499/UIDB/50019/2020), through the scholarship UI/BD/154616/2023 and through UIDP/50019/2020 (https://doi.org/10.54499/UIDP/50019/2020), LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020) and DL57/2016/CP1479/CT0030 (https://doi.org/10.54499/DL57/2016/CP1479/CT0030). M.F. Pereira acknowledges financial support from the FCT project (grant No. FCT/UIDB/ 04683/2020-ICT).

A general petrologic model for the transformation of chromitite in the FeO–MgO–Al2O3–Cr2O3–SiO2–H2O (FMACrSH) system is presented based in mass-balance and thermodynamic constraints. In the model, the transformation of chromitite reaches the common Cr-spinel+chlorite assemblage of transformed chromitites upon reaction with external fluid. This metasomatic process takes place in two major sequential steps involving a net-transfer reaction of olivine consumption first ensued by Cr-spinel+chlorite dissolution–precipitation. The first step is completed early in the hydration/metasomatic process producing new Cr-spinel (+chlorite±brucite) with restricted composition close to the composition of reacting mantle Cr-spinel as a function of Cr-spinel/olivine ratio and the stoichiometric coefficients of olivine and Cr-spinel in the net-transfer reaction. The second transformation step, triggered upon exhaustion of olivine, is protracted and continuously produces increasing chlorite and decreasing Cr-spinel contents, the latter with continued more deviated composition from reacting mantle Cr-spinel, as a function of continued infiltration of external fluid. The mass-balance model does not prejudice transformation under isothermal-isobaric conditions, heating, or cooling, but thermodynamic calculations confirm that all these thermal scenarios are possible for the generation of the predicted mineral assemblages and compositions. These calculations demonstrate that extreme Cr-spinel compositions are a strong function of decreasing spinel volume upon reaction progress at reaction sites under strongly overstepped conditions. The application of the model to mantle chromitites of the Cadomian Calzadilla metaophiolite (Ossa-Morena Complex, SW Iberia) allows reinterpreting the thermal scenario for chromitite transformation in a context of prograde metamorphism at near-isothermal-isobaric conditions. Proposals of cooling during transformation of regionally metamorphosed chromitites should be revisited in light of the petrologic model offered.

ABSTRACTThe Rocciavrè massif is a large eclogitized ophiolitic fragment exposed in the Western Alps (Piemonte, Italy) exhibiting an almost complete sequence of the subducted Liguro‐Piemont lithosphere. Raman spectroscopy on carbonaceous material in metasediments from Rocciavrè and the juxtaposed Orsiera massif indicates maximum temperatures in the range ~510°C–550°C, whereas thermodynamic modelling in mafic lithologies reveals peak burial metamorphic conditions of 550°C–590°C/2.2–3.0 GPa for both units, suggesting the absence of a metamorphic gap between them. Late Jurassic (ca. 151–158 Ma) zircons extracted from Rocciavrè metagabbros reflect the crystallization age near the seafloor, and no alpine metamorphic rims have been detected. The garnet‐omphacite‐rutile–dominated Fe‐Ti metagabbros are crosscut by a variety of high‐pressure vein systems, including garnet‐rich, omphacite‐rich, omphacite‐quartz–rich, glaucophane‐quartz–rich and winchite‐actinolite‐talc veins. Vein textures, mineral assemblages and mineral compositions suggest the formation of garnet‐rich and omphacite‐rich veins at conditions close to peak burial and the successive formation of omphacite‐quartz–rich and glaucophane‐quartz–rich types by reopening former omphacite‐rich veins at eclogite‐ to epidote‐blueschist‐facies conditions along the exhumation path. In contrast, winchite‐actinolite‐talc veins are interpreted as retrograde greenschist‐facies features. In situ U‐Pb dating of monazite constrains the age omphacite‐quartz–rich veining at 40.4 ± 0.2 Ma. Major and trace element mapping of vein assemblages shows various zoning patterns of omphacite and rutile crystals for a large variety of elements (e.g., Fe, Mg, Mn, Sr, Li, U and Cr). Aqueous primary fluid inclusions trapped in vein‐filling and host‐rock minerals have intermediate to high salinity values, interpreted to reflect the partial signature of hydrothermal alteration preserved up to eclogite‐facies conditions. High fluid inclusion salinity values associated with the presence of N2 (± CO2) suggest the presence of fluids produced by local dehydration reactions at peak burial. In contrast, some inclusions from glaucophane‐quartz–rich veins contain a low to intermediate salinity CO2‐CH4–bearing fluid interpreted as reflecting a sedimentary contribution and a larger scale of fluid circulation. In addition, the mineralogy of winchite‐actinolite‐talc veins associated with high‐salinity values suggests an ultramafic signature. The successive steps of vein formation are interpreted to record the evolution from a closed to open chemical system during exhumation, with late sedimentary and ultramafic fluid contributions that witness the mobility of fluids within the mafic sequence and transport distances likely reaching the kilometre scale. The Rocciavrè massif, which shares a similar metamorphic history to the Monviso Lago Superiore Unit further south, enables a precise characterization of fluid–rock interaction processes in subduction from eclogite‐facies to greenschist‐facies conditions.

Subduction zones act as pivotal engines for global element cycling. Titanium (Ti) isotopes, exhibiting massdependent fractionation, emerge as a robust geochemical tracer for deciphering complex processes in these dynamic settings. However, the behavior of Ti and its isotopes during metamorphic dehydration and partial melting of deeply subducted continental lithosphere remains poorly constrained. This study addresses this issue through an integrated investigation of age, elemental signatures, and Ti isotopic compositions of rutile from quartz-bearing and granitic felsic veins, eclogites, paragneisses, and orthogneisses within the North Qaidam orogen-a paleo-continental subduction zone where eclogite boudins are embedded in gneissic matrices. Petrochronological U-Pb dating reveals distinct temporal records: rutile in eclogites and gneisses yields metamorphic ages of 439 f 3 Ma to 430 f 11 Ma, aligning with regional eclogite-facies metamorphism and implicating rutile growth during dehydration of subducted continental crust. In contrast, rutile from quartz- and felsic veins documents protracted melt/fluid activities spanning 433 f 3 Ma to 408 f 1 Ma, reflecting melt/fluid generation under eclogite-facies conditions and subsequent exhumation. High-resolution Ti isotope and trace element analyses demonstrate minimal intra-sample isotopic variability among rutile grains within individual eclogites or gneisses, suggesting negligible Ti isotope fractionation during metamorphic dehydration. Similarly, rutile in eclogite-hosted felsic veins exhibits delta 49Ti values indistinguishable from their host eclogites or adjacent gneisses, further negating significant isotopic fractionation during partial melting. However, inter-sample delta 49Ti variations correlate systematically with whole-rock geochemical proxies: negative correlations with epsilon Nd(t) and positive correlations with (87Sr/86Sr)i highlight protolith heterogeneity as the dominant control on Ti isotopic signatures. These findings collectively demonstrate that Ti isotopic compositions in deep subduction-related systems primarily inherit protolith characteristics rather than reflecting process-driven fractionation. Consequently, Ti isotopes serve as powerful tracers for identifying melt/fluid sources in subduction zones. Notably, cold subduction regimes promote localized Ti mobility via eclogite-derived melts/fluids, while warmer settings facilitate widespread Ti activation through partial melting of gneiss-eclogite mixtures, as evidenced by abundant rutile-bearing veins spanning 427-408 Ma. The study underscores that continental subduction zones-spanning thermal gradients from cold to warm-exhibit melt-mediated Ti mobilization influenced by melt abundance, source heterogeneity, and prolonged melt-crystal interaction. These insights from the North Qaidam orogen advance our understanding of Ti cycling in continental subduction systems globally, emphasizing the interplay between protolith inheritance and tectonic thermal regimes in governing element redistribution.

Petrology, geochemistry and geochronology of a metapelite (sillimanite-garnet-biotite-plagioclase-quartz) from the vicinity of the Archean Mercara Shear Zone in Coorg, S. India show that metamorphism at temperatures > 850 °C occurred between 2700–3300 Ma (Phase equilibria, thermobarometry, U-Pb dating of zircons and Lu-Hf dating of garnets). Subsequently, the rocks experienced thermal events at lower temperatures at 2400-2600 Ma as well as at 600-640 Ma (U-Pb dates from rutile). There are indications of multiple episodes of metasomatic/ (high temperature) hydrothermal activity during the Archean events. Residence of the rocks at lower temperatures between the high temperature events is indicated by the kinetics of dissolution of zircon in melt. Taken together, this history shows that (a) P-T-t evolution in this Archean collisional setting happened along an overall clockwise path but not in a single continuous loop - episodes at high temperatures were interspersed with residence at cooler temperatures in between, (b) subtle effects of metamorphism that occurred at temperatures below the peak temperature could help to resolve some controversies related to tectonothermal reconstructions in the region (e.g. whether signatures of both - amalgamation of Dharwar and Coorg cratons and activity along an equivalent of the Betsimisaraka suture zone in east-central Madagascar may be present in the region), and (c) the duration of high-temperature events (several 100 million years at 800 °C) are consistent with an early Earth peel-back style of plate tectonics, rather than modern day plate tectonics, operating in the region at the time.

It is now widely known that the fluids play a major role in the formation and evolution of the Earth´s crust. Fluids and melts formed by dehydration-melting reaction of muscovite, biotite and amphibole during prograde metamorphism can have a profound effect on the trace element and isotope composition on the crustal sources of magmas.However, while the phase relations of these reactions as well as the textural and mineralogical evidence of fluid-rock interaction in high-grade metamorphic complexes are well studied, the mechanisms of the onset of fluid and melt generation by the dehydration melting along with the transport and geochemical impact of these mobile phases onto anhydrous minerals at the grain scale remain unclear.We investigated the mechanisms of local reactions involved in the incipient dehydration/melting processes at different temperatures, the sequence of reactions and the interaction of the fluid and/or melt with the anhydrous mineral phases, by conducting analogue heating experiments of rock cylinders in a vertical furnace. The rock samples used in the experiments were granitoids and gneisses from the Iberian Massif (Spain) with variable content of biotite and muscovite. They were cut into cylinders with dimensions of about 3x20 cm and placed into a vertical furnace. The experiments were done under thermal gradient at sub- and supersolidus conditions (600-1200oC) at ambient pressure in an inert atmosphere of N2 and last up to 8 days.First results show that significant compositional and textural changes in biotite and muscovite were produced in experimental runs at temperatures >850oC. Muscovite experienced dehydration melting breakdown to ultrabasic, very peraluminous melts with higher Na and lower K than the starting muscovite, small grains of aluminosilicates and large vesicles. Biotite underwent subsolidus dehydration, resulting in the formation of spinel and/or Fe-Ti oxides and alkali-rich aqueous fluid. Notably, K-feldspar did not nucleate at the dehydration site; instead, excess K and other incompatible elements (Li, Rb, Cs, Ba) were transported by fluids released from biotite and muscovite. These fluids subsequently induced metasomatic reactions in plagioclase, transforming it into K-feldspar. Additionally, Ca released from plagioclase contributed to the formation of titanite after ilmenite. The metasomatic changes were facilitated by fluid migration along micropores that were present in the starting plagioclase, highlighting the intricate processes involved in mica driven metamorphism and metasomatism. A second type of melt was generated with increasing temperature, characterized by higher silica and more granitic-like compositions, suggesting the involvement of quartz and feldspars in the melting reactions.Although the experimental pressure conditions are much lower than those inside the crust, these analogous experiments allow us to investigate the mechanism by which fluids and melts are segregated from the reaction sites and the influence of rock texture. In these analogue experiments the breakdown of hydrous minerals is enhanced because they are outside their P-T stability fields. Besides, the formation of gas is maximized since its solubility in the melt is very low. Therefore, it allows us to investigate the importance of vesiculation in the creation of pathways for fluid and melt migration.

The Cadomian basement of SW Iberian Massif appears in the so-called Ossa-Morena Complex, which contains several allochthonous terranes that are key for deciphering the evolution of the Gondwana margin during the Neoproterozoic. In this context, the Calzadilla Ophiolite represents the remnants of an oceanic Ediacaran lithosphere formed by a boninitic mafic crust that overlies a serpentinized ultramafic section, the latter being the biggest outcrop of Cadomian serpentinites in Europe. In this work, the petrological and geochemical nature of the mantle section is analysed, as well as the tectonothermal evolution of both, the mafic crust and the ultramafic section. The algebraic analysis of whole rock geochemistry of the serpentinites indicates that the protolith corresponds to harzburgite. The geochemical fingerprint of the serpentinites, as well as the primary spinel crystals composition, shows that peridotites underwent high degrees of partial melting (up to 16%) in a forearc setting. Thermodynamic modelling and average PT methods were applied to serpentinites and mafic dykes from the ultramafic section as well as to amphibolites from the mafic section, both of which experienced progradation up to 4-5 kbar and 525-575 degrees C. According to the data, the protoliths of the Calzadilla Ophiolite were formed in a peri-Gondwanan arc system developed during the subduction of oceanic crust below Gondwana. The roll-back of the subducting slab would induce the opening of the forearc basin at c. 602 Ma, protolith age of amphibolites from crustal section, with an extensive partial melting of the mantle wedge and the formation of boninitic-affinity magmas that constitute the mafic section of the Calzadilla Ophiolite. Metasomatism and serpentinization of the mantle wedge would occur via melt- and fluid-rock interactions. A compression-dominated stage at c. 540 led to the emplacement of the Calzadilla Ophiolite onto the Cadomian magmatic arc, with the development of the amphibolite-facies metamorphic imprint. [GRAPHICS] .

The Neoproterozoic Bou Azzer Ophiolite, located in the Pan-African orogen of the Anti-Atlas, Morocco, exposes a well-preserved mantle section of the ophiolitic sequence. Although ultramafic rocks are the most common rock types in the sequence, they have received less attention in previous studies. In this work, we investigate the petrogenesis of these ultramafic rocks in detail, including their petrology, mineralogy, and whole rock geochemistry. The mineral assemblages are mainly formed by antigorite with minor lizardite, chlorite, and accessory Cr-spinel. The only preserved primary mantle mineral is Cr-spinel, which shows unaltered cores with Cr# between 0.61 and 0.71 and alteration rims to ferrian chromite and magnetite. Whole rock analyses indicate almost total serpentinization and, locally, strong carbonation (mass loss of ignition between 11.94 and 22.20 wt%). The non-carbonated samples preserve protolith signatures, with decreased MgO/SiO2 suggesting metasomatic processes related to serpentinization. Immobile trace elements compare well with fore-arc peridotites, while melting modelling indicates intense and polyphasic melting events. We propose polyphase melting in a subduction-initiation setting, with a first stage of 14-23% anhydrous melting, forming fore-arc basalts, and a second stage of 15-25% melting with 1 wt% H2O, forming boninites. The Bou Azzer ultramafic rocks represent the fore-arc region of a supra-subduction zone, challenging previous interpretations that suggested a back-arc position.

In the region of Pichilemu (Central Chile), the exhumed roots of the Carboniferous paleo-accretionary wedge developed during subduction of an oceanic realm underneath the western Gondwana margin are exposed. We focus on the areas of Infiernillo beach and Punta de Lobos: the former representing a well-characterized transitional blueschist-facies coherent stack of metabasitic lithologies interlayered at the centimetre- to metre-scale within metasedimentary rocks and glaucophanites, whereas the latter corresponds to a massive, up to hundreds of metres thick, strained metapillow lavas edifice surrounded (not interlayered) by metasedimentary rocks. We provide new field, geochemical, Raman thermometry and zircon SHRIMP U-Th-Pb geochronology on the lithologies forming the Infiernillo-Punta de Lobos complex aiming at characterizing the sedimentary sources and the extent of mechanical mixing. Field observations suggest that the Infiernillo-Punta de Lobos complex can be considered as part of the same successions, subducted and basally accreted almost coherently at high pressure-low temperature (HP-LT) conditions. The geochemistry signal in metasedimentary rocks and maximum deposition ages point to a marked forearc source for all the metasedimentary and glaucophanite lithologies, whereas massive metapillow lavas and greenschists might represent volcanic products and oceanic roughnesses erupted near the trench when the oceanic material approached the Gondwana margin at c. 329 Ma. It is suggested that the interlayering now observed in the field is mostly the consequence of sedimentary and/or volcanic processes close to the trench environment, with minor tectonic reworking of the pre-subduction structures (e.g. lithological contacts). Our results suggest that the Punta de Lobos edifice is closely related to the subduction of large volumes of continent-derived, trench-filling sediments towards HP-LT conditions. Thus, in line with previous experimental and numerical simulations, we emphasize the potential role of oceanic roughness as stress perturbations that could contribute to the subduction of thick sedimentary sequences, and discuss their implications for seismogenesis.

Detailed textural and compositional study of calc-alkaline lamprophyres and minettes from Zeneta, SE Spain Neogene Volcanic Province (NVP) are used to unravel the magma sources and differentiation processes involved in their formation. The presence of xenocrysts of various origins indicates a hybrid nature involving mantle-derived alkaline lamproitic and continental crust-derived granitic parental magmas. A new U-Th-Pb zircon age of Zeneta minettes allows contextualizing their generation in time and space during regional lamproite magma intrusion and crustal anatexis in the NVP. Interaction and mixing of these compositionally contrasted magmas resulted in the formation of hybrid calc-alkaline lamprophyre minette. This process is reflected by the phases that crystallized from the hybrid magma, particularly phlogopite, and by the whole-rock composition of the less fractionated rocks. The calculated contribution of lamproitic and crustal-derived magma end-members in minette formation are 30–40% and 60–70%, respectively. Mixing of end-member magmas of contrasting rheological properties was possible only in a calculated thermal-window of 1025–1125°C, after cooling of intruding lamproitic magma and heating of host granitic anatectic region. The calculated thermal window fits with the estimated rheological properties of Zeneta minettes and the crystallization temperature of early minette-derived phlogopite. Furthermore, fractional crystallization was identified for the first time in the Zeneta minettes, based on the observed whole-rock line of descent from the less evolved mixed magmas through intermediate to felsic minettes and associated textural and compositional features of late crystallized phases. Polytope Vector Analysis allowed an integrated quantitative characterization of magma differentiation, including magma mixing and subsequent fractional crystallization. The identification of mixing and fractionation processes in calc-alkaline minettes genesis, which otherwise does not show spatial and temporal association in the field with granitic bodies, opens a new scenario to be considered in understanding the petrogenesis of these rocks.