Alkali metasomatism is a fundamental process in alkaline‑carbonatitic systems and plays a critical role in rare earth element (REE) enrichment. In the Gakara region of western Burundi, along the western branch of the East African Rift, fenitization is manifested by pervasive albitization of Mesoproterozoic gneissic basement rocks. This study documents, for the first time, the occurrence of albitites in the Nyamikole area and establishes a metasomatic continuum from partially albitized rocks (white aplites) to nearly monomineralic albitites. Petrographic observations and mineral chemistry demonstrate extensive replacement of K-feldspar, plagioclase, quartz, and micas by albite, accompanied by significant quartz dissolution. The albitites are dominated by albite (>80 modal%) with minor tremolite, zircon, and clay minerals. Whole-rock isocon mass-balance analysis indicates substantial losses of SiO₂, K₂O, and K-feldspar-hosted large-ion lithophile elements (Rb, Ba, Pb), together with gains in Na₂O, Al₂O₃, MgO, TiO₂, high-field-strength elements (Th, U, Nb, Ta, Hf, Y, Zr), light REEs (LaSm), V, and Sc. In contrast, Co and Cr were depleted, whereas CaO, Li, Tm, and Lu remained relatively immobile. Albitites contain significantly higher total REE concentrations (ΣREE = 399–606 ppm) than white aplites (34–279 ppm) and the host gneisses (66–161 ppm), indicating efficient REE enrichment during alkali metasomatism. Zircon UPb dating of the albitites yields an age of 1381 ± 12 Ma, interpreted as the crystallization age of the gneissic protolith. These results demonstrate that structurally focused Na-rich hydrothermal fluids drove extensive mineralogical and geochemical re-equilibration, emphasizing the importance of alkali metasomatism in crustal modification and critical metal enrichment within alkaline–carbonatitic systems.
The petrogenesis of intraplate magmas and the nature of their mantle sources remain subjects of ongoing debate. The Central Asian Orogenic Belt (CAOB), situated distal to active plate margins, hosts Mesozoic-Cenozoic intraplate alkaline basalts that provide exceptional insights into deep mantle processes in stable continental interiors. This study investigates the Late Cretaceous-Paleogene olivine basalt and basanite from the Tuoyun Basin in the southwestern CAOB using an integrated approach involving Os-He isotopes and melt inclusion analyses to constrain their mantle source characteristics and magmatic evolution. Results show that both the Tuoyun olivine basalts and basanites exhibit distinctly radiogenic Os isotope compositions, with (187Os/188Os)i values of olivine basalts spanning from 0.2644 to 0.3718 and basanites from 0.2020 to 0.2644, significantly higher than those of the depleted mantle. Olivine-hosted 3He/4He ratios (2.69 ± 0.16 to 4.97 ± 0.64 Ra, mean = 3.46 Ra) are markedly lower than those of MORB (8 ± 2 Ra), indicating a mantle source modified by subduction materials. The combined He-Os isotopic systematics, which are unaffected by crustal contamination, degassing or post-magmatic processes, preclude any significant contribution from the lower mantle; instead, they require the involvement of recycled oceanic crust in the mantle source. Quantitative Os-Sr modeling suggests a contribution of approximately 15–25% from silica-depleted pyroxenite. Melt inclusions in olivine phenocrysts document systematic compositional variations from low-Si/high-alkali (nephelinitic) to high-Si/low-alkali (basaltic) melts. In combination with previously published Zn isotope data that exhibit co-variations with SiO2, Na2O + K2O, Th, and Nb, these observations collectively suggest that melt-peridotite interaction occurred during magma ascent. Ultimately, we propose that the primary melt of Tuoyun alkaline basalts was derived from silica-deficient pyroxenite sources (recycled Paleo-Asian oceanic crust), and then underwent melt-rock reaction with the surrounding mantle peridotite, producing the observed compositional spectrum from nephelinite to basalt.
UPb zircon dating from some Variscan granites of the Central Western Carpathians (CWC) revealed the presence of relict zircons of Ediacaran/Cambrian age. Relict zircon crystals yielding an age of ~557 Ma with ɛHf(500) values (+9.6 to −5.5) are preserved in the Variscan tonalite aged ca. 353 Ma. Variscan magmatic ages in this tonalite show a narrower range of zircon ɛHf(350) values, from +4.0 to −2.2, suggesting that the granite protolith is a recycled Ediacaran/Cambrian magmatic source. Several relict zircon crystals with Cambrian ages of 535 and 527 Ma were also found in nearby Variscan granitic massifs dated to ~353 and ~ 351 Ma, respectively. Metaluminous diorite (~359 Ma) with a significant mantle signature having zircon ɛHf(350) = +8.3 to +0.2 is devoid of inherited zircons, while in the surrounding granodiorite, the relict zircon cores are ubiquitous. Variable proportions of crustal and mantle sources in the Variscan granites are indicated by Hf zircon isotopes and also from whole-rock ɛNd(350) values ranging from +1.6 to −5.9. Several Variscan zircons were detected in a peraluminous S-type metagranite with a high content of Ordovician relict zircons (~460 Ma, Cenerian orogeny), clearly providing evidence, supported by whole-rock geochemical analyses, of low-temperature partial melting of a Central Western Carpathian (CWC) crustal rocks during the Cenerian period. The preserved Ediacaran/Cambrian relict zircon crystals indicate rapid emplacement of Variscan granites driven by slab-breakoff geodynamics from the MASH (melting, assimilation, storage, homogenization) zone to the Ordovician and Upper Devonian wedge during a collisional Variscan granitic magmatism.
The Takab Ophiolite Assemblage (TOA) in northwestern Iran represents a critical fragment of oceanic lithosphere emplaced within the peri-Gondwanan terranes. However, its age, petrogenesis, and tectonic setting have been the subject of controversy. Here, we present new zircon UPb geochronological, whole-rock geochemical, and Sr-Nd-Pb isotopic data from mafic rocks of the TOA to constrain these aspects. Zircon UPb dating of a gabbro yields a crystallization age of 525.3 ± 4.7 Ma (Early Cambrian), linking the TOA to the early Paleozoic Proto-Tethys ophiolites. The gabbros are tholeiitic and enriched in large-ion lithophile elements (LILEs; e.g., Ba and Pb) and depleted in high-field-strength elements (HFSEs; e.g., Nb, Ta), with moderate initial 87Sr/86Sr ratios (0.70492–0.70586) and near-chondritic to slightly positive εNd(t) values (−0.9 to +3.2). These characteristics indicate derivation from a depleted mantle source that was metasomatized by subduction-related fluids and/or melts. Combined Sr-Nd-Pb isotopic data indicate contributions from both an enriched mantle (EMII-type) reservoir and slab-derived components. The mantle section of the TOA is characterized by high-Cr# chromites and refractory peridotites with geochemical features typical of highly depleted mantle domains, whereas the gabbroic crust records back-arc basin magmatism. This apparent duality is interpreted to reflect the inherent heterogeneity of the mantle wedge beneath a back-arc basin, in which variably depleted mantle domains were subsequently metasomatized by subduction-related melts and fluids, rather than representing distinct tectonic stages. Pyroxenite veins cutting the peridotites provide evidence for melt-rock interaction within the mantle wedge, while the broad detrital zircon age spectra of associated Ediacaran metasedimentary rocks support a peri-continental setting that received sediment from both the arc and the Gondwanan hinterland. The integrated data support a supra-subduction zone origin for the TOA, specifically a back-arc basin that formed along the northern Gondwanan margin during the Early Cambrian, in response to rollback of the Proto-Tethys slab. This model links the TOA to early Paleozoic extensional events along the northern Gondwanan margin, and provides new insights into Proto-Tethys subduction geodynamics and the evolution of peri-Gondwanan terranes in Iran.
The Kribi Alkaline Igneous Complex (south Cameroon), within the Yaoundé Domain of the Central African Fold Belt (CAFB), provides evidence of significant Pan-African magmatic and tectonothermal events. Zircon U-Pb ages (ca. 602-591 Ma) constrain crystallization of the nepheline syenites during the D2 Pan-African event. 40Ar/39Ar analyses of individual amphibole and biotite crystals from ten samples yield ages ranging from 556.9 ± 1.1 Ma to 456.3 ± 1.0 Ma (2σ). These results document an intense D3 tectono-metamorphic activity between 558.7 and 550.5 Ma, followed by intermittent reactivations until ~ 518 Ma, and a later D4 phase initiated around 496 Ma, peaking at ca. 482-476 Ma and persisting down to ca. 456 Ma. This chronology reflects a sustained post-collisional activity within the Pan-African orogenic system. Geochemically, the nepheline syenites and associated mafic rocks are silica-undersaturated, metaluminous, and display miaskitic mineralogy. Their isotopic (εNd(0.6 Ga) = –1.9 to –8.4) and trace-element signatures (e.g., Nb/La, La/Yb, Th/Ta, Nb/Yb, Th/Nb, Zr/Hf, Nb/Ta, Zr/Sm, Hf/Sm) indicate derivation from an enriched, metasomatized subcontinental lithospheric mantle (SCLM) infiltrated by carbonatitic melts. Subsequent fractional crystallization processes further evolved the magmas. Zircon trace-element chemistry supports this petrogenetic model. Elevated HFSE and LREE concentrations reflect both magmatic differentiation and late-stage hydrothermal enrichment. Estimated crystallization temperatures (≈ 611-1000 °C) and variable redox states fO2 (ΔFMQ = –1.85 to +3.74) point to high-temperature and volatile-rich magmatic conditions during emplacement. Overall, these results highlight that the alkaline magmatism developed along a craton margin during transition from contractional to transcurrent tectonics, followed by Neoproterozoic-Cambrian structural reactivation.
Mukherjee and Venkatesh integrate petrographic, fluid inclusion microthermometry, Raman spectroscopy and stable C-O-S isotope data for the Bhukia gold-graphite‑copper deposit. They propose a hybrid magmatic-metamorphic hydrothermal system evolving through progressive fluid-rock interaction in carbonate-rich hosts and classify it as transitional within the MIAC-IOCG-ISCG framework. While the study provides useful local insights, re-analysis of the reported microthermometric ranges (~171–389 °C), salinity values (up to ~37 wt% NaCl equiv.), fluid inclusion assemblage textural criteria, carbon‑oxygen isotopic bimodality and sulfur isotope uniformity raises several important questions. These concern the temporal coherence of fluid pulses, the quantitative contribution of organic carbon as a redox buffer, and the precise application of the MIAC model to a disseminated-style system. Such scrutiny, including quantitative re-evaluation against established standards and detailed suggestions for reactive transport simulation, helps refine genetic models for carbonate-hosted critical metal systems and identifies specific areas where additional constraints would strengthen the original conclusions.
Mantle xenoliths from the Mercaderes–Río Mayo volcanic area (SW Colombia) provide rare evidence of carbonate–silicate interactions within the sub-arc mantle of the Northern Andean Volcanic Zone. We describe metasomatized high-pressure garnet-peridotites containing up to 51 vol% carbonate, mainly aragonite, calcite, and dolomite, indicating extensive Ca‑carbonate metasomatism. Microstructural observations, together with mineral chemistry, stable isotope data and thermodynamic models, suggest that these carbonates originated from carbonatitic melts derived from the decarbonation or partial melting of subducted Nazca sediments. Thermobarometric calculations indicate equilibration at 3.0–3.7 GPa and 1200–1250 °C, consistent with a hot mantle wedge environment. Reaction textures between carbonates and silicates indicate a multistage carbonation history, involving initial olivine consumption, followed by carbonate partial melting and melt–rock interaction with orthopyroxene during exhumation. The associated CO₂ release may have promoted rapid, buoyancy-driven magma ascent. Preservation of aragonite and disequilibrium microstructures indicates ascent rates on the order of hours to days, far exceeding those of ductile or porous flow.
The Neoproterozoic mafic–ultramafic rocks of G. Muqsim, G. Balamhindit, and G. Abu Dahr constitute well-preserved fragments of dismembered ophiolitic sequences in the Southern Eastern Desert (SED) of Egypt as part of the Arabian–Nubian Shield (ANS). This study integrates new mineralogical, whole-rock, and in-situ geochemical data to constrain low-T CO2-rich metasomatism and its imprints during serpentinization and carbonation. The ultramafic assemblage comprises serpentinized and metasomatized peridotites, serpentinites hosting chromitite lenses, and ophiocarbonates after harzburgite protoliths, whereas the mafic assemblage is represented mainly by ophiolite metagabbros. Despite pervasive metasomatic overprinting, the immobile elements of ultramafic rocks, integrated with compositions of primary olivine (Fo91–93), orthopyroxene (Opx Mg#: 91–94), clinopyroxene (Cpx), and Cr-spinel (Cr#: 0.6–0.8), suggest generation of their peridotite protoliths in a forearc setting. For both chromitites and their host peridotites, equilibrium melts calculated from primary Cr-spinel compositions display MORB-like affinities for G. Abu Dahr and boninitic affinities for G. Balamhindit and G. Muqsim ultramafic rocks, suggesting their formation at different stages during forearc evolution. Low-T CO2-rich metasomatism is evidenced by hydrous inclusions in Cr-spinel and the occurrence of thin vein networks of carbonate, talc‑carbonate matrix, amphibole, and malachite, together with extensive serpentinization and carbonation of the ultramafic rocks. In contrast, clinopyroxenite veinlets within the G. Abu Dahr chromitites record localized melt-related metasomatism associated with melt impregnation. The metasomatic overprint is most pronounced in the G. Muqsim harzburgites, which are characterized by elevated U/Th ratios (0.43–0.6), enrichment in LREE and fluid-mobile elements (FME: Li, Rb, Pb, Sr, U, and Th). This is also evidenced by the high occurrence of carbonate matrix and low values of olivine Fo (Fo86–89), Ni and Mn, and low Opx Mg# (0.87–0.89), Ca and Cr, but high spinel Ti (TiO2: 1.11 wt%). In-situ analyses of G. Muqsim metasomatized olivine and Opx reveal enrichment in LREE and several FME relative to their primary counterparts. The investigated ultramafic rocks display U-shaped REE patterns, high Ba/La ratios (up to 104), and low Th/Nd ratios (0.3–1.2), whereas the associated metagabbros and their constituent amphibole and plagioclase are enriched in LILE (e.g., Li, Cs, Rb, Ba, Pb, and Sr) and LREE relative to HFSE (e.g., Nb, Ta, Zr, and Hf) and HREE, respectively. Collectively, the investigated ophiolitic rocks and the associated chromitites record low-T CO2-bearing slab-derived fluid metasomatism during forearc evolution, with localized melt-related metasomatism.
This study investigates the diamondiferous Newlands kimberlite (South Africa, ~144 Ma), a Group II orangeite characterized by a prolific variety of garnet xenocrysts despite a scarcity of intact mantle xenoliths. By analyzing the geochemical signatures of Cr-pyrope populations, this study reconstructs the dynamic evolution and stratified nature of the Kaapvaal lithospheric mantle.The deepest section (~ 5.0 GPa; 145–155 km) is represented by Cr-pyrope type 4, which exhibits ultra-depleted CaO and trace-element profiles consistent with high-pressure exsolution from residual orthopyroxene. These garnets testify to a highly depleted harzburgitic mantle at the incipient stages of metasomatism, providing a redox-favourable environment for diamond stability. In contrast, Cr-pyropes type 3 and 2 record a progressive transition towards a more oxidized, lherzolitic composition. This evolution is driven by continuous melt-rock interactions involving metasomatic fluids with increasing contributions from inferred recycled oceanic components, leading to the enrichment of V, U, and Fe3+.At shallower depths (3.5–2.0 GPa), Cr-pyrope type 1 reflects a re-fertilized lherzolite matrix, while the presence of Cr-Ca-rich (green) garnets suggests localized interactions with melted subducted oceanic crust. Furthermore, the occurrence of pyrope and almandine marks the mantle-crust boundary at ~1.3–1.5 GPa, aligning with geophysical Moho estimates in the region. CS isotopic data obtained with a tentative, early-stage approach, support the influence of recycled crustal materials throughout the mantle column. Ultimately, these findings demonstrate that garnet xenocryst geochemistry is a robust tool for deconstructing the complex thermochemical history of the subcontinental lithospheric mantle.
Post-collisional Cu-Au fertile porphyries, characterized by high K2O contents, elevated K2O/Na2O ratios, and adakite-like affinity, are widely distributed across the southeastern (SE) Tibetan Plateau. However, the petrogenesis of these economically significant porphyries remains debated. One prominent but not universally accepted model proposes that these fertile porphyries originated from hybrid magmas derived from both juvenile lower crust and metasomatized lithospheric mantle sources. To test this model, we conducted systematic dehydration partial melting experiments on a composite source material comprising juvenile lower crust-derived garnet amphibolite and metasomatized lithospheric mantle-derived lamprophyre. Experiments were performed at 2.0–2.5 GPa and 800–1050 °C, simulating magma generation at the lower crust-upper mantle boundary. The experimental melts span a compositional spectrum from intermediate to granitic. Crucially, melts generated at 2.2 GPa/950 °C and 2.5 GPa/(850–1000 °C) display geochemical signatures in agreement with those of fertile porphyries in the region. These include: (1) elevated SiO2, K2O and total alkalis (Na2O + K2O) contents and K2O/Na2O ratios, (2) enrichment in large-ion lithophile elements (LILEs) and light rare earth elements (LREEs), (3) pronounced negative Nb-Ta-Ti anomalies, and (4) adakite-like geochemical affinity. Moreover, these experimental melts contain high volatile (H2O-Cl-S) contents, consistent with estimates for natural fertile porphyries. Our results demonstrate that mixing of crustal and mantle components represents a viable petrogenetic model that can account for both the geochemical characteristics and volatile-rich nature of post-collisional fertile porphyry magmas in the SE Tibetan Plateau. This model offers important constraints on the origin of fertile porphyries in post-collisional settings.
The Ivrea-Verbano Zone (IVZ, western Italian Alps) exposes a section of the lower continental crust and mantle where carbonatites have recently been identified. This study integrates petrographic, mineral chemical, whole-rock geochemical, and isotopic data to constrain their origin and evolution. The investigated carbonatites consist of a calcite-dominated matrix hosting xenocrystals and xenoliths of silicate country rocks. Diagnostic textures, including fluidal structures, reaction rims, and euhedral accessory phases (e.g., apatite, pyrite, thorite), record a tightly a coupled magmatic-metasomatic evolution. We show that carbonate melts and silicate xenoliths generated calc-silicate assemblages dominated by anorthite and Ca-rich clinopyroxene, through localized antiskarn processes. CO2-rich fluids played a critical role in mobilizing HFSE and REEs, which were subsequently incorporated into newly formed mineral phases. Crucially, whole-rock geochemistry and stable isotope signatures are inconsistent with a typical mantle carbonatite magma and instead support derivation from carbonate-rich protoliths subsequently modified by melt-rock interaction. Phase equilibria constraints crystallization at ∼900 °C and 9-13 kbar, confirming formation under lower crustal conditions. Our results demonstrate that IVZ carbonatites are not mantle-derived but represent crustal melts formed by carbonate syntaxis of high-grade marbles. We further argue that micro-antiskarn phenomena are primary processes, but fundamental drivers controlling mineral assemblages and trace element redistribution during melt-rock interaction. This integrated model establishes a unified genetic framework in which syntaxis and localized antiskarn processes operate simultaneously, offering a robust alternative to traditional mantle-based models. More broadly, our findings redefine the mechanisms of carbonatite formation in the lower crust and highlight the critical role of metasomatism in generating chemical and texturally complex carbonatitic systems.
Metamorphic fluid produced by devolatilization of greenstone belt metavolcanic and metasedimentary rocks plays a critical role in the formation of Archean lode gold-only deposits. In this study, mass balance and phase equilibria modelling were combined with trace element analyses and 87Sr/86Sr composition of hydrothermal scheelite and apatite from gold-mineralized reefs of the Neoarchean Hutti gold deposit in southern India. These approaches were integrated to constrain the nature of fluid and to quantify the relative contributions of metabasalt- and metapelite-derived fractions involved in mineralization. Variable REE patterns, Eu anomalies, and trace elements concentrations enabled classification of scheelite into four types and apatite into three, reflecting fluid composition. Low Mo concentrations and high Sr/Mo ratios in scheelite indicate a reduced, metamorphic fluid, with little magmatic input. The bulk of the measured 87Sr/86Sr ratios of apatite from Hutti range from 0.70140 to 0.70860 (mean: 0.70343 ± 0.00193, 2σ) while those in scheelite span 0.70156 to 0.70818 (mean: 0.70327 ± 0.00181, 2σ). We modelled the stepwise release of Sr and the 87Sr/86Sr ratios of fluids expunged during prograde metamorphism of metabasalt and metapelite using a mass balance approach. The model estimates the isotopic compositions of fluids resulting from mixing varying proportions of metabasaltic (0.70109 ± 0.00185) and metapelitic (0.71186 ± 0.00453) inputs across different metamorphic grades. Mixing calculations indicate that 70–95% metabasalt-derived fluid mixed with 30–5% metapelite-derived fluid can explain the 87Sr/86Sr ratios of most scheelite and apatite. The metapelitic component was crucial for explaining the gold and sulphur budget, although the bulk of the fluid itself was derived from metabasalt.
This contribution documents the field relationships, petrography, whole-rock geochemistry, and geochronology of rare earth element (REE)-enriched pegmatites at the Hot Ridge occurrence in northern Saskatchewan, Canada. These pegmatites intrude metasedimentary rocks of the Wollaston Group and are characterized by a K-feldspar + plagioclase + quartz ± biotite mineral assemblage. The REE mineralization is hosted in monazite with a strong spatial association to biotite segregations having ∑LREE up to 7716 ppm. The pegmatites display strong peraluminous compositions and lack any associated coeval granitic plutons, supporting an origin through anatexis of the surrounding metasedimentary sequence rather than late-stage magmatic differentiation. Monazite grains exhibit complex internal textures, ranging from oscillatory-zoned cores to patchy, altered domains, reflecting a shift from huttonite-dominated to cheralite-dominated substitution. Textural and chemical analysis of the monazite combined with UPb geochronology constrain formation and emplacement of the pegmatites to ca. 1815–1810 Ma, coincident with peak thermal metamorphism associated with the Trans-Hudson orogen in the Wollaston Domain. Post-peak fluid-mediated dissolution and reprecipitation reactions (ca. 1810–1760 Ma) and extended cooling (< 1760 Ma) are also recorded in the monazite. The petrographic assemblage, petrogenesis, and geophysical expression of the pegmatites at Hot Ridge are analogous to other pegmatites across the central Wollaston Domain suggesting a regionally pervasive crustal melting event resulting in the formation of these REE-enriched pegmatites.
Post-peak metasomatic reworking of rocks is a common process in high-grade terranes, and granites can serve as an active reservoir of fluids for metasomatism. This study reports a unique scenario of fluid-induced reworking of Neoarchean enderbite enclaves in the massive Paleoproterozoic leucogranite of the Mahalapye Complex at the northern edge of the Kaapvaal Craton, southern Africa. UPb zircon geochronology attests a crystallization age of 2624 ± 85 Ma for the enderbite, whereas whole-rock geochemical characteristics support the sanukitoid (sensu-lato) affinity of the rock. The enderbite enclaves were likely incorporated by leucogranite from the adjacent Archean terrane. Leucogranite dykes produced metasomatic haloes in the enclaves. In the interior of the enclaves distant from the contact with leucogranite, orthopyroxene is preserved being partially replaced by biotite. In contrast, orthopyroxene is absent in the rock proximal to the contact with leucogranite, while it contains coexisting Al-bearing tremolite and cummingtonite along with biotite. A scenario of interaction of the enderbite with fluids released from the leucogranite is envisaged. Conventional thermobarometry and PERPLE_X phase-equilibria modelling in terms of water and potassium activities indicate that the metasomatic front developed at temperatures 750–700 °C and pressure around 2 kbar. Bulk-rock geochemical characteristics indicate clear effect of metasomatic modification.
