Metasomatic Fluids Research Articles

Late Triassic Felsic and Mafic Magmatism in the South Qinling Orogen, Central China: Insights from the Petrology, Zircon U-Pb Geochronology, and Geochemistry of the Huoshaodian Pluton

The petrology, geochemistry, and zircon U-Pb chronology of the Huoshaodian pluton in the Liuba area of the western part of the South Qinling tectonic belt are investigated in this study. The Huoshaodian pluton consists of gabbro, quartz diorite, and granodiorite, and the dominated rock type is quartz diorite. The results indicate that the Huoshaodian pluton belongs to the calc-alkaline series. In the chondrite-normalized REE, all of the samples showed similar patterns, with an enrichment of light REEs and depletion of heavy REEs, but they showed slight differences in the degrees of Eu anomalies. The primitive mantle-normalized trace element diagram reveals an enrichment of large-ion lithophile elements (LILEs) and light rare earth elements (LREEs), as well as depleted high field strength elements (HFSEs). The zircon U-Pb dating results reveal that the gabbro, quartz diorite, and granodiorite have crystallization ages of 214.9 ± 0.58 Ma, 215.0 ± 1.2 Ma, and 215.4 ± 1.9 Ma, respectively, indicating that the Huoshaodian pluton was emplaced during the late Triassic period (214.9–215.4 Ma). In terms of petrogenesis, the gabbro of the Huoshaodian pluton originates from a transitional lithospheric mantle that has undergone fluid metasomatism and partial melting. Specifically, it originated through 1%–2% garnet spinel peridotite undergoing partial melting. In addition, the gabbro underwent a slight degree of contamination by crustal materials during its ascent and intrusion, with some continental crust material being incorporated. The quartz diorite and granodiorite of the Huoshaodian pluton are formed through partial melting processes occurring within the normal lower crust. Combined with the previous studies on the early Mesozoic tectonic evolution of the South Qinling, this study proposes that the formation mechanism of the Huoshaodian pluton may be as follows: in the early Triassic, the Mianlue Ocean subducted northward beneath the Qinling microblock, resulting in a large-scale continental-continental collision between the North China Block and the Yangtze Block; when the oceanic crust subducted to a certain depth, the detachment of the subducting slab triggered the upwelling of mantle material. The heat from mantle-derived magma caused the partial melting of the mafic lower crust, while the mafic magma entered into the upper granitic magma chamber and began to mix. Due to the high viscosity contrast and temperature difference between the two end-member magmas, incomplete mixing led to the formation of a melt with distinct adakitic characteristics and a mafic melt representing mantle-derived material.

A Genetic Model for the Biggenden Gold-Bearing Fe Skarn Deposit, Queensland, Australia: Geology, Mineralogy, Isotope Geochemistry, and Fluid Inclusion Studies

The Biggenden gold-bearing Fe skarn deposit in southeast Queensland, Australia, is a calcic magnetite skarn that has been mined for Fe and gold (from the upper portion of the deposit). Skarn has replaced volcanic and sedimentary rocks of the Early Permian Gympie Group, which formed in different tectonic settings, including island arc, back arc, and mid-ocean ridge. This group has experienced a hornblende-hornfels grade of contact metamorphism due to the intrusion of the Late Triassic Degilbo Granite. The intrusion is a mildly oxidized I-type monzogranite that has geochemical characteristics intermediate between those of granitoids typically associated with Fe-Cu-Au and Sn-W-Mo skarn deposits. The skarn mineralogy indicates that there was an evolution from prograde to various retrograde assemblages. Prograde garnet (Adr11-99Grs1-78Alm0-8Sps0-11), clinopyroxene (Di30-92Hd7-65Jo0-9), magnetite, and scapolite formed initially. Epidote and Cl-bearing amphibole (mainly ferropargasite) were the early retrograde minerals, followed by chlorite, calcite, actinolite, quartz, and sulfides. Late-stage retrograde reactions are indicated by the development of nontronite, calcite, and quartz. Gold is mainly associated with sulfide minerals in the retrograde sulfide stage. The fluids in equilibrium with the ore-stage calcites had δ13C and δ18O values that indicate deposition from magmatically derived fluids. The calculated δ18O values of the fluids in equilibrium with the skarn magnetite also suggest a magmatic origin. However, the fluids in equilibrium with epidote were a mixture of magmatic and meteoric water, and the fluids that deposited chlorite were at least partly meteoric. δD values for the retrograde amphibole and epidote fall within the common range for magmatic water. Late-stage chlorite was deposited from metasomatic fluids depleted in deuterium (D), implying a meteoric water origin. Sulfur isotopic compositions of the Biggenden sulfides are similar to other skarn deposits worldwide and indicate that sulfur was most probably derived from a magmatic source. Based on the strontium (87Sr/86Sr) and lead (206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb) isotope ratios, the volcanic and sedimentary rocks of the Gympie Group may have contributed part of the metals to the hydrothermal fluids. Lead isotope data are also consistent with a close age relationship between the mineralization at Biggenden and the crystallization of the Degilbo Granite. Microthermometric analysis indicates that there is an overall decrease in fluid temperature and salinity from the prograde skarn to retrograde alterations. Fluid inclusions in prograde skarn calcite and garnet yield homogenization temperatures of 500 to 600 °C and have salinities up to 45 equivalent wt % NaCl. Fluid inclusions in quartz and calcite from the retrograde sulfide-stage homogenized between 280 and 360 °C and have lower salinities (5–15 equivalent wt % NaCl). In a favored genetic model, hydrothermal fluids originated from the Degilbo Granite at depth and migrated through the shear zone, intrusive contact, and permeable Gympie Group rocks and leached extra Fe and Ca and deposited magnetite upon reaction with the adjacent marble and basalt.

Subduction zone rocks oxidized by supercritical fluid: Constraints from an ultrahigh-pressure eclogite-vein system in the Dabie orogen, China

The role of subducting slab-derived supercritical fluids in transferring oxidized components in subduction zones remains poorly constrained. We investigated the oxidation state and effect of supercritical fluids by a combined study of petrology and geochemistry on an ultrahigh-pressure eclogite-vein system from the Dabie orogen, China. These samples record the composition and evolution of supercritical fluids at subarc depths. Multiphase crystal inclusions trapped within minerals in the vein encompass calcite, anhydrite, and magnetite, suggesting that the supercritical fluids contain such cations as S6+, C4+, and Fe3+. Whole-rock and mineral compositions of the eclogites vary systematically with distance from the vein, suggesting that the eclogites adjacent to the vein were metasomatized by vein-forming supercritical fluids. The garnet-clinopyroxene oxybarometric results indicate that the metasomatism of supercritical fluids resulted in the elevation of oxygen fugacity in the eclogites. The metasomatized eclogites exhibit decreases in Fe and S contents, Fe3+/Fetotal ratios, and modes of omphacite and pyrite. This observation suggests that the supercritical fluids dissolving omphacite and pyrite were accompanied by a Fe-S redox coupling process during fluid-rock reactions. This process further releases Fe3+ and S6+ from the eclogites to supercritical fluids. In combination with the mineral composition of the vein, it is inferred that sulfur was likely transported farther by the supercritical fluids. Therefore, some supercritical fluids were oxidizing at the subducting slab-mantle wedge interface and thus can oxidize the metasomatized rocks. Therefore, supercritical fluids are efficient carriers of oxidized components from the subducting slab to the mantle wedge, eventually contributing to the oxidization of arc magma sources.

Petrogenesis and Geochronology of Late Devonian Intrusive Rocks in Eastern Tianshan, Xinjiang, China: Subduction Constraints of the North Tianshan Ocean

We conducted a study on the petrology, geochemistry, and zircon U–Pb dating of Late Devonian intrusive rocks in the Tulargen area of the Eastern Tianshan Orogenic Belt, Xinjiang, China. These intrusive rocks primarily consist of gabbro (382 ± 5 Ma), tonalite (370.9 ± 2.7 Ma), and biotite monzogranite (362.8 ± 4.4 Ma). Gabbro belongs to the low-K calc-alkaline series of quasi-aluminous rocks, with a high Al2O3 content (16.46–20.34 wt.%) and Mg# value (64.55–67.73). Tonalite and biotite monzogranite, which belong to the high-K calc-alkaline series, are metaluminous or weakly peraluminous and also exhibit high Al2O3 contents (14.6–15.87 wt.%) and Mg# values (40.12–62.47). These rocks are enriched in light rare-earth and large-ion lithophile elements (Rb, Ba, and K) and depleted in heavy rare-earth and high-field-strength elements (e.g., Ta, Nb, and Ti), characteristics typical of island-arc magmatic rocks. Gabbro melts are primarily derived from the mantle and result from the partial melting of a depleted mantle that has undergone fluid metasomatism due to subducted plates. Tonalite exhibits high 176Hf/177Hf and εHf(t) values, with a younger two-stage model age (tDM2) derived from partial juvenile crust melting. The source magma of the biotite monzogranite originated from partial metabasalt melting at a medium crustal depth combined with a new lower crustal material. We concluded that the Late Devonian intrusive rocks in this area formed within the island-arc tectonic setting are associated with the subduction of the North Tianshan Ocean.

Rodingitization of Neoproterozoic Al-Barramiya ophiolite metagabbro in the Eastern Desert of Egypt, Arabian-Nubian Shield

ABSTRACT The present work studies the rodingite that is recorded for the first time as being associated to Al-Barramiya Neoproterozoic ophiolite at Eastern Desert of Egypt, Arabian-Nubian Shield (ANS). Al-Barramiya ophiolite is one of the most important ophiolitic sequences exposed in the ANS. It is affected by different styles of alterations included carbonatization, listvenitization, chloritization and rodingitization. The Neoproterozoic ophiolitic rocks of Al-Barramiya district consists of serpentinized peridotite and metagabbros. Serpentinized peridotites are highly altered to assemblages of talc-carbonate rocks, listvenite and magnesite, while metagabbro is partly converted into rodingite. The field studies indicate that serpentinite and rodingite in Al-Barramiya area display high-strain cataclastic deformation. Rodingite consists of garnet, diopside, vesuvianite and chlorite with minor epidote, prehnite, actinolite and opaque minerals. The garnets and vesuvianite are mostly concentrated near the peripheries of the studied rodingite masses, while clinopyroxene, chlorite, epidote and prehnite gradually increase inwards due to progressive decreasing water/rock ratio as fluids originally buffered by serpentinite move towards equilibrium with gabbroic bulk chemistry. Some rodingite samples preserve a few relics of igneous textures of ophiolitic metagabbros. Garnets are distinguished into andradite, hydroandradite and grossular. Pyroxene is represented mainly by secondary clinopyroxene beside primary fresh relics of diopside; the latter has high Mg# (0.92–0.97) similar to ophiolitic clinopyroxene in the ANS. Rodingite is enriched in CaO, Fe2O3 and MgO but depleted in SiO2, Al2O3 and Na2O compared to associated metagabbro. The association of rodingite with serpentinites and metagabbros suggests that Al-Barramiya rodingite is genetically related to serpentinization of ultramafic rocks. The rodingitization occurred during ocean floor alteration contemporaneous with the serpentinization process. The superposition of cataclastic deformation on the altered rocks indicates that alteration preceded obduction and occurred on the seafloor of the suprasubduction zone where Al-Barramiya ophiolite was originally emplaced. During serpentinization the olivine in the peridotite broke down and released Mg2+ in the metasomatic fluids and clinopyroxene broke down producing Ca-rich fluids. These fluids react with the mafic protoliths and enrich them in MgO and CaO. Also, serpentinization fluids react with the plagioclase of the metagabbro, releasing Ca from the mafic source and concentrated it in the rodingitized samples. The positive Eu anomalies beside high Sr contents of the rodingite can be inherited from a plagioclase-rich protolith such as ophiolitic metagabbro. The enrichment of rodingite in Th and U indicates two distinct solutions affected the gabbroic protolith because the alteration of depleted peridotite to serpentinite cannot be an efficient source to enrich these elements. The present study indicates that the rodingitization process underwent lower greenschist facies conditions associating metamorphism that caused serpentinization and released Ca-rich metasomatic fluids.

Heterogeneous water distribution in between peridotite xenoliths from Kaapvaal Craton kimberlites: Constraints on diamond barren nature of kimberlites

Nominally anhydrous mantle minerals (olivine, pyroxenes, garnets, etc.) in 11 peridotite xenoliths from four different uneconomic and economic Kaapvaal Craton kimberlite pipes (Matsoku, Thaba Putsoa, Pipe 200 and Bultfontein) have been investigated using Fourier transform infrared spectroscopy (FTIR). All xenoliths contain accessories of garnet, diopside, chromite, and phlogopite. High orthopyroxene content (>30 mol vol.%) in most xenoliths from all kimberlites and its interconnected channel-like nature hint towards hydrous siliceous fluid metasomatism. Peridotite xenoliths from uneconomic kimberlites show development of phlogopite and clinopyroxene (± chromite) forming veins and in garnet rims suggesting metasomatism by alkaline silico-carbonatite (possibly kimberlite-related) melt. The xenoliths contain significant H2O in olivine (17–62 ppm), orthopyroxene (21–230 ppm), and clinopyroxene (87–833 ppm), whereas garnets are dry and only show IR absorbance bands at > 3,670 cm−1 for contamination of hydrous minerals. Compared to the economic kimberlites in the Kaapvaal Craton, the uneconomic kimberlite xenoliths from this study have lower orthopyroxene and olivine H2O content. In the xenoliths affected by garnet breakdown metasomatism, the H2O content of orthopyroxene and olivine is higher and lower, respectively. The structural hydroxyl distribution profile across olivine and higher inter-mineral water partition coefficient, suggest diffusion of hydrogen and possible re-equilibration. Statistical analysis of the olivine spectra suggests that hydrogen bands at 3540, 3624, 3638, and 3672 cm−1 are a good discriminant of economic and uneconomic kimberlites and in literature, they are associated with metasomatism, weathering-associated processes, high water activity, and oxygen fugacity. The lower water concentration in xenoliths from uneconomic kimberlite from the margin of the craton than the economic kimberlites from the interior of the Kaapvaal Craton and identified metasomatism hints towards dehydration of xenoliths by water-poor and CO2-rich melts in tectonized cross-lithospheric zones causing diamond resorption and may be responsible for the diamond-poor nature of uneconomic kimberlites in northern Lesotho.

Magmatic (Cr-Ni-PGE) and secondary/hydrothermal (emerald-peridot-rodingite-nephrite jade) mineralization associated with mafic-ultramafic rock complexes of Pakistan

Mafic magmatism has repeatedly taken place in Pakistan throughout its geologic history from the earliest Proterozoic to Recent. Much of it, however, does not have associated metallic mineralization. This paper concerns mineralization associated with mafic-ultramafic plutonic rock occurrences in the western- and northern suture zones of Pakistan, which demarcate the collision boundary between the northwestern Indian plate and Eurasian blocks. We discuss the petrology of the ophiolites and basaltic magma-related chromite deposits, showings of PGE and Ni minerals, and secondary mineral deposits (emerald and peridot gems, rodingite, and nephrite jade) associated with mafic-ultramafic complexes of the two suture zones. Chromite with high Cr#s (68 to 85) occurs in fair quantity and is mostly considered to be genetically related to basaltic magma of subduction-related environments. PGE and nickeliferous minerals are insignificant in quantity; PGE's are formed by a higher degree of partial melting and melt-rock reaction, whereas Ni-sulphides are the product of serpentinization of the ultramafic rocks. Emerald occurs in several localities in the ISZ, whereas the peridot has so far been reported from only one locality. Emerald is associated with Mg-carbonate ± quartz ± fuchsite ± talc ± serpentine, and peridot (Fo 89-92 ) is associated with chrysotile ± magnesite ± talc ± magnetite ± ludwigite. Both are formed via hydrothermal alteration of ultramafic rocks through the introduction of hydrous fluids incorporating CO 2 , Be, B, K, Al, and possibly Na. In view of the undeformed nature of the emerald and peridot and the 23 Ma age of emerald formation, the mineralization appears to post-date deformation and may be of Late Oligocene-Early Miocene age. The metasomatising fluids were either derived from the subducting Indian plate crust underneath the Kohistan arc or were related to 23-26 Ma silicic magmatism in this area of the NW Indian plate. The nephrite jade in the ophiolite mélange is hosted in serpentinites and has disseminated grains of chrome spinel with elevated Fe/(Mg + Fe) ratios (0.10–0.18), implying low-grade metamorphism of ultramafic rocks (protolith of serpentinite) with loss of Al and introduction of SiO 2 during accompanying metasomatism. The rodingite in Dargai ophiolite could have formed by the action of seawater channeled through fractures and cracks in rocks. However, their mineral assemblages indicate that they formed as a result of the influx of Ca, Al, and Na-bearing hydrothermal fluids related to the intrusion of gabbros/sheeted dykes in the serpentinized ultramafic rocks, resulting in Ca-metasomatism.

Magmatic degassing and fluid metasomatism promote compositional variation from I-type to peralkaline A-type granite in the late Cretaceous Fuzhou felsic complex, SE China

Abstract A-type granites generally have much lower water, higher temperature, and incompatible element concentrations than I-type granitoids. Yet it remains unclear why I-A-type granitic complexes occur in convergent plate margins. Here we conduct geochemical analyses on apatite and mafic minerals from the late Cretaceous I-A-type granitic complex in Fuzhou area, SE China, aiming to decipher differentiation, fluid metasomatism, and degassing that primarily control the compositional diversity of felsic magmas. Apatites in both rock types are F-rich and show large H2O and δD variations, i.e., 341–3892 ppm H2O and –325 to +336‰ δD in I-type granitoids; 67–1366 ppm H2O and –251 to +1439‰ δD in A-type granites. H2O in apatite is negatively correlated with La/Sm and Sr/Y in the I-type granitoids, whereas it is positively correlated with Ce and total rare earth element (REE) concentrations in the A-type granites. Once H2O increases up to hundreds of ppm, both rock types show a rapid decrease of H2O/Ce, an increase of F/Cl, and extensive H isotope fractionation. Arfvedsonite occurs as a late crystallizing mineral in the A-type granite and has much higher contents of Na2O, K2O, F, and high field strength elements (HFSE) than hornblende in the I-type granitoids, indicating the addition of F-HFSE-rich alkaline fluids during its magmatic evolution. The consumption of arfvedsonite and formation of aegirine further indicate the role of fluid metasomatism and H2 degassing via a reaction of 3Na3Fe5Si8O22(OH)2 + 2H2O = 9NaFeSi2O6 + 2Fe3O4 + 6SiO2+5H2. The combined geochemical data demonstrate that the systematic differences in mineral assemblage, whole-rock composition, magma temperature, H2O content, and δD of apatite between the I- and A-type granites are likely attributed to varying degrees of differentiation, fluid metasomatism and magmatic degassing. The I-type granitoids experienced hornblende, biotite, plagioclase, K-feldspar, and apatite fractionation and close-system degassing. The A-type granite was likely formed from the I-type monzogranitic magma that was metasomatized by the mantle-derived F-HFSE-rich alkaline fluids to produce the peralkaline magma, which further experienced K-feldspar + plagioclase + biotite + apatite fractionation and open-system degassing. Further numerical estimation indicates that the primary magma of Fuzhou granitic complex contained ~3.0 wt% H2O, and the lower water content of A-type granite was likely attributed to strong degassing during its emplacement. Our results indicate that some peralkaline A-type granites can be generated from relatively water-poor I-type granitic magmas by fluid metasomatism and degassing.

Omphacitite formation and fluid-rock interaction processes in an intra-slab eclogite-facies shear zone

We document the formation of omphacitites in the Monviso Lago Superiore Unit (W. Alps) where eclogite-facies, serpentinite-bearing shear zones host ample evidence for intense intra-slab fluid flow. In the first locality studied, a serpentinite-hosted jadeitite block is rimmed by strained omphacitite (with lawsonite pseudomorphs and minor phengite). U-Th-Pb zircon dating yields a protolith age of c.150 Ma, thus pointing to a burial-related replacement of a former Tethyan seafloor material, and revealed a lack of alpine rims. Multi-mineral Rb-Sr dating of the minerals forming this omphacitite rind yields an isochron age of 41.6 ± 0.7 Ma, corresponding to the early exhumation path still within the eclogite-facies. In this case, omphacitization led to the nearly complete replacement of the former prograde jadeitite through solution-precipitation and was followed by crystal-plastic deformation processes.In various Monviso localities, Fe-Ti metagabbro blocks (commonly exhibiting brecciated garnetite fragments) display evidence for intense fluid-rock interactions leading to the formation of decimeter-thick, weakly strained omphacite-dominated rinds (with minor amounts of lawsonite, clinochlore and talc) around the blocks at the contact with the surrounding serpentinite. Partly dissolved zircon crystals separated from omphacitite rimming an eclogitic Fe-Ti metagabbro block only yielded middle-Jurassic protolith ages. This indicates that omphacitite rinds did not overgrow the eclogite blocks but rather replaced metasomatically the garnet-rich eclogitic assemblage. The omphacitization overprint occurred independently of the nature of the protolith (namely, forming after a jadeitite or after an Fe-Ti eclogite) as a consequence of infiltration of externally-derived fluids and subsequent element sequestration from the incoming fluid phase. These observations highlight the extraordinary ability of eclogite-facies metasomatic fluids to nearly totally overprint rocks as resistant and impermeable as Monviso eclogite breccias or jadeitites.

Mineralogy and Geochemistry of Jasperoid Veins in Neoproterozoic Metavolcanics: Evidence of Silicification, Pyritization and Hematization

The Wadi Ranga sulfidic jasperoids in the Southern Eastern Desert (SED) of Egypt are hosted within the Neoproterozoic Shadli metavolcanics as an important juvenile crustal section of the Arabian Nubian Shield (ANS). This study deals with remote sensing and geochemical data to understand the mechanism and source of pyritization, silicification, and hematization in the host metavolcanics and to shed light on the genesis of their jasperoids. The host rocks are mainly dacitic to rhyolitic metatuffs, which are proximal to volcanic vents. They show peraluminous calc-alkaline affinity. These felsic metatuffs also exhibit a nearly flat REE pattern with slight LREE enrichment (La/Yb = 1.19–1.25) that has a nearly negative Eu anomaly (Eu/Eu* = 0.708–0.776), while their spider patterns display enrichment in Ba, K, and Pb and depletion in Nb, Ta, P, and Ti, reflecting the role of slab-derived fluid metasomatism during their formation in the island arc setting. The ratios of La/Yb (1.19–1.25) and La/Gd (1.0–1.17) of the studied felsic metatuffs are similar to those of the primitive mantle, suggesting their generation from fractionated melts that were derived from a depleted mantle source. Their Nb and Ti negative anomalies, along with the positive anomalies at Pb, K, Rb, and Ba, are attributed to the influence of fluids/melt derived from the subducted slab. The Wadi Ranga jasperoids are mainly composed of SiO2 (89.73–90.35 wt.%) and show wide ranges of Fe2O3t (2.73–6.63 wt.%) attributed to the significant amount of pyrite (up to 10 vol.%), hematite, goethite, and magnetite. They are also rich in some base metals (Cu + Pb + Zn = 58.32–240.68 ppm), leading to sulfidic jasperoids. Pyrite crystals with a minor concentration of Ag (up to 0.32 wt.%) are partially to completely converted to secondary hematite and goethite, giving the red ochre and forming hematization. Euhedral cubic pyrite is of magmatic origin and was formed in the early stages and accumulated in jasperoid by epigenetic Si-rich magmatic-derived hydrothermal fluids; pyritization is considered a magmatic–hydrothermal stage, followed by silicification and then hematization as post-magmatic stages. The euhedral apatite crystals in jasperoid are used to estimate the saturation temperature of their crystallization from the melt at about 850 °C. The chondrite (C1)-normalized REE pattern of the jasperoids shows slightly U–shaped patterns with a slightly negative Eu anomaly (Eu/Eu* = 0.43–0.98) due to slab-derived fluid metasomatism during their origin; these jasperoids are also rich in LILEs (e.g., K, Pb, and Sr) and depleted in HFSEs (e.g., Nb and Ta), reflecting their hydrothermal origin in the island arc tectonic setting. The source of silica in the studied jasperoids is likely derived from the felsic dyke and a nearby volcanic vent, where the resultant Si-rich fluids may circulate along the NW–SE, NE–SW, and E–W major faults and shear zones in the surrounding metavolcanics to leach Fe, S, and Si to form hydrothermal jasperoid lenses and veins.

Origin and multi-episodic melt/fluid interactions revealed by the subducted serpentinites in the North Qilian Belt, NW China: Implications for the compositional heterogeneity of the fossil oceanic mantle

In nature, serpentinites act as reservoirs of water and fluid-mobile elements and play an essential role in arc magmatism, fluid metasomatism, and elemental circulation in subduction zones. This paper presents new mineral and whole-rock geochemical analyses of the Qingshuigou serpentinite massif and discusses its petrogenesis, tectonic setting, and mantle metasomatism. The chemical composition (e.g., REE, Y, Mg, and Al) and petrological observations (e.g., pseudomorph and hourglass textures) of serpentines demonstrate that the serpentinites’ protoliths belong to the peridotites. Their degrees of partial melting were estimated to be 15–20 % using Yb, Cr, Ti, and HREEs, indicating that these peridotite protoliths were residual in origin after high degrees of mantle melting. They were relics of the subducted lithospheric mantle in the subduction zone, based on their geochemical similarities with subducted serpentinites. Notably, the Qingshuigou serpentinites show several geochemical fingerprints of melt-rock interactions, such as the U-shaped REE patterns, elevated Zr/Ti, Zr/Hf, and Nb/Ta ratios; this led us to propose that the peridotite protoliths of the serpentinites were not merely melting remnants but were significantly modified by melt metasomatism. Also, they experienced two stages of fluid/rock interactions in the subduction zone: early-stage serpentinization at shallow depths and lizardite/chrysotile to antigorite transition at an elevated depth. Combined with the published data of ophiolitic peridotites from the North Qilian Belt, the compositional heterogeneity of these peridotites and associated serpentinites might be associated with the nature of melt/fluid metasomatism under different geodynamic backgrounds.

Timing of multiple fluid pulses recorded by garnet and accessory minerals in metarodingites

Metarodingites are commonly garnet-bearing metasomatized mafic dykes within serpentinized mantle rocks. Garnet in metarodingites has the potential to preserve compositional and chronological information about the entire metamorphic-metasomatic evolution, from ocean floor formation to deep subduction. In this study, we investigate the chemical and chronological record of garnet, titanite, and zircon from metarodingites and garnet veins in chloritized blackwall, from the Zermatt-Saas zone in the Western European Alps. Garnet in the chlorite-rich metasomatic rind (blackwall) displays re-crystallization and new growth textures and therefore represents a second generation of garnet (Grt2), after the one in the metarodingite core (Grt1). Major and trace element mapping shows that Grt2 is richer in andradite and Ti-andradite components compared to the more grossular-rich Grt1 in the metarodingite. Moreover, we found that in both Grt1 and Grt2, Ti and U content are positively correlated, with U abundances up to 1.5 μg⋅g−1. Grt2 does not show rare earth element (REE) zonation, suggesting that REE transport was facilitaed by the presence of a fluid phase. We propose that the metasomatizing fluids from which Grt2 precipitated, were serpentinite-derived high-temperature brines capable of transporting Fe3+, Ti and REE.LA-ICPMS U–Pb dating of metarodingite garnet Grt1 samples yielded overlapping ages between 43.6 ± 0.9 and 44.1 ± 1.3 Ma, which are consistent with previous estimates of peak metamorphic conditions. Our data indicate that the metarodingite Grt1 grew within <2 Myr, suggesting a rapid burial rate of the unit or fast and pulsed garnet growth aided by the presence of fluids in the intergranular medium. Titanite from the blackwall sample yields an age of 45.0 ± 1.7 Ma and zircon rim an age of 48.0 ± 1.1 Ma, indicating that fluid release and chloritization occurred at peak conditions. U–Pb ages of garnet in the blackwall samples (Grt2) are significantly younger, yielding ages of 40.1 ± 0.9 Ma and 38.4 ± 0.8 Ma, respectively. The presence of multiple generations of metasomatic garnet in the blackwall suggests that periods of increased rock permeability occurred repeatedly, plausibily aided by deformation, as supported by the consistency of our multi-mineral geochronology. We show how garnet from metarodingites and blackwalls records the metamorphic-metasomatic history and can be used to obtain information on (de)hydration reactions and fluid flow during subduction.

Metal remobilization at the Nibao gold deposit in Youjiang basin, SW China: Implications from mineral texture, trace element and sulfur isotope compositions of sulfides and TiO2 polymorphs

Multistage mineralization overprinting is important for the formation of giant hydrothermal gold deposits. The Youjiang basin in SW China has undergone at least three major deformation phases in the Mesozoic, but their relationship with the large-scale gold accumulation in the region remains enigmatic. Here, we study the large Nibao gold deposit (reserve > 70 t Au) in the northern part of the basin, which has both fault-controlled and stratabound orebodies. We identified a pre-ore stage and syn-ore stage including four tectono-hydrothermal (T-H) substages (i.e., T-H stage 1, 2, 3 and 4) for the local mineralization at Nibao. The pre-ore stage is dominated by coarse-grained pyrite coexisting with TiO2 polymorphs in the Middle-Late Permian basaltic rocks. The T-H substage 1 is characterized in that the sedimentary pyrites are brecciated in bedding-parallel faults. Fluorination and silicification occur in the T-H substage 2, which is re-brecciated at the T-H substage 3 forming the highest gold grade ore in high-angle thrust fault. The T-H substage 4 presents as fault reactivation within minor calcite and pyrite deposition. Mineralogy and in-situ trace element analyses reveal that mineral dissolution and reprecipitation (incl. dolomite, auriferous pyrite and TiO2 polymorphs) is ubiquitous during the multistage tectono-hydrothermal process, and the continuous alteration overprinting may have promoted gold accumulation. This replacement process of different generations of auriferous pyrite is also recorded by their inherited sulfur isotopes, giving a narrow δ34S (−3.1 to + 2.1 ‰) range. Anatase is the main accessory phase of TiO2 polymorphs, as identified with LA-ICP-MS trace element and laser Raman spectroscopic analyses. Moreover, the anatase can been divided into two types (type-I and II) based on their texture, alteration and chemistry. The type-I anatase commonly occurring in the stratabound ore is characterized by higher Zr (mean 1,730 ppm), V (mean 1,412 ppm), and Cr (mean 110 ppm) contents than type-II that coexists with zoned pyrite in the fault-controlled ore (mean 26 ppm, 187 ppm, and 38 ppm, respectively). Additionally, W (mean 636 ppm) and Mg (10,021 ppm) in type-II are much higher than type-I ones (mean 135 ppm W and 476 Mg). These features show that the sedimentary-stage Zr-V-Cr-rich type-I anatase was dissolved via fluid metasomatism, and reprecipitated as the hydrothermal-stage W-Mg-rich and Zr-poor anatase, with gold gradually enriched. Besides, the anatase phase of TiO2 polymorphs also indicates that the Nibao gold deposit was formed under low pressure–temperature condition. Overall, we propose that the Nibao is a Carlin-type gold deposit, dominated by multistage tectono-hydrothermal events.

The formation of volatile‐bearing djerfisherite in reduced meteorites

AbstractEnstatite meteorites, both aubrites and enstatite chondrites, formed under exceptionally reducing conditions, similar to the planet Mercury. Despite being reduced, the MESSENGER mission showed that the surface of Mercury is more enriched in volatiles (e.g., S, Na, K, Cl) than previously thought. To better understand the mineral hosts of these volatiles and how they formed, this work examines the chemistry and petrographic settings of a rare, K‐bearing sulfide called djerfisherite within enstatite chondrites and aubrites. The petrographic settings of djerfisherite within aubrites suggest this critical host of Cl formed after both the crystallization of troilite and exsolution of daubréelite. Djerfisherite is commonly observed as a rim on other sulfides and in contact with metal. We present an alteration model for djerfisherite formation in aubrite meteorites, whereby troilite and Fe‐Ni metal are altered through anhydrous, alkali‐ and Cl‐rich fluid metasomatism on the aubrite parent body to produce secondary djerfisherite. Moreover, we observe a loss of volatiles in djerfisherite within impact melted regions of the Miller Range 07139 EH3 chondrite and the Bishopville aubrite and explore the potential for impact devolatilization changes to sulfide chemistry on other reduced bodies in the Solar System. Vapor or fluid phase interactions are likely important in the formation of volatile‐rich phases in reduced systems. While most Na and K on the mercurian surface is expected to be hosted in feldspar, djerfisherite is likely a minor, but critical, reservoir for K, Na, and Cl. Djerfisherite present on reduced bodies, such as Mercury, may represent sulfides formed via late‐stage, primary metasomatism.

Altered spinels act as a mirror of multi-episodic fluid metasomatism in the forearc mantle: An example from the Minhe ophiolite in Qilian Orogen, NW China

Forearc mantle peridotites commonly undergo complex melt/fluid metasomatism, which inevitably has a considerable influence on their geochemical compositions at the whole-rock scale. Serpentinites predominantly comprise the Minhe ophiolite, which is a fragment of the Proto-Tethys Ocean that outcrops in the Qilian Orogen in NW China. These serpentinites contain zoned spinels that vary compositionally from the centers to the margins, with a spinel core overgrown by ferritchromite layer and magnetite at the outermost rim. The mineral chemistry of the spinel cores (i.e., Cr#, Mg#, Cr2O3, and Al2O3) suggests that the protolith of the serpentinites was an oceanic mantle remnant in a forearc setting. Moreover, the ferritchromite exhibited increases in Fe, Mn, Sc, and Ni contents and decreases in Mg, Cr, Al, V, Zn, and Ga contents. Considering the linear correlation among elements (e.g., Co, Ni, and Zn) and the absence of mineral inclusions, the ferritchromite likely formed through ionic substitutions induced by slab fluids. The magnetite was enriched in Cr, Ni, V, Zn, Co, and Cu, but did not have a magmatic origin owing to low V concentrations. All evidence indicates that the peridotites underwent at least two episodes of fluidic metasomatism in the subduction zone. During early slab subduction, aqueous fluids released from the dehydration of the subducted slab directly serpentinized the forearc mantle peridotites, thereby altering the olivine and orthopyroxene to lizardite/chrysotile. During this process, certain mobile elements (e.g., Fe, Ni, and Mn) in the fluids and/or olivine serpentinization were incorporated into existing spinels, favoring ferritchromite growth. The second fluidic episode was dependent on prograde metamorphism at greater depths in the subduction channel, which not only produced antigorite via the recrystallization of lizardite and chrysotile, but also drove the fluids towards a more oxidizing state, thereby facilitating the growth of Cr-magnetites. Subsolidus re-equilibration controlled by fluid infiltration further promoted the inter-diffusion of elements (e.g., Cr, V, Co, Ni, Cu, and Zn) in the zoned spinels.

In Situ Geochemical Evaluation of Retrograde Hydration Effects in the Peri-Siberian Forearc Mantle (Khara-Nur and Alag-Khadny Peridotite Complexes)

In order to assess the geochemical effects of retrograde metamorphic rehydration, fluid metasomatism, and the fluid-mobile elements (FMEs) budget in the case of oceanic and continental subduction, we report the petrography, bulk, and in situ LA-ICP-MS trace-element data for the two poorly studied ophiolites in the northern (Khara-Nur, Eastern Sayan, Russia) and central (Alag-Khadny accretionary wedge, SW Mongolia) parts of the peri-Siberian orogenic framing. Both complexes are relics of the ancient oceanic mantle, which was subjected to processes of partial melting, metasomatism, and retrograde metamorphism. Typical mineral assemblages include olivine + orthopyroxene + chlorite + tremolite ± secondary olivine (640–800 °C), olivine + antigorite ± secondary clinopyroxene (&lt;640 °C), and olivine + chrysotile ± secondary clinopyroxene (&lt;250 °C) and are stable at pressures up to 2 GPa. Hydration and partial serpentinization of mantle peridotites lead to tremolite formation after orthopyroxene, followed by olivine replacement by antigorite. Serpentine-group minerals (antigorite and chrysotile) were distinguished by Raman spectroscopy, and the contents of incompatible elements (mobile and immobile in fluids) in metamorphic minerals (tremolite, antigorite, and chrysotile) were examined in situ by LA-ICP-MS. The behavior of conservative HFSE (Zr, Nb, Ta, and Ti) and—in part—HREE does not distinguish between the two types (oceanic and continental) of subduction environments. Different patterns of FMEs (Cs, Rb, Ba, U, Sb, Pb, Sr, and LREE) enrichment in metaperidotites reflect variations in the slab fluid composition, which was primarily governed by the contrasting nature of subducted lithologies. The affinity of Alag-Khadny to the subduction of a continental margin is recorded by increased FME contents and selective enrichment by some moderately mobile elements, such as U, Th, and LREE, with respect to the oceanic-type subduction environment of Khara-Nur. Distinct patterns of FME enrichment in tremolite and antigorite from two complexes indicate different sequences of fluid-induced replacement, which was controlled by Opx composition. We demonstrate that evaluation of the initial composition of precursor minerals affected by multi-stage melting and melt metasomatism should be considered with care to estimate the differential fluid overprint and associated elemental uptake from subduction fluids.

Zircon in tin granite as tracer for fluid metasomatism and Sn mineralization

Most tin deposits in the world are genetically related to tin granite and form during complex magmatic-hydrothermal processes. Zircon is a common accessory mineral in granite and related ore systems and can host a number of ore metals, such as Sn, W, Nb, Ta, U and Th, in its crystal lattice. However, whether metal enrichment/depletion can trace ore-forming processes is still unclear. Here, we report that the metal concentrations in various types of zircons from the Mopanshan tin granites in the southern Great Xing'an Range (Northern China) can be used as good indicators of fluid metasomatism and Sn mineralization. Two lithological zones are developed in the Mopanshan pluton, including porphyritic syenogranite (PG) in the center and fine-grained syenogranite (FG) at the margin. Zircons in the PG (PGZ-1, PGZ-2, and PGZ-3) are all magmatic in origin, while zircons in the FG can be categorized into magmatic zircons (FGZ-1 and FGZ-2) and metasomatic zircons (FGZ-3). The PGZ-1 and FGZ-1 grains are transparent prismatic crystals with bright oscillatory zonation, whereas the PGZ-2 grains are murky crystals with dark oscillatory zonation. PGZ-3 and FGZ-2 grains occur as overgrowths of previously formed zircon (PGZ-1, PGZ-2, and FGZ-1) or as brown individual crystals, showing dark and homogeneous cathodoluminescence (CL) textures. The metasomatic FGZ-3 grains are translucent-opaque porous crystals with vermicular CL zonation and commonly replace FGZ-2. The trace element compositions of magmatic zircons are completely melt controlled, providing a record of magmatic evolution as a constant decrease in Zr/Hf ratios and a gradual increase in Th, U, Nb, and Ta contents. Moreover, the structure of magmatic zircons transforms from a crystalline state to an amorphous state as a consequence of radioactive decay of U and Th. A coupled dissolution-reprecipitation process is proposed for the formation of metasomatic FGZ-3. The reactive fluid is the magmatic fluid that exsolved from the melt in the late magmatic stage. The magmatic fluid replaced biotite and rare earth phosphates (mainly monazite and apatite) enclosed within biotite, resulting in significant amounts of Nb, Ta, Sn, P, Al, Ca, Fe, and REEs, which subsequently reacted with the FGZ-2 zircons to leach Th, U, Y, and HREEs. Eventually, REEs, Y, Th, and U in the fluid combined with P to form monazite and xenotime, while the other elements partially precipitated with the crystallization of the FGZ-3 zircons. Although the alteration of biotite only released approximately 190 ppm of Sn into the fluid, this is still a significant Sn source for the Sn deposits surrounding the Mopanshan pluton, taking the granite size (∼50 km2) and the volume proportion of biotite (∼5%) into account. Furthermore, based on previous studies on tourmaline from the Mopanshan granite and regional geochemistry, it may be inferred that the addition of wall rock components may also play an important role in Sn mineralization.

Heterogeneity of the late Mesozoic lithospheric mantle beneath the Jiaodong Peninsula, eastern North China Craton: a synthesis from geochemistry and Sr-Nd-Pb isotopes of mafic dykes

ABSTRACT The Late Mesozoic lithospheric mantle evolution of the Jiaodong Peninsula is crucial for understanding the destruction of the eastern North China Craton (NCC) and large-scale gold mineralization, but it still remains highly controversial. Here, we comprehensively compiled published Late Mesozoic mafic dyke magmatism data from the Jiaodong Peninsula to provide key constraints on the nature and evolution of the lithospheric mantle. Mafic dykes are widely emplaced in the Jiaodong Peninsula and have been divided into two groups (Jiaobei terrane and Sulu orogenic belt); they mainly comprise lamprophyre, dolerite, and gabbro. The ages of the mafic dykes in the Jiaobei terrane are predominantly between 120 and 125 Ma, and those of the Sulu orogenic belt are between 110 and 120 Ma. The Jiaobei and Sulu mafic dykes both have high contents of Mg and compatible elements (Cr, Ni), as well as low SiO2 content; they are characterized by arc-like mafic magma, including enrichment in Large Ion Lithophile Elements and Light Rare Earth Elements, depletion in High Field Strength Elements, high (87Sr/86Sr)i, and low εNd(t), implying a metasomatized lithospheric mantle source. However, the Jiaobei mafic dykes have relatively higher (87Sr/86Sr)i, lower εNd(t), and a larger range of (206Pb/204Pb)i, (208Pb/204Pb)i, and (207Pb/204Pb)i compared with the Sulu mafic dykes. The Jiaobei and Sulu mafic dykes exhibit Ba/Rb ratios of 1.29‒64.39 (average 29.3) and 6.61‒108.28 (average 33.1), respectively, indicating the presence of hornblende in the melts. The high Fe/Mn (>60) and Zn/Fe (>12) ratios of Jiaobei and Sulu mafic dykes indicate the mixed mantle source of peridotite and pyroxenite. But the content of pyroxenite in the mantle source of Jiaobei terrane is higher with wider range of Fe/Mn and Zn/Fe ratios. These features indicate the heterogeneity of the mantle sources of the mafic dykes in the Jiaobei and Sulu regions. It is suggested that both regions originated from the partial melting of hornblende-bearing peridotite, with the addition of more pyroxenite to the mantle source of the Jiaobei terrane compared to Sulu orogenic belt. The mafic dykes have the low Nb/Zr (Jiaobei: 0.024–0.229; Sulu: 0.030–0.166) and Th/Zr ratios (Jiaobei: 0.017–0.081; Sulu: 0.010–0.068), and the high Rb/Y (Jiaobei: 0.347–12.162; Sulu: 0.224–9.600) and low Nb/Y ratios (Jiaobei: 0.202–1.486; Sulu: 0.140–1.322). It implies that the mantle source area in both regions was metasomatized by subducted slab-derived fluids. The increase in (La/Yb)N and decrease in Hf/Sm implies carbonate metasomatism in both regions. In addition, some Jiaobei and Sulu mafic dykes have high Ti/Eu ratios, implying that the silicate metasomatism occurred in the mantle source of both regions. Based on the regional evolution, the lithospheric mantle source in the two regions beneath Jiaodong Peninsula experienced carbonated fluid metasomatism resulting from the subduction of the Palaeo-Asian Ocean plate beneath the northern NCC (ca. ~250 Ma), silicate melt-related metasomatism from subduction of the Yangtze Craton plate beneath the southern NCC (ca. 240‒220 Ma), and oceanic slab-derived fluid metasomatism from the subducted Palaeo-Pacific plate (at 180‒110 Ma). It is believed that the mantle source of Jiaobei terrane had undergone stronger carbonated fluid metasomatism and silicate melt-related metasomatism, compared to Sulu orogenic belt. Moreover, the continental fluid-related metasomatism only occurred in the Sulu orogenic belt. During the Late Mesozoic, the eastern NCC experienced large-scale lithospheric thinning, leading to upwelling of the asthenospheric mantle. The mafic dykes in Jiaobei and Sulu, show different geochemical features, were derived from partial melting of the metasomatized lithospheric mantle through upwelling of the asthenospheric mantle.

Petrogenesis and REE mineralogical characteristics of Shitouping granites in southern Jiangxi Province: Implication for HREE mineralization in South China

The enrichment of heavy rare earth elements (HREE) in the granitic host-rocks is crucial for generating ion-adsorption HREE deposits in South China. The Shitouping pluton, located in the southern part of Jiangxi Province, South China, develops large-scale HREE deposits in its weathering profile identified by recent geological surveys. These granites can be divided into medium to coarse-grained biotite monzogranite (MGBG), medium to coarse-grained syenogranite (MGSG) and fine-grained alkali feldspar granite (FGAG) based on field observations and mineral assemblages. New geochronologic results show that this composite pluton intruded during the Early Cretaceous (ca. 140 Ma). Three granitic units displaying comparable whole-rock Nd [εNd(t) = −4.5 to −3.4] and zircon Hf compositions [εHf(t) = −4.4 to +1.2], combined with mineral association, hydrothermally altered zircon and whole-rock geochemistry, indicate they were derived from partial melting of hybridized crustal source, followed by intense fractional crystallization and fluid metasomatism in the highly evolved granitic system. The MGSG and FGAG have higher Rb, Nb, HREE contents and Rb/Sr ratios, as well as lower Nb/Ta, K/Rb ratios in comparison with the MGBG, implying they are more evolved units. The overlapping magmatic zircon trace element compositions between MGBG and MGSG, coupled with the subvertical contact boundary between the FGAG and the MGBG, suggest that the generation of Shitouping pluton should be attributed to the periodic magma replenishment, remobilization, and differentiation process in the long-lived magmatic system rather than simple fractional crystallization from a single parental magma. Hydrothermally altered zircon with elevated concentration of HREE and the paragenetic relationship between LREE-phosphate and HREE minerals in the MGSG suggest that the fluid metasomatism promotes the migration and accumulation of HREE. This is consistent with the high HREE concentration recorded in the MGSG, which contains abundant HREE minerals associated with fluorites. Thus, this study highlights that magma differentiation and fluid metasomatism during the late magmatic stage could achieve the relatively high HREE contents in granites to favor the formation of ion-adsorption HREE deposits.

Lawsonite and Garnet Oxygen Isotope Record of Fluid‐Rock Interaction During Subduction

AbstractDuring the subduction of an oceanic plate, fluids are released from metabasaltic crust, metasediment, and serpentinite under high‐pressure/low‐temperature conditions. Although some fluids may eventually leave the slab, some participate in metamorphic reactions within the slab during subduction and exhumation. To identify fluid sources and other controls influencing mineral composition, we report the in situ‐measured δ18O of lawsonite and garnet in blueschist‐ to eclogite‐facies rocks from 10 subduction zones that represent various field settings, including mélanges, structurally coherent terranes, and an eclogite xenolith derived from a subducted plate. Lawsonite records distinct δ18O depending on the host rock type and other rock types that were fluid sources during lawsonite growth. In general, lawsonite in metabasalt (7.6 ± 0.2–14.8 ± 1.1‰) is isotopically lighter than in metasediment (20.6 ± 1.4–24.1 ± 1.3‰) but heavier than in metagabbro (4.0 ± 0.4–7.9 ± 0.3‰). The extent of δ18O fractionation was evaluated for lawsonite–fluid and lawsonite–garnet pairs as a function of temperature (T). Results demonstrate that variations of &gt;1.7‰ in lawsonite and &gt;0.9‰ in garnet are not related to changing T. More likely, the relative contributions of fluids derived from isotopically heavier lithologies (e.g., sediments) versus lighter lithologies (e.g., ultramafic rocks) are the major control. Monte Carlo simulations were performed to investigate the sources of metasomatic fluids and the water/rock ratio that formed lawsonite‐bearing metasomatite. Results indicate that δ18OLws and δ18OGrt record interactions with fluids sourced from diverse lithologies (sediment, serpentinite), further supporting that δ18OLws is a useful indicator of subduction fluid‐rock interactions.