Mineral Thermobarometry Research Articles

Titanite: A source, thermodynamic, and temporal proxy for W-Mo-Cu Skarn mineralization at the Seridó Mineral Province, Northeastern Brazil

New zircon U-Pb geochronology (SHRIMP) and geochemistry combined with geochronology (LA-ICP-MS) of titanite data are provided to investigate the metallic sources (whether igneous or sedimentary) and the temporal evolution of one of the most relevant Precambrian metallogenic provinces of the South American Platform, the Seridó Mineral Province. The granites of Serra de João do Vale pluton, calc-silicate “skarnoid” rocks and paragneiss related to the Bonito W-Mo-Cu exoskarn deposit were investigated. It was analyzed titanites of magmatic (Serra de João do Vale), hydrothermal (calc-silicate rocks) and metamorphic (paragneiss) origin. The magmatic titanite is wedge-shaped with larger grain size (up to 250 μm), richer in HFSE elements, Al and Fe, and manifests negative Eu anomalies (Eu/Eu × averaging 0.31–0.60). The hydrothermal titanite is found alongside epidote and calcite, have subhedral shapes and medium grain size (up to 150 μm). The metamorphic titanite is anhedral, generally smaller than 100 μm and appear as thin coronae in ilmenite or as inclusions in biotite. Both metasomatic and metamorphic titanites manifest positive Eu anomalies (Eu/Eu × averaging 1.20 and 1.06, respectively) and are enriched in Ti, Rb and U in relation to the magmatic. Conventional mineral thermobarometry (Al-in-titanite independent barometer, and Zr-in-titanite thermobarometer) provided pressures between 400 and 550 MPa and temperatures between 720 and 787 °C for the crystallization history of the granite, coeval with the regional peak metamorphism at 568–624 °C and 300–500 MPa (Ti in biotite independent barometer and biotite-muscovite thermobarometer). Conditions for the titanite-epidote calc-silicate assemblage were constrained through thermodynamic modelling in NCKFMAST + COH pseudosection at 439 ± 35 °C and 190 ± 50 MPa. The zircon U-Pb age registered the magmatism between 585.5 and 580.5 Ma. Titanite U-Pb ages from granite and calc-silicate rocks timed the closure of U-Pb system at the isotherm of 550 °C between 562 and 557 Ma. (20 m.y. after the intrusion), which marks the metasomatic process. The average W content in paragneisses and calc-silicate titanites (35.8–35.5 ppm) is 4 times higher than in the granite (6.76 ppm), reaching a maximum of 328 ppm. Mo and Cu mirrors this behavior, with concentrations that exceed nearly double that of titanites from Serra de João do Vale granites, peaking at 308 ppm of Mo and 162 ppm of Cu. Metal contents in titanite disclose the metasiliciclastic rocks of the Jucurutu Formation as, at least partially, a source of metals (mainly for W, but also Cu and Mo) in the skarn mineralization during the terrane exhumation at the end of Ediacaran period.

Iron–Titanium Oxide–Apatite–Sulfide–Sulfate Microinclusions in Gabbro and Adakite from the Russian Far East Indicate Possible Magmatic Links to Iron Oxide–Apatite and Iron Oxide–Copper–Gold Deposits

Mesozoic gabbro from the Stanovoy convergent margin and adakitic dacite lava from the Pliocene–Quaternary Bakening volcano in Kamchatka contain iron–titanium oxide–apatite–sulfide–sulfate (ITOASS) microinclusions along with abundant isolated iron–titanium minerals, sulfides and halides of base and precious metals. Iron–titanium minerals include magnetite, ilmenite and rutile; sulfides include chalcopyrite, pyrite and pyrrhotite; sulfates are represented by barite; and halides are predominantly composed of copper and silver chlorides. Apatite in both gabbro and adakitic dacite frequently contains elevated chlorine concentrations (up to 1.7 wt.%). Mineral thermobarometry suggests that the ITOASS microinclusions and associated Fe-Ti minerals and sulfides crystallized from subduction-related metal-rich melts in mid-crustal magmatic conduits at depths of 10 to 20 km below the surface under almost neutral redox conditions (from the unit below to the unit above the QFM buffer). The ITOASS microinclusions in gabbro and adakite from the Russian Far East provide possible magmatic links to iron oxide–apatite (IOA) and iron oxide–copper–gold (IOCG) deposits and offer valuable insights into the early magmatic (pre-metasomatic) evolution of the IOA and ICOG mineralized systems in paleo-subduction- and collision-related geodynamic environments.

The noble gas signature of the 2021 Tajogaite eruption (La Palma, Canary Islands)

Here, we characterize the temporal evolution of volatiles during the Tajogaite eruption by analyzing the elemental (He-Ar-CO2-N2) and isotopic (He-Ar-Ne) composition of fluid inclusions (FI) in phenocrysts (olivine+pyroxene) identified in erupted lavas. Our 2021 lava samples identify substantial temporal variations in volatile composition. We show that, during the 2021 Tajogaite eruption, the He-CO2-N2 concentrations in FI increased since October 15th; this increase was accompanied by increasing 40Ar/36Ar ratios (from ∼300 to >500), and paralleled a major shift in bulk lava chemistry, with increasing Mg contents (Mg#, from 47 to 52 to 55–59), CaO/Al2O3 (from 0.65 to 0.74 to 0.75–0.90), Ni and Cr, and decreasing TiO2, P2O5 and incompatible elements. The olivine core composition also became more forsteritic (from Mg# = 80–81 to Mg# = 84–86). Mineral thermobarometry and FI barometry results indicate that the eruption was sustained by magmas previously stored in at least two magma accumulation zones, at respectively ∼6–12 km and 15–30 km, corroborating previous seismic and FI evidence. We therefore propose that the compositional changes seen throughout the eruption can be explained by an increased contribution - since early/mid-October - of more primitive, less-degassed magma from the deeper (mantle) reservoir. Conversely, Rc/Ra values (3He/4He ratios corrected for atmospheric contamination) remained constant throughout the whole eruption at MORB-like values (7.38 ± 0.22 Ra, 1σ), suggesting an isotopically homogeneous magma feeding source. The Tajogaite He isotope signature is within the range of values observed for the 1677 San Antonio lavas (7.37 ± 0.17Ra, 1σ), but is more radiogenic than the 3He/4He values (>9 Rc/Ra) observed in the Caldera de Taburiente to the north. The 3He/4He ratios (6.75 ± 0.20 Ra, 1σ) measured in mantle xenoliths from the San Antonio volcano indicate a relatively radiogenic nature of the mantle beneath the Cumbre Vieja ridge. Based on these results and mixing modeling calculations, we propose that the homogeneous He isotopic signatures observed in volatiles from the Tajogaite/San Antonio lavas reflect three component mixing between a MORB-like source, a radiogenic component and small additions (6–15%) of a high 3He/4He reservoir-derived (>9Ra) fluid components. The simultaneous occurrence of high 3He/4He (>9Ra)- and MORB-like He signatures in northern and southern La Palma is interpreted to reflect small-scale heterogeneities in the local mantle, arising from spatially variable proportions of MORB, radiogenic, and high 3He/4He components.

Early Paleozoic Tectonothermal Evolution of the Khan-Khukhay Metamorphic Complex (Northern Mongolia)

A reconstruction of metamorphic stages of Moren and Erzin metamorphic Complexes in the Khan-Khuhay block of Nothern Mongolia presented in the paper. Based on it a general geodynamic history of Sangilen terrain of Tuva-Mongolia Massif discussed. Reconstructed “clock-wise” P-T trend demonstrated two stage of metamorphism, the first of which is due to collision metamorphism with P-T parameters of 9 kbar, 740°C, the second one is due to regional magmatic episode with P-T parameters of 6-7 kbar, 860–880°C. Geochemical and petrological peculiarities of rocks in the Khan-Khuhay block demonstrates resemblance with metamorphic block of Western Sangilen in South-East Tuva. In the first time quartz monzodiorite intrusive bodies founded within Khan-Khuhay block. Their geochemical peculiarities are similar to Ordovician gabbro-monzodiorite intrusives of Western Sangilen. Quartz monzodiorite intrusives of Khan-Khuhay are likely a small apophysis of a deep-crust intermediate magma chamber and can present probable thermal source of metamorphic stage M2 in the Khan-Khuhay block. Numerical thermo-mechanical model of metamorphism in the Khan-Khuhay block explain heating in the thickening crust during collision due to higher radiogenic heat source of 1.52 μW/m3. Using computer modeling was show that the P-T parameters of second metamorphic stage can realize only by means of magmatic heat from monzodiorite intrusion. Reconstruction of P-T parameters using mineral thermobarometry, similarity of geochemical characteristics, as well as the results of thermomechanical modeling allow us to conclude that the Erzin and Moren complexes have undergone a joint tectono-metamorphic evolution.

Previously melt-depleted mantle beneath the Cascades Range arc

Mantle peridotite xenoliths from Lorena Butte, Simcoe Mountains volcanic field, Washington, USA, have been reported with radiogenic 187Os/188Os (≤0.149) and elevated oxidation states, which have been interpreted to reflect Farallon slab contributions to the mantle wedge. To examine subarc processes beneath the Cascades arc, mineral thermobarometry and bulk-rock compositions, including highly siderophile element (HSE) abundances and 187Os/188Os, were obtained for Lorena Butte harzburgite xenoliths. Harzburgites have 187Os/188Os ratios (0.1231 ± 20; 2 SD; n = 10) that only marginally overlap and are, on average, 7% less radiogenic than the previously reported xenoliths, with estimates of oxygen fugacity (ΔFMQ = +1.0 ± 0.5) within the range of both subduction-related and abyssal peridotites. Lorena Butte harzburgites experienced significant shallow-level melt depletion (>15%) prior to incorporation into the mantle wedge. After incorporation into the lithosphere, the peridotites were erupted within trachybasalt of intraplate origin. This heritage suggests that only low-Os harzburgites preserve radiogenic 187Os/188Os from subduction fluids and that previously melt-depleted mantle forms an important component of mantle wedges. Furthermore, the extent of melt depletion experienced by the peridotites is not necessarily coupled with mineral thermobarometry, which can be affected by melt infiltration and metasomatism.

A dunite fragment in meteorite Northwest Africa (NWA) 11421: A piece of the Moon’s mantle

Abstract A centimeter-sized fragment of dunite, the first recognized fragment of Moon mantle material, has been discovered in the lunar highlands breccia meteorite Northwest Africa (NWA) 11421. The dunite consists of 95% olivine (Fo83), with low-Ca and high-Ca pyroxenes, plagioclase, and chrome spinel. Mineral compositions vary little across the clast and are consistent with chemical equilibration. Mineral thermobarometry implies that the dunite equilibrated at 980 ± 20 °C and 0.4 ± 0.1 gigapascal (GPa) pressure. The pressure at the base of the Moon’s crust (density 2550 kg/m3) is 0.14–0.18 GPa, so the dunite equilibrated well into the Moon’s upper mantle. Assuming a mantle density of 3400 kg/m3, the dunite equilibrated at a depth of 88 ± 22 km. Its temperature and depth of equilibration are consistent with the calculated present-day selenotherm (i.e., lunar geotherm). The dunite’s composition, calculated from mineral analyses and proportions, contains less Al, Ti, etc., than chondritic material, implying that it is of a differentiated mantle (including cumulates from a lunar magma ocean). The absence of phases containing P, Zr, etc., suggests minimal involvement of a KREEP component, and the low proportion of Ti suggests minimal interaction with late melt fractionates from a lunar magma ocean. The Mg/Fe ratio of the dunite (Fo83) is significantly lower than models of an overturned unmixed mantle would suggest, but is consistent with estimates of the bulk composition of the Moon’s mantle.

Origin of the Paleoproterozoic basaltic dikes from the central and eastern Dharwar Craton and sills and volcanics from the adjoining Cuddapah Basin, southern India

Reverse fractionation modeling considering energy-constrained assimilation-fractional crystallization is performed to estimate primary magma compositions, degree of crustal contamination, pressure–temperature of equilibrium with mantle, and potential temperatures for the origin of the Paleoproterozoic (~ 2.37–1.88 Ga) basaltic dikes in central and eastern Dharwar Craton and sills and volcanics in the adjoining Cuddapah Basin, southern India. Mineral thermobarometry indicates that the dikes crystallized at upper crustal conditions (~ 1–6 kbar/ ~ 1120–1210 °C). Hence, the reverse fractionation calculations are performed at low pressures by adding olivine + plagioclase + clinopyroxene, olivine + plagioclase and only olivine in equilibrium with melt, and simultaneously subtracting an upper crustal partial melt in small steps until the melt is multiply saturated with lherzolite at a high pressure. The results indicate that the basalts are 5–30% contaminated, and their enriched light rare earth element (REE) patterns can be attributed to upper crustal assimilation. The upper crust was pre-heated to 665–808 °C during dike emplacement. The primary magmas of all basalts were last equilibrated with spinel lherzolite at 10–16.5 kbar/1291–1366 °C, and they resemble pooled polybaric incremental melts generated along a ~ 1450 °C adiabat. The estimated mantle potential temperatures (1293–1515 °C) are similar to Paleoproterozoic ambient mantle temperatures. All basalts and their primary magmas show lower chondrite-normalized DyN/YbN ratios than the plume-derived mid-Proterozoic Mackenzie dikes of Canadian Shield, and the primary magmas show flat REE patterns indicating spinel lherzolite melting. The low estimated potential temperatures, low DyN/YbN ratios, and a spinel-bearing mantle source are at odds with an origin of the basalts from mantle plumes.

Geochemical variation in biotite from the Devonian South Mountain Batholith, Nova Scotia: Constraints on emplacement pressure, temperature, magma redox state and the development of a magmatic vapor phase (MVP)

Abstract Here we report the composition of biotite from the peraluminous Devonian South Mountain Batholith (SMB) of southwestern Nova Scotia (Canada), the largest intrusive body within the Appalachian orogen. The batholith was emplaced in two phases: an early (379–375 Ma) granodiorite-monzogranite suite (Stage 1) and a later (375–372 Ma) more-evolved monzogranite-leucogranite suite (Stage 2). Biotite analyses (major and minor elements) were obtained on 55 unmineralized samples representing 11 plutons. Regardless of the stage of pluton emplacement, biotite is commonly interstitial to alkali feldspar, quartz and plagioclase, indicating similar timing of biotite saturation. This suggests that biotite chemistry records conditions at similar extents of magma evolution for the chosen suite of samples. Biotite compositions are Fe-rich, with Fe/(Fe+Mg) ranging from 0.6 to 0.98, and Al-rich, with IVAl ranging from 2.2 to 2.9 atoms per formula unit (apfu; 22 oxygen basis), the latter reflecting the coexistence of other Al-rich phases, such as muscovite, garnet, aluminosilicates, and cordierite. Biotite anion sites are dominated by OH (>3 apfu), followed by F (~0.3 apfu) and Cl (≤0.02 apfu), with a general trend of decreasing OH, increasing F and a marked decrease in Cl, with increasing differentiation. Pressure (P) is estimated from the Al content of biotite to be between 280–430 MPa, consistent with a range of 240 to <470 MPa derived from phase equilibria and fluid inclusion microthermometry combined with mineral thermobarometry. Temperature (T) calculated from the Ti content of biotite ranges from 603–722 °C. Comparison of P-T estimates with water-saturated granite phase relations suggest minimum water contents of 6–7 wt% for the SMB magmas. The redox state of the SMB was estimated by comparing biotite Fe#-Ti relations with compositions calculated using the MELTS thermodynamic model, as experiments have shown that biotite Fe# increases with decreasing fO2 at a given extent of crystallization. Results of MELTS modeling for the most primitive magmas of the SMB sample suite indicate that the observed biotite Fe#-Ti variation is consistent with crystallization at FMQ to FMQ-1, with more oxidizing conditions suggested for the most strongly differentiated samples. To constrain the origin of the biotite anion site variation, a quantitative model using biotite-melt exchange coefficients (KD) derived from existing experimental data was used to track the change in biotite OH-F-Cl abundances as a function of crystallization, with or without an extant magmatic vapor phase (MVP). The model reproduces the relative OH, F, and Cl abundances in biotite, and suggests that SMB crystallization occurred in the presence of a MVP. The relatively reduced redox state of the SMB, similar to other peraluminous granitoid occurrences worldwide, aligns with other measures of fO2 for the SMB, including the occurrence of primary ilmenite. The observed correspondence between the estimated fO2 and that imposed by graphite-gas equilibrium suggests a role for reduced carbon in the generation and evolution of the SMB. This is consistent with evidence for SMB interaction with graphite-bearing felsic granulites of the underthrust Avalon terrane, and assimilation of carbonaceous and sulfidic metasediments during pluton ascent and emplacement. Reducing conditions and development of a MVP have implications for granophile element concentration processes in the SMB magmatic system. Low fO2 during crystallization affects the mineral/melt partitioning and solubility of the redox-sensitive elements Sn, W, U, and Mo, serving to suppress early SnO2 precipitation, and cause both an increase in W/Mo and an overall buildup of all four elements in evolving SMB liquids. Available experimental data indicate that reducing conditions also shifts DMVP/melt to favor partitioning into the melt phase. Therefore, early vapor exsolution under reducing conditions also lessens the extraction efficiency of these redox-sensitive elements to the MVP, further underscoring the role of extensive crystallization as an important metal enrichment process.

EVOLUTION OF MINERAL ASSEMBLAGES IN GRANULITES FROM BUNGER HILLS, EAST ANTARCTIСA AS A RESULT OF THEIR GEODYNAMIC SETTING

The paper discusses the mineral composition of metapelitic granulites. Two phases of Mesoproterozoic metamorphism are suggested from mineral thermobarometry, isochemical phase diagram modeling, and geochronology. It is shown that the early metamorphism was ultra-high temperature (UHT) grade and associated with collisional tectonics during the Rodinia supercontinent amalgamation. The later metamorphism was amphibolite-facies and taken place in a post-collisional extensional tectonic regime.

Magmatic and metamorphic history of the Proterozoic Lesser Himalayan Crystallines from Bomdila area, Arunachal Pradesh, NE Lesser Himalaya, India: Constraints from whole rock and mineral chemistry

The excellent exposures of plentiful Proterozoic granitic bodies in the north‐east Himalaya offer a chance to explore the evolution of continental crust in the Early Proterozoic period. The present comprehensive study from Bomdila area, western Arunachal Pradesh, NE India documents petrogenesis of Palaeoproterozoic Bomdila granite gneiss and metamorphic history of the region using the whole rock and mineral chemistry, and suitable mineral thermobarometry. The gneisses, with quartz, k‐feldspar, plagioclase, and mica mineralogy, are characterized by enrichment in rare earth elements (total REE up to 290 ppm), Rb, Ba, Th, and depletion in mafic elements (Ni, Cr, V, Sc). The gneisses are subalkaline and peraluminous with A/CNK (Al2O3/CaO + Na2O + K2O) > 1.1, indicating S‐type nature of the granites. The gneisses show fractionated REE patterns (LaN/YbN = up to 22) with LREE enrichment and HREE depletion. Geochemical characteristics such as enriched SiO2, Al2O3, alkalis, LREE, high incompatible element ratios (Rb/Sr and Ba/Sr), and depleted mafic contents with noticeable negative Eu anomalies (Eu/Eu* = 0.2 – 0.6) suggest an origin of gneisses through high‐temperature (>800°C) shallow crustal melting of a pelitic source similar to the metasedimentary enclaves within the gneisses. Mineral thermobarometric calculations suggest a tightly constrained P–T condition of 7.5–8.6 kbar and 560–590°C for the gneiss and schist from the study area. Almost identical P–T conditions of metamorphism from different lithologies attests that the Bomdila area may have attained an upper amphibolitic facies condition, possibly concomitant to the Himalayan Orogeny. Though rudimentary, temperature estimates from Ti‐in biotite thermometer suggest an increase in metamorphic grade towards the Main Central Thrust.

Petrological evidence of rapid evolution of the magma plumbing system of Bezymianny volcano in Kamchatka before the December 20th, 2017 eruption

We used petrological data on samples of the December 20th, 2017 eruption of the Bezymianny volcano (Kamchatka, Russia) to investigate the evolution of the magmatic system during the repose period and the following reactivation of the volcano. The eruptive products are diverse in bulk rock and matrix glass compositions, but are similar in phenocryst proportion, composition, and zoning. Three rock types are distinguished by the presence of either silica-phase (cristobalite or tridymite) or their absence. Rocks without silica phase (Silica-phase Free Host Rocks) are characterized by ~55–56 wt% bulk SiO2 content and have predominantly glassy groundmass with rare microlites in dacitic matrix glass (65.5–70.5 wt% SiO2 and 2.5–3.5 wt% K2O). Cristobalite-bearing rocks are characterized by ~56.6–57.5 wt% bulk silica content and highly crystalline groundmass with many small microlites and rhyolitic matrix glass (76.5–78 wt% SiO2 and 3.8–4.2 wt% K2O) between them. Tridymite-bearing rocks are characterized by ~58.8 wt% bulk silica content and glassy groundmass with rare microlites in rhyolitic matrix glass (77–77.5 wt% SiO2 and ~3.6 wt% K2O).Rhyolite-MELTS simulation and mineral thermobarometry indicate that silica-phase-bearing rock last equilibrated under low-pressure conditions (~15 Mpa) corresponding to the conduit depths, whereas rocks without silica phase crystallized at higher pressure corresponding to shallow magma storage depths (>40–60 Mpa). Bulk rock mass-balance calculations indicate that silica-phase-bearing magmas were produced by 15% of crystal fractionation from the magma without silica phase. Based on this petrological data, we reconstruct the evolution of the magmatic system as follows. During the volcano repose period, undisturbed magma fractionated and stratified due to buoyant ascent of fluid and melt to the chamber roof. Rejuvenation of magmatic system caused pushing the differentiated magma from the top magmatic reservoir to the conduit, where silica phases crystallized. We argue that cristobalite-bearing rocks are likely the remnants of conduit plug, whereas tridymite-bearing rocks represent magma trapped in the conduit under the plug. The renewal of explosive activity in December 2017 was triggered by magma influx from deeper levels of the plumbing system shortly before the explosion, which is supported by the presence of amphibole-bearing mafic enclaves of mid-crustal origin (520–850 Mpa).

Constraints on assembly of Tongariro and Ruapehu andesite magmas based on Sr-isotope compositions of plagioclase and groundmass

Sub-crystal scale 87Sr/86Sr analysis of plagioclase phenocrysts and groundmass in andesites reveal cryptic disequilibrium that is not observable in bulk rock-based studies, shedding light on magma assembly. Isotopic, chemical and textural features demonstrate that plagioclase phenocrysts in continental arc andesites from Tongariro and Ruapehu volcanoes are a heterogenous population of various cognate and non-cognate origins. The studied rocks span a wide age range (~270 ka) and have different vent locations but their plagioclase share many characteristics. Most phenocrysts comprise of a relic core surrounded by one or more mantling overgrowths of various textures demonstrating a multi-stage growth during crustal transit. The total compositional range found in a single thin section is wide (~An45–90). Some zoned phenocrysts have a comparable wide range. Inter-crystal 87Sr/86Sr diversity is considerable with differences up to 0.7044–0.7058 within a hand-sized sample. Individual phenocrysts can be isotopically zoned with rim-core differences up to 0.0005, and contrasting isotopic zonation can occur in phenocrysts of the same sample. Overall, more than half the phenocrysts are non-cognate as indicated by their isotopic differences. Phenocrysts with relic calcic (>An85) interiors and low 87Sr/86Sr ratio (<0.7050) are probably antecrysts and xenocrysts from deep mafic cumulates and/or forerunner intrusions. Other phenocrysts have relic sodic interiors (<An55) with high 87Sr/86Sr ratios (~0.7055) compared to the accompanying groundmass, and were probably entrained from country rock and/or differentiated crystal mush in the crust. Phenocrysts most likely cognate because of their concordant composition and isotopic ratio with the groundmass, exhibit oscillatory zoning and lack relic cores, and others display decompression-induced replacement of calcic plagioclase with more sodic compositions. These textures reflect the dynamic processes of magmatic ascent and temporary storage. In some andesites the groundmass (host magma) has a greater mantle-like isotopic affinity than that of the phenocrysts, while in other andesites the reverse is true. This reflects the degree of mingling between the host magma and crustal melts during ascent. The cryptic isotopic disequilibrium revealed highlights the problem of petrogenetic modelling and mineral thermobarometry that assess equilibrium among the phases on elemental abundance. Such concordance could be coincidental rather than co-magmatic.

>1.8 Ga cold subduction of lithospheric mantle: Evidences from the Fengzhen olivine-bearing garnet pyroxenite xenoliths in Trans-North China Orogen

Whether modern-style subduction exists in Paleoproterozoic has been hotly debated because of the scarcity of robust petrological evidence. Here, we present a comprehensive study of olivine-bearing garnet pyroxenite xenoliths hosted in the Paleoproterozoic Fengzhen carbonatite. The carbonatite is located in the conjugate area between the Paleoproterozoic Trans-North China Orogen (TNCO) and the Khondalite Belt in North China Craton (NCC) with a dated age of 1810 ± 3 Ma. Petrographically, four-stages of evolution have been identified in the studied olivine-bearing garnet pyroxenite: 1) the formation of the protolith spinel websterite (S1), 2) the prograde metamorphism of spinel-lherzolite facies to garnet-lherzolite facies (M1), 3) retrograde metamorphism to Ariegite subfacies (M2) with formation of symplectite-I, and 4) Seiland subfacies (M3) forming symplectite-II. The clinopyroxene in the xenoliths display high Mg# (Mg2+/(Mg2++Fe2+)*100 = 89–94), strongly depleted HREE (heavy REE) and HFSE (high field strength elements; e.g., Nb, Zr, Ti) and enriched LREE (light REE) and LILE (large ion lithophile elements; e.g., Th and U). Similarly, the whole-rock chemistry exhibits high Mg# (85–88) and high Al2O3 + CaO (20.2–21.9 wt%) with enrichment in LREE and LILE (Th, U) and depletion in HREE and HFSE (e.g., Nb, Ta, Zr, Hf, Ti). The geochemical signatures imply that they might originate from the refractory mantle wedge in subduction zone and have been metasomatized by crust-derived melts. Major mineral thermobarometry yields the peak PT conditions of 26–33 kbar and 890–962 °C corresponding to a cold subduction geothermal gradient (307 ± 35 °C/GPa) with a depth up to 110 km for olivine-bearing garnet pyroxenites, which is also consistent with the results by zirconium-in-rutile and REE thermobarometers. This studies suggest that that the modern-style subduction of the lithospheric mantle was initiated at least since 1.8 Ga.

Mineral thermobarometry and its implications for petrological constraints on Mesoarchean granitoids from the Carajás Province, Amazonian Craton (Brazil)

This paper describes the first investigation of the crystallization parameters of the Mesoarchean granitoids from Ourilândia do Norte, which is located in the midwestern part of the Carajás Province. These rocks are divided into three major groups: (i) sanukitoids composed of amphibole-bearing granitoids and associated intermediate to mafic rocks; (ii) potassic granites composed of biotite monzogranites and associated high-Ti granodiorite; and (iii) subordinated TTG-affinity granitoids (amphibole-bearing trondhjemites and biotite-bearing tonalitic xenoliths). These rocks were formed from different sources at different depths, including enriched mantle (sanukitoids), continental crust (monzogranite), mantle-crust mixture (high-Ti granodiorite) and oceanic crust (TTG-affinity granitoids). Potassic granites and porphyritic sanukitoid facies have high magnetite contents, and consequently high magnetic susceptibility (MS), compared to the equigranular sanukitoid facies and trondhjemite. Thermometers based on apatite saturation in whole-rock indicate liquidus temperatures above 800 °C for all granitoids, reaching 959 °C in the high-Ti granodiorite. Amphibole-based and Si-sensitive geothermometers indicate solidus temperatures from 785 to 738 °C for sanukitoids and 892 to 772 °C for trondhjemite, with estimated emplacement pressures varying from 297 to 80 MPa and 580 to 263 MPa, respectively. Because the potassic granites display an amphibole-free assemblage, no geothermobarometric estimate based on amphibole composition was possible. However, potassic granites are associated with coeval sanukitoids (2.88 Ga), which indicates similar pressure conditions and emplacement depths for these rocks. Crystallization constraints from the Ourilândia sanukitoids are comparable to those of the geochemically similar Rio Maria sanukitoids of the Carajás Province, although the pressure estimates and water content show differences. The Fe/(Fe + Mg) ratios of both biotite and amphibole indicate high to intermediate redox conditions for the sanukitoids and potassic granites, which are mostly above that of nickel-nickel oxide (+1.0 < NNO < + 1.9), although less oxidized conditions are estimated for the trondhjemite (– 0.5 < NNO < + 1.0). High water content was estimated for the sanukitoid-related granitoids (> 5%) relative to the TTG-affinity granitoids (< 4%) using amphibole-based water calibrations. Compared with sanukitoids from other Archean cratons, the Ourilândia sanukitoids crystallization constraints show similarities with the more oxidized members from the Baltic Shield, the Closepet-type granites and the Matok-type sanukitoids but differ from the reduced redox conditions of the Elan-type granitoids from the East Sarmatian orogen.

Crystallization and melt extraction of a garnet-bearing charnockite from South China: Constraints from petrography, geochemistry, mineral thermometry, and rhyolite-MELTS modeling

AbstractSince granitic rocks in high-grade terranes commonly undergo amphibolite-granulite facies meta-morphic overprint, recovering magmatic records from the metamorphic modification remains a major challenge. Here, we report an early Paleozoic, garnet-bearing Yunlu charnockite that outcropped in the Yunkai terrane of the Cathaysia block from South China and underwent amphibole-grade metamorphic overprint in the late Devonian. Field observation, micro-texture, and mineral geochemistry combined with diffusion modeling constrain that the metamorphic overprint with an extremely short duration of ~0.2–0.5 Ma only influences a narrow rim of &amp;lt;100 μm for most minerals. The magmatic information can be retrieved by combining rhyolite-MELTS modeling with mineral thermobarometry using mineral core compositions to quantitatively estimate magmatic pressure, temperature, and melt H2O contents. Rhyolite-MELTS modeling results are evaluated by comparison with experimentally determined phase relations for a peraluminous granite with ~69.83 wt% SiO2 at a pressure of ~500 MPa. The comparison suggests that the modeling reproduces phase relationships of feldspars and quartz within 20–60 °C when the melt H2O contents are below 7.0 wt%, but fails to account properly for all the phases when the melt H2O contents are higher than 7.0 wt%. The modeling results using reconstructed primary magma composition of the Yunlu charnockite combined with the orthopyroxene-garnet-plagioclase-quartz thermobarometry and fluid inclusion analyses suggest that the magma was emplaced at a pressure of ~600 MPa, a temperature of &amp;gt;900 °C, and an initial H2O content of ~4.0 wt% with rare CO2 components. The orthopyroxene-garnet, biotite-garnet, and biotite-orthopyroxene thermometers yield a consistent temperature range of 770–820 ± 60 °C, which is significantly higher than the H2O-saturated solidus temperature of ~630 °C estimated from experimental results and two-feldspar thermometry. These results indicate that the early crystallized minerals (e.g., garnet, orthopyroxene, and some euhedral biotite) of the Yunlu charnockite equilibrate at higher temperatures with crystallinities of ~30–45%, rather than the H2O-saturated solidus conditions. We thus propose a hypothesis of melt extraction at 780–820 °C in a deep-seated, slowly cooling, partially crystalline magma reservoir. The melt extraction physically segregates the early crystallized minerals from residual interstitial melts, which inhibits element diffusion equilibration between these minerals and interstitial melts. Granite thermometry commonly yields a large range of temperature estimations, which may be related to melt extraction events. Our study shows that melt extraction recorded in granites can be identified by combining micro-texture, mineral thermometry and rhyolite-MELTS modeling, which further provides quantitative insights into the fractionation process of silicic magmas.

Физико-химическое моделирование условий формирования мафического гранулита (оазис Бангера, Восточная Антарктида)

The article presents the results of mineral thermobarometry and physico-chemical simulation of the formation of garnet-enstatite gneiss from the Mesoproterozoic metamorphic suite of the Bunger Hills, East Antarctica. As a result, the water activity, temperature and pressure of rock formation were estimated. It is shown that the peak temperature of metamorphism could reach 900 °С or more. Such temperature conditions indicate the prerequisites for the occurrence of UHT-metamorphism during the formation of the East Antarctic Shield.

Mineral chemistry and thermo-barometry of intrusives from northern side of Shahroud river around Zardkuh in Guilan province (northern Iran)

The study area is located in northern Iran and is part of the Alborz mountain range. The exposed rock units in the study area are mostly Eocene volcanic rocks with some Oligomiocene intrusive masses included and locally Paleocene, Jurassic, Permian and Carboniferous sedimentary outcrops. The intrusive rocks of the study area are mostly gabbro and olivine gabbro in terms of mineralogical composition, but intermediate types such as porphyry monzonite are also rarely seen. Based on the results of microprobe analysis, pyroxenes have augite and diopside compositions, orthopyroxenes are hypersthenes, biotites have magnesium biotite and phlogopite compositions, olivines have hyalosiderite compositions and amphiboles are pargasite and ferropargasite compositionally. The results of temperature and pressure evaluation indicate a temperature range of 650 to 1200 ° C and pressure range &gt;2-5 KB. Based on the chemistry of clinopyroxene mineral of the mentioned intrusions, magma that made these intrusions is of the sub-alkaline type and is formed in a volcanic arc tectonic setting.

Modeling of Mineral Parageneses and Thermobarometry of Metavolcanic Rocks of the Ruker Group in the Southern Prince Charles Mountains, East Antarctica

Diverse geological complexes that form the crystalline basement of the East Antarctic Platform crop out in the Prince Charles Mountains. The Ruker Group is a member of the metasedimentary complexes that make up the Paleoproterozoic through Neoproterozoic suprastructure of the Ruker Terrane. It is divided into two sequences consisting of highly deformed and greenschist facies metamorphosed sedimentary and volcanic rocks. The mineral and major element compositions of metavolcanic rocks have been studied and physicochemical modeling of mineral parageneses has been performed to reconstruct the P–T parameters of metamorphism. The dependence of the mineralogy of metabasic schists on the protolith composition and the ratio of components in the H2O–CO2 fluid involved in the phase reactions were analyzed. The calculated mole fraction of CO2 in the fluid equilibrated with carbonate-bearing parageneses is 0.13–0.27. It is assumed that chloritoid schist consists of metamorphosed laterites derived from basalt. Modeling and the data of chlorite–phengite thermobarometry indicate that the Ruker Group rocks were metamorphosed under conditions of the high-pressure part of the greenschist facies (300–450°C, 7–8 kb). These conditions are significantly higher than the stable continental geotherm and are close to those in the zone of a slow subduction geotherm. According to the available geological data, a similar geodynamic setting could be caused by the evolution of the Neoproterozoic intraplate sedimentary basin in connection with deep-sinking basement blocks as a result of tectonic aggregation during the closure of the basin.

Tectonic evolution of the Dadongcha Formation, Ji'an group, Qinghe area, Tonghua, NE China: Implications from geochronological and geochemical evidence of metapelites

The Jiao–Liao–Ji Belt (JLJB) is an important Paleoproterozoic orogenic belt that has been a focus of research to understand the Precambrian evolution of the North China Craton (NCC). The widespread Paleoproterozoic Ji’an group, which was exposed in the northern margin of JLJB, is the key to solving controversies in the tectonic setting of this special belt. An intensive study on metamorphic reactions, zircon U–Pb ages, mineral thermobarometry, geochemistry, and quantitative phase equilibria modeling (pressure–temperature pseudosections) of typical metapelites from the Dadongcha Formation of the Ji’an group provides important insight to understand the tectonic setting and processes of the Paleoproterozoic JLJB. The analyzed metapelites are exposed in Toudao and Qinghe towns in NE China, include garnet–biotite schist, garnet–biotite–plagioclase gneiss, garnet–sillimanite–cordierite–biotite gneiss, and hypersthene-spinel–cordierite–biotite gneiss. Garnets contain mineral inclusions such as sillimanite, biotite, feldspar, and quartz and retain the typical cordierite reactive edge structures. The protoliths of metapelites are sets of claystone or miscellaneous sandstone with an average chemical composition that is close to that of the upper continental crust (UCC). Chemical Index of Alteration (CIA) and Index of Compositional Variability (ICV) values suggest a high degree of weathering of their sources and low compositional maturity of clastic sediments. The chondrite-normalized rare earth element (REE) pattern of analyzed samples displays characteristics of medium fractionation in light and heavy REEs, and strong enrichment in light REEs. The primitive mantle-normalized trace element pattern shows enrichment of large ion lithophile elements (LILEs) such as Ba, and the content of high field strength elements (HFSEs) such as Zr and Hf is relatively low; furthermore, Nb, Ta, P, and Ti are distinctly depleted. The emphasis is on the metamorphism of metapelites, which shows a clockwise P–T path. In combination with zircon U–Pb dating, a general evolution model for the metapelites was drawn. The metapelites were formed in an active continental margin environment where the evolved island arcs developed around 1.95–2.02 Ga, then were involved in the collision between the Longgang and Langrim blocks in the eastern part of the NCC, and returned to the surface after undergoing regional metamorphism with widespread anatexis around 1.8–1.95 Ga.

Fluids in High-Pressure Granulites

Properties of fluids in high-pressure granulites were studied in HP granulites (~8.7–11 kbar, ~800–900°С) and syngranulite fluid infiltration-driven HP metasomatites (~11–9 kbar, ~920–850°С) from the Lapland granulite belt of the Fennoscandian Shield. The study involved large-scale mapping of the rocks, microthermometry of mineral-hosted fluid inclusions, multiequilibrium mineral thermobarometry, and calculations of H2O activity based on mineral equilibria. The mafic pyroxene granulites and syngranulite metasomatites (quartz blastomylonites with orthopyroxene, sillimanite, and garnet; veins and vein-like bodies of orthopyroxene–garnet and diopside–scapolite rocks) contain similar assemblages of syngenetic fluid inclusions (which are hosted mostly in quartz and also in garnet, orthopyroxene, and scapolite) of contrasting chemical composition: nearly pure СО2 (distinctly predominant), brines (the dominant salts are CaCl2 and NaCl), and N2 ± H2O. These three types of inclusions coexist in the same generations of early inclusions: rarer primary (p) and predominant primary–secondary (ps). The CO2 inclusions have either high or low densities, and the N2 inclusions are of low density. The brine inclusions show a wide range of total salt contents (up to 30–35 wt%) and variable concentration proportions of the dominant salts: p-inclusions with a salinity of 20 wt% CaCl2 + 10 wt% NaCl; ps-inclusions with a salinity of 5 wt% CaCl2 + 20 wt% NaCl; p- and ps-inclusions with a salinity of 5–23 wt% NaCl eq; and p-inclusions with halite (up to 35 wt% NaCl). In general, CaCl2 is the predominant salt component in the early p- and ps-inclusions of the rocks. Considered together, currently available data (including Sr, Nd, and O isotope systems) on these rocks indicate that the external fluid flow during the origin of granulites was evidently of mantle origin. At the peak P–T parameters, the inclusions were entrapped from a heterogeneous fluid in which immiscible water–salt and CO2-rich fluids, which initially contained N2, coexisted. Data on the chemical composition and salt concentrations of the fluids, $${{a}_{{{{{\text{H}}}_{2}}{\text{O}}}}}$$ = 0.40–0.51, are compared with the theoretically predicted phase state of the fluids and the properties of the coexisting immiscible fluid phases at the estimated P–T parameters of granulite petrogenesis on the basis of numerical models in the H2O–CO2–NaCl and H2O–CO2–CaCl2 ternary systems. The location of the tie-lines and solvus were calculated to subsequently use for the thermodynamic prediction. The paper discusses similarity and the reasons for the difference between the theoretical compositions of the generated fluid phases and the composition of fluid inclusions, geochemical consequences of the heterogenization of granulite fluids (the formation of concentrated alkaline brines and a potentially acidic CO2-rich fluid phase, the values of the mass and volume fractions of these phases depending on variations in the composition of the initial homogeneous fluid, etc.). It follows that an extensive region of the compositions of aqueous fluids with different concentrations of CO2 and Na and Ca chlorides exists at the P–T parameters of HP granulites in which originally homogeneous fluid splits into compositionally contrasting fluid phases with different properties. This region of coexisting immiscible fluids significantly expands with increasing CaCl2 concentration. Hence, the lower crust at the level of the HP granulite facies may be the region where high-temperature immiscible fluids are generated. One of these fluids is a denser phase of alkaline brines, and the other is a less dense potentially acidic phase of H2O–CO2 fluids rich in CO2. Ascending along regional permeable zones, these fluid phases of deep origin can play an important role in magmatic, metamorphic, metasomatic, and ore-forming processes in the middle and upper crust.