Monazite Grains Research Articles

Apatite and Monazite Geochemistry Record Magmatic and Metasomatic Processes in Rare Earth Element Mineralization at Mountain Pass, California

Abstract The largest rare earth element (REE) deposit in the United States is a carbonatite intrusion at Mountain Pass in the Mojave Desert, California. Despite a clear spatiotemporal association of alkaline silicate and carbonatite intrusions at Mountain Pass, a genetic model of their mutual formation has not been resolved. The Mountain Pass carbonatite has long been upheld as an example of a primary magmatic body, but this has not been investigated in detail at the mineral scale. This study investigates the geochemistry of apatite and monazite grains from the alkaline silicate and carbonatite stocks and dikes of the Mountain Pass district to elucidate the magmatic history of the intrusive suite and identify the potential role of fluids in REE mineralization. Three apatite populations are identified in the alkaline silicate rocks. A primary magmatic apatite group supports intrusion of the stocks as separate pulses of magma derived from a spatially extensive metasomatized mantle source region. The second group implicates the role of a regional fluid that mobilized light REEs from apatite grains. Low Sr concentrations and negative Eu anomalies in cores of a minor group of inherited apatite support assimilation of crustal material in the formation of the intrusive suite. Analyses of monazite and apatite grains from the carbonatite orebody also reveal a mix of primary magmatic and metasomatic (fluid related) minerals. Compositional similarities between primary phosphates in the carbonatite and alkaline silicate rocks support a genetic link between the intrusions. The impact of fluids on mineralization in the carbonatite orebody indicates the Mountain Pass carbonatite should not be classified as a purely magmatic REE deposit.

Pressure–Temperature–Time Evolution of a Polymetamorphic Paragneiss With Pseudomorphs After Jadeite From the HP–UHP Gneiss‐Eclogite Unit of the Variscan Erzgebirge Crystalline Complex, Germany

ABSTRACTA quartz‐rich paragneiss from the Variscan Erzgebirge Crystalline Complex (ECC) was studied in detail because of abundant millimetre‐sized and clearly oriented pseudomorphs after a sodic mineral interpreted to have been jadeite. This mineral, or pseudomorphs after it, is rarely found in extensive high‐pressure (HP)–ultrahigh‐pressure (UHP) terranes worldwide despite reported pressure–temperature (P–T) conditions suitable for the formation of jadeite in common paragneisses and orthogneisses. In the studied rock, which contains abundant large and oriented potassic white mica flakes and minor millimetre‐sized garnet grains, the pseudomorphs consist of clusters of small albite grains with thin phengitic muscovite flakes in between. X‐ray maps for Ca and Mg in garnet demonstrate that an early generation of this mineral (Gt1) was corroded and subsequently overgrown by a Ca‐richer generation (Gt2). White mica is phengite with maximum Si contents of 3.42 atoms per formula unit. P–T conditions of 0.85 GPa and 650°C and 1.7 GPa and 660°C were derived for the formation of Gt1 and Gt2 rim + Si‐rich phengite, respectively, using pseudosection modelling. The latter conditions representing the pressure peak experienced by the paragneiss are compatible with the original presence of jadeite and possibly paragonite as well. This metamorphic peak occurred at 338.4 ± 2.3 (2σ) Ma based on in situ dating of monazite grains with the electron microprobe. A single monazite age of 386.4 ± 10.5 (2σ) Ma is related to the formation of Gt1. Thus, a Late Devonian metamorphism is suggested here for the first time to have occurred in ECC gneisses before the major HP event in the Early Carboniferous. Furthermore, the study demonstrates that the eclogite‐facies gneisses of the Gneiss‐Eclogite Unit of the ECC experienced peak pressures of not more than 2 GPa in contrast to recent proposals of an extensive UHP area in this unit. In addition, it is suggested that the localized occurrence of UHP rocks surrounded by other lithologies otherwise lacking evidence for UHP conditions should be interpreted with caution with respect to their regional extent and significance.

Geochronology and geochemistry of a Neoproterozoic syn-tectonic granitic pluton in the Gari-Gombo area, East Cameroun: Implications for petrogenesis and tectonic evolution

Granites are widespread in many Precambrian orogenic belts worldwide; therefore, they can provide insights into orogenic processes and associated magmatism. Zircon UPb age, monazite Th-U-total Pb age and whole-rock geochemical data for a granite pluton from the Gari-Gombo area in the Adamawa-Yade domain of the Central African Fold Belt (CAFB) in East Cameroon are presented. The granite is composed dominantly of perthitic K-feldspars, quartz, plagioclase and minor biotite with accessory monazite, apatite and zircon. LA-ICP-MS zircon UPb dating yielded an age at ca 631–620 Ma, which is interpreted as age of emplacement that coincides with the onset of D2 Pan-African deformation. Monazite grains in Gari-Gombo granite follow strictly the huttonite substitution trend in Th + U vs Si coordinates. Monazites give consistent Neoproterozoic ages of 630 ± 4 Ma and 602 ± 4 Ma, indicating that growth history and crystallization age of monazites also correlate well with the Pan-African plutonism and granulite facies metamorphism (ca 614–600 Ma) in the Gari-Gombo area. The Gari-Gombo pluton samples show high-K calc-alkaline magnesian, slightly peraluminous signature, high SiO2 (70.16–78.80 wt%), K2O (4.39–5.38 wt%), and Rb (165–248 ppm), and low P2O5 ≤ 0.01 wt% and Sr (146–222 ppm) contents. They have highly-fractionated REE pattern ((La/Yb)N = 6.17–148.18), moderately Eu negative anomalies (Eu/Eu* = 0.53–0.93) and the obviously Nb and Ti negative anomalies. These geochemical features suggest that the Gari-Gombo pluton is a highly fractionated I-type granite generated by partial melting of older meta-igneous materials at middle to lower crustal levels. The 2.9 and 0.95 Ga inherited zircon grains identified within the studied granites further confirm the existence of ancient crust in this region.

Phosphate geochronometer minerals: Crystal chemistry and radiation disorder, methodological issues of their microprobe non-isotope U–Th–Pb<sub>tot</sub> dating

Research subject. Phosphate mineral geochronometers – the international reference sample of Trebilcock monazite from pegmatites with the age of 272 ± 2 Ma, as well as samples of monazite from pegmatites of the Shartash massif and monazite, cheralite and xenotime from leucogranite of the Peshcherninsky stock and diorite of the Khomutinsky massif, Middle Urals. Methods. The composition of minerals was studied using CAMECA SX100 microprobe; Raman spectra were obtained using LabRAM HR800 Evolution confocal spectrometer. Research aim. Study of the internal texture of the grains of phosphate minerals on the basis of their elemental and spectroscopic mapping; analysis of the mineral crystal chemistry and estimation of auto-irradiation doses; microprobe non-isotopic U–Th–Pbtot dating of phosphate minerals; development of the appropriate algorithm for using analytical techniques. Results. It has been shown that the studied monazites belong to the cerium variety with ThO2 content from 1.1 to 17.2; UO2 – from 0 to 0.8; PbO – from 0.01 to 0.23 wt % (detection limits 160, 230, and 110 ppm). When analyzing the PbO content, the background line was interpolated into models of linear background (Trebilcock monazite, monazite and cheralite of the Peshcherninsky stock) and exponential background (monazite of the Shartash massif). It has been shown that for monazite, both huttonite and cheralite types of isomorphism are realized; the non-stoichiometric parameter of its composition β = (Si + Ca)/(Th + U + Pb + S) lies in the range of 0.95–1.05, which indicates the preservation of the U–Th–Pb-system. The analysis of BSE-images, intensity distribution maps of the Th Mα and Pb Mα RE lines, compositional point analyses and the results of spectroscopic mapping of the parameters of the ν1(PO4) vibrational mode testify to high homogeneity of Trebilcock monazite and pronounced zoning of the Ural monazites. It has been shown that the parameters of the ν1(PO4) vibrational mode in monazites are determined by the superposition of two factors, i.e. chemical and radiation disorder. The data on U, Th, and Pb content for different zones of monazite grains were used to perform non-isotopic U–Th–Pbtot dating: weighted average age values for the zones were obtained, and isochron plotting was made on the ThO2* vs. PbO diagram. The datings obtained based on the Trebilcock sample are in satisfactory agreement with the literature. Conclusions. The dating of monazite from leucogranite of the Peshcherninsky stock and the Shartash massif are in agreement with the U-Pb isotopic dating of zircon. The physical and chemical characteristics of cheralite, xenotime, and zircon in samples from the Peshcherninsky stock were analyzed. The U–Th–Pbtot dating of cheralite, xenotime, and zircon was attempted. The described algorithm and analytical methods were used at the Geoanalitik Common Use Center for microprobe non-isotopic dating of phosphate minerals.

Late Neoproterozoic to early Cambrian high-grade metamorphism from Mikir Hills (Assam-Meghalaya gneissic Complex, northeast India): Implications for eastern Gondwana assembly

Mikir Hills region, which represents the eastern segment of the Assam-Meghalaya Gneissic Complex (AMGC) in northeast India, constitutes part of the Eastern Gondwana. The Mikir Hills preserves multiple metamorphic and magmatic events ranging from Early Mesoproterozoic to Early Cambrian. Out of these events, documenting the late Neoproterozoic to early Cambrian tectonothermal events is helpful in correlating the continental blocks of Eastern Gondwana. We present an integrated study involving field relations, petrology, P–T history and zircon-monazite geochronology of hitherto poorly studied pelitic and quartzo-feldspathic gneisses from the Mikir Hills region. These gneisses have experienced at least three deformation events (D1, D2 and D3) with dominant foliation indicated by ENE–WSW striking and shallow-moderately dipping (<40°) S2 gneissic foliation. The peak metamorphism in pelitic and quartzo-feldspathic gneisses is characterized by garnet(core)–K-feldspar–sillimanite–plagioclase–biotite–rutile–quartz–ilmenite–melt and garnet–plagioclase–K-feldspar–biotite–quartz–ilmenite–melt assemblages, respectively. The application of thermobarometric methods constrains the peak P–T conditions of 7.5–8.4 kbar at 674–778 °C and 6.7–7.4 kbar at 601–618 °C for pelitic and quartzo-feldspathic gneisses, respectively. These results are consistent with the values estimated using phase equilibria modelling and melt reintegration approach. The results of pseudosection modelling suggests a clockwiseP–Tpath for pelitic gneisses involving migmatisation during peak metamorphism followed by near isothermal decompression from 8.0 to 8.6 kbar at 768–780 °C to 4.0–5.0 kbar at 720–765 °C. In contrast, quartzo-feldspathic gneisses preserved slightly lower peak P–T conditions at 3.8–4.6 kbar and 590–650 °C. The U–Pb zircon dating of migmatised pelitic and quartzo-feldspathic gneisses yielded concordant ages of1647 ± 11 Ma and 1590 ± 7 Ma, respectively. These dates represent the inherited igneous protolith components, possibly equivalent to the Mesoproterozoic granulite facies metamorphism in the western AMGC. The rarely preserved cores of monazite in pelitic gneisses yielded an older population of1058 ± 35 Ma, most likely representing a weak tectonic imprint associated with the amalgamation of India with Western Australia and East Antarctica in the Rodinia assembly. However, the majority of monazite grains in pelitic and quartzo-feldspathic gneisses show high Th/U ratios with ages between 496 ± 7 Ma and 467 ± 16 Ma, indicating the timing of migmatisation that is contemporary with voluminous ∼ 500 Ma granite magmatism in and around the Mikir Hills. The similarities inP–T–thistoriesestimated in this study (eastern AMGC) and those obtained from the Sonapahar-Umpretha region (central AMGC) confirm that these domains experienced common tectonometamorphic history during Pan-Africanorogeny. The dominance of Late Neoproterozoic migmatisation and magmatism in the Mikir Hills region indicate that the eastern AMGC represent an active convergent margin with Western Australia and East Antarctica and evolved as a hot orogen during the assembly of Western and Eastern Gondwana continental fragments.

Pressure-temperature-time evolution of the Milin pelitic schists: Implications for the tectonic evolution of the eastern Indus-Yarlung suture zone, eastern Himalaya

Abstract We report an integrated comprehensive dataset composed of petrography, pressure-temperature (P-T) calculations, monazite U-Th-Pb ages, and trace-element data from pelitic schists in the eastern Indus-Yarlung suture zone in the Milin area of the eastern Himalaya. These rocks represent the exposure of subduction-related rocks within the eastern Indus-Yarlung suture zone accretionary complex. The dominant mineral assemblages of the pelitic schists are garnet + kyanite + staurolite + biotite + quartz and garnet + kyanite + staurolite + biotite + paragonite + sillimanite with quartz, plagioclase, and ilmenite assemblages. Phase equilibrium modeling of sillimanite-bearing pelitic schists yielded peak P-T conditions of ~670–680 °C at ~8.6 kbar, similar to that of kyanite-bearing schists (~670 °C, ~8.8 kbar). Monazite grains with complex internal structures retained variable ages ranging from 28 Ma to 15 Ma, which correlate systematically with changes in the concentrations of Y, Th, U, and heavy rare earth elements and ratios of Th/U. Combined with petrologic analysis, we conclude that the pelitic schists experienced a long-lived prograde metamorphism from ca. 28 Ma to ca. 22 Ma. Peak metamorphism occurred in the period 22–21 Ma, followed by quasi-isothermal decompression until 15 Ma. The discrepancies among metamorphic P-T-t paths in the eastern Indus-Yarlung suture zone indicate the presence of not only collision-related regional metamorphism at medium P-T conditions, but also subduction-related high-pressure–low-temperature terranes in the Milin region. These two domains experienced different P-T evolution and tectonic histories and were juxtaposed in the early Neogene during the India-Asia continental collision.

Refining granite generation by interrogation of zircon and monazite U-Th-Pb and Hf/Nd-O isotopes

In-situ geochemical and isotopic analyses of zircon and monazite can provide compelling and detailed insights into the source and evolution of their hosting granite. Here we investigated the U-Th-Pb and Hf/Nd-O isotopic signatures of zircon and monazite in two petrological zones (i.e., fine- to medium-grained zone and medium-grained zone) of the Laojunshan granite pluton in the Qinling Orogen, central China, to better constrain their source nature and evolution processes. Co-existing magmatic zircon and monazite in both zones gave identical crystallization ages of ca. 113 Ma. In the fine- to medium-grained zone, both magmatic and xenocrystic zircons were identified, which respectively have εHf(t) values of −6.5 to −5.0 and − 17.4 to +5.1, and δ18OVSMOW values of +5.25‰ to +5.78‰ and + 5.17‰ to +9.15‰. The age distribution pattern and HfO isotopic characteristics of the xenocrystic zircons are similar to those of detrital zircons from the wall rock, i.e., meta-sedimentary rocks in the Kuanping unit. Monazite grains in this zone have relatively low SiO2, ThO2 but high CaO contents, dominated by cheralite-type substitution. These monazites have εNd(t) values of −5.8 to −4.6 and δ18O values of +3.79‰ to +5.66‰, except one grain (εNd(t) = −10.4 to −9.6, δ18O = +9.22‰ to +9.81‰), which is likely a xenocrystic crystal assimilated from wall rocks. The identical UPb age but contrasting NdO isotopic compositions between the xenocrystic monazite and the magmatic monazite grains indicate that the former might have experienced solid-state recrystallization process and completely lost radiogenic Pb. Both zircon and monazite geochemical and isotopic signatures consistently indicate that crustal assimilation occurred in the fine- to medium-grained granite zone. In contrast, in the medium-grained zone, only magmatic zircons and monazites were identified. The magmatic zircons have εHf(t) values of −3.2 to +2.7, and δ18O values of +4.65‰ to +6.36‰. The magmatic monazites are characterized by huttonite-type substitution with high SiO2, ThO2 but low CaO contents. Their εNd(t) values vary from −5.7 to +1.6 (mostly < −4.4) and δ18O values from +4.65‰ to +6.36‰. Given that the isotopic features of zircons and monazites in this zone are consistent with their whole-rock isotope features and no inherited/xenocrystic grains were observed, the zircons and monazites should record the source isotope compositions. The presence of highly radiogenic Nd isotopes in monazite coupled with its normal mantle-like O isotopes suggest minor involvement of juvenile mantle-derived components in the magma source. Accordingly, we advocate combined study of zircon and monazite, which can better track granite source nature and evolution processes than individual mineral.

Microstructural and isotopic analysis of shocked monazite from the Hiawatha impact structure: development of porosity and its utility in dating impact craters

U–Pb geochronology of shocked monazite can be used to date hypervelocity impact events. Impact-induced recrystallisation and formation of mechanical twins in monazite have been shown to result in radiogenic Pb loss and thus constrain impact ages. However, little is known about the effect of porosity on the U–Pb system in shocked monazite. Here we investigate monazite in two impact melt rocks from the Hiawatha impact structure, Greenland by means of nano- and micrometre-scale techniques. Microstructural characterisation by scanning electron and transmission electron microscopy imaging and electron backscatter diffraction reveals shock recrystallisation, microtwins and the development of widespread micrometre- to nanometre-scale porosity. For the first time in shocked monazite, nanophases identified as cubic Pb, Pb3O4, and cerussite (PbCO3) were observed. We also find evidence for interaction with impact melt and fluids, with the formation of micrometre-scale melt-bearing channels, and the precipitation of the Pb-rich nanophases by dissolution–precipitation reactions involving pre-existing Pb-rich high-density clusters. To shed light on the response of monazite to shock metamorphism, high-spatial-resolution U–Pb dating by secondary ion mass spectrometry was completed. Recrystallised grains show the most advanced Pb loss, and together with porous grains yield concordia intercept ages within uncertainty of the previously established zircon U–Pb impact age attributed to the Hiawatha impact structure. Although porous grains alone yielded a less precise age, they are demonstrably useful in constraining impact ages. Observed relatively old apparent ages can be explained by significant retention of radiogenic lead in the form of widespread Pb nanophases. Lastly, we demonstrate that porous monazite is a valuable microtexture to search for when attempting to date poorly constrained impact structures, especially when shocked zircon or recrystallised monazite grains are not present.

From ID-TIMS U-Pb dating of single monazite grain to APT-nanogeochronology: application to the UHT granulites of Andriamena (North-Central Madagascar)

The causes of U-Pb isotopic discordance documented by Paquette et al. (2004) in monazite grains from the ultra-high temperature (UHT) granulite of the Andriamena unit of Madagascar are re-evaluated in the light of nanoscale crystal-chemical characterization utilising Atom Probe Tomography (APT) and state-of-the-art Scanning Transmission Electron Microscopy (STEM). APT provides isotopic (208Pb/232Th) dating and information on the chemical segregation of trace elements (e.g., Pb) in monazite at nanoscale. Latest generation of STEM allows complementary high-resolution chemical and structural characterization at nanoscale. In situ isotopic U–Pb dating with Secondary Ion Mass Spectrometry (SIMS) on 25 monazite grains and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) on zircon have been employed to refine the age spectra. Monazite and zircon grains located in quartz and garnet formed with the peak UHT metamorphic assemblage, which is partially overprinted by retrograde coronitic textures. Zircon grains hosted in garnet and in quartz yield concordant U–Pb ages at 2758 ± 28 Ma and 2609 ± 51 Ma, respectively whereas monazite grains hosted in quartz and garnet show a discordant Pb* loss trend on the Concordia diagram recording disturbance at 1053 ± 246 Ma that is not seen by the zircon, underlining the importance of combining the use of monazite and zircon to understand the history of polymetamorphic rocks. The Pb*-loss trend of monazite is related to petrographic position, with less Pb* lost from monazite hosted in quartz and garnet than monazite hosted in the coronitic reaction texture domains. STEM shows that the garnet- and quartz-hosted monazite grains contain more Pb-bearing nanophases than monazite grains located in the coronitic textures. An inverse correlation between the number of Pb-bearing nanophases and the percentage of Pb*-loss in monazite grains demonstrates that Pb* is retained in the grain in the form of nanophases. The formation of Pb-bearing nanophases limits Pb*-loss at the grain scale and therefore allows the preservation of early events. 208Pb/232Th ratios obtained with APT in monazite located in quartz and garnet and excluding Pb*-bearing nanophases indicate a mean age of 1059 ± 129 Ma corresponding to a disturbance event hitherto undetected in the geochronological record of the Andriamena unit. Thus, geochronology with APT allows access to information and the definition of events that may be blurred or obscured when examined at lower spatial resolution.

Microstructural Deformation and the Age of Monazite-(Ce) from Diatectite Granite in the Jarva-Varaka Structure (Kola Region, Russia)

Microstructural deformation and the age of monazite (Ce) from diatectite granite of the presumably impact Jarva-Varaka structure in the Kola Region (northeastern Fennoscandian Shield) are presented. Biotite diatectite granite forms lenses in the aluminous gneisses of the Kola group hosting the 2.5-Ga-layered Jarva-Varaka Massif (JVM). A sample of biotite granite was collected northeast of the Jarva-Varaka Massif near the earlier described pseudotachylitic breccias. BSE images revealed primary domains in monazite grains with rhythmic euhedral zoning and secondary altered domains. Backscattered electron diffraction maps of monazite grains document the development of deformation twins along {100} and {001} and plastically deformed domains with a maximum misorientation of up to 10°. Newly formed areas of recrystallization (neoblasts) cut the twins and plastically deformed domains. Monazite yielded a U-Pb age of 2706 ± 10 Ma (ID-TIMS method), which defines the crystallization age of the host diatectite granite coeval to the 2.76–2.70 Ga metamorphism of the Kola gneisses. A similar age of 2734 ± 139 Ma (ThO2*–PbO isochron) was obtained for primary monazite domains by the chemical U-Th-total Pb isochron method (CHIME). Domains altered under late hydrothermal processes yield CHIME ages of 1796–1723 Ma. Monazite neoblastic domains are close to primary domains in chemical composition and yielded CHIME ages of 2550–2519 Ma, reflecting probably an influence of the JVM formation. The data obtained are insufficient to confirm the impact origin of the Jarva-Varaka structure, which requires further investigation.

New Insights into the Genesis of Dibrova U-Th-REE Mineral Deposit (West Azov Megablock, Ukraine) Using Monazite Chemistry

This paper investigates the monazite grains from the Dibrova rare-earth-thorium-uranium (U-Th-REE) mineral deposit within the Azov Megablock of Ukrainian Shield. U-Th-REE mineralization is associated with K-feldspar-quartz metasandstones and metagritstones (hereafter quartzites) and pegmatoids. The latter possibly represent products of ultrametamorphism/granitization of initially sedimentary clastic rocks during tectono-magmatic activation during the Paleoproterozoic. Ores are composed of quartz as a principal mineral, feldspar, sillimanite, muscovite, monazite, brannerite, uraninite, zircon, rutile, and sulfides. The purpose of this work was to obtain insights into the genesis of the mineral deposit by studying the monazite grains, their chemistry, and ages. Petrographic research work was carried out that included studying/analyzing the monazites from various monazite-bearing rocks (quartzites, pegmatoid, and biotite schist samples). A variety of methods and tools were used, including optical microscopy study, X-ray fluorescence (XRF) mapping of selected samples, as well as scanning electron microscope (SEM) and electron microprobe (EPMA) characterization of monazites, including U-Th-Pb monazite chemical dating. U-Pb-Th chemical electron microprobe dating of the monazites yielded two major distinct monazite age groups at 3.0–2.8 Ga and 2.2–2.0 Ga. The first age group corresponds to the time of formation of the Archean granitoids, which served as a source of monazite for its clastic sedimentation during the Paleoproterozoic in the Dibrova suite sediments. The second age group corresponds to the reprecipitation (i.e., remobilization) of monazite during the Paleoproterozoic tectono-magmatic activation. The location of the mineral deposit within the deep mantle-crustal Devladivska shear zone is another favorable factor for the remobilization and transport of metals. New data on the age of mineralization yield a more complete understanding of the geological history and formation of the complex polyphase rare-earth-uranium-thorium Dibrova mineral deposit.

U–Th–PbTotal monazite geochronology of the felsic volcanic rocks from the Hutti greenstone belt: Evidence of magmatic and hydrothermal events

Petrochronology of monazite in volcanic rocks of the Hutti deposit in the Eastern Dharwar Craton (India) has been studied to understand the magmatic and hydrothermal events that have affected the area. The volcanic rocks occurring in the Hutti greenstone belt are bimodal in nature and contain both felsic and metabasalt. The felsic volcanic rocks in the area preserve accessory minerals like monazite, allanite, and apatite, whereas the same is absent in metabasalts. Petrographic observation of felsic volcanic rock shows corona growth of allanite and apatite (in addition to huttonite) clustering around monazite grains. This corona texture, termed allanite cluster, occurs parallel to the shear planes (C‐plane) of the felsic volcanic rock and characterizes late‐stage hydrothermal activity in the area. Texturally, monazite in the felsic volcanic rock occurs included within arsenopyrite (&lt;1 μm), and in association with apatite, allanite, and occasionally with huttonite/thorite (~10–40 μm), in which monazite grains preserve geochemical heterogeneity in both intra‐ and inter‐grains. The U–Th–PbTotal dating of monazite associated with the allanite cluster shows two age peaks (both from intra‐ and inter‐grains) at 2658 ± 17 Ma (older) and 2534 ± 38 Ma (younger), in which the older age represents the magmatic/crystallization age of the studied felsic volcanic rock overprinted by a later hydrothermal event in the study area. Based on monazite geochemistry‐geochronology, the present study deduces two texturally constrained events related to the primary crystallization of monazite (~2.65 Ga) and subsequent alteration of the same during a later hydrothermal event (ca. 2.53 Ga) that has affected the felsic volcanic rock in the Hutti gold deposit.

An extended Neoarchaean to Neoproterozoic history of the Sandmata Complex (Aravalli Craton, northwestern India): Insights from metamorphic evolution and zircon-monazite geochronology of high-grade quartzofeldspathic gneisses

The Sandmata Complex in the Aravalli Craton of northwestern India experienced syn- to post-collisional high-grade metamorphism and migmatization related to the 1.9–1.7 Ga Aravalli orogeny. Garnet-bearing quartzofeldspathic gneisses are an important rock type in the Sandmata Complex. Their prograde assemblage is represented by garnet cores and inclusion minerals Bt + Pl + Qz, whereas the assemblage Pl + Kfs + Qz + Btmatrix + Sil + Ilm, along with chemically homogeneous garnet porphyroblasts, represents peak metamorphism. Post-peak retrograde metamorphism is reflected in the growth of muscovite, chlorite and biotite, and diffusion adjustments in garnet rims. Mineral paragenesis, mineral chemistry, conventional thermobarometry, and phase equilibrium modelling indicate P–T conditions of 6.7–10.2 kbar at 725–830 °C for peak metamorphism, and 4.8–6.9 kbar at 450–610 °C for post-peak retrogression. The P–T conditions determined from the quartzofeldspathic gneisses define a near-isobaric cooling P–T path. In contrast, previously studied high-grade pelitic gneisses and metagranitoids from the Sandmata Complex define an isothermal decompression P–T path. U–Pb dating of magmatic zircon cores in the quartzofeldspathic gneisses provides the crystallization age of their felsic protoliths as ∼ 2.52 Ga. Zircon and monazite geochronology show that peak metamorphism occurred at ∼ 1.90–1.78 Ga and thus shortly preceded the emplacement of the Gyangarh-Asind igneous suite and Anjana granite, previously dated at ∼ 1.72–1.64 Ga. A model of oceanic crust subduction followed by continent–continent collision during the Aravalli orogeny can explain granulite facies metamorphism in the Sandmata Complex. The combined results of this and previous studies support the operation of modern-style plate tectonic processes involving subduction and continental collision during Palaeoproterozoic time. We also identify a younger (∼0.90–0.78 Ga) metamorphic episode, preserved mostly in recrystallized monazite grains elongated parallel to the final deformation fabric, likely related to the South Delhi orogeny.

Unravelling the complex sub-ice geology of the Wilkes Subglacial Basin region of East Antarctica from marine sediment provenance analyses

AbstractDeciphering the sub-ice geology in the Wilkes Subglacial Basin region is important for understanding solid earth-ice sheet evolution and for assessing geological ties between East Antarctica and formerly contiguous Australia. We analyse marine sediment samples derived from drill site U1359 of Integrated Oceanic Drilling Program Expedition 318. Our study reports for the first time that the inland sediment source area comprises a complex mafic igneous terrain and a metamorphosed Precambrian subglacial basement. Pyroxene geochemical analyses confirm the presence of tholeiitic to calc-alkaline basalts. The high-grade part of the subglacial terrain contains upper amphibolite to granulite facies rocks that are comparable to Archaean to Palaeoproterozoic rocks exposed in the Terre Adélie Craton and the formerly adjacent Gawler Craton in Australia. Chemical Th-U-total Pb isochron method (CHIME) ages extracted from a subhedral monazite grain associated with the low-grade biotite-muscovite schist rock fragment provide a unimodal age of 799 ± 13 Ma. Rare occurrences of 800 Ma age in the Terre Adélie Craton and/or George V Coast provide evidence for the presence of at least one late Neoproterozoic magmato-metamorphic event in the interior of Wilkes Land. The affinity of the unexposed geological domains of Wilkes Land, East Antarctica, with their Australian counterparts is discussed in the context of the Rodinia supercontinent.

Low-Temperature Fluorocarbonate Mineralization in Lower Devonian Rhynie Chert, UK

Rare earth element (REE) fluorocarbonate mineralization occurs in lacustrine shales in the Lower Devonian Rhynie chert, Aberdeenshire, UK, preserved by hot spring silicification. Mineralization follows a combination of first-cycle erosion of granite to yield detrital monazite grains, bioweathering of the monazite to liberate REEs, and interaction with fluorine-rich hot spring fluids in an alkaline sedimentary environment. The mineral composition of most of the fluorocarbonates is referable to synchysite. Mineralization occurs at the surface, and the host shales subsequently experience maximum temperatures of about 110 ℃. Most fluorocarbonate mineralization originates at much higher temperatures, but the Rhynie occurrence emphasizes that low-temperature deposits are possible when both fluorine and REEs are available from granite into the sedimentary environment.

The mineral composition and formation conditions of quartz-beryl mineralization of the Pervomaiskoe molybdenum deposit, Dzhida ore field, Southwestern Transbaikalian region

The article is devoted to determination of the mineral composition, physicochemical parameters, and sources of the substance of quartz-beryl mineralization of the Pervomaiskoe ore deposit, that is distinguished as an independent stage of the mineral formation. The main minerals are quartz and beryl; phenakite has first been identified for the Pervomaiskoe deposit. Ore minerals are represented by molybdenite, pyrite, and rare grains of cassiterite, chalcopyrite, rutile, aikinite, wulfenite, columbite, monazite, and xenotime. Results of studying the fluid inclusions have showed that homogenization temperatures of the fluid inclusions in the beryl vary in the range from 265 to 191°C. The ore-forming solutions are characterized by a low salinity of 5.9-8.6 NaCl equiv. Homogenization temperatures of fluid inclusions in the quartz correspond to 281-250°C. Data on the isotopic composition show the participation of meteoric waters in the ore-forming process.

Ore-fluid source of multiphase gold mineralization at Tuanshanbei in the central Jiangnan Orogen (NE Hunan, South China): Insight from geology, quartz H-O and monazite in-situ Nd isotope compositions

The Jiangnan Orogen is China’s third largest gold province, and has undergone complex orogenic processes and tectonic overprinting, forming multistage magmatism, deformation, metamorphism, and gold mineralization. Our field studies indicate that the Tuanshanbei gold ores (central Jiangnan Orogen) have two generations of auriferous quartz-ankerite-pyrite-arsenopyrite veins (Q2 and Q3), with the latter containing abundant ankerite and base metal sulfides. Both Q2 and Q3 veins are younger than the emplacement of the Tuanshanbei granodiorite vein-ore host, and cut by post-ore diabase dikes. Q2 veins were likely associated with the Early Devonian near N-S-directed shortening, along sub-horizontal EW-/WNW-striking transpressive faults, whereas Q3 veins (hosting ∼ 70% of the total Au resource), were primarily hosted in the Early Triassic moderately-/steeply-dipping NW-striking tensional/transtensional faults and moderately-dipping NE-/NNE-striking transpressive faults, associated with NW-SE-directed shortening. Discrete structural styles and mineral assemblages from Q2 and Q3 suggest superimposing gold mineralization at Tuanshanbei. To determine the source(s) of the ore-forming aqueous-carbonic fluids, we conducted mineralogical and in-situ LA-(MC)-ICP-MS Nd-isotope analyses on two generations of hydrothermal monazite, and H-O isotope analyses on the quartz from Q2 and Q3 veins.The results suggest that the Tuanshanbei ore fluids have δD = −66.7 to −57.6‰ (Q2) and −66.8 to −66.2‰ (Q3), and calculated δ18OH2O = 4.5–9.7‰ (Q2) and 6.1–7.8‰ (Q3). The H-O isotopic data suggest a metamorphic and/or magmatic water source for the ore fluids. Hydrothermal monazite (coexists with native gold and auriferous sulfides) from Q2 and Q3 veins displays subtle heterogeneous BSE response along cracks and grain margin, suggesting alteration there. The 147Sm/144Nd and 143Nd/144Nd ratios of altered parts of Q2 and Q3 monazite grain are significantly perturbed via dissolution–recrystallization by the hydrothermal fluids, whereas their initial Nd isotope compositions seem to be less affected. The unaltered Q2 and Q3 monazite samples have similar initial εNd values (Q2: −8.07 to −7.36, Q3: −10.87 to −9.99) to the Neoproterozoic Cangxiyan Group greenschist–amphibolite-facies metamorphic rocks. Geological and structural evidence suggest both Q2 and Q3 ore-forming events were related to metamorphism, typical of orogenic gold deposits. We interpreted that various parts of the basement were metamorphosed (to near the greenschist-amphibolite-facies boundary) at different times, during which the gold-bearing metamorphic fluids produced migrated into the same structural conduits and deposited the ores there in two ore-forming episodes.

Multi-stage alteration at Nifty copper deposit resolved via accessory mineral dating and trace elements

The sediment hosted Nifty prospect is one of the most prominent Cu deposits in the Neoproterozoic Paterson Province, which girdles the eastern margin of the Archean Pilbara Craton in Western Australia. The timing of mineralization at Nifty has proved challenging to constrain despite several attempts to date it using a range of isotopic methods, including muscovite 40Ar/39Ar (total fusion) and solution ICP-MS apatite U–Pb geochronology. The region preserves a protracted and complex geological history, with potential for several generations of fluid flow/mineralisation, which necessitates a texturally-controlled geochronology approach. Here, we report in situ apatite and monazite U–Pb isotopes complemented with trace elements from both mineralized veins and matrix of the sedimentary host-rock collected via laser ablation split stream inductively coupled plasma mass spectrometry (LASS-ICP-MS). Apatite grains from pyrite- and quartz-bearing veins are enriched in middle rare earth elements (MREE) with prominent convex-upward chondrite-normalized REE profiles. This chemical signature is similar to hydrothermal apatite from the Olympic Dam high-grade bornite Cu deposit, commonly associated with MREE-enriched, lower salinity fluids, and alkaline pH conditions capable of mobilizing Cu. Hydrothermal apatite from pyrite-bearing veins associated with euhedral, concentrically zoned pyrite yields lower intercept ages of ca 810 Ma, whereas hydrothermal apatite from quartz-pyrite veins associated with pyrite replacement microstructures have variable apparent ages. Monazite grains on the margins of pyrite-bearing veins (along micro-cracks) and monazite associated with chalcopyrite in veinlets yield a weighted mean 238U/206Pb age of ca 640 Ma. These results necessitate at least two distinct hydrothermal/fluid flow events related to the formation of the Nifty Cu deposit, with the former being temporally associated with ca 830 Ma mafic intrusions. Evidence for the latter hydrothermal event is cryptic, being highly localised at a grain scale, but is broadly coeval with ca 640 Ma granitic magmatism linked to Cu–Au mineralization elsewhere in the Paterson Orogen (e.g., Telfer & Winu deposits).

Magnetite‐fluorapatite geochemistry and monazite U–Pb geochronology of the Mohuldih uranium deposit, Singhbhum Shear Zone, eastern India

In this study, textural relations coupled with mineral chemistry of hydrothermal magnetite, fluorapatite, monazite and allanite from the Mohuldih uranium deposit in the Singhbhum Shear Zone (SSZ) is used to characterize the nature of the fluid during various events of alteration and uranium precipitation. These minerals have two distinct textural types, the earlier of which are coeval with the uraninite mineralization. The later type formed during subsequent hydrothermal overprint as a result of coupled dissolution‐reprecipitation of earlier fluorapatite and mobilization of light rare earth elements. Uranium‐lead dating of texturally‐constrained hydrothermal monazite grains yields two major concordant age clusters at 1855 ± 7 Ma and 963 ± 10 Ma and several discordant analyses. Two spot analyses furnish concordant ages of 1656 ± 33 Ma and 1438 ± 35 Ma that are identical within error of the 207Pb/206Pb ages (1628–1643 Ma and ca. 1392 Ma) of several discordant data points. Similar ages have been reported by other studies from the rocks of the SSZ and are therefore considered to be geologically meaningful. The oldest age of ca. 1855 Ma obtained from the texturally early core regions of monazite corresponds to the earliest stage of uraninite mineralization. The two main alteration types can be interpreted based on the known metamorphic events in the SSZ. A combination of high‐temperature calcic iron ± sodic and high‐temperature potassic iron alteration accompanied the M1 metamorphic event and the low‐temperature silicification/K‐Al alteration during the M2 metamorphic event. A comparative study between our data on magnetite and fluorapatite compositions with those from global iron oxide‐Cu‐gold (IOCG) and iron oxide‐apatite deposits classifies Mohuldih as an IOCG‐type deposit.

Multi-stage metamorphic and metasomatic imprints on apatite-monazite-xenotime assemblages in a set of small iron oxide-apatite (IOA) ore bodies, Prins Karls Forland, Svalbard

On Prins Karls Forland, Svalbard Archipelago, a set of small iron oxide-apatite (IOA) ore bodies have been discovered within a crustal shear zone, which deformed the polymetamorphosed Neoproterozoic metasedimentary rocks. The ores have various styles and grades of deformation and distinct mineral assemblages whose compositions record a multi-stage tectonothermal and metasomatic history. These IOA ore bodies can be subdivided into fluorapatite-bearing and predominant low-Th monazite in the upper section of the shear zone and F-Cl apatite-bearing and predominant high Th-monazite in the structurally lower higher-grade deformed part. The first stage of alteration for these ore bodies resulted in metasomatic alteration of the apatite and liberation of REE and P redeposited as monazite and xenotime. The transport of dissolved REE and P was likely enhanced by deformation. The second stage of alteration had a distinct impact on the individual ore bodies, which resulted in the Th-enrichment of a small subset of the monazite grains in the upper section of the shear zone. In the lower section of the shear zone most of the monazite was replaced by high Th monazite. Here the original fluorapatite is enriched in Cl, Mn, and Sr, most probably due to interaction with CaCl2-rich fluids enriched in Sr and Mn that was scavenged from the hosting metasediments and altered metagabbros. Contrasting textures, mineral assemblages, and the geochemistry of the ores from distinct localities reflect involvement of compositionally different fluids from the gabbroic rocks and surrounding metasedimentary rocks during the protracted tectonothermal evolution of Prins Karls Forland. Therefore, it is concluded that the IOA ore bodies most likely resulted due to the fractionation of Fe, P, Ca, and REE from hypersaline fluids associated with the gabbros. Once deposited, these IOA ore bodies were subsequently altered during at least one and perhaps two later metamorphic events.