Prograde destruction and formation of monazite and allanite during contact and regional metamorphism of pelites: petrology and geochronology

The conditions at which monazite and allanite were produced and destroyed during prograde metamorphism of pelitic rocks were determined in a Buchan and a Barrovian regional terrain and in a contact aureole, all from northern New England, USA. Pelites from the chlorite zone of each area contain monazite that has an inclusion-free core surrounded by a highly irregular, inclusion-rich rim. Textures and ²⁰⁸Pb/²³²Th dates of these monazites in the Buchan terrain, obtained by ion microprobe, suggest that they are composite grains with detrital cores and very low-grade metamorphic overgrowths. At exactly the biotite isograd in the regional terrains, composite monazite disappears from most rocks and is replaced by euhedral metamorphic allanite. At precisely the andalusite or kyanite isograd in all three areas, allanite, in turn, disappears from most rocks and is replaced by subhedral, chemically unzoned monazite neoblasts. Allanite failed to develop at the biotite isograd in pelites with lower than normal Ca and/or Al contents, and composite monazite survived at higher grades in these rocks with modified texture, chemical composition, and Th-Pb age. Pelites with elevated Ca and/or Al contents retained allanite in the andalusite or kyanite zone. The best estimate of the time of peak metamorphism at the andalusite or kyanite isograd is the mean Th-Pb age of metamorphic monazite neoblasts that have not been affected by retrograde metamorphism: 364.3Dž.5 Ma in the Buchan terrain, 352.9NJ.9 Ma in the Barrovian terrain, and 403.4LJ.9 Ma in the contact aureole. Some metamorphic monazites from the Buchan terrain have ages partially to completely reset during an episode of retrograde metamorphism at 343.1Nj.1 Ma. Interpretation of Th-Pb ages of individual composite monazite grains is complicated by the occurrence of subgrain domains of detrital material intergrown with domains of material formed or recrystallized during prograde and retrograde metamorphism.     

Direct dating of mid‐crustal shear zones with synkinematic allanite: new in situ U‐Th‐Pb geochronological approaches applied to the Mont Blanc massif

Dating the timing of motion on crustal shear zones is of tremendous importance for understanding the assembly of orogenic terranes. This objective is achieved in this paper by combining petrological and structural observations with novel developments in in situ U‐Th‐Pb geochronology of allanite. A greenschist facies shear zone within the Mont Blanc Massif is documented. Allanite is synkinematic and belongs to the mylonitic assemblage. LA‐ICP‐MS U‐Th‐Pb isotope analyses of allanite reveal high contents and highly radiogenic isotopic compositions of the common‐Pb component. The use of measured Pb‐isotope compositions of associated minerals (feldspars and chlorite) is critical for accurate common‐Pb correction, and provides a powerful mechanism for linking allanite growth to the metamorphic assemblage. A mean ²⁰⁸Pb/²³²Th age of 29.44 ± 0.95 Ma is accordingly taken for synkinematic allanite crystallisation under greenschist facies conditions. This age reflects the timing of the Mont Blanc underthrusting below the Penninic Front and highlights the potential of directly dating deformation with allanite.     

Trace-element distributions in silicates during prograde metamorphic reactions: implications for monazite formation

To assess the petrogenetic relationship between monazite and major silicates during prograde metamorphism, REE were measured across coexisting zoned silicates in garnet through kyanite‐grade pelitic schists from the Great Smoky Mountains, western Blue Ridge terrane, southern Appalachians, to establish REE concentrations and distributions before and after the monazite‐in isograd, and to identify the role major silicates play in the formation of monazite. Results indicate significant scavenging of light rare‐earth elements (LREE) from silicates during the monazite‐in isograd reaction; however, the absolute concentration of LREE hosted in the silicates was insufficient to produce monazite in the quantity observed in these schists. Monazite must have formed mainly from either the dissolution of allanite or some other source of concentrated LREE (possibly adsorbed onto grain boundaries), even though direct evidence for allanite is lacking in a majority of the samples. Laser‐ablation ICP‐MS analyses and theoretical thermodynamic calculations show that monazite may have formed as a result of contributions from both allanite and major silicates. Allanite breakdown initially formed monazite, and monazite production drew LREE liberated from allanite, major silicates and possibly from crystal boundaries. In many rocks the reaction was further promoted by the staurolite‐in reaction, allowing for rapid, isogradic monazite growth.     

Ree minerals of alkaline metasomatic rocks in the Main Sayan Fault

The composition of accessory REE minerals (allanite, chevkinite, fergusonite, and REE carbonates) in alkaline metasomatic rocks of the Main Sayan Fault (quartz-albite-microcline-riebeckite-aegirine, quartzalbite-microcline-magnetite, and clinopyroxene-albite) was studied using back-scattered scanning electron microscopy. Chevkinite occurs only in quartz-albite-microline metasomatic rock. The paragenesis of allanite and titanite is stable in clinopyroxene-albite metasomatic rocks. Allanite and fergusonite are typical of all zones of the metasomatic column. Chevkinite and allanite are often altered due to interaction with hydrothermal fluid and lose some amount of LREE. Secondary bastnaesite, synchysite, and ancylite are formed after allanite, while secondary monazite is developed after chevkinite. Presumably, the low-temperature alteration of allanite and chevkinite under effect of F^?, CO ₃ ^2? , and P ₄ ^3? -bearing fluids had not any significant manifestation in the total REE content in metasomatic rocks.     

Prograde metamorphic sequence of REE minerals in pelitic rocks of the Central Alps: implications for allanite–monazite–xenotime phase relations from 250 to 610 °C

The distribution of REE minerals in metasedimentary rocks was investigated to gain insight into the stability of allanite, monazite and xenotime in metapelites. Samples were collected in the central Swiss Alps, along a well‐established metamorphic field gradient that record conditions from very low grade metamorphism (250 °C) to the lower amphibolite facies (～600 °C). In the Alpine metapelites investigated, mass balance calculations show that LREE are mainly transferred between monazite and allanite during the course of prograde metamorphism. At very low grade metamorphism, detrital monazite grains (mostly Variscan in age) have two distinct populations in terms of LREE and MREE compositions. Newly formed monazite crystallized during low‐grade metamorphism (<440 °C); these are enriched in La, but depleted in Th and Y, compared with inherited grains. Upon the appearance of chloritoid (～440–450 °C, thermometry based on chlorite–choritoid and carbonaceous material), monazite is consumed, and MREE and LREE are taken up preferentially in two distinct zones of allanite distinguishable by EMPA and X‐ray mapping. Prior to garnet growth, allanite acquires two growth zones of clinozoisite: a first one rich in HREE + Y and a second one containing low REE contents. Following garnet growth, close to the chloritoid–out zone boundary (～556–580 °C, based on phase equilibrium calculations), allanite and its rims are partially to totally replaced by monazite and xenotime, both associated with plagioclase (± biotite ± staurolite ± kyanite ± quartz). In these samples, epidote relics are located in the matrix or as inclusions in garnet, and these preserve their characteristic chemical and textural growth zoning, indicating that they did not experience re‐equilibration following their prograde formation. Hence, the partial breakdown of allanite to monazite offers the attractive possibility to obtain in situ ages, representing two distinct crystallization stages. In addition, the complex REE + Y and Th zoning pattern of allanite and monazite are essential monitors of crystallization conditions at relatively low metamorphic grade.     

Recurrent rare earth element mineralization in the northwestern Okcheon Metamorphic Belt,Korea: SHRIMP U–Th–Pb geochronology,Nd isotope geochemistry,and tectonic implications

Rare earth element (REE) mineralization is hosted within Neoproterozoic alkaline metaigneous rocks in the northwestern part of the Okcheon Metamorphic Belt (OMB), a polymetamorphosed fold-and-thrust belt transecting the Paleoproterozoic Gyeonggi and Yeongnam Massifs in the southern Korean Peninsula. The principal carrier phase of REEs is allanite. Allanite grains can be subdivided into several types based on the texture and mineral assemblage including quartz, K-feldspar, biotite, britholite, apatite, fergusonite, andradite, magnetite, zircon, titanite and fluorite. Electron microprobe analysis of allanite clearly distinguishes sample-to-sample variations in total REEs, Ca, Al, Fe and Y but the textural varieties from each rock sample do not show marked differences in those elements. Sensitive high-resolution ion microprobe dating of allanite and zircon reveals a complex history of multistage mineralization. Allanite grains from REE ores yielded Late Ordovician (444.6 ± 8.0 Ma), Permian to Triassic (ca. 300–220 Ma) and Early Jurassic (199–183 Ma) ²⁰⁸Pb/²³²Th ages. These multiple age components often coexist in single grains showing slight differences in backscattered electron brightness. The Ordovician components have distinctly higher Th/U than the younger domains in the same rock sample. The cores and rims of zircon from a syenite hosting REE ore bodies yielded Neoproterozoic (858.2 ± 6.3 Ma) and Early Jurassic (ca. 190 Ma) ²⁰⁶Pb/²³⁸U ages, respectively. The Early Jurassic ages (194–187 Ma) also obtained from zircon grains from granites taken from dykes occurring close to the ores and a drill core indicate the correspondence between granitic magmatism and REE mineralization. The Neoproterozoic zircon inheritance (weighted mean = 853.9 ± 3.8 Ma) in these granites is in sharp contrast to the dominant Paleoproterozoic inherited zircon from the widespread earliest Middle Jurassic granites enclosing the mineralized zone. The geotectonic significance of the Late Ordovician event recorded in the allanite, as well as in detrital zircon from the OMB, is still unclear but its temporal coincidence with intraplate volcanism and arc-related igneous activity, respectively, reported from the southwestern edge of the adjacent Taebaeksan Basin and the southwestern Gyeonggi Massif is noteworthy. The following Permian–Triassic and Early Jurassic mineralization events are probably linked to the continental suturing between the North and South China blocks and subsequent post-orogenic magmatism, and arc magmatism resulting from the paleo-Pacific plate subduction, respectively. Sub-grain Sm–Nd isotopic analyses of allanite by laser ablation multiple collector ICPMS yielded initial ε_Nd values plotting along the Nd isotopic evolution path of the Neoproterozoic metaigneous rocks, indicating that REEs originating from the host rock have been recycled during multistage mineralization events. The profound differences in inherited zircon ages and Nd isotopic compositions between the Early and Middle Jurassic granites may reflect the presence of a major thrust-bounded crustal structure beneath the OMB.     

Discovery of a c.370 Ma granitic gneiss clast from the Hwanggangri pebble-bearing phyllite in the Okcheon metamorphic belt, Korea

The chemical Th–U total Pb isochron method (CHIME) of dating was carried out on accessory minerals in samples from the Okcheon metamorphic belt in Korea. Dated minerals include xenotime and monazite with overgrown mantles in a granitic gneiss clast from the Hwanggangri Formation, metamorphic allanite in garnet-bearing muscovite–chlorite schist of the Munjuri Formation, and polycrase and monazite in post-tectonic granite from the Hwanggangri area. Overgrowth of mantles took place at 369 ± 10 Ma on c. 1750 Ma cores of xenotime and monazite in the granitic gneiss. Allanite, occurring in textural equilibrium with peak metamorphic minerals, yields a CHIME age of 246 ± 15 Ma that is discriminably older than the polycrase (170 ± 6 Ma) and monazite (170 ± 3 Ma) ages of the post-tectonic granite. These chronological data suggest that some of the metasedimentary rocks in the belt formed through a single stage of metamorphism at c. 250 Ma from post-370 Ma sediments. Late Permian age signatures have also been reported from the Precambrian Gyeonggi and Yeongnam massifs that border the Okcheon metamorphic belt, and indicate that parts of the basement massifs and the metamorphic belt were affected by the same regional metamorphic event.     

Assessing the origin of old apparent ages derived by Pb stepwise leaching of vein-hosted epidote from Mount Isa,northwest Queensland,Australia

Epidote metasomatism affected large areas of tholeiitic metabasalts of the ~1,780 Ma Eastern Creek Volcanics in the Western Fold Belt of the Proterozoic Mount Isa inlier. Hydrothermal epidote generally occurs in quartz veins parallel to or boudinaged within the dominant S₂ fabrics which formed during the regional metamorphic peak at ~1,570 Ma associated with the Isan orogeny. Previously published stable isotopic and halogen data suggest that the fluids responsible for epidote formation are metamorphic in origin (with an evaporitic component). Application of the Pb stepwise leaching technique to the epidote does not separate radiogenic Pb⁴⁺ and common Pb²⁺, generating little spread in ²⁰⁶Pb/²⁰⁴Pb (between 16.0 and 30.5). The causes for this relatively low range are twofold: There is little radiogenic Pb in the epidotes (the most radiogenic steps account for <1 % of Pb released) and both Pb²⁺ and uranogenic Pb⁴⁺ substitute into the same site in the epidote crystal lattice. Consequently, age regressions using the Pb stepwise leaching data give ages between 150 and 1,500 myrs older than the host rocks and over 450 myrs older than the thermal metamorphic peak. These old ages are attributed to chemical inheritance from the host metabasalts, via radiogenic Pb release by breakdown of phases such as zircon, monazite, titanomagnetite, and ilmenite during metamorphism. This idea is supported by trace element data and chrondrite-normalized rare earth element patterns that are similar to both the metabasalts and epidotes (except for a variable Eu anomaly in the latter). Relatively high fO₂ during vein formation (Fe³⁺ dominates in the epidote crystal lattice) would allow the incorporation of Th⁴⁺ and exclusion of U⁶⁺ and would explain elevated Th/U ratios (up to 12) in epidote compared with the host metabasalts. Non-incorporation of U would explain the relatively low U/Pb ratios and non-radiogenic character of the epidote. This process may provide a source of metal for the small U deposits around Mount Isa and may also suggest a relationship between U mineralization and regional Cu mobilization during the Isan orogeny. Our work suggests that non-conventional geochronometers should be used only if additional geological information and geochemical data (e.g., mineral chemistry, trace elements) are available to evaluate any resulting age calculations.     

Radioactive quartz-pebble conglomerates from western margin of Bonai granite pluton,Sundargarh district,Orissa — A new find

Several radioactive quartz-pebble conglomerate (QPC) occurrences at the western margin of Archaean Bonai granite and overlying Iron Ore Group (IOG) rocks have recently been located over a total strike length of 8–10 km intermittently in a NE-SW to E-W trend with steep dips due north-west to north in parts of Sundargarh district of Orissa. The QPC samples have analysed up to 0.039% U₃O₈ and 0.035% ThO₂ with high concentration of Y (74 to 518 ppm), La(<100 to 880 ppm), Cr ( 126 to 633 ppm), Zr (137 to 1250 ppm) and Pb (31 to 581 ppm). Cellulose Nitrate (CN) film studies of few QPC samples indicated adsorbed uranium over goethite and infiltrated ferruginous material (limonite), secondary uranium as encrustation and fracture filling and discrete sub-rounded grains of monazite, zircon, allanite and rare xenotime in the matrix of QPC as radioactive phases. Higher content of Th over U, elevated concentration of Y and La in QPC eliminates the possibility of its low temperature product by epigenetic processes. Poor correlation of U with elements like Pb, Y, Zr, La and Cr can be explained due to surficial leaching of uranium from QPC after its deposition as reflected by adsorbed U over iron-oxides and low U/Th ratio in QPC in the area.     

Monazite and xenotime U–Th–Pb geochronology by ion microprobe: dating highly fractionated granites at Xihuashan tungsten mine, SE China

Zircon, monazite, and xenotime have proven to be valuable chronometers for various geological processes due to their commonly high-U–Th and low common Pb contents. However, zircons that have crystallized in highly fractionated granites often have such high-U contents that radiation damage can lead to scattered U–Pb ages when measured with secondary ion mass spectrometry (SIMS). In this study, monazite and xenotime were separated from a number of highly fractionated granites at the Xihuashan tungsten mine, Southeast China, for alternative dating methods by SIMS. For monazite analysis, obvious excess ²⁰⁴Pb signal (mainly from interference of ²³²Th¹⁴⁴Nd¹⁶O₂ ⁺⁺) was observed in high-Th (>2 wt%) monazite, which hinders ²⁰⁴Pb-based common Pb corrections. A ²⁰⁷Pb-based common Pb correction method was used instead. By employing power law relationships between Pb⁺/U⁺ versus UO₂ ⁺/U⁺, Pb⁺/Th⁺ versus ThO₂ ⁺/Th⁺ and suitable exponentials, monazites with ThO₂ contents in the range of ~3–19 % do not exhibit this matrix effect. Independent SIMS U–Pb ages and Th–Pb ages of three phases of Xihuashan granite samples were consistent with each other and yielded dates of 158.7 ± 0.7, 158.0 ± 0.7, and 156.9 ± 0.7 Ma, respectively. Xenotime does show marked matrix effects due to variations of U, Th, and Y [or total rare earth element (REE), referred as ΣREE hereafter] contents. Suitable correction factors require end-member standards with extremely high or low U, Th, and Y (or ΣREE) contents. No excess ²⁰⁴Pb was observed, indicating that the ²⁰⁴Pb-based common Pb correction method is feasible. Independent ²⁰⁷Pb/²⁰⁶Pb ages can be obtained, although multi-collector mode is necessary to improve precision. The main difficulties with dating xenotime are when high-Th (U) mineral inclusions are ablated. We can identify when this occurs, however, by comparing the measured UO₂ ⁺/U⁺ and ThO₂ ⁺/Th⁺ with those in xenotime standards. Three xenotime samples from the first phase of Xihuashan granite yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb date of 159.5 ± 4.4 Ma (MSWD = 1.0) and a ²⁰⁶Pb/²³⁸U date of 159.4 ± 0.9 Ma (MSWD = 1.6), which are consistent with monazite U–Pb and Th–Pb ages from the same granites. This study demonstrates that monazite and xenotime are better SIMS chronometers for highly fractionated granites than zircon, which can yield doubtful ages due to high-U contents.     

Petrogenesis of the Xihuashan Granite in Southern Jiangxi Province,South China: Constraints from Zircon U-Pb Geochronology,Geochemistry and Nd Isotopes

Mesozoic granitic intrusions are widely distributed in the Nanling region,South China.Yanshanian granites are closely connected with the formation of tungsten deposits.The Xihuashan granite is a typica...     

The effect of allanite inclusions on U–Pb step-leaching ages and Sm–Nd isotope systematics of garnet from the Ogcheon metamorphic belt,South Korea

The Ogcheon metamorphic belt consists primarily of metasedimentary and metavolcanic rocks that have experienced polyphase tectonometamorphism since the Neoproterozoic. Peak metamorphism reaching up to lower-amphibolite facies produced ubiquitous garnet porphyroblasts in pelitic and mafic schists. To determine the timing of their formation, step-leaching experiments were undertaken for five garnet fractions separated from pelitic and quartz-hornblende-garnet schists. The U–Pb ages from three samples are identical within 2σ errors, ranging from 291 ± 41 Ma to 276 ± 29 Ma. The quasi-linearity of leachates in ²³⁸U–²⁰⁶Pb and ²⁰⁸Pb–²⁰⁶Pb diagrams suggests that U and Pb are released from a single mineral phase and that minor chemical fractionation between U and Pb may have occurred during the leaching experiment. Deviations of residues and bulk garnet fractions from the linear trend are attributed to partial dissolution of refractory inclusions of detrital zircon. Th/U ratios of leachates are in the range of 3.4–12, much higher than those of pure garnet, and suggest the contribution of allanite. Negative relationships in the Sm–Nd isochron diagram and similar ¹⁴⁷Sm/¹⁴⁴Nd ratios between whole rock and garnet corroborate the influence of light rare earth element (LREE)-rich allanite on the Sm–Nd isotopic system. Simple mass-balance calculations indicate that only a trace amount (0.35 modal%) of allanite inclusions should govern the U–Th–Pb systematics of garnet. Petrographic evidence together with the consistency in U–Pb ages suggests that allanite is a product of prograde metamorphism. Thus, peak metamorphism responsible for the growth of allanite-bearing garnet porphyroblasts in the Ogcheon metamorphic belt is best estimated to be Early Permian.     

Age and pyrite Pb-isotopic composition of the giant Sukhoi Log sediment-hosted gold deposit, Russia

Sukhoi Log is one of the largest gold deposits in Russia (1100 t Au at 2.45 g/t). Like many other sediment-hosted gold deposits throughout the world, Sukhoi Log preserves textural, structural and geochemical evidence for multiple generations of Au enrichment and pyrite growth.The deposit is located in the Lena gold province of Siberia, on the edge of the Siberian Craton and occurs in the core of a recumbent anticline in a Neoproterozoic black shale and quartz-rich siltstone-sandstone turbidite succession. Temporal constraints on pyrite paragenesis at the deposit have been determined using laser ablation inductively coupled mass spectrometry (LA-ICPMS) measurements of U, Th and Pb isotopes in pyrite, monazite and zircon. LA-ICPMS age determinations on detrital zircons indicate the host rocks were deposited after 600 ± 10 Ma and derived from a mixture of Palaeoproterozoic and Neoproterozoic sources. The U, Th and Pb isotopic systematics indicate the cores of large monazite crystals, which predate obvious tectonic fabric development in the host rocks, began growing at 573 ± 12 Ma. The rims of the same monazite crystals formed at 516 ± 10 Ma, during peak metamorphism and deformation. Small monazite crystals also grew in the sedimentary rocks during the Devonian (374 ± 20 Ma) and the Carboniferous or Early Permian (288 ± 22 Ma), possibly in response to fluid movements triggered by synchronous granite intrusion in the area. Multi-collector and quadrupole LA-ICPMS Pb isotopic determination on pyrite, combined with overprinting criteria, show that the earliest (stratiform) Pb and Au-bearing pyrite formed prior to metamorphism—possibly during sedimentation or early diagenesis (575-600 Ma). Small Au-rich pyrite nodules preserved as cores to folded bedding-parallel pyrite-quartz veins probably grew during late diagenesis or early metamorphism. Large pyrite euhedra, which overgrow the strong axial planar cleavage in the host rocks, have more radiogenic Pb-isotopic compositions and formed either late during or after deformation. Framboidal pyrite that is overgrown by both the late diagenetic-early metamorphic and syn- to post-metamorphic pyrite has the most radiogenic Pb-isotopic composition suggesting exchange with radiogenic Pb in the matrix may have continued until late in the history of the deposit.The dating and Pb isotopes support a multistage origin for the gold deposit with Au first introduced during or prior to growth of the earliest stratiform pyrite and progressively re-concentrated (with or without addition of further gold) during later metamorphic events.     

Monazite U-Pb dating of staurolite grade metamorphism in pelitic schists

A study of the occurrence of and relations between rare-earth element (REE) minerals in pelitic schists indicates that monazite forms at or near the P and T of the staurolite isograd. Samples at staurolite grade from the Silurian Perry Mountain Formation in the Rumford quadrangle of Maine yield monazite in sufficient quantities to permit accurate dating of the metamorphic events forming the monazites. The bulk chemistry of the metapelites, as seen in the major element abundances and REE patterns, does not vary significantly across the study area. Thus the appearance and disappearance of REE phases is assumed to reflect changes in metamorphic grade. In a sample from the biotite zone, scanning electron microscope and microprobe studies show allanite and monazite intimately associated on a 10 m scale. The texture suggest that metastable detrital monazite breaks down, distributing its REE components to allanite. From samples below staurolite grade in which monazite is not present, our observations suggest that REEs are partitioned into allanite. At or near the staurolite isograd monazite forms as a metamorphic mineral, initiating its role as a geochronometer. Garnet-biotite geothermometry on samples at this grade from this and other studies places constraints on the minimum temperature necessary to form monazite: 525° C±25°C at 3.1±0.25 kbar. A total of 15 separates from nine schist samples ranging up to sillimanite grade have been dated. Each date is remarkably concordant, even though petrologic and textural studies by previous workers have shown that the rocks in the area have been affected by at least three metamorphic episodes. Calculations indicate insignificant Th disequilibrium in these monazites. The conditions associated with the metamorphic events suggest that monazite remains closed to lead loss provided that subsequent metamorphisms are at or below sillimanite grade. Two distinct metamorphic events are resolved, one at around 400 Ma and one at about 370 Ma. The latter was due to thermal effects of a nearby pluton that yields concordant monazite ages of 363 Ma. This work suggests that in addition to dating plutonism and high-grade metamorphism, monazite should be viewed as a reliable geochronometer for moderate metamorphism of pelitic schists.     

Ages and compositions of primary and secondary allanite from the Lala Fe–Cu deposit,SW China: implications for multiple episodes of hydrothermal events

Numerous Fe–Cu deposits in southwestern China form the Kangdian Iron-Oxide Copper-Gold (IOCG) metallogenic Province. These deposits have a close association of Fe-oxides and Cu-sulfides formed at different stages, which are possibly related to multiple hydrothermal events. In this paper, U–Pb dating and chemical analyses on allanite from different stages of the Lala deposit were used to constrain timing and origin of such events. Allanite occurs as disseminated grains or patches in Fe–Cu ores and is closely associated with chalcopyrite, molybdenite, calcite and minor titanite, postdating magnetite and apatite. High-resolution backscattered electronic (BSE) imaging, electron microprobe compositions and X-ray scanning profiles demonstrate that REE-rich primary allanite was replaced by later, relatively porous and REE-poor secondary allanite. Such a replacement was promoted by interaction between primary allanite and fluid fluxes infiltrating the minerals, following an exchange scheme of REE³⁺ + Fe²⁺ → Ca²⁺ + Al³⁺. The secondary allanite has higher Fe³⁺/(Fe³⁺+Fe²⁺) ratios and U contents, indicating involvement of relatively oxidized fluids during alteration. The alteration has also produced unidentified secondary REE minerals in fractures, indicating re-deposition of some of the removed REEs. The primary and secondary allanites are dated by in situ LA-ICP-MS technique and have U–Pb ages of 1,067 ± 41 Ma and 880–850 Ma, respectively. The ~1.07 Ga primary allanite was contemporaneous with the main Mo–Cu–LREE mineralization with a molybdenite Re–Os age of ~1.08 Ga. The 880–850 Ma secondary allanite is comparable with the Ar–Ar ages (890–830 Ma) of biotite from hosting schists and undeformed sulfide veins occurring throughout the Kangdian Province, suggesting that such an event was possibly syn-deformational and represents a younger hydrothermal event. Occurrences of both primary and secondary allanites suggest that the mineralization may have involved multiple tectonothermal events including the ~1.05–1.1 Ga intra-plate and subsequent 960–740 Ma arc magmatism in the Kangdian region.     

REE partitioning between monazite and garnet: Implications for petrochronology

Rare‐earth element and Y partitioning between garnet and monazite was measured in metamorphic rocks from western Norway to provide more confidence in tying monazite U/Th–Pb dates to P–T conditions recorded in garnet. A subset of samples has low‐Y garnet mantles and low‐Y monazite cores that gave Y‐partitioning temperatures similar to independently determined metamorphic temperatures. In combination with previously published data, these monazite–garnet pairs have temperature‐dependent partitioning of the HREE from Dy to Lu, and nonsystematic partitioning of the LREE from La–Gd. The temperature‐dependent partitioning must be considered when using HREE to assess which portions of garnet and monazite might have coexisted, but experiments are needed to place the dependence on a firm footing.     

Dating of Columbite-tantalite and Monazite from Pegmatites of the Kawadgaon–Challanpara Area,Bastar Craton,Central India

The results of geochronological studies on columbite-tantalite and monazite from the rare metal pegmatites of the Kawadgaon–Challanpara area in Bastar craton, central India are presented. Columbite-tantalite yielded U-Pb concordia upper intercept age of 1978±16 Ma (MSWD = 0.18). Radiogenic ²⁰⁷Pb*/²⁰⁶Pb* (T_7/6) ages on 4 out of 5 columbite-tantalite vary in a narrow range of 1903 to 2077 Ma and are similar to U-Pb age, whereas, one sample shows younger ²⁰⁷Pb*/²⁰⁶Pb*(T_7/6) age of 1728 Ma. Younger Pb-Pb age of 1744 ± 250 Ma (MSWD = 150) has also been indicated by these columbite-tantalite samples. Four out of five monazite samples define Pb-Pb errorchron age of 2050±370 Ma (MSWD = 165) and radiogenic ²⁰⁷Pb*/²⁰⁶Pb* (T_7/6) ages on 3 out of 5 monazites show a narrow range of 1983 to 2083 Ma. Other two samples show younger ²⁰⁷Pb*/²⁰⁶Pb*(T_7/6) ages as 1254 Ma and 1592Ma. Both monazite and columbite-tantalite indicate disturbance in Pb and U isotopic systematics as revealed by high MSWD. However, selected samples from both monazite and columbite-tantalite indicate age of their formation as c. 2000 Ma. Younger ages, i.e., 1254 to 1744 Ma are indicative of later geological disturbances. Reported age of c. 2000 Ma is comparable to Rb-Sr date of pegmatitic muscovite (1850-2330 Ma) from this area and is younger to intrusive granites of c. 2500 Ma. By analogy, therefore, it may be inferred that the age of the rare element mineralization may be ~2000 Ma old, and linked with younger granitic activity that spanned over the period from 2300 to 2100 Ma in the Bastar craton.     

Timescales of partial melting in the Himalayan middle crust: insight from the Leo Pargil dome, northwest India

The Leo Pargil dome (LPD) in northwest India exposes an interconnected network of pre-, syn-, and post-kinematic leucogranite dikes and sills that pervasively intrude amphibolite-facies metapelites of the mid-crustal Greater Himalayan sequence. Leucogranite bodies range from thin (5-cm-wide) locally derived sills to thick (2-m-wide) crosscutting dikes extending at least 100 m. Three-dimensional exposures elucidate crosscutting relations between different phases of melt injection and crystallization. Combined laser ablation inductively coupled plasma mass spectrometry U–Th/Pb geochronology and trace element analysis on well-characterized monazite grains from nineteen representative leucogranites yields a large, internally consistent data set of approximately 700 U–Th/Pb and 400 trace element analyses. Grain-scale variations in age correlate with trace element distributions and indicate semi-continuous crystallization of monazite from 30 to 18 Ma. The youngest U–Th/Pb ages in a given sample are consistent with the outcrop-scale crosscutting relations, whereas older ages within individual samples record inheritance from partially crystallized melt and source metapelites. U–Th/Pb isotopic and trace element data are incorporated into a model of melting within the LPD that involves (1) steady-state equilibrium batch melting of compositionally homogeneous metapelitic sources; (2) pulses of increased melt mobility lasting 1–2 m.y. resulting in segregation of melt from its source and amalgamation into mixed magmas; and (3) rapid emplacement and final crystallization of leucogranite bodies. Melt systems in the LPD evolved from locally derived, in situ melt in migmatitic source rocks into a vast network of dikes and sills in the overlying non-migmatitic host rocks.     

Fluid-mediated partial alteration in monazite: the role of coupled dissolution–reprecipitation in element redistribution and mass transfer

Monazite [(Ce,LREE,Th,U,Ca)(P,Si)O₄], with complex zoning in Th and other elements, is commonly observed in metamorphic and igneous rocks. The hypothesis that this alteration is a product of fluid-mediated element mass transfer has been tested in the piston-cylinder press (CaF₂ assembly, cylindrical graphite oven) at 1,000 MPa and 900°C and in cold seal autoclaves on a hydrothermal line at 500 MPa and 600°C. Experiments included a relatively homogeneous monazite-(Ce) (7–8 wt% ThO₂) from a heavy mineral sand plus a series of alkali-bearing fluids including 2N NaOH, 2N KOH, and Na₂Si₂O₅ + H₂O. Experiments were conducted using BSE imaging, EMP analysis, and both TEM and HRTEM. A subset of monazite grains from each experiment show evidence of partial alteration in the form of areas enriched in Th + Si with sharp curvilinear compositional boundaries extending from the grain rim into the monazite interior. These ThSiO₄-enriched textures are similar to those commonly seen in natural examples of metasomatised monazite in both magmatic and metamorphic rocks. In the Na₂Si₂O₅ + H₂O experiments, scarce inclusions of britholite formed in the altered monazite. The altered monazite is also characterised by strong depletion in Pb, Ca, and Y. Thorium and Si mobility, coupled with the formation of britholite inclusions, during partial alteration in the monazite grain is considered to be the product of fluid-aided coupled dissolution–reprecipitation as opposed to solid-state diffusion. Since other fluids, including NaCl and KCl brines, do not result in the formation of these textures, the experimental replication of ThSiO₄-enriched areas in the monazite strongly suggests that similar textures in monazite observed in nature are fluid induced, specifically by alkali-bearing fluids. If true, complex metasomatically induced textures in monazite could yield information concerning the nature of the fluid responsible for their formation as well as allow for the dating of the metasomatic event, presuming that all the original radiogenic Pb has been removed.     

海南石碌铁矿独居石的成因类型、化学定年及地质意义

海南石碌铁矿是我国最大的富赤铁矿矿床,同时伴生有钴、铜等多金属矿产。轴向北西-南东向的复式向斜是石碌铁、钴铜矿体的主要控矿构造,富铁矿和钴铜矿的形成与该褶皱变形及伴随的韧性剪切和高温塑性流动有着密切的关系。为获得该构造变形的年代学信息和证实构造变形对成矿物质的富集影响,本文开展了石碌铁矿近矿围岩—石碌群第六层透辉石透闪石岩中独居石的显微结构观察和电子探针化学Th-U-Pb定年(CHIME法)。显微结构观察发现独居石往往沿岩石面理定向分布,且具典型的球冠结构,表现为围绕独居石核部向外依次出现磷灰石、褐帘石、绿帘石同心环。电子探针分析结果表明这些独居石为Ce-La-Nd磷酸盐[(Ce,La,Nd,Th)PO4],具富钍独居石端元组分。ThO 2含量范围(0.78%~4.61%)、稀土特征以及独居石的产出特征均暗示了其为同构造变质成因。电子探针CHIME化学定年结果表明独居石的年龄变化范围为614~397Ma,并具有两个峰值年龄:即主峰值ca.455Ma和次峰值ca.564Ma。低的ThO 2(0.78%~1.65%),PbO(0.02%~0.04%)和CaO(0.50%~0.97%)含量,以及高的Th/U比值(23.06~53.11)暗示了构成ca.564Ma的独居石是早期剪切变形事件的产物。而在随后剪切变形过程中独居石在低角闪岩相变质条件下以及碱性变质流体诱导下发生了溶解-再沉淀,形成了具ca.455Ma年龄的补丁状成分区。该过程引起了U-Pb体系的局部重置,形成的独居石具有变化较大的ThO 2(0.92%~4.61%)、PbO(0.01%~0.08%)和CaO(0.28%~1.58%)含量范围以及Th/U值(24.83~52.86)。在剪切变形之后,早期变质成因的独居石在绿片岩相退变质作用过程中及富Ca、Fe、Si、Al流体参与的条件下,经不平衡反应形成了磷灰石-褐帘石-绿帘石球冠物,反应机制以独居石和球冠矿物间的元素扩散动力学为主。该反应暗示了REE、Y、Th等元素发生了迁移,并可能引起边部独居石的部分Pb丢失。结合华南的构造演化,年龄谱主峰值455Ma代表了与华南加里东造山运动有关的区域变质和动力变质作用事件年龄,是加里东运动在海南岛的响应;次峰值年龄564Ma对应着冈瓦纳泛非事件,暗示了华南在晚新元古代-早古生代与冈瓦纳大陆具有亲缘性,华南加里东运动引起陆内造山过程可能与冈瓦纳大陆的聚合碰撞事件有关。因此,晚新元古代-早古生代造山事件对海南岛构造演化历史具重要影响。此外,该构造运动使石碌群发生褶皱变形,伴随产生的变质流体使铁、钴铜成矿元素进一步活化和富集,对石碌铁、钴铜矿的富集有着重要影响。