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Metamorphism and partial melting of the lower crust is commonly assumed to cause depletion in heat producing elements (HPEs; K, U, Th). In the deep crust, volumetrically subordinate metasedimentary layers, which are source to crustal granites (sensu lato), host the majority of Th ± U, primarily within the REE + Th + U + Y phosphate mineral monazite. We examine the spatial and temporal distribution of Th within monazite grains in metasedimentary rocks from the lower crustal section of the Ivrea–Verbano Zone (Italy), using textural, compositional and geochronological data. We link this to outcrop and regional scale trends described by in‐field gamma‐ray spectrometry data (in‐field GRS) for the purpose of understanding how Th distribution is controlled by progressive metamorphism and partial melting. In‐field GRS data shows that the whole rock budget of Th does not change between granulite facies rocks and their unmelted equivalents but is significantly lower in rocks that have undergone more significant melt loss at ultra‐high temperature (UHT) conditions. Concurrently, the bulk Th budget of monazite increases with metamorphic grade to granulite facies conditions and is greatly reduced in UHT samples. Monazite geochronology returns dates mostly in the range 240–320 Ma with two main peaks at circa 290 and 270 Ma. Textural and chemical constraints indicate that these dates record the timing of pre‐peak to peak metamorphic conditions. Amphibolite facies monazite compositional zones are absent from granulite facies monazite, in contrast to examples from lower‐pressure terranes. This is consistent with the expanded stability of allanite relative to monazite with increasing pressure having an important role in determining the internal structure, composition and extent of inheritance of monazite in going from amphibolite facies to granulite facies rocks. We propose high‐pressure granulites should preserve less monazite inherited from amphibolite facies conditions than low‐pressure granulites. Monazite is preserved at all metamorphic grades and presents a mineralogical mechanism for retaining Th in residual deep crust during partial melting and after melt loss. [ABSTRACT FROM AUTHOR]

Thorium-Pb crystallization ages of hydrothermal monazites from the western, central and eastern Tauern Window provide new insights into Cenozoic tectonic evolution of the Tauern metamorphic dome. Growth domain crystallization ages range from 22.3 ± 0.6 Ma to 7.7 ± 0.9 Ma. Three major periods of monazite growth are recorded between ~ 22-19 (peak at 21 Ma), 19-15 (major peak at 17 Ma) and 13-8 Ma (major peaks at 12, 10 and 8 Ma), respectively interpreted to be related to prevailing N-S shortening, in association with E-W extension, beginning strike-slip movements, and reactivation of strike-slip faulting. Fissure monazite ages largely overlap with zircon and apatite fission tracks data. Besides tracking the thermal evolution of the Tauern dome, monazite dates reflect episodic tectonic movement along major shear zones that took place during the formation of the dome. Geochronological and structural data from the Pfitschtal area in the western Tauern Window show the existence of two cleft generations separated in time by 4 Ma and related to strike-slip to oblique-slip faulting. Moreover, these two phases overprint earlier phases of fissure formation. [ABSTRACT FROM AUTHOR]

Most monazite reference materials (RMs) for in situ U‐Pb geochronology are rich in Th; however, many hydrothermal ore deposits contain monazite that is low in trace element contents, including Th, U and Pb. Because of potential problems with matrix effects and the lack of appropriate matrix‐matched RMs, such variations can bias dating of hydrothermal deposits. In this paper, we describe a polycrystalline low‐U and low‐Th Diamantina monazite from the Espinhaço Range, SE Brazil. It has a U‐Pb ID‐TIMS weighted mean 207Pb*/235U ratio of 0.62913 ± 0.00079, 206Pb*/238U of 0.079861 ± 0.000088 and 207Pb*/206Pb* of 0.057130 ± 0.000031, yielding a weighted mean 206Pb*/238U date of 495.26 ± 0.54 Ma (95% c.l.). In situ dates acquired with different methods (LA‐(Q, SF, MC)‐ICP‐MS and SIMS) are within uncertainty of the ID‐TIMS data. U‐Pb LA‐(Q, MC)‐ICP‐MS runs, using Diamantina as a primary RM, reproduced the ages of other established RMs within < 1% deviation. The LA‐MC‐ICP‐MS analyses yielded homogeneous Sm‐Nd isotopic compositions (143Nd/144Nd = 0.511427 ± 23, 2s; 147Sm/144Nd = 0.1177 ± 13, 2s) and εNd(495 Ma) of −18.7 ± 0.5 (2s). SIMS oxygen isotope determinations showed measurement reproducibility better than ± 0.3‰ (2s), confirming Diamantina's relative homogeneity at test portion masses below 1 ng. [ABSTRACT FROM AUTHOR]

The timing of crustal melting and cooling has been investigated across the migmatites of the Greater Himalayan Crystalline Complex (GHC) in the Nyalam region, central Himalaya. Monazite U-Pb ages vary from 32 to 14 Ma and are linked to metamorphic conditions on the basis of monazite internal zoning, mineral inclusions, and changes in heavy rare earth element and Y composition. Metamorphic temperatures were estimated by Zr-in-rutile thermometry and cooling rates were further constrained by rutile U-Pb ages. The results reveal two distinct blocks within the GHC of the Nyalam region. The upper GHC experienced higher peak metamorphic temperatures (730-750°C) and a higher degree of melting (15-25%). Partial melting was dominated by muscovite dehydration melting, which lasted from ~32 to 25 Ma, possibly until ~20 Ma. The lower GHC experienced lower peak metamorphic temperatures (640-675°C) and a lower degree of melting (0-10%) mainly via H2O-saturated melting from 19 to 16 Ma. At different times, both upper and lower blocks experienced initial slow cooling (rates 35 ± 8 and 10 ± 5°C Ma-1, respectively) followed by rapid cooling (100 ± 20°C Ma-1). The documented diachronous metamorphism implies the presence of the 'High Himalayan Thrust' that was active at ~25-16 Ma within the GHC of the central Himalaya. Different degrees and durations of partial melting in the investigated section suggest that a channel flow process dominated the exhumation of the upper GHC migmatites at 25-16Ma, whereas a critical taper process dominated the exhumation of the relatively lower-grade lower GHC rocks and cooled upper GHC migmatites at 16-10 Ma. We suggest that propagating thrusts along large tectonic boundaries together with low-viscosity lateral crustal flow could contribute to exhumation of high-grade metamorphic rocks in the Himalaya and other similar collisional orogens. [ABSTRACT FROM AUTHOR]

Monazite forms at sub-solidus conditions in a variety of metamorphic rocks and has been proven to be reactive to fluids, and thus is a potential monitor of fluid–rock interaction. As monazite can preserve multiple growth zones, in order to explore the potential of monazite as a fluid tracer in metamorphic conditions, microbeam analysis is required. We performed oxygen isotope analysis of monazite using the SHRIMP ion microprobe and newly characterized standards, for which we obtained laser fluorination δ 18 O values (USGS-44069 monazite 7.67 ± 0.26‰ and Itambé monazite 0.46 ± 0.20‰). Reproducibility of δ 18 O ion microprobe analyses for USGS-44069 and Itambé monazites is in the order of 0.4–0.6‰, standard deviation at 95% c.l., similar to what is routinely obtained for silicates. This reproducibility is comparable to that of the analyses of experimental P-rich glasses, which are assumed to be homogeneous and free of geological imperfections. The variable composition of natural monazite has the potential to produce matrix effects during ion microprobe measurements. Monazite grains from the Malagasy syenite (Madagascar) and the Dora Maira whiteschists (Italy) display a scatter in δ 18 O values that show a negative correlation with Th content and is related to the huttonite [ThSiO 4 ] and cheralite [CaTh(PO 4 ) 2 ] substitutions in monazite. The matrix effect on oxygen isotope measurements can be significant and is estimated to produce a shift in δ 18 O of circa − 0.85 or − 1.9‰ for every 10 wt.% Th introduced by the huttonite and cheralite components, respectively. Corrections for this matrix effect are proposed on the basis of the natural samples investigated. Oxygen isotope fractionation factors for monazites of different compositions, cheralite and huttonite were calculated with the increment method. The results suggest that the substitution of trivalent LREE by tetravalent Ce and Th results in consistent enrichment of 18 O in the monazites, whereas the substitution of tetravalent Th by divalent Ca results in the depletion of 18 O in cheralite-rich monazite. Monazites from high-grade metasediments (Mount-Stafford, Central Australia) preserve inherited cores, but are homogeneous in oxygen composition. This suggests that diffusion may efficiently erase the oxygen isotope signature in monazite that experienced ~ 800 °C metamorphism. [ABSTRACT FROM AUTHOR]

The Kokchetav complex in Kazakhstan contains garnet-bearing gneisses that formed by partial melting of metasedimentary rocks at ultrahigh-pressure (UHP) conditions. Partial melting and melt extraction from these rocks is documented by a decrease in KO and an increase in FeO + MgO in the restites. The most characteristic trace element feature of the Kokchetav UHP restites is a strong depletion in light rare earth elements (LREE), Th and U. This is attributed to complete dissolution of monazite/allanite in the melt and variable degree of melt extraction. In contrast, Zr concentrations remain approximately constant in all gneisses. Using experimentally determined solubilities of LREE and Zr in high-pressure melts, these data constrain the temperature of melting to ~1,000 °C. Large ion lithophile elements (LILE) are only moderately depleted in the samples that have the lowest U, Th and LREE contents, indicating that phengite retains some LILE in the residue. Some restites display an increase in Nb/Ta with respect to the protolith. This further suggests the presence of phengite, which, in contrast to rutile, preferentially incorporates Nb over Ta. The trace element fractionation observed during UHP anatexis in the Kokchetav gneisses is significantly different from depletions reported in low-pressure restites, where generally no LREE and Th depletion occurs. Melting at UHP conditions resulted in an increase in the Sm/Nd ratio and a decoupling of the Sm-Nd and Lu-Hf systems in the restite. Further subduction of such restites and mixing with mantle rocks might thus lead to a distinct isotopic reservoir different from the bulk continental crust. [ABSTRACT FROM AUTHOR]

Dating metamorphism in external domains of orogenic belts is a challenging task due to the few chronometers available and common inheritance or disturbance of the isotopic systems at low-temperature metamorphic conditions. In the external domains of the Central Alps (Urseren zone), the occurrence of distinct populations of monazite and allanite in one single outcrop offers a unique chance to evaluate the potential of these two REE-minerals to date successive metamorphic stages within a collisional cycle. The studied outcrop (locality of Garvera) exposes Mesozoic metasediments that were metamorphosed under greenschist facies conditions (T<450°C). Under these conditions, REE-mineral grains (or domains) are typically small (~5–50μm), and in this case also occur as porphyroblast in pelites at the contact with carbonate layers or veins (allanite grains >200μm). Based on the texture and mineral assemblages, REE-mineral growth conditions were attributed to be detrital (Mnz1), syn-kinematic (Mnz2 and Aln1), post-kinematic at the thermal peak (Aln2) and retrograde during late-stage deformation (Mnz3). To constrain temporally the successive crystallisation conditions, REE-minerals have been dated using SHRIMP ion microprobe. U–Th–Pb analyses show that the Alpine REE-minerals have a high Th/U and while most of the 206Pb is non-radiogenic (f 206 that can reach up to 99% for allanite), Th–Pb ages can be obtained using 206Pbc-corrected Th–Pb isochron. The age of different generations of allanite and monazite constrains the timing of successive collisional stages: for the prograde accretion at 22.5±1.5Ma (Mnz1 and Aln1), thermal peak at 19.3±2.0Ma (Aln2) and late stage hydrothermal veining at 13.6±1.4Ma (Mnz3). These novel geochronological data confirm the regional diachroneity through the Central Alps with younger ages (burial, thermal peak and exhumation) toward the external domains. The similarity between metamorphic ages of samples taken in the Urseren zone and the nearby crystalline massifs suggests that the metasedimentary cover of the Urseren zone remained closely juxtaposed to the external crystalline basement during the Alpine collision cycle. The monazite age of the late stage vein testifies for active tectonic deformation at 13.6±1.5Ma, which likely played a crucial control on the exhumation of the external massifs in the Middle Miocene. [ABSTRACT FROM AUTHOR]

Abstract: The Main Central Thrust Zone (MCTZ) is a key tectonic feature in the architecture of the Himalayan chain. In the Arun valley of the eastern Nepal Himalaya, the MCTZ is a strongly deformed package of amphibolite- to granulite-facies metapelitic schist and granitic orthogneiss. This package is tectonically interposed between the underlying, low-grade, Lesser Himalaya sequences and the overlying, high-grade and locally anatectic, Higher Himalayan Crystallines (HHC). The MCTZ is characterized by a well documented inverted metamorphism from the Grt–Bt zone, across the Ky-in, St-in and -out, Kfs-in, Ms-out and Sil-in isograds. Partial melting with local occurrence of migmatitic segregations has been rarely reported from the highest structural levels of the MCTZ. While it is widely accepted that thrusting along the MCT occurred during the Miocene, geochronological data constraining the timing of crustal anatexis in the upper portion of the MCTZ are still lacking. In order to understand the link between partial melting in the MCTZ and the Miocene activation of the MCT, we present the P–T–time evolution of a kyanite-bearing anatectic gneiss occurring at the highest structural levels of the MCTZ, along the Arun–Makalu transect (eastern Nepal). Microstructural observations combined with P–T pseudosection analysis show that dehydration partial melting occurred in the kyanite-field. After reaching peak conditions at about 820°C, 13kbar, the studied sample experienced decompression accompanied by cooling down to 805°C, 10kbar, which caused in situ melt crystallization. SHRIMP monazite and zircon geochronology provides evidence that the anatexis affecting the upper portion of the MCTZ occurred during Early Oligocene (∼31Ma). These results demonstrate that in the upper MCTZ, at least in the eastern Himalaya, crustal anatexis was earlier than, and not a consequence of, decompression linked to exhumation along the MCT. [Copyright &y& Elsevier]

Abstract: We present a petrological and geochronological study of “silvery micaschists” that crop out in the northern side of the Gran Paradiso Massif, Western Alps, with the aim of constraining P–T conditions and age of the Alpine high-pressure metamorphism. The studied “silvery micaschists”, which are the products of metasomatic alteration of granitoids along ductile shear zones, consist of Mg-chlorite, talc, chloritoid, minor phengite, and accessory minerals. Microstructural relationships indicate the following prograde sequence in the growth of U–Th bearing accessory minerals: florencite → monazite → allanite. Thermobarometric calculations indicate that the Mg-chlorite+talc+chloritoid+allanite peak assemblage was stable at P =1.9–2.7 GPa and T =515–600 °C, while monazite formed earlier at pressures over 2.0 GPa. SHRIMP dating of allanite yielded 33.7±1.6 Ma, interpreted as the age of the high-pressure metamorphic peak. Prograde monazite yielded an age of 37.4±0.9 Ma, implying a minimum duration of ∼2–4 Ma for the Alpine subduction event. Combining our ages with previous constraints, it can be concluded that the initial exhumation of the Gran Paradiso Massif occurred at a fast rate higher than 2 cm/year, and slowed down to ∼1 cm/year in the final stages. In a regional context, the new geochronological data align the subduction of the Gran Paradiso Massif with the other Internal Crystalline Massifs of the Western Alps. This supports a subduction model marked by alternating compressional events, related to the accretion of continental terranes, and extensional events, related to the episodic retreat of subduction zone hinges. [Copyright &y& Elsevier]

The formation, age and trace element composition of zircon and monazite were investigated across the prograde, low-pressure metamorphic sequence at Mount Stafford (central Australia). Three pairs of inter-layered metapelites and metapsammites were sampled in migmatites from amphibolite-facies (T ∼600°C) to granulite-facies conditions (T ∼800°C). Sensitive high-resolution ion microprobe U–Pb dating on metamorphic zircon rims and on monazite indicates that granulite-facies metamorphism occurred between ∼1795 and 1805 Ma. The intrusion of an associated granite was coeval with metamorphism at 1802 ± 3 Ma and is unlikely to be the heat source for the prograde metamorphism. Metamorphic growth of zircon started at T ∼750°C, well above the pelite solidus. Zircon is more abundant in the metapelites, which experienced higher degrees of partial melting compared with the associated metapsammites. In contrast, monazite growth initiated under sub-solidus prograde conditions. At granulite-facies conditions two distinct metamorphic domains were observed in monazite. Textural observations, petrology and the trace element composition of monazite and garnet provide evidence that the first metamorphic monazite domain grew prior to garnet during prograde conditions and the second in equilibrium with garnet and zircon close to the metamorphic peak. Ages from sub-solidus, prograde and peak metamorphic monazite and zircon are not distinguishable within error, indicating that heating took place in less than 20 Myr. [ABSTRACT FROM AUTHOR]

Abstract: We report the results of monazite/melt partitioning experiments conducted in the piston-cylinder apparatus at 10–50kbar and 750–1200°C, using a synthetic granite mix with approximately 10wt.% H 2 O and doped with trace-elements in proportions corresponding to the composition of monazite. Monazite was produced in all experiments, generally in the form of small grains. Electron microprobe and laser ablation-ICP-MS analyses were carried out on the resulting “monazite–melt” mixes from these experiments, and the composition of the crystallized monazite calculated using regression analysis. The concentrations of LREE and Th in the melts coexisting with monazite increase sharply with increasing temperature. Monazite solubility decreases by 35–40% as pressure increase from 10 to 30kbar. Monazite solubility in granitic melts with an Alumina Saturation Index above 0.85 and FeO + CaO + MgO < 3wt.% can be described by the following equation: Where H 2 O is in weight percent, T is in Kelvin, P in kbar and ∑ LREE is the sum of La–Sm in ppm; X mnz LREE is the molar ratio of LREE to the sum of all cations (REE, Th, U) in monazite. REE, Th, U, Y, V and As partition into monazite, whereas other trace elements (Li, Be, B, Sc, Ti, Mn, Sr, Zr, Nb, Ba, Hf, Ta and Pb) have monazite/melt partition coefficients less than unity. Monazite shows the greatest preference for LREE from La to Nd, with a progressive decrease in partition coefficients for Sm and the HREE. The partition coefficients for Th are 30% higher than those for the LREE, and Th/LREE ratios are independent of pressure and temperature. Partition coefficients for U are 4–23 times lower than for the LREE. The new experimental data provide a numerical basis for modeling the behavior of LREE, Th and U during fractional crystallization of granitic magmas, as well as the melting in the presence of monazite, both within the continental crust, and in subduction zones. [Copyright &y& Elsevier]

Abstract: Monazite is a common accessory mineral in various metamorphic and magmatic rocks, and is widely used for U–Pb geochronology. However, linking monazite U–Pb ages with the PT evolution of the rock is not always straightforward. We investigated the behaviour of monazite in a metasedimentary sequence ranging from greenschist facies phyllites into upper amphibolites facies anatectic gneisses, which is exposed in the Eocene Chugach Metamorphic Complex of southern Alaska. We investigated textures, chemical compositions and U–Pb dates of monazite grains in samples of differing bulk rock composition and metamorphic grade, with particular focus on the relationship between monazite and other REE-bearing minerals such as allanite and xenotime. In the greenschist facies phyllites, detrital and metamorphic allanite is present, whereas monazite is absent. In lower amphibolites facies schists (~550–650°C and ≥3.4kbar), small, medium-Y monazite is wide-spread (Mnz1), indicating monazite growth prior and/or simultaneous with growth of garnet and andalusite. In anatectic gneisses, new low-Y, high-Th monazite (Mnz2) crystallised from partial melts, and a third, high-Y, low-Th monazite generation (Mnz3) formed during initial cooling and garnet resorption. U–Pb SHRIMP analysis of the second and third monazite generations yields ages of ~55–50Ma. Monazite became unstable and was overgrown by allanite and/or allanite/epidote/apatite coronas within retrograde muscovite- and/or chlorite-bearing shear zones. This study documents polyphase, complex monazite growth and dissolution during a single, relatively short-lived metamorphic cycle. [Copyright &y& Elsevier]

The prograde sequence of rare earth minerals recorded in metapelites during regional metamorphism reveals a series of irreversible reactions among silicates and phosphates. In individual samples from the northern Lepontine (Central Alps), allanite is partly replaced by monazite at 560-580 °C. Relic allanite retains its characteristic growth zoning acquired at greenschist facies conditions (430-450 °C). Coexisting monazite and allanite were dated in situ to delimit in time successive stages of the Barrovian metamorphism. In situ sensitive high-resolution ion microprobe (SHRIMP) U-Tb-Pb dating of allanite (31.5 ± 1.3 and 29.2 ± 1.0 Ma) and monazite (18.0 ± 0.3 and 19.1 ± 0.3 Ma) constrains the time elapsed between 430-450 °C and 560-580 °C, which implies an average heating rate of 8-15 °C/m.y. Combined with new fission track ages (zircon, 10-9 Ma; apatite, 7.5-6.5 Ma), metamorphic rates of the entire orogenic cycle, from prograde to final cooling, can be reconstructed. [ABSTRACT FROM AUTHOR]

Abstract: The Central Zone of the Limpopo Belt (South Africa) underwent high-grade metamorphism at ∼2.7–2.5 and ∼2.03 Ga. Quartz-rich, garnet-, cordierite-, biotite- and orthoamphibole-bearing, feldspar-free gneisses from the western Central Zone reached granulite-facies conditions (∼800 °C at ∼8–10 kbar) followed by decompression. Garnet from one such sample shows significant zonation in trace elements but little zonation in major elements. Zoning patterns suggest that the early prograde breakdown of REE-rich accessory phases contributed to the garnet trace element budget. Monazite from the sample yields a SHRIMP weighted mean 207Pb–206Pb age of 2028±3 Ma, indistinguishable from a SHRIMP zircon age of 2022±11 Ma previously measured on metamorphic overgrowths on ∼2.69 Ga igneous zircon cores. New zircon and monazite formed before, or at, the metamorphic peak, and occur as inclusions in garnet. Monazite appears to have formed through the breakdown of early allanite±xenotime±apatite. Trace element zoning patterns in garnet and the age of accessory phases are most consistent with a single tectonometamorphic event at ∼2.03 Ga. The plagioclase and K-feldspar-free composition of the garnet–cordierite–orthoamphibole gneisses requires open system processes such as intense hydrothermal alteration of protoliths or advanced chemical weathering. In the studied sample, the ∼2.69 Ga igneous zircons show a prominent negative Eu anomaly, suggesting equilibrium with plagioclase, or plagioclase fractionation in the precursor magma. In contrast, the other minerals either show small negative (∼2.03 Ga monazite), no (∼2.02 Ga zircon and garnet) or positive Eu anomalies (orthoamphibole). This suggests that the unusual bulk compositions of these rocks were set in after ∼2.69 Ga but before the peak of the ∼2.03 Ga event, most probably while the protoliths resided at shallow or surficial crustal levels. [Copyright &y& Elsevier]