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Jean-louis Paquette - One of the best experts on this subject based on the ideXlab platform.

  • Syn-deformation fluid-assisted growth of Monazite during renewed high-grade metamorphism in metapelites of the Central Rhodope (Bulgaria, Greece)
    Chemical Geology, 2014
    Co-Authors: Amélie Didier, Valérie Bosse, Pierre Gautier, Jean-louis Paquette, Z. Cherneva, M. Georgieva, Ianko Gerdjikov
    Abstract:

    We present textural, chemical and U-Th-Pb-age data on Monazites from garnet-kyanite (Grt-Ky) metapelites, from the Chepelare Shear Zone (Bulgaria) and the Nestos Shear Zone (Greece), in the Central Rhodope. Samples from both locations have experienced two stages of high temperature metamorphism during Alpine times. The first event involved mid-Mesozoic granulite facies dehydration melting. The second event involved mid- Cenozoic lower-grade fluid-assisted partial melting. The latter is well expressed in adjacent felsic rocks but had limited impact on the Grt-Ky metapelites. Most samples display evidence for strong ductile shearing in the presence of fluids.Monazite is present in the highly foliated matrix and as inclusions in garnet and kyanite. Unlike the inclusions, matrix Monazites display features of fluid-assisted dissolution-recrystallization. Y-poor domainswith U-Th-Pb ages of between ca. 115 and 165 Ma represent the largest part of the grains. Y-rich domains with mid- Cenozoic ages occur as rims, or as small satellite grains surrounding the Mesozoic grains. The Cenozoic Monazite domains crystallized at the expense of the Mesozoic ones and simultaneously incorporated Y provided by the fluid-assisted resorption of garnet. An age of ca. 36Ma is obtained for the samples of both shear zones, interpreted as dating the main episode of Monazite growth during the Cenozoic. Similar ages exist for the crystallization of leucosomes and pegmatites in the adjacent migmatitic gneisses, indicating that the fluids responsible for the precipitation of the Cenozoic Monaziteswere probably released during the crystallization of nearby anatectic melts. Together with associated rutile and biotite, many newly grown Monazites show a preferred orientation paralleling the matrix foliation. This supports the hypothesis of dynamic dissolution-precipitation as an efficient mean to promote renewed Monazite crystallization during ductile deformation of the host rock at ca. 36 Ma.

  • The role of fluids in the Monazite record during successive partial melting events: a textural, chemical and in situ dating study in Grt-Ky gneisses of the Central Rhodope (Bulgaria, Greece)
    Mineralogical Magazine, 2012
    Co-Authors: Amélie Didier, Valérie Bosse, Pierre Gautier, Zlatka Cherneva, Milena Georgieva, Jean-louis Paquette, Ianko Gerdjikov
    Abstract:

    Monazite is considered to be resistant to diffusive Pb loss at high temperatures and thus, it is particularly adapted to record various stages during a sequence of high-temperature geological events. This study focuses on Grt-Ky gneisses from the lower part of the metamorphic pile in the central part of the Rhodope Metamorphic Complex, in the areas of Chepelare (Bulgaria) and Sidironero (Greece). The outcrops of both regions have experienced a polycyclic evolution during Alpine times, with at least two stages of high temperature metamorphism. According to P-T estimates, the first event involved granulite facies dehydration melting that produced peritectic garnet and kyanite together with a K-rich melt. The second event relates to widespread fluid-assisted partial melting. The latter is well known in adjacent rocks, where it is dated at ~36-50 Ma, but is poorly expressed in our samples, which well preserve the early granulite facies assemblages. Monazite is present in all samples, included in early porphyroblasts such as garnet and kyanite or in the matrix. Matrix Monazites are associated with white mica, rutile and biotite (Greek part) and sillimanite and biotite (Bulgarian part). Matrix Monazite grains show fluid-assisted dissolution-recrystallisation features with pronounced Y-zoning correlated to age domains. Y-poor domains, dated by LA-ICPMS method (208Pb/232Th ages) between 130 and 155 Ma, represent the largest part of each grain, while Y-rich domains, dated between 40 and 50 Ma, occur either as thin discontinuous rims (< 15 μm) or as small single grains surrounding the Mesozoic grains or filling white mica cleavages. The low P and REE-content of the surrounding minerals suggests that the Cenozoic domains essentially crystallized at the expense of the Mesozoic domains. In addition, garnet being the Y-richest mineral in the samples, its fluid-assisted resorption is the most likely mean to provide Y involved in the Cenozoic domains. The origin of the fluid is not clearly defined: external fluid infiltration or fluid produced by the recrystallisation of H2O-bearing minerals. Regardless, this fluid interaction was responsible for the partial dissolution of Mesozoic Monazite grains, as well as for garnet resorption, and the precipitation of newly-formed Y-rich Monazite during mid-Cenozoic times.

  • Low-temperature alteration of Monazite: Fluid mediated coupled dissolution-precipitation, irradiation damage, and disturbance of the U-Pb and Th-Pb chronometers
    Chemical Geology, 2012
    Co-Authors: Anne-magali Seydoux-guillaume, Jean-marc Montel, Emilie Janots, Valérie Bosse, Bernard Bingen, Philippe De Parseval, Jean-louis Paquette, Richard Wirth
    Abstract:

    Low-temperature alteration of Monazite is documented in three centimeter-sized Monazite crystals from Norway (Arendal), Madagascar (Ambato), and Sri Lanka. The three crystals have different chemical compositions, especially in their U, Th, Y and Pb contents and have 208Pb/232Th ages ranging from 491 to 900 Ma. Optical microscope (OM), Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) images and electron microprobe analyses (EPMA) show that all three preserve a similar patchy texture. This texture is interpreted as resulting from an alteration reaction in which unaltered Monazite (Mnz1) reacts to form a secondary, Th-U(Y)-depleted, high-Th/U, Monazite (Mnz2) accompanied by thorite/huttonite (ThSiO4), thorianite (ThO2) and xenotime (YPO4), the proportions of which are dependent upon the initial composition of the Monazite (Mnz1). Images reveal variably intense internal fracturing, with cracks filled with Th-rich ± Fe-rich phases. Monazite-xenotime thermometry demonstrates that the pristine Monazites (Mnz1) interacted with a low-temperature fluid. The alteration process is interpreted to follow a mechanism of fluid-mediated coupled dissolution-precipitation. Chemical dating with the electron microprobe shows no Th-U-Pb age differences between primary and secondary Monazites, except in the case of the Ambato Monazite, in which altered domains yield older (750 Ma) apparent ages than the pristine Mnz1 domains. U-Pb and Th-Pb isotope dating using LA-ICP-MS yields ages consistent with electron probe dates for pristine Mnz1 zones. However, disturbance of these systems in the altered Monazite domains leads to variable age results for these, depending on sample. In the case of Sri Lanka and Arendal, only 208Pb/232Th dates provide a reasonable estimate of the age of alteration, which are constrained to be 450 and 864 Ma, respectively. U/Pb systems are disturbed due to common Pb contamination (up to 40%) and U fractionation relative to Th during alteration, responsible for depletion of U in altered Monazites (and increase of Th/U). In contrast, for the Ambato Monazite, both the U-Pb and Th-Pb systems were affected and yield inconsistent older dates for altered zones. This is attributed to significant common Pb contamination (up to 80%), which affects all Pb isotopes and explains why electron probe ages are erroneous. Th-U-silicate contamination during measurement, resulting from the presence of a numerous nano-phases and nano-fractures filled with Th-U-silicates that are visible only under TEM, also contributes to the anomalously old ages for these disturbed (Mnz2) domains. These results demonstrate the important role of radiation damage effects, in particular swelling-induced fracturing, and the essential role of porosity and cracks, which allow fluid (charged with elements) migration through Monazite during low-temperature alteration.

  • high resolution 5 μm u th pb isotope dating of Monazite with excimer laser ablation ela icpms
    Chemical Geology, 2007
    Co-Authors: Jean-louis Paquette, M. Tiepolo
    Abstract:

    Abstract Monazite [(LREE)PO 4 ], a common accessory mineral in magmatic and metamorphic rocks, is complementary to zircon in U–Th–Pb geochronology. Because the mineral can record successive growth phases it is useful for unravelling complex geological histories. A high spatial resolution is required to identify contrasted age domains that may occur at the crystal-scale. Bulk mineral techniques such as ID-TIMS, applied to single Monazite grains recording multiple overgrowths or isotope resetting can result in partly scattered discordant analytical points that produce inaccurate intercept ages. Laser ablation (LA)-ICPMS has been demonstrated to be a useful technique for U–Th–Pb dating of zircons, and this study tests its analytical capabilities for dating Monazite. A sector field high resolution ICPMS coupled with a 193 nm ArF excimer laser ablation microprobe is capable of achieving a high spatial resolution and producing stable and reliable isotope measurements. The U–Th–Pb systematic was applied to Monazite grains from several samples: a lower Palaeozoic lens from high-grade terrains in Southern Madagascar, Neogene hydrothermal crystals from the Western Alps, a Palaeoproterozoic very high temperature granulite from central Madagascar and a Variscan leucogranite from Spain, directly on a polished thin section. The major aim was to compare and/or reproduce TIMS and EMP ages of Monazite from a variety of settings and ages. The three independent 206 Pb/ 238 U, 207 Pb/ 235 U and 208 Pb/ 232 Th ratios and ages were calculated. Isotope fractionation effects (mass bias, laser induced fractionation) were corrected using a chemically homogeneous and U–Pb concordant Monazite as external standard. This study demonstrates that excimer laser ablation (ELA)-ICPMS allows U–Th–Pb dating of Monazite with a high level of repeatability, accuracy and precision as well as rapidity of analysis. A spatial resolution almost comparable to that of EMP in terms of crater width (5 μm) produced precise 208 Pb/ 232 Th, 206 Pb/ 238 U and 207 Pb/ 235 U ratios for dating Palaeozoic to Precambrian Monazites. The advantages of (ELA)-ICPMS isotope dating are precision, accuracy and the ability to detect discordance. In the case of late Miocene hydrothermal Monazites from the Alps, a larger spot size of 25 μm diameter is required, and precise and accurate ages were obtained only for 208 Pb/ 232 Th systematics. Results from the Variscan granite show that in situ U–Th–Pb dating of Monazites with (ELA)-ICPMS is possible using a 5 μm spot directly on thin sections, so that age data can be placed in a textural context.

  • High resolution (5 μm) U–Th–Pb isotope dating of Monazite with excimer laser ablation (ELA)-ICPMS
    Chemical Geology, 2007
    Co-Authors: Jean-louis Paquette, M. Tiepolo
    Abstract:

    Monazite [(LREE)PO4], a common accessory mineral in magmatic and metamorphic rocks, is complementary to zircon in U–Th–Pb geochronology. Because the mineral can record successive growth phases it is useful for unravelling complex geological histories. A high spatial resolution is required to identify contrasted age domains that may occur at the crystal-scale. Bulk mineral techniques such as ID-TIMS, applied to single Monazite grains recording multiple overgrowths or isotope resetting can result in partly scattered discordant analytical points that produce inaccurate intercept ages. Laser ablation (LA)-ICPMS has been demonstrated to be a useful technique for U–Th–Pb dating of zircons, and this study tests its analytical capabilities for dating Monazite. A sector field high resolution ICPMS coupled with a 193 nm ArF excimer laser ablation microprobe is capable of achieving a high spatial resolution and producing stable and reliable isotope measurements. The U–Th–Pb systematic was applied to Monazite grains from several samples: a lower Palaeozoic lens from high-grade terrains in Southern Madagascar, Neogene hydrothermal crystals from the Western Alps, a Palaeoproterozoic very high temperature granulite from central Madagascar and a Variscan leucogranite from Spain, directly on a polished thin section. The major aim was to compare and/or reproduce TIMS and EMP ages of Monazite from a variety of settings and ages. The three independent 206Pb/238U, 207Pb/235U and 208Pb/232Th ratios and ages were calculated. Isotope fractionation effects (mass bias, laser induced fractionation) were corrected using a chemically homogeneous and U–Pb concordant Monazite as external standard. This study demonstrates that excimer laser ablation (ELA)-ICPMS allows U–Th–Pb dating of Monazite with a high level of repeatability, accuracy and precision as well as rapidity of analysis. A spatial resolution almost comparable to that of EMP in terms of crater width (5 μm) produced precise 208Pb/232Th, 206Pb/238U and 207Pb/235U ratios for dating Palaeozoic to Precambrian Monazites. The advantages of (ELA)-ICPMS isotope dating are precision, accuracy and the ability to detect discordance. In the case of late Miocene hydrothermal Monazites from the Alps, a larger spot size of 25 μm diameter is required, and precise and accurate ages were obtained only for 208Pb/232Th systematics. Results from the Variscan granite show that in situ U–Th–Pb dating of Monazites with (ELA)-ICPMS is possible using a 5 μm spot directly on thin sections, so that age data can be placed in a textural context

Christian Nicollet - One of the best experts on this subject based on the ideXlab platform.

  • Eocene ultra-high temperature (UHT) metamorphism in the Gruf complex (Central Alps): constraints by LA-ICPMS zircon and Monazite dating in petrographic context
    Journal of the Geological Society, 2018
    Co-Authors: Christian Nicollet, Valérie Bosse, Maria Spalla, Federica Schiavi
    Abstract:

    The Gruf complex in the Lepontine Alps is one of the rare occurrences of Phanerozoic ultra-high temperature (UHT) metamorphism in the world, but its age is still a matter of debate. Herewe present LA-ICPMS dating in a petrographic context of zircon and Monazite from a UHT restitic granulite. Zircons and Monazites are both included in large crystals and in retrograde symplectites. In such restitic rocks, partial melting or fluid interactions are unlikely, precluding resetting of the Monazite chronometers. Zircon cores yield Permian ages, which are interpreted as the age of charnockitization. They are sometimes surrounded by a narrow rim at 32 Ma. Monazites are strongly zoned, but all yield a 31.8 ± 0.3 Ma age interpreted as the time of complete (re-)crystallization during the UHT paragenesis. We propose that the zircons dated a post-Hercynian metamorphism which is responsible for the widespread formation of granulites in the Southern Alps and the crust differentiation. This fluidabsent melting event produced refractory lithologies, such as restites in charnockites. We suggest that Gruf UHT paragenesis is alpine in age and crystallized from these refractory lithologies.We conclude that the lower restitic crust produced in the Permian had the ability to achieve UHT conditions during the fast exhumation and heating related to lithospheric thinning in Alpine time.

  • Eocene ultra high temperature (UHT) metamorphism in the Gruf complex (Central Alps): constraints by LA-ICPMS zircon and Monazite dating in petrographic context.
    2018
    Co-Authors: Christian Nicollet, Valérie Bosse, Maria Spalla
    Abstract:

    The Gruf complex in the Lepontine Alps is one of the rare occurrences of Phanerozoic UHT metamorphism in the world but its age is still a matter of debate. Here we present LA-ICPMS dating in petrographic context of zircon and Monazite from an UHT restitic granulite. Zircons and Monazites are both included in large crystals and in retrograde symplectites. In such restitic rocks, partial melting or fluid interactions are unlikely precluding resetting of the Monazite chronometers. Zircon cores yield Permian ages interpreted as age of charnockitisation. They are sometimes surrounded by a narrow rim at 32 Ma. Monazites are strongly zoned, but all yield a 31.8 ± 0.3 Ma 15 age interpreted as the time of complete (re-)crystallisation during the UHT paragenesis. We propose that the zircons dated a post Hercynian metamorphism which is responsible of the widespread formation of granulites in the Southern Alps and the crust differentiation. This fluid-absent melting event produced refractory lithologies such as restites in charnockites. We suggest that Gruf UHT paragenesis is alpine in age and cristallised from these refractory lithologies. We conclude that the lower restitic crust produced at the Permian time had the ability to achieve UHT conditions during the fast exhumation and heating related to lithospheric thinning in Alpine time.

  • Two ultra high temperature (UHT) metamorphic events in the Gruf complex (Central Alps) ? Constraints by in situ dating of zircon and Monazite
    2017
    Co-Authors: Christian Nicollet, Valérie Bosse, Iole Spalla
    Abstract:

    The Gruf complex in the Lepontine Alps is one of the two occurrences of Phanerozoic UHT metamophism in the world. This area is thus of major interest to understand the geodynamic signification of such extreme metamor-phic conditions. However, the age of the UHT metamorphism is currently a matter of debate. Based on zircon U/Pb dating, Galli et al. (2013, Swiss J. Geosc. p33 and ref herein) have proposed a Permian age. Minerals of the charnockitic paragenesis are included within the zircon cores. Rims of these zircon grains yield 34-29 Ma ages interpreted as dating the Alpine amphibolite facies migmatisation. A different interpretation is proposed by Liati and Gebauer (2003, Schweiz. Miner. Petrog., p159) who consider that the zircon Alpine rims grew during the UHT metamorphic event. Based on Monazite dating, Schmitz et al. (2009, Eur. J. Mineral., p927) follow this interpretation , whereas Galli et al. (2013) suggest that the Alpine age is the result of Monazite resetting processes during Alpine migmatisation. In order to try to solve this controversy, we have realized LA-ICPMS in situ dating on zircon and Monazite from a restitic granulite within charnockite showing the typical well preserved UHT mineral assemblage (Spr-Al-rich Opx-Sil-Crd-Grt-Bt ± mesoperthite-Qtz-Spl). Only complex symplectitic Crd-Spl-Spr-Opx assemblages correspond to the beginning of the retrograde evolution. Both zircons and Monazites are included in the large crystals from the UHT assemblage as well as in the late symplectites. In such restitic rocks, a significant fluid interaction is unlikely, precluding a fluid mediated resetting of the Monazite. U/Pb ages in zircon and Th/U/Pb ages in Monazite measured in the same sample confirm the ages previously measured. Zircon cores yield Permian ages (from around 250 to 304 Ma), sometimes surrounded by a narrow rim at 33.2 ± 1.2 Ma. Intermediate ages may reflect mixing between core and very thin rim. Monazites are present in the core of large Spr, Opx, Crd crystals or form clusters of small grains in the late symplectites. All the grains are strongly zoned in Th, U and Y, but all yield a 31.8 ± 0.3 Ma age interpreted as the time of complete (re-)crystallisation of the Monazite in equilibrium with the UHT paragenesis. In agreement with Galli et al. (2013), these results show that the Permian age preserved in the zircon cores is related to the charnockitisation. But the Monazite age also demonstrates that the Spr-Opx-Sil UHT paragenesis in the restites in the charnockites equilibrated at 32 Ma, in agreement with Liati and Gebauer (2003) and Schmitz et al. (2009). We propose that this typical UHT paragenesis cristallised from refractory lithologies such as restites or schlieren in the charnockites. The refractory character was acquired during the previous metamorphic event, although it is difficult to precise what were the mineral assemblages in these rocks during the Permian (U?)HT metamorphism.

  • electron microprobe dating of Monazites from high grade gneisses and pegmatites of the kerala khondalite belt southern india
    Chemical Geology, 1998
    Co-Authors: Ingo Braun, Jean-marc Montel, Christian Nicollet
    Abstract:

    Monazites of five samples (one leptynitic garnet–biotite gneiss, one khondalite, one augen gneiss and two pegmatites) from the central and northern part (Ponmudi Unit) of the Kerala Khondalite Belt (KKB) in southern India were analyzed with the electron microprobe dating technique. Monazites in the augen gneiss and the pegmatites yield grain sizes between 200–800 μm, Th abundances are rather low (<10 wt.%) and the distribution of Th, U and Pb within single grains is fairly homogeneous. Contrasting to this, Monazites in the leptynitic gneiss and the khondalite are small (<150 μm). They often display very complex Th–U–Pb patterns and contain high Th concentrations up to 20 wt.%. The statistical treatment of individual ages from the investigated samples revealed three populations of Lower Proterozoic (∼1.9 Ga), Upper Proterozoic (∼580 Ma) and Ordovician age (∼470 Ma) as well as Mid Proterozoic ages between 0.8–1.7 Ga which are not regarded to be of geological significance. Lower Proterozoic ages are preserved in the cores of Monazites from leptynitic gneisses and khondalites. They fairly agree with Sm–Nd model ages for similar rocks of the KKB and give a minimum age for first Monazite growth or complete homogenization. The prominent Pan-African population with mean values between 540 and 580 Ma is present in the leptynitic gneiss, the khondalite and the augen gneiss and in line with published isotope ages for the KKB. The Ordovician population finally marks the emplacement of granitic pegmatites subsequent to the Pan-African high-grade metamorphic event. There is an obvious discrepancy between khondalites and leptynitic gneisses on the one hand and augen gneisses on the other concerning the presence of Lower Proterozoic ages. While these are abundant in the former, often rimmed by Upper Proterozoic ages, they are completely absent in the latter. It appears unlikely that Lower Proterozoic ages were completely reset during a Pan-African event exclusively in the augen gneisses while they were preserved in leptynitic gneisses and khondalites. It is further concluded that the augen gneisses are of magmatic origin and were derived from porphyritic granites. Thus, the Upper Proterozoic age of 605±37 Ma calculated for the investigated sample approximates the time of magma emplacement, which slightly precedes the peak stage of Pan-African high-grade metamorphism in the KKB, and of Monazite crystallization from the granitic melt. A characteristic feature of the investigated Monazites is the resetting of Lower Proterozoic and Pan-African ages to significantly younger values due to partial Pb loss. Monazites not shielded by other minerals (e.g., garnet) suffered selective mobilization of Pb along fractures or at their rims while Th and U concentrations remained almost unchanged. The results presented in this study indicate that this was mainly due to fluid-rock interaction. It is concluded that thermal diffusion of Pb even at the suggested temperatures of 900°C only had minor influence on the Th–U–Pb composition in Monazite and that the closure temperature for this system must be significantly higher than previously assumed (∼750°C).

  • Electron microprobe dating of Monazite
    Chemical Geology, 1996
    Co-Authors: Jean-marc Montel, Christian Nicollet, Suzanne Foret, Michèle Veschambre, Ariel Provost
    Abstract:

    Because Monazite is extremely rich in U and Th, radiogenic Pb ( * Pb) accumulates very quickly, and reaches, in about 100 Ma, a level where it is possible to analyse it with the electron probe. Assuming that common Pb is negligible, and that partial loss of Pb has not occurred, the simultaneous measurement of U, Th, and Pb allows to obtain a geologically meaningful age from a single electron probe analysis. Here we present the results of two years of systematical investigations aiming to define both the limits and potential of this method. A specific statistical method to deal with the large number of data which can be obtained on a single sample is described, and several guidelines, illustrated by examples, are suggested to optimize the method. Electron probe measurements carried out on samples of known age, from 200 Ma to 3.1 Ga, yield ages that always fall inside the confidence interval of the isotopically determined age, demonstrating that this method is reliable. The younger age limit is approximately 100 Ma, although it can be younger in some favourable cases. In old Monazites, extremely high *Pb contents have been found (up to 5 wt%) indicating that Monazite can tolerate high radiation doses without experiencing lead loss. The final precision on the age, for a 'normal' Monazite, is + 30-50 Ma, for a total counting time of 600 s. A complete dating procedure can be completed in less than 1 h. First results indicate that old ages can be preserved in Monazite, either in small relict cores in crystals, or by the coexistence of several generations of Monazites in a sample. This method has all the advantages of the electron probe: it is non-destructive, has an excellent spatial resolution (Monazites as small as 5 I~m can be dated), and because it is possible to work on normal polished thin-sections, the petrographical position of the dated crystal is known. This method offers a large number of geologists access to an in-situ dating technique at moderate cost.

Michel Ballèvre - One of the best experts on this subject based on the ideXlab platform.

  • U–Pb zircon and Monazite age constraints on granulite-facies metamorphism and deformation in the Strangways Metamorphic Complex (central Australia)
    Contributions to Mineralogy and Petrology, 2003
    Co-Authors: Andreas Möller, Bas J. Hensen, Richard A. Armstrong, Klaus Mezger, Michel Ballèvre
    Abstract:

    The age of Proterozoic granulite facies metamorphism and deformation in the Strangways Metamorphic Complex (SMC) of central Australia is determined on zircon grown in syn-metamorphic and syn-deformational orthopyroxene-bearing, enderbitic, veins. SHRIMP zircon studies suggest that M_1–M_2 and the correlated periods of intense deformation (D_1–D_2) are part of a single tectonothermal event between 1,717±2 and 1,732±7 Ma. It is considered unlikely that the two metamorphic phases (M_1, M_2) suggested by earlier work represent separate events occurring within 10–25 Ma of each other. Previous higher estimates for the age of M_1 granulite metamorphism in the SMC (Early Strangways event at ca. 1,770 Ma) based on U–Pb zircon dating of granitic, intrusive rocks, are not believed to relate to the metamorphism, but to represent pre-metamorphic intrusion ages. Conventional multi-grain U–Pb Monazite analyses on high-grade metasediments from three widely spaced localities in the western SMC yield ^207Pb/^235U ages between 1,728±11 and 1,712±2 Ma. The age range of the Monazites corresponds to the SHRIMP zircon ages in the granulitic veins and is interpreted to record Monazite growth (prograde in the metasedimentary rocks). The data imply a maximum time-span of 30 Ma for high-grade metamorphism and deformation in the SMC. There is, thus, no evidence for an extremely long period of continuous high-temperature conditions from 1,770 to ca. 1,720 Ma as previously proposed. The results firmly establish that the SMC has a very different high-grade metamorphic history than the neighbouring Harts Range, where upper amphibolite facies metamorphism in the Palaeozoic caused widespread growth or recrystallization of Monazite.

  • u pb zircon and Monazite age constraints on granulite facies metamorphism and deformation in the strangways metamorphic complex central australia
    Contributions to Mineralogy and Petrology, 2003
    Co-Authors: Andreas Möller, Klaus Mezger, Michel Ballèvre, Bas Hensen, Richard Armstrong
    Abstract:

    The age of Proterozoic granulite facies metamorphism and deformation in the Strangways Metamorphic Complex (SMC) of central Australia is determined on zircon grown in syn-metamorphic and syn-deformational orthopyroxene-bearing, enderbitic, veins. SHRIMP zircon studies suggest that M1–M2 and the correlated periods of intense deformation (D1–D2) are part of a single tectonothermal event between 1,717±2 and 1,732±7 Ma. It is considered unlikely that the two metamorphic phases (M1, M2) suggested by earlier work represent separate events occurring within 10–25 Ma of each other. Previous higher estimates for the age of M1 granulite metamorphism in the SMC (Early Strangways event at ca. 1,770 Ma) based on U–Pb zircon dating of granitic, intrusive rocks, are not believed to relate to the metamorphism, but to represent pre-metamorphic intrusion ages. Conventional multi-grain U–Pb Monazite analyses on high-grade metasediments from three widely spaced localities in the western SMC yield 207Pb/235U ages between 1,728±11 and 1,712±2 Ma. The age range of the Monazites corresponds to the SHRIMP zircon ages in the granulitic veins and is interpreted to record Monazite growth (prograde in the metasedimentary rocks). The data imply a maximum time-span of 30 Ma for high-grade metamorphism and deformation in the SMC. There is, thus, no evidence for an extremely long period of continuous high-temperature conditions from 1,770 to ca. 1,720 Ma as previously proposed. The results firmly establish that the SMC has a very different high-grade metamorphic history than the neighbouring Harts Range, where upper amphibolite facies metamorphism in the Palaeozoic caused widespread growth or recrystallization of Monazite.

Valérie Bosse - One of the best experts on this subject based on the ideXlab platform.

  • Eocene ultra-high temperature (UHT) metamorphism in the Gruf complex (Central Alps): constraints by LA-ICPMS zircon and Monazite dating in petrographic context
    Journal of the Geological Society, 2018
    Co-Authors: Christian Nicollet, Valérie Bosse, Maria Spalla, Federica Schiavi
    Abstract:

    The Gruf complex in the Lepontine Alps is one of the rare occurrences of Phanerozoic ultra-high temperature (UHT) metamorphism in the world, but its age is still a matter of debate. Herewe present LA-ICPMS dating in a petrographic context of zircon and Monazite from a UHT restitic granulite. Zircons and Monazites are both included in large crystals and in retrograde symplectites. In such restitic rocks, partial melting or fluid interactions are unlikely, precluding resetting of the Monazite chronometers. Zircon cores yield Permian ages, which are interpreted as the age of charnockitization. They are sometimes surrounded by a narrow rim at 32 Ma. Monazites are strongly zoned, but all yield a 31.8 ± 0.3 Ma age interpreted as the time of complete (re-)crystallization during the UHT paragenesis. We propose that the zircons dated a post-Hercynian metamorphism which is responsible for the widespread formation of granulites in the Southern Alps and the crust differentiation. This fluidabsent melting event produced refractory lithologies, such as restites in charnockites. We suggest that Gruf UHT paragenesis is alpine in age and crystallized from these refractory lithologies.We conclude that the lower restitic crust produced in the Permian had the ability to achieve UHT conditions during the fast exhumation and heating related to lithospheric thinning in Alpine time.

  • Eocene ultra high temperature (UHT) metamorphism in the Gruf complex (Central Alps): constraints by LA-ICPMS zircon and Monazite dating in petrographic context.
    2018
    Co-Authors: Christian Nicollet, Valérie Bosse, Maria Spalla
    Abstract:

    The Gruf complex in the Lepontine Alps is one of the rare occurrences of Phanerozoic UHT metamorphism in the world but its age is still a matter of debate. Here we present LA-ICPMS dating in petrographic context of zircon and Monazite from an UHT restitic granulite. Zircons and Monazites are both included in large crystals and in retrograde symplectites. In such restitic rocks, partial melting or fluid interactions are unlikely precluding resetting of the Monazite chronometers. Zircon cores yield Permian ages interpreted as age of charnockitisation. They are sometimes surrounded by a narrow rim at 32 Ma. Monazites are strongly zoned, but all yield a 31.8 ± 0.3 Ma 15 age interpreted as the time of complete (re-)crystallisation during the UHT paragenesis. We propose that the zircons dated a post Hercynian metamorphism which is responsible of the widespread formation of granulites in the Southern Alps and the crust differentiation. This fluid-absent melting event produced refractory lithologies such as restites in charnockites. We suggest that Gruf UHT paragenesis is alpine in age and cristallised from these refractory lithologies. We conclude that the lower restitic crust produced at the Permian time had the ability to achieve UHT conditions during the fast exhumation and heating related to lithospheric thinning in Alpine time.

  • Two ultra high temperature (UHT) metamorphic events in the Gruf complex (Central Alps) ? Constraints by in situ dating of zircon and Monazite
    2017
    Co-Authors: Christian Nicollet, Valérie Bosse, Iole Spalla
    Abstract:

    The Gruf complex in the Lepontine Alps is one of the two occurrences of Phanerozoic UHT metamophism in the world. This area is thus of major interest to understand the geodynamic signification of such extreme metamor-phic conditions. However, the age of the UHT metamorphism is currently a matter of debate. Based on zircon U/Pb dating, Galli et al. (2013, Swiss J. Geosc. p33 and ref herein) have proposed a Permian age. Minerals of the charnockitic paragenesis are included within the zircon cores. Rims of these zircon grains yield 34-29 Ma ages interpreted as dating the Alpine amphibolite facies migmatisation. A different interpretation is proposed by Liati and Gebauer (2003, Schweiz. Miner. Petrog., p159) who consider that the zircon Alpine rims grew during the UHT metamorphic event. Based on Monazite dating, Schmitz et al. (2009, Eur. J. Mineral., p927) follow this interpretation , whereas Galli et al. (2013) suggest that the Alpine age is the result of Monazite resetting processes during Alpine migmatisation. In order to try to solve this controversy, we have realized LA-ICPMS in situ dating on zircon and Monazite from a restitic granulite within charnockite showing the typical well preserved UHT mineral assemblage (Spr-Al-rich Opx-Sil-Crd-Grt-Bt ± mesoperthite-Qtz-Spl). Only complex symplectitic Crd-Spl-Spr-Opx assemblages correspond to the beginning of the retrograde evolution. Both zircons and Monazites are included in the large crystals from the UHT assemblage as well as in the late symplectites. In such restitic rocks, a significant fluid interaction is unlikely, precluding a fluid mediated resetting of the Monazite. U/Pb ages in zircon and Th/U/Pb ages in Monazite measured in the same sample confirm the ages previously measured. Zircon cores yield Permian ages (from around 250 to 304 Ma), sometimes surrounded by a narrow rim at 33.2 ± 1.2 Ma. Intermediate ages may reflect mixing between core and very thin rim. Monazites are present in the core of large Spr, Opx, Crd crystals or form clusters of small grains in the late symplectites. All the grains are strongly zoned in Th, U and Y, but all yield a 31.8 ± 0.3 Ma age interpreted as the time of complete (re-)crystallisation of the Monazite in equilibrium with the UHT paragenesis. In agreement with Galli et al. (2013), these results show that the Permian age preserved in the zircon cores is related to the charnockitisation. But the Monazite age also demonstrates that the Spr-Opx-Sil UHT paragenesis in the restites in the charnockites equilibrated at 32 Ma, in agreement with Liati and Gebauer (2003) and Schmitz et al. (2009). We propose that this typical UHT paragenesis cristallised from refractory lithologies such as restites or schlieren in the charnockites. The refractory character was acquired during the previous metamorphic event, although it is difficult to precise what were the mineral assemblages in these rocks during the Permian (U?)HT metamorphism.

  • Comment la Monazite peut enregistrer et préserver l’âge d’événements métamorphiques successifs de haut grade : l’exemple des métapélites à Grt-Ky du Rhodope Central (Bulgarie, Grèce)
    2014
    Co-Authors: Valérie Bosse, Amélie Didier, Pierre Gautier, Zlatka Cherneva, Milena Georgieva, Ianko Gerdjikov
    Abstract:

    Le faible taux de diffusion du Pb dans la structure cristalline de la Monazite, même à température élevée, en fait un géochronomètre robuste susceptible de préserver les âges d’événements métamorphiques successifs. Nous présentons des données texturales, chimiques et des âges U-Th-Pb in situ dans les Monazites des métapélites à Grt-Ky des zones de cisaillement de Chepelare (Bulgarie) et Nestos (Grèce) dans le Rhodope central. Les échantillons étudiés ont subi deux épisodes de métamorphisme à haute température : le premier est responsable d’une fusion partielle anhydre dans le faciès des granulites au Mésozoïque, le second implique une fusion partielle hydratée au Cénozoïque. Une déformation ductile intense en présence de fluides est visible dans la majorité des échantillons. La Monazite est présente dans la matrice foliée ainsi qu’en inclusion dans le grenat et le disthène. Les Monazites de la matrice présentent des évidences de dissolution-recristallisation en présence de fluides. La majeure partie des grains est formée de domaines pauvres en Y donnant des âges entre 115 et 165 Ma. Des domaines riches en Y présentant des âges cénozoïques forment les bordures des grains matriciels ou de petits grains satellites entourant les grains mésozoïques. Les Monazites cénozoïques cristallisent au dépend des Monazites mésozoïques et incorporent simultanément l’Y provenant de la dissolution partielle du grenat. Dans les deux zones de cisaillement, la croissance des Monazites cénozoïques est datée à 36 Ma. Des âges identiques ont été obtenus dans les leucosomes et pegmatites des gneiss migmatitiques environnants indiquant que les fluides responsables de la précipitation des Monazites cénozoïques provenaient probablement de la cristallisation de liquides anatectiques adjacents. Ces résultats montrent l’efficacité des processus de dissolution-précipitation dynamique qui favorisent la cristallisation d’une nouvelle génération de Monazite pendant la déformation à 36 Ma.

  • Syn-deformation fluid-assisted growth of Monazite during renewed high-grade metamorphism in metapelites of the Central Rhodope (Bulgaria, Greece)
    Chemical Geology, 2014
    Co-Authors: Amélie Didier, Valérie Bosse, Pierre Gautier, Jean-louis Paquette, Z. Cherneva, M. Georgieva, Ianko Gerdjikov
    Abstract:

    We present textural, chemical and U-Th-Pb-age data on Monazites from garnet-kyanite (Grt-Ky) metapelites, from the Chepelare Shear Zone (Bulgaria) and the Nestos Shear Zone (Greece), in the Central Rhodope. Samples from both locations have experienced two stages of high temperature metamorphism during Alpine times. The first event involved mid-Mesozoic granulite facies dehydration melting. The second event involved mid- Cenozoic lower-grade fluid-assisted partial melting. The latter is well expressed in adjacent felsic rocks but had limited impact on the Grt-Ky metapelites. Most samples display evidence for strong ductile shearing in the presence of fluids.Monazite is present in the highly foliated matrix and as inclusions in garnet and kyanite. Unlike the inclusions, matrix Monazites display features of fluid-assisted dissolution-recrystallization. Y-poor domainswith U-Th-Pb ages of between ca. 115 and 165 Ma represent the largest part of the grains. Y-rich domains with mid- Cenozoic ages occur as rims, or as small satellite grains surrounding the Mesozoic grains. The Cenozoic Monazite domains crystallized at the expense of the Mesozoic ones and simultaneously incorporated Y provided by the fluid-assisted resorption of garnet. An age of ca. 36Ma is obtained for the samples of both shear zones, interpreted as dating the main episode of Monazite growth during the Cenozoic. Similar ages exist for the crystallization of leucosomes and pegmatites in the adjacent migmatitic gneisses, indicating that the fluids responsible for the precipitation of the Cenozoic Monaziteswere probably released during the crystallization of nearby anatectic melts. Together with associated rutile and biotite, many newly grown Monazites show a preferred orientation paralleling the matrix foliation. This supports the hypothesis of dynamic dissolution-precipitation as an efficient mean to promote renewed Monazite crystallization during ductile deformation of the host rock at ca. 36 Ma.

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  • high resolution 5 μm u th pb isotope dating of Monazite with excimer laser ablation ela icpms
    Chemical Geology, 2007
    Co-Authors: Jean-louis Paquette, M. Tiepolo
    Abstract:

    Abstract Monazite [(LREE)PO 4 ], a common accessory mineral in magmatic and metamorphic rocks, is complementary to zircon in U–Th–Pb geochronology. Because the mineral can record successive growth phases it is useful for unravelling complex geological histories. A high spatial resolution is required to identify contrasted age domains that may occur at the crystal-scale. Bulk mineral techniques such as ID-TIMS, applied to single Monazite grains recording multiple overgrowths or isotope resetting can result in partly scattered discordant analytical points that produce inaccurate intercept ages. Laser ablation (LA)-ICPMS has been demonstrated to be a useful technique for U–Th–Pb dating of zircons, and this study tests its analytical capabilities for dating Monazite. A sector field high resolution ICPMS coupled with a 193 nm ArF excimer laser ablation microprobe is capable of achieving a high spatial resolution and producing stable and reliable isotope measurements. The U–Th–Pb systematic was applied to Monazite grains from several samples: a lower Palaeozoic lens from high-grade terrains in Southern Madagascar, Neogene hydrothermal crystals from the Western Alps, a Palaeoproterozoic very high temperature granulite from central Madagascar and a Variscan leucogranite from Spain, directly on a polished thin section. The major aim was to compare and/or reproduce TIMS and EMP ages of Monazite from a variety of settings and ages. The three independent 206 Pb/ 238 U, 207 Pb/ 235 U and 208 Pb/ 232 Th ratios and ages were calculated. Isotope fractionation effects (mass bias, laser induced fractionation) were corrected using a chemically homogeneous and U–Pb concordant Monazite as external standard. This study demonstrates that excimer laser ablation (ELA)-ICPMS allows U–Th–Pb dating of Monazite with a high level of repeatability, accuracy and precision as well as rapidity of analysis. A spatial resolution almost comparable to that of EMP in terms of crater width (5 μm) produced precise 208 Pb/ 232 Th, 206 Pb/ 238 U and 207 Pb/ 235 U ratios for dating Palaeozoic to Precambrian Monazites. The advantages of (ELA)-ICPMS isotope dating are precision, accuracy and the ability to detect discordance. In the case of late Miocene hydrothermal Monazites from the Alps, a larger spot size of 25 μm diameter is required, and precise and accurate ages were obtained only for 208 Pb/ 232 Th systematics. Results from the Variscan granite show that in situ U–Th–Pb dating of Monazites with (ELA)-ICPMS is possible using a 5 μm spot directly on thin sections, so that age data can be placed in a textural context.

  • High resolution (5 μm) U–Th–Pb isotope dating of Monazite with excimer laser ablation (ELA)-ICPMS
    Chemical Geology, 2007
    Co-Authors: Jean-louis Paquette, M. Tiepolo
    Abstract:

    Monazite [(LREE)PO4], a common accessory mineral in magmatic and metamorphic rocks, is complementary to zircon in U–Th–Pb geochronology. Because the mineral can record successive growth phases it is useful for unravelling complex geological histories. A high spatial resolution is required to identify contrasted age domains that may occur at the crystal-scale. Bulk mineral techniques such as ID-TIMS, applied to single Monazite grains recording multiple overgrowths or isotope resetting can result in partly scattered discordant analytical points that produce inaccurate intercept ages. Laser ablation (LA)-ICPMS has been demonstrated to be a useful technique for U–Th–Pb dating of zircons, and this study tests its analytical capabilities for dating Monazite. A sector field high resolution ICPMS coupled with a 193 nm ArF excimer laser ablation microprobe is capable of achieving a high spatial resolution and producing stable and reliable isotope measurements. The U–Th–Pb systematic was applied to Monazite grains from several samples: a lower Palaeozoic lens from high-grade terrains in Southern Madagascar, Neogene hydrothermal crystals from the Western Alps, a Palaeoproterozoic very high temperature granulite from central Madagascar and a Variscan leucogranite from Spain, directly on a polished thin section. The major aim was to compare and/or reproduce TIMS and EMP ages of Monazite from a variety of settings and ages. The three independent 206Pb/238U, 207Pb/235U and 208Pb/232Th ratios and ages were calculated. Isotope fractionation effects (mass bias, laser induced fractionation) were corrected using a chemically homogeneous and U–Pb concordant Monazite as external standard. This study demonstrates that excimer laser ablation (ELA)-ICPMS allows U–Th–Pb dating of Monazite with a high level of repeatability, accuracy and precision as well as rapidity of analysis. A spatial resolution almost comparable to that of EMP in terms of crater width (5 μm) produced precise 208Pb/232Th, 206Pb/238U and 207Pb/235U ratios for dating Palaeozoic to Precambrian Monazites. The advantages of (ELA)-ICPMS isotope dating are precision, accuracy and the ability to detect discordance. In the case of late Miocene hydrothermal Monazites from the Alps, a larger spot size of 25 μm diameter is required, and precise and accurate ages were obtained only for 208Pb/232Th systematics. Results from the Variscan granite show that in situ U–Th–Pb dating of Monazites with (ELA)-ICPMS is possible using a 5 μm spot directly on thin sections, so that age data can be placed in a textural context