Generations Of Monazite Research Articles

Physico-chemical control on the REE minerals in chloritoid-grade metasediments from a single outcrop (Central Alps, Switzerland)

This study assesses the effects of fluid, whole-rock composition and oxygen fugacity, on the texture and composition of monazite, allanite, and xenotime. For this purpose, these were investigated in 13 mono-metamorphic metasediments from a single locality of the Central Alps (Switzerland), which record greenschist facies conditions (T~400–450°C). Two of the samples contain hydrothermal veins dominated by quartz and calcite+quartz, respectively. In metasediments devoid of veins, the light rare earth elements (LREE) are concentrated in allanite in all samples except for one metamarl (Ga06). Allanite formation is texturally coeval with apatite, chloritoid and xenotime, during the main tectono-metamorphic stage. Allanite formation implies significant mass transfer of Ca and P via a fluid phase, which is not clearly related to advective transport. In Ga06, elongate monazite grains have a detrital core rimmed by newly formed monazite. Significant arsenic contents are found in newly formed monazite, xenotime and apatite. Monazite texture and composition suggest (re)crystallization by pressure solution, at an oxygen fugacity sufficient to partly oxidize As, S, U, and Fe. Whether or not monazite is preserved appears to be but weakly dependent on the Ca/Al ratio and thus whole-rock composition at greenschist facies conditions. Samples with veins show peculiar features. Along carbonate-bearing veins allanite occurs as porphyroblasts overgrowing the main foliation of the host rock. As similar allanite porphyroblasts occur in calcite-bearing metamarl, their formation is attributed to environments rich in CO2-bearing fluid. A new generation of monazite is found in and along retrograde quartz veins.This study demonstrates that, at least in metasediments up to chloritoid-grade, REE minerals record fluid/rock interaction that occurred at different deformation stages. Arsenic concentrations in REE phosphates appear to reflect conditions of elevated oxygen fugacity. In cases where such conditions are not inherited from the sedimentary protolith, the oxidation reflects a hydrothermal event, the age of which may be directly datable by U/Th–Pb of As-rich monazite.

Th–U–total Pb geochronology of authigenic monazite in the Adelaide rift complex, South Australia, and implications for the age of the type Sturtian and Marinoan glacial deposits

The Adelaide rift complex in South Australia contains the type sections for Sturtian and Marinoan glacial deposits. The litho- and chemo-stratigraphy of these deposits play a central role in evaluating global Neoproterozoic ice age hypotheses and Rodinia supercontinent reconstructions, but reliable depositional age constraints have been extremely limited. We report results of in situ Th–U–total Pb (electron microprobe) dating of detrital and authigenic monazite in two samples from the Umberatana Group (Sturtian Holowilena Ironstone and pre-Marinoan Enorama Shale) in the Central Flinders Ranges. Several texturally and chemically distinct detrital and authigenic populations are recognized. Detrital dates range from 1600 Ma to 760 Ma and most relate to well-known orogenic or igneous events in surrounding cratonic regions. Authigenic monazite grew in three or more pulses ranging from 680 Ma to 500 Ma. The date of 680 ± 23 Ma (2 σ) for the earliest generation of authigenic monazite in sandstone from the Enorama Shale (1) provides an estimate for the age of the base of the Trezona carbon isotopic anomaly just beneath the Marinoan glacial deposits, (2) provides an absolute minimum age constraint on the underlying Sturtian glacial deposits, and (3) supports proposed correlations between type Marinoan deposits and precisely dated glacial deposits in Namibia and China, which bracket the presumed Marinoan equivalents between 655 and 635 Ma. This age is inconsistent with a Re–Os isochron age of 643 ± 2.4 Ma (2 σ) on shales near the bottom of the Sturtian–Marinoan interglacial succession, stratigraphically > 3000 m below the Enorama Shale sample, and militate against the hypothesis that the type Marinoan is correlative with the 580 Ma Gaskiers glaciation. Monazite growth near 600 Ma and again at about 500 Ma probably represent hydrothermal fluid-flow events, the latter of which also corresponds to the well-known Delamerian Orogeny during which the Adelaide sediments were folded into their present structural pattern.

Two types of metamorphic monazite with contrasting La/Nd, Th, and Y signatures in an ultrahigh-pressure metapelite from the Pohorje Mountains, Slovenia: Indications for pressure-dependent REE exchange between apatite and monazite?

Two monazite generations (M1; M2) were distinguished in a kyanite-garnet gneiss from the UHP terrain of the Pohorje Mountains, Slovenia. P-T estimates reveal a peak event at 760 °C/2.6 GPa and isothermal decompression down to 700 °C/0.6 GPa. M1 type provides a Th-U-Pb mean date of 100 ± 6 Ma, ThO 2 contents between 3–7 wt%, Y 2 O 3 values 1.2 MPa), probably close to the P-T peak. M2 type is slightly younger than M1 (74 ± 16 Ma), and has a lower La/Nd (0.3–0.9), lower ThO 2 (0.1–5 wt%), and higher Y 2 O 3 (up to 3.2 wt%). Most M2 monazites occur as tiny needles within apatite (subtype M2-a) or along apatite margins (M2-b). Parasitic growth of M2-a and -b from apatite is supported by its low ThO 2 ( 2 (5 wt%) and were supplied from an apatite and M1 source. Elevated yttrium suggests that M2 formed during decompression, when garnet was consumed and Y was released. These observations imply that at UHP conditions MREE-rich apatite coexisted with low-MREE M1 monazite and reacted during decompression to Ca-F-apatite plus MREE-rich M2 monazite. This provides strong arguments that REE-partitioning between apatite and monazite is pressure-dependent.

Monazite as an indicator of formation conditions of quartz veins at the Zhelannoe deposit, the Subpolar Urals

At the Zhelannoe quartz deposit, the content of monazite attains 0.5 wt % in unaltered sericitolite and 18 wt % in hydrothermally altered sericitolite. Two monazite generations, including four varieties, characterize the sequence of formation and alteration of sericitolite bodies at the Zhelannoe deposit. Monazite of the first generation occurs in unaltered sericitolite as prismatic and tabular crystals characterized by (Nd,Ce) > La and enrichment in HREEs and ThO2 (5–16 wt %). Its formation is accompanied crystallization of milk white quartz. Monazite of the second generation occurs in altered sericitolite as the product of recrystallization of the first-generation monazite. The large drusy crystals of second-generation monazite were formed similarly with Alpine-type veins. Monazite of the second generation is characterized by Ce > (La,Nd), low contents of HREEs and ThO2 (0.5–7 wt %) and high contents of CaO and SO3 (up to 3–5 wt %). Monazite of the second generation appeared as a result of local superimposed processes and is a characteristic feature of the Zhelannoe deposit.

Reaction microtextures of monazite: Correlation between chemical and age domains in the Nazaré Paulista migmatite, SE Brazil

Back-scattered imaging, X-ray element mapping and electron microprobe analyzer (EMPA) chemical dating reveal complex compositional and age zoning in monazite crystals from different layers and textural positions in a garnet-bearing migmatite in SE Brazil. Y-rich (variable Y 2O 3, averaging 2.5 wt.%) relict cores are preserved in mesosome and melanosome monazite, and correspond to 793 ± 6 Ma inherited crystals possibly generated in a previous metamorphic event. These cores are overgrown and widely replaced by two generations of monazite, which are present in all migmatite layers. The first, also Y-rich (average 2.5 wt.% Y 2O 3), was produced at ~ 635 Ma during prograde metamorphism under subsolidus conditions, while the second has an Y-poor (< 1.5 wt.% Y 2O 3), low Th/U signature, and precipitated from low Y and HREE anatectic melts produced by reactions in which garnet was inert. Quartz-rich trondhjemitic leucosome represents lower temperature melt (bearing some subsolidus quartz and garnet with included monazite) formed at temperatures below muscovite breakdown; its Y-poor monazite indicates an age of 617 ± 6 Ma. Granitic leucosomes formed close to peak metamorphic conditions ( T > 750 °C) above muscovite breakdown have their slightly younger character confirmed by a 609 ± 7 Ma low-Y monazite age. A similar 606 ± 5 Ma age was obtained for low-Y monazite rims and domains in mesosome and melanosome, and reflects the time of monazite saturation in interstitial granitic melt that was trapped in these layers. Our results confirm that inherited monazite crystals can be preserved during partial melting at temperatures above muscovite breakdown. Moreover, careful textural control aided by X-ray chemical mapping may allow monazite generated at different stages in a ~ 25 Myr prograde metamorphic path to be identified and dated using an electron microprobe.

Geological setting of the Guelb Moghrein Fe oxide-Cu-Au-Co mineralization, Akjoujt area, Mauritania

Abstract The Guelb Moghrein Fe oxide–Cu–Au–Co deposit is located at the western boundary of the West African craton in NW Mauritania. The wall rocks to the mineralization represent a meta-volcanosedimentary succession typical of Archaean greenstone belts. Two types of meta-volcanic rocks are distinguished: (1) volcanoclastic rocks of rhyodacite–dacite composition (Sainte Barbe volcanic unit), which form the stratigraphic base; (2) tholeiitic andesites–basalts (Akjoujt meta-basalt unit). The trace element signature of both types is characteristic of a volcanic arc setting. A small meta-pelitic division belongs to the Sainte Barbe volcanic unit. A meta-carbonate body, which contains the mineralization, forms a tectonic lens in the Akjoujt meta-basalt unit. It can be defined by the high X Mg (=36) of Fe–Mg carbonate, the REE pattern and the δ 13 C values of −18 to −17‰ as a marine precipitate similar to Archaean banded iron formation (BIF). Additionally, small slices of Fe–Mg clinoamphibole–chlorite schist in the meta-carbonate show characteristics of marine shale. This assemblage, therefore, does not represent an alteration product, but represents an iron formation unit deposited on a continental shelf, which probably belongs to the Lembeitih Formation. The hydrothermal mineralization at 2492 Ma was contemporaneous with regional D 2 thrusting of the Sainte Barbe volcanic unit and imbrications of the meta-carbonate in the upper greenschist facies. This resulted in the formation of an ore breccia in the meta-carbonate, which is enriched in Fe, Ni, Co, Cu, Bi, Mo, As and Au. Massive sulphide ore breccia contains up to 20 wt% Cu. The ore fluid was aqueous–carbonic in nature and either changed its composition from a Mg-rich oxidizing to an Fe-rich reducing fluid or the two fluid types mixed at the trap site. All lithologies at Guelb Moghrein were deformed by D 3 thrusting to the east in the lower greenschist facies. The mobility of REE in the retrogressed rocks explains the formation of a second generation of hydrothermal monazite, which was dated at c . 1742 Ma. Archaean rocks of the West African craton extend to the west to Guelb Moghrein. The active continental margin was deformed and mineralized in the Late Archaean–Early Proterozoic and again reactivated in the Mid-Proterozoic and Westphalian, showing that the western boundary of the craton was reactivated several times.

EMP chemical ages of monazites from Central Zone of the eastern Kunlun Orogen: Records of multi-tectonometamorphic events

Two generations of monazite were identified in one mica schist sample from the central zone of the eastern Kunlun Orogen at the northern margin of the Qinghai-Tibet Plateau. The first generation was found in the cores of garnet porphyroblasts with relatively high Y2O3 contents (averaging 1.012±0.088 wt%). The second generation occurs either as inclusions in the rims of garnet porphyroblasts, matrix kyanite, plagioclase and quartz, or as separate crystals associated with matrix biotite and muscovite mostly having a relatively low Y2O3 content (averaging 0.479±0.100 wt%). Electron microprobe (EMP) Th-U-Pbtotal chemical dating for two grains of the first generation monazite yields average ages of 455 ± 18 Ma and 420 ± 20 Ma, respectively; six grains of the second generation gave an average age of 246.1 ± 3.8 Ma. These ages suggest that the core and rim of the garnet porphyroblasts and their associated assemblages record two events of significant tectonometamorphism. The formation of the Ordovician monazite is related to the tectonometamorphic event responsible for continuous amalgamation of the Gondwanaland in the Late Pan-African period or for consumption of the Proto-Tethys, whereas the development of the Triassic monazite is related to the tectonometamorphic event for consumption of the Paleo-Tethys.

The tectonic significance of (U,Th)/Pb ages of monazite inclusions in garnet from the Himalaya of central Nepal

Textural observations, chemical zoning, and 196 in situ LA-ICP-MS 232Th/ 208Pb and 238U/ 206Pb dates of monazite inclusions in Greater Himalayan garnets demonstrate that inclusions are frequently composite grains composed of multiple generations of monazite. Cenozoic prograde and retrograde monazite are commonly intergrown, and both of these components are frequently intergrown with lower Paleozoic monazite. Because zones in most inclusions are too small to date, and because some inclusions show little zoning of Y and Th, chemical characterization alone is often insufficient to guide interpretation of the crystallization significance of Th/Pb dates. Comparison of 238U/ 206Pb and 232Th/ 208Pb dates thus is critical for interpreting the tectonic significance of dates derived from these composite inclusions. Most Cenozoic monazite crystallized between 42–29 and 22–12 Ma, and these age ranges may record prograde and retrograde metamorphism of Greater Himalayan rocks, respectively. Dates younger than ∼ 12 Ma are discordant, without exception. The < 12 Ma dates come from the eastern side of the Annapurna Range and record limited monazite growth during retrograde metamorphism and metasomatism of these Greater Himalayan rocks. Microcracks allow communication between the interior and exterior of garnet, affecting monazite inclusions by facilitating Pb loss, dissolution, recrystallization, and intergrowth of younger monazite.

Microprobe Monazite Geochronology: Understanding Geologic Processes by Integrating Composition and Chronology

Monazite is a light rare earth element (LREE)-bearing phosphate mineral that is present in a wide variety of rock types, has an extremely variable composition reflecting host rock conditions, and is a robust geochronometer that can preserve crystallization ages through a long history of geological events. Monazite crystals typically contain distinct compositional domains that represent successive generations of monazite, which in turn, can provide a detailed record of the geologic history of its host rocks. The electron microprobe can be used to characterize the geometry of compositional domains, analyze the composition of each domain, and, when carefully configured, determine the U-Th-total Pb age for domains as small as 5 μm in width. These data allow the monazite to be linked with, and place timing constraints on, silicate processes in the host rocks. Current applications span a broad range of geologic processes in igneous, metamorphic, hydrothermal, and sedimentary rocks.

In situ U–Pb dating and element mapping of three generations of monazite: Unravelling cryptic tectonothermal events in low-grade terranes

In situ U–Pb dating of monazite and xenotime in sedimentary rocks from the mid-Archean Soanesville Group in the Pilbara Craton, yields ages for provenance, diagenesis and multiple low-grade metamorphic events. Detrital monazite and xenotime grains give dates >3250 Ma, whereas diagenetic xenotime provides a new minimum age of 3190 ± 10 Ma for deposition of the basal Soanesville Group, previously constrained between ∼3235 Ma and ∼2955 Ma. Metamorphic monazite provides evidence for three episodes of growth: at 2.88, 2.16 and 1.65 Ga. Element mapping of monazite for La, Sm, Y and Th reveals distinct cores and rims in some crystals that were used to guide the placement of analytical spots during in situ U–Pb dating by sensitive high-resolution ion microprobe (SHRIMP). Specifically, La and Sm distributions closely correlate with different generations of monazite. The presence of two generations in single monazite crystals highlights the need for characterizing mineral chemistry prior to geochronology. It also shows the importance of using in situ dating techniques rather than methods that rely on the analysis of entire, potentially multi-aged, crystals. The ages recorded by metamorphic monazite span more than one billion years and are interpreted to record cryptic tectonothermal events within the craton. The 2.88 Ga age coincides with a phase of regional deformation, metamorphism and gold mineralization along a major crustal lineament, whereas the most common monazite age population (at 2.16 Ga) corresponds with the migration of a foreland fold-and-thrust belt across the craton. The youngest age (1.65 Ga) coincides with an episode of tectonic reworking in the Capricorn Orogen along the southern Pilbara margin. The prolonged history of monazite growth may, in part, relate to channelized fluid flow during reactivation of long-lived N- to NE-trending crustal structures that transect the craton. Despite repeated episodes of metamorphism, the isotopic system in each generation of monazite remained unperturbed, yielding precise dates. The ability of monazite to record three separate events, and in some instances two events in a single crystal, distinguishes it from most other low-temperature mineral chronometers, which are readily reset during metamorphic overprinting. Low-temperature monazite geochronology can provide a detailed isotopic history of cryptic thermal events and reveal the temporal and spatial patterns of far-field fluid flow related to tectonic processes. The previously unrecognized history of crustal fluid flow in the Pilbara Craton has implications for chemical, mineralogical and isotopic studies seeking to understand conditions on the early Earth.

Formation of monazite and rhabdophane at the expense of allanite during Alpine low temperature retrogression of metapelitic basement rocks from Crete, Greece: Microprobe data and geochronological implications

Accessory LREE-bearing minerals have been investigated in a sequence of Palaeozoic amphibolite facies metapelites in eastern Crete, Greece, which were overprinted by Alpine low-T metamorphism (~ 300 °C). The series is dominated by Al–Ca rich metapelite layers, which contain Palaeozoic allanite, secondary Alpine monazite with low Y contents (< 0.6 wt.%) or rhabdophane. Microstructural evidence suggests that during the Alpine overprint widespread growth of rhabdophane and Alpine monazite occurred at the expense of allanite. Allanite was obviously unstable and reactive at the given low-T conditions. Rhabdophane, a hydrated REE phosphate with low EMPA analytical totals (90–95%), high Y and Ca contents (1.5–8 wt.% Y 2O 3, up to 10 wt.% CaO) and high concentrations of common lead, formed instead of monazite in zones with increased fluid activity. Only in some quartz-rich layers of metapelites, which are allanite-free, is primary Palaeozoic monazite to be found. In such special variants, Alpine monazite or rhabdophane appear to be absent and the LREEs remained trapped in the Palaeozoic monazite. Not even the outermost rims of the old monazites experienced alteration during the Alpine retrograde metamorphism, and preserve undisturbed Palaeozoic Th–U–Pb ages. This observations made in this study confirm other recent work according to which, in many cases, monazite and not allanite is the stable LREE phase under low-grade metamorphic conditions. Rocks with primary allanite contents can be considered particularly fertile for the production of monazite at low metamorphic grades. In contrast, rocks without allanite, but with primary monazite, are unfertile for the growth of a secondary monazite generation. A composite, lithologically variable basement with a low temperature overprint will therefore often include both, remnant and new monazite, although the two monazite generations will commonly occur in different lithologies and rarely together in the same samples. Therefore, if a cunning sampling strategy is applied, monazite geochronology is capable of yielding the ages of both primary and secondary events in such settings.

Three metamorphic monazite generations in a high-pressure rock from the Bohemian Massif and the potentially important role of apatite in stimulating polyphase monazite growth along a PT loop

Based on microstructural and compositional criteria, three generations of monazite can be identified in a peraluminous, garnet-rich, high-pressure metamorphic rock from the Bohemian Massif. The first monazite generation (M1) formed probably on the prograde part of the clockwise PT loop, a second (M2) during the high-pressure stage (26 ± 3 kbar and 830 ± 30 °C) and a third generation (M3) during subsequent near-isothermal decompression of the rock to 8 ± 2 kbar. The high-pressure, M2 monazites are volumetrically dominant and form 100–300 μm grains. They are characterised by an unusual Sr–brabantite component (SrO up to 2 wt.%) and grew together with accessory Sr-rich fluorapatites (SrO up to 33 wt.%). The Sr contents of both minerals are indicative of high-pressure conditions and result from the breakdown of plagioclase and the release of Sr that occurred during this process. M3 monazites occur exclusively as small grains (10–100 μm) within and at the rims of apatite. Microstructures show that they grew at the expense of the apatite extracting LREEs from the latter under retrograde conditions. These “parasite monazites” in apatite are characterised by very low Th contents, which is consistent with having been derived primarily from an apatite source. M1 monazites are represented by resorbed, irregular shaped cores, which are occasionally preserved within the Sr–brabantite rich, high-pressure monazites (M2). These M1 cores are generally Sr-poor, indicating that they should have formed prior to the high-pressure stage, in the presence of plagioclase. The dissolution of M1 monazite and the reprecipitation of a Th, U-enriched M2 monazite during the high-pressure stage implies that LREEs were taken up by the high-pressure Sr-rich fluorapatite to some extent, and not completely processed for the formation of the M2 monazite. This would also explain the widespread retrograde growth of M3 monazite within the apatite. The ability of apatite to act as an interim host for LREEs at peak-PT conditions could be one of the reasons why monazite growth during regional metamorphism is often periodic.

Dating metamorphic reactions and fluid flow: application to exhumation of high‐P granulites in a crustal‐scale shear zone, western Canadian Shield

AbstractThe Legs Lake shear zone is a crustal‐scale thrust fault system in the western Canadian Shield that juxtaposes high‐pressure (1.0+ GPa) granulite facies rocks against shallow crustal (&lt; 0.5 GPa) amphibolite facies rocks. Hangingwall decompression is characterized by breakdown of the peak assemblage Grt + Sil + Kfs + Pl + Qtz into the assemblage Grt + Crd + Bt ± Sil + Pl + Qtz. Similar felsic granulite occurs throughout the region, but retrograde cordierite is restricted to the immediate hangingwall of the shear zone. Textural observations, petrological analysis using P–T/P–MH2O phase diagram sections, and in situ electron microprobe monazite geochronology suggest that decompression from peak conditions of 1.1 GPa, c. 800 °C involved several distinct stages under first dry and then hydrated conditions. Retrograde re‐equilibration occurred at 0.5–0.4 GPa, 550–650 °C. Morphology, X‐ray maps, and microprobe dates indicate several distinct monazite generations. Populations 1 and 2 are relatively high yttrium (Y) monazite that grew at 2.55–2.50 Ga and correspond to an early granulite facies event. Population 3 represents episodic growth of low Y monazite between 2.50 and 2.15 Ga whose general significance is still unclear. Population 4 reflects low Y monazite growth at 1.9 Ga, which corresponds to the youngest period of high‐pressure metamorphism. Finally, population 5 is restricted to the hydrous retrograded granulite and represents high Y monazite growth at 1.85 Ga that is linked directly to the synkinematic garnet‐consuming hydration reaction (KFMASH): Grt + Kfs + H2O = Bt + Sil + Qtz. Two samples yield weighted mean microprobe dates for this population of 1853 ± 15 and 1851 ± 9 Ma, respectively. Subsequent xenotime growth correlates with the reaction: Grt + Sil + Qtz + H2O = Crd. We suggest that the shear zone acted as a channel for fluid produced by dehydration of metasediments in the underthrust domain.

Time constraints on deformation and metamorphism from EPMA dating of monazite in the Proterozoic Robertson River Metamorphics, NE Australia

EPMA dating of monazite confirms the relative timing of a succession of Foliation Intersection Axis trends in porphyroblasts (FIAs) and two periods of progressive metamorphism intervened by retrogression in the Robertson River Metamorphics (NE Australia). A complete pressure-temperature-deformation-time (P-T-D-t) path has been deciphered involving an early clockwise P-T loop followed by high temperature, low-pressure metamorphism. Successive generations of monazite were identified using microstructure and FIA trends and then dated. The succession of four FIAs trending ENE–WSW (FIA1), E–W (FIA2), N–S (FIA3), NE–SW (FIA4) plus four matrix foliations, S 1– S 4 were accompanied by three periods of garnet and two periods of staurolite growth. Monazite grew in response to most of these events. Isopleth thermobarometery, using P-T pseudosections in the MnNCKFMASH system for garnet porphyroblasts selected based on FIA trends in their core, reveal that pressure increased progressively from 3–4 kb at 530–550 °C to 6–7 kb at 600–620 °C during medium pressure-temperature metamorphism. This was accompanied by changing bulk shortening directions from NNW-SSE to N–S to E–W (perpendicular to FIAs 1, 2 and 3). The development of FIA1 and FIA2 occurred between 1655 ± 2.2 Ma and 1625 ± 15 Ma. FIA3 developed over 30–50 Ma periods from 1625 ± 15 Ma to 1586 ± 6 Ma. Retrogressive metamorphism, decompression and exhumation occurred over a ∼45 Ma period. Further metamorphism accompanied regional granite intrusion and generated lower pressure–higher temperature metamorphism during the development of FIA4 (NW-SE bulk shortening) at 1542 ± 8 Ma. This period possibly extended for another 30–50 Ma based on the monazite ages obtained from the youngest foliations in the matrix. Correlation of these data with other NE Australian Proterozoic Inliers suggests that they were all once part of a single orogen that developed from ca. 1650 to 1500 Ma.

Genesis of monazite and Y zoning in garnet from the Black Hills, South Dakota

The paragenesis of monazite in metapelitic rocks from the contact aureole of the Harney Peak Granite, Black Hills, South Dakota, was investigated using zoning patterns of monazite and garnet, electron microprobe dating of monazite, bulk-rock compositions, and major phase mineral equilibria. The area is characterized by low-pressure and high-temperature metamorphism with metamorphic zones ranging from garnet to sillimanite zones. Garnet porphyroblasts containing euhedral Y annuli are observed from the garnet to sillimanite zones. Although major phase mineral equilibria predict resorption of garnet at the staurolite isograd and regrowth at the andalusite isograd, textural and mass balance analyses suggest that the formation of the Y annuli is not related to the resorption-and-regrowth of garnet having formed instead during garnet growth in the garnet zone. Monazite grains in Black Hills pelites were divided into two generations on the basis of zoning patterns of Y and U: monazite 1 with low-Y and -U and monazite 2 with high-Y and -U. Monazite 1 occurs in the garnet zone and persists into the sillimanite zone as cores shielded by monazite 2 which starts to form in the andalusite zone. Pelites containing garnet porphyroblasts with Y annuli and monazite 1 with patchy Th zoning are more calcic than those with garnet with no Y annuli and monazite with concentric Th zoning. Monazite 1 is attributed to breakdown of allanite in the garnet zone, additionally giving rise to the Y annuli observed in garnet. Monazite 2 grows in the andalusite zone, probably at the expense of garnet and monazite 1 in the andalusite and sillimanite zones. The ages of the two different generations of monazite are within the precision of chemical dating of electron microprobe. The electron microprobe ages of all monazites from the Black Hills show a single ca. 1713 Ma population, close to the intrusion age of the Harney Peak Granite (1715 Ma). This study demonstrates that Y zoning in garnet and monazite are critical to the interpretation of monazite petrogenesis and therefore monazite ages.

In situ chemical dating of tectonothermal events in the French Variscan Belt

Abstract In situ U–Th–Pb geochronology on monazite using Electron Probe Micro Analyser has been performed on migmatite in the southern French Variscan Massif Central in order to decipher its complex history. After the Early Visean (340 Ma) nappe stacking, the Cévennes area experienced a regional migmatization already dated 330–325 Ma in northern Cévennes. In these rocks two monazite populations are recognized on the basis of petrology texture and geochemistry. The oldest monazite generation that appears as inclusion in K‐feldspar is dated at 331 ± 4 Ma. This age complies with that of the crustal melting experienced by the Cévennes metamorphic series. The youngest monazite generation is interstitial and gives an age of 320 ± 5 Ma. A hydrothermal origin, coeval with the peraluminous magmatism that predates the formation of the Late Carboniferous Velay Dome is proposed as a working hypothesis to account for the formation of this second monazite generation.

Five generations of monazite in Langtang gneisses: implications for chronology of the Himalayan metamorphic core

AbstractMonazite grains from Greater Himalayan Sequence gneisses, Langtang valley, Nepal, were chemically mapped and then dated in situ via Th–Pb ion‐microprobe analysis. Correlation of ages and chemistry reveals at least five different generations of monazite, ranging from c. 9 to &gt;300 Ma. Petrological models of monazite chemistry provide a link between these generations and the thermal evolution of these rocks, yielding an age for the melting of Greater Himalayan rocks within the Main Central Thrust sheet (c. 16 Ma), and for the timing of thrust sheet emplacement that are younger than commonly viewed. Chemical characterization of monazite is vital prior to chronological microanalysis, and many ages previously reported for monazite from the Greater Himalayan Sequence are interpretationally ambiguous.

Fast Cooling and Exhumation of the Valhalla Metamorphic Core Complex, Southeastern British Columbia

High-grade, migmatitic paragneisses (820°C, 8 kbar peak conditions) of the Valhalla metamorphic core complex experienced the retrograde net transfer reaction (ReNTR): garnet + melt + K-feldspar = sillimanite + biotite + plagioclase during cooling and crystallization of the melt, resulting in an increase in the Fe/(Fe + Mg) of both garnet and biotite at their margins. Diffusion profiles generated by this increase in Fe/(Fe + Mg) have been modeled numerically. Optimal fits were obtained with an initial, short period of rapid cooling (> 200°C/m.y. for > 0.5 m.y.) followed by slower (but still rapid) cooling at 20-30°C/m.y. for 10-20 m.y. Four generations of monazite have been observed. Monazite inclusions within garnet contain only generations 1 and 2, and generations 3-4 are found only in matrix monazites. Generations 1-2 record ages of 75-85 Ma, which is believed to represent the prograde metamorphism of the complex. A single age of ˜105 Ma may represent an early contact metamorphic episode. Third-generation monazite is 60 ±2 Ma, and reflects near-peak conditions. Generation 4 monazite, which occurs only on the outer rims of monazite, has not been dated, but is believed to represent monazite produced during melt crystallization. A hornblende K-Ar age of 58±2 Ma suggests a cooling rate of several tens to several hundreds of degrees/m.y. immediately following the metamorphic peak. The extremely rapid cooling (hundreds of degrees/m.y.) at ˜60 Ma must have been tectonically induced and it is suggested that transport of the hot Valhalla complex onto cooler basement up a thrust ramp along the underlying Gwillim Creek shear zone was the cause. Two-dimensional thermal modeling indicates that thrust transport of several centimeters/year up a 20° ramp will produce the observed cooling rates. The subsequent slower cooling rate (tens of degrees/m.y.) is believed to have been the result of extensional unroofing by low-angle normal faulting along the Slocan Lake fault.

Four generations of accessory-phase growth in low-pressure migmatites from SW New Hampshire

Mineral compositions and reaction textures found in migmatitic gneisses from near Gilsum, New Hampshire, constrain both peak metamorphic pressure ( P ) and temperature ( T ) conditions and the P - T path. Large K-feldspar porphyroblasts indicate isobaric heating of the samples at P P > 4 kb on the cooling path. Four generations of monazite have been identified, three of which have been linked to specific whole-rock reactions. Monazite (4) (the last generation) was produced with xenotime ± apatite during melt crystallization and consumption of garnet and cordierite. Monazite (3) grew in a xenotime-absent mineral assemblage as garnet + muscovite reacted to form sillimanite + biotite. Monazite (2) grew in a xenotime-bearing ± garnet + biotite + chlorite assemblage, as xenotime and chlorite were consumed during garnet production. Monazite (1) has not been linked to a specific reaction; it may be detrital, record an earlier metamorphic event, or represent disequilibrium overgrowths of xenotime. YAG-monazite, YAG-xenotime, and monazite-xenotime thermometry for monazite generations (2)–(4) yield temperatures consistent with major-phase thermometers.

Idades Th-U-PbTde monazita e geotermobarometria de rochas metapelíticas de alto grau do sistema orogênico Itabuna-Salvador-Curaçá, BA, Brasil: um estudo em microssonda eletrônica

Electron microprobe Th-U-PbT dating of monazite was performed on aluminous gneiss samples from the northern Jacurici and southern Ilhéus areas of the Itabuna-Salvador-Curaçá Orogenic System. Studied samples are spinel-sillimanite-cordierite-garnet gneiss and corundum-spinel-sillimanite-garnet gneiss that equilibrated during a high temperature metamorphism at T-P conditions of about 835°C/6.9 kbar and 890°C/7.4 kbar, respectively. Microprobe ages of monazite in equilibrium with these mineral associations are interpreted as true crystallization ages and date granulite facies metamorphism. Both samples gave ages of about 2.07 and 2.08 (± 0.02) Ga, in agreement with the known geochronological pattern in this system. The Ilhéus sample contains a younger, contrasting monazite generation (e.g., with distinct REE patterns and Th/Gd ratios), dated at ca. 1.97 (± 0.04) Ga. Based on the occurrence of the corundum-sillimanite-quartz association in the sample and information from the literature, we suggest that this age may be related to a regional decompression event and/or a thermal overprint due to late intrusive magmatism (1.9 - 2.0 Ga) in the orogenic system.