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.     

Pressure and temperature conditions for crystallization of metamorphic allanite and monazite in metapelites: a case study from the Miyar Valley (high Himalayan Crystalline of Zanskar,NW India)

The textural and chemical evolution of allanite and monazite along a well‐constrained prograde metamorphic suite in the High Himalayan Crystalline of Zanskar was investigated to determine the P–T conditions for the crystallization of these two REE accessory phases. The results of this study reveals that: (i) allanite is the stable REE accessory phase in the biotite and garnet zone and (ii) allanite disappears at the staurolite‐in isograd, simultaneously with the occurrence of the first metamorphic monazite. Both monazite and allanite occur as inclusions in staurolite, indicating that the breakdown of allanite and the formation of monazite proceeded during staurolite crystallization. Staurolite growth modelling indicates that staurolite crystallized between 580 and 610 °C, thus setting the lower temperature limit for the monazite‐forming reaction at ~600 °C. Preservation of allanite and monazite inclusions in garnet (core and rim) constrains the garnet molar composition when the first monazite was overgrown and subsequently encompassed by the garnet crystallization front. Garnet growth modelling and the intersection of isopleths reveal that the monazite closest to the garnet core was overgrown by the garnet advancing crystallization front at 590 °C, which establishes an upper temperature limit for monazite crystallization. Significantly, the substitution of allanite by monazite occurs in close spatial proximity, i.e. at similar P–T conditions, in all rock types investigated, from Al‐rich metapelites to more psammitic metasedimentary rocks. This indicates that major silicate phases, such as staurolite and garnet, do not play a significant role in the monazite‐forming reaction. Our data show that the occurrence of the first metamorphic monazite in these rocks was mainly determined by the P–T conditions, not by bulk chemical composition. In Barrovian terranes, dating prograde monazite in metapelites thus means constraining the time when these rocks reached the 600 °C isotherm.     

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

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

Phase equilibria of bastnaesite,allanite, and monazite: Bastnaesite-out isograde in metapelites of the Vorontsovskaya group,Voronezh crystalline massif

Paleoproterozoic metapelites of the Vorontsovskaya structure contain accessory REE phosphates (monazite, xenotime, and REE-apatite), fluorine-carbonates (bastnaesite and synchysite), and silicate (allanite). Analysis of phase equilibria involving REE-bearing minerals indicates that bastnaesite is stable only in the greenschist facies and decomposes with the synthesis of monazite at temperatures below the staurolite isograde (490–500°C) at a pressure of 3 kbar. Monazite first appears in the greenschist facies, and its stability expands with increasing temperature, including the granulite facies. A diversity of reaction textures suggests that the mineral is formed in the garnet zone by a reaction of bastnaesite with apatite and by the partial decomposition of REE-bearing chlorite. Monazite is produced in the garnet and staurolite zones by a reaction of allanite with apatite and by a decomposition reaction of REE-bearing apatite.     

Bulk chemical control on metamorphic monazite growth in pelitic schists and implications for U–Pb age data

Several petrographic studies have linked accessory monazite growth in pelitic schist to metamorphic reactions involving major rock‐forming minerals, but little attention has been paid to the control that bulk composition might have on these reactions. In this study we use chemographic projections and pseudosections to argue that discrepant monazite ages from the Mount Barren Group of the Albany–Fraser Orogen, Western Australia, reflect differing bulk compositions. A new Sensitive High‐mass Resolution Ion Microprobe (SHRIMP) U–Pb monazite age of 1027 ± 8 Ma for pelitic schist from the Mount Barren Group contrasts markedly with previously published SHRIMP U–Pb monazite and xenotime ages of c. 1200 Ma for the same area. All dated samples experienced identical metamorphic conditions, but preserve different mineral assemblages due to variable bulk composition. Monazite grains dated at c. 1200 Ma are from relatively magnesian rocks dominated by biotite, kyanite and/or staurolite, whilst c. 1027 Ma grains are from a ferroan rock dominated by garnet and staurolite. The latter monazite population is likely to have grown when staurolite was produced at the expense of garnet and chlorite, but this reaction was not intersected by more magnesian compositions, which are instead dominated by monazite that grew during an earlier, greenschist facies metamorphic event. These results imply that monazite ages from pelitic schist can vary depending on the bulk composition of the host rock. Samples containing both garnet and staurolite are the most likely to yield monazite ages that approximate the timing of peak metamorphism in amphibolite facies terranes. Samples too magnesian to ever grow garnet, or too iron‐rich to undergo garnet breakdown, are likely to yield older monazite, and the age difference can be significant in terranes with a polymetamorphic history.     

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

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

Time‐scale of deformation and intertectonic phases revealed by P–T–D–t relationships in the orogenic middle crust of the Orlica‐Śnieżnik Dome,Polish/Czech Central Sudetes

A section of the orogenic middle crust (Orlica‐?nie?nik Dome, Polish/Czech Central Sudetes) was examined to constrain the duration and significance of deformation (D) and intertectonic (I) phases. In the studied metasedimentary synform, three deformation events produced an initial subhorizontal foliation S1 (D₁), a subsequent subvertical foliation S2 (D₂) and a late subhorizontal axial planar cleavage S3 (D₃). The synform was intruded by pre‐, syn‐ and post‐D₂ granitoid sheets. Crystallization–deformation relationships in mica schist samples document I_1–2 garnet–staurolite growth, syn‐D₂ staurolite breakdown to garnet–biotite–sillimanite/andalusite, I_2–3 cordierite blastesis and late‐D₃ chlorite growth. Garnet porphyroblasts show a linear Mn–Ca decrease from the core to the inner rim, a zone of alternating Ca–Y‐ and P‐rich annuli in the inner rim, and a Ca‐poor outer rim. The Ca–Y‐rich annuli probably reflect the occurrence of the allanite‐to‐monazite transition at conditions of the staurolite isograd, whereas the Ca‐poor outer rim is ascribed to staurolite demise. The reconstructed P–T path, obtained by modelling the stability of parageneses and garnet zoning, documents near‐isobaric heating from ~4 kbar/485 °C to ~4.75 kbar/575 °C during I_1–2. This was followed by a progression to 4–5 kbar/580–625 °C and a subsequent pressure decrease to 3–4 kbar during D₂. Pressure decrease below 3 kbar is ascribed to I_2–3, whereas cooling below ~500 °C occurred during D₃. In the dated mica schist sample, garnet rims show strong Lu enrichment, oscillatory Lu zoning and a slight Ca increase. These features are also related to allanite breakdown coeval with staurolite appearance. As Lu‐rich garnet rims dominate the Lu–Hf budget, the 344 ± 3 Ma isochron age is ascribed to garnet crystallization at staurolite grade, near the end of I_1–2. For the dated sample of amphibole–biotite granitoid sheet, a Pb–Pb single zircon evaporation age of 353 ± 1 Ma is related to the onset of plutonic activity. The results suggest a possible Devonian age for D₁, and a Carboniferous burial‐exhumation cycle in mid‐crustal rocks that is broadly coeval with the exhumation of neighbouring HP rocks during D₂. In the light of published ages, a succession of telescoping stages with time spans decreasing from c. 10 to 2–3 Ma is proposed. The initially long period of tectonic quiescence (I_1–2 phase, c. 10 Ma) inferred in the middle crust contrasts with contemporaneous deformation at deeper levels and points to decoupled P–T–D histories within the orogenic wedge. An elevated gradient of ~30 °C km^?1 and assumed high heating rates of c. 20 °C Ma^?1 are explained by the protracted intrusion of granitoid sheets, with or without deformation, whereas fast vertical movements (2–3 Ma, D₂ phase) in the crust require the activity of deformation phases.     

Monazite behaviour and age significance in poly-metamorphic high-grade terrains: A case study from the western Musgrave Block, central Australia

Integrated, in situ textural, chemical and electron microprobe age analysis of monazite grains in a migmatitic metapelitic gneiss from the western Musgrave Block, central Australia has identified evidence for multiple events of growth and recrystallisation during poly-metamorphism in the Mesoproterozoic. Garnet + sillimanite-bearing metapelite underwent partial melting and segregation to palaeosome and leucosome during metamorphism between 1330 and 1296 Ma, with monazite grains in leucosome recording crystallisation at 1300 Ma. Monazite breakdown during melting is inferred to have occurred in the palaeosome. During a subsequent granulite facies event at 1200 Ma, deformation and metamorphism of leucosome and palaeosome resulted in partial disturbance of ages and potential minor growth on 1300 Ma monazite in leucosome. Growth of new, high-Y (+HREE) monazite in palaeosome domains occurred during garnet breakdown in the presence of sillimanite to cordierite and spinel, as a result of post-peak isothermal decompression. Diffusive enrichment of resorbed garnet rims in Y + HREE suggests garnet breakdown occurred slower than volume diffusion of REE. Monazite in both palaeosome and leucosome were subsequently partially to penetratively recrystallised during a retrogression event that is suggested to have occurred at 1150–1130 Ma. The intensity of recrystallisation and disturbance of ages appears linked to proximity to retrogressed garnet porphyroblasts and their occurrence in the relatively reactive or ‘fertile’ local environments provided by the palaeosome/mesosome volumes, which caused localised changes in retrogressive fluids towards compositions more aggressive to monazite. Like reaction textures, it is apparent that domainal equilibrium and reaction may control or at least strongly influence monazite REE and U–Th–Pb chemistry and hence ages.     

Polymetamorphic evolution of pelitic schists and evidence for Permian low-pressure metamorphism in the Vepor Unit, West Carpathians

Phase equilibrium modelling and monazite microprobe dating were used to characterize the polymetamorphic evolution of metapelites from the northern part of the Vepor Unit, West Carpathians. Three generations of garnet and associated metamorphic assemblages found in these rocks correspond to three distinct metamorphic events related to the Variscan orogeny, a Permian phase of crustal extension and the Alpine orogeny. Variscan staurolite‐bearing and Alpine chloritoid‐bearing assemblages record medium‐temperature and medium‐pressure regional metamorphisms reaching 540–570 °C/5–7.5 kbar and 530–550 °C/5–6.5 kbar respectively. The Permian metamorphic assemblage involves garnet, andalusite, sillimanite, biotite, muscovite, plagioclase and corundum and locally forms silica‐undersaturated andalusite‐biotite‐spinel coronas around older staurolite. The transition from andalusite to sillimanite indicates a prograde low‐pressure and medium‐temperature metamorphism characterized by temperature increase from 500 to 650 °C at ～3 kbar. As accessory monazite is abundant in the rocks, an attempt was made to derive its age of formation by means of electron microprobe‐based Th‐U‐Pb chemical dating. Despite the polymetamorphic nature of the metapelites, the monazite yielded uniform Permian ages. Microstructures confirm that monazite was formed in relation to the low‐pressure and medium‐temperature paragenesis, and the weighted average ages obtained for two different samples are 278 ± 5 and 275 ± 12 Ma respectively. The virtual lack of Variscan and Alpine monazite populations points to interesting aspects concerning the growth systematics of monazite in metamorphic rocks.     

Contrasting response of monazite and zircon to a high-T thermal overprint

Contact metamorphism in the aureole of the 1322 Ma Makhavinekh Lake Pluton, northern Labrador, affected monazite and zircon in the adjacent 1850 Ma metapelitic gneisses. Transformation of regional garnet and sillimanite to lower-pressure symplectitic intergrowths of cordierite, orthopyroxene, and spinel was accompanied by resorption of inherited monazite inclusions in garnet coupled with the appearance of coronitic high-Y monazite rims. In situ ion-microprobe dating is used to show that high-Y rims formed during contact metamorphism. Liberation of Y and HREE from garnet also gave rise to new xenotime growth. The coronitic nature of monazite overgrowths reflects the diffusion-controlled nature of net-transfer reactions whereas its higher Y composition reflects equilibration with xenotime at peak T (> 800 °C) conditions in the inner aureole. Very thin overgrowths on inherited zircon were also encountered, but only where zircon is surrounded by the symplectitic assemblage, reflecting liberation of Zr from garnet. Although these overgrowths are too thin to date using conventional ion-microprobe techniques, well-developed triple junctions between zircon and orthopyroxene suggests that they grew in textural equilibrium with the contact metamorphic assemblage.

In contrast to monazite, inherited zircon remained intact during contact metamorphism, exhibiting no change in morphology (other than the growth of thin rims) or internal zoning throughout the aureole. However, inherited sector-zoned zircons of anatectic origin display evidence for intracrystalline Pb redistribution in the inner aureole. In these samples, ion-microprobe analyses encountered heterogeneous Pb signals and a dispersion of ²⁰⁷Pb / ²⁰⁶Pb dates away from the well constrained 1850 Ma age of regional metamorphism. Whereas analyses from the outer aureole faithfully record the age of regional metamorphism, those from the inner aureole are normally and reversely discordant and distributed along a line collinear with a 1850 to 1322 Ma discordia. This disturbance is correlated with proximity to the pluton implying that Pb was mobile in the zircon lattice during contact metamorphism. Most grains are characterized by apparent Pb loss from low-U domains and apparent Pb gain in higher-U domains. These data are interpreted to reflect recovery of strained crystalline domains leading to expulsion of Pb* that was able to efficiently diffuse into higher-U domains that were partly amorphous prior to rapid reheating in the inner aureole.     

Monazite and xenotime petrogenesis in the contact aureole of the Makhavinekh Lake Pluton,northern Labrador

High-temperature (700–900°C) metamorphism in the contact aureole of the Makhavinekh Lake Pluton (MLP), northern Labrador, led to the growth of monazite and xenotime during progressive replacement of regional garnet-bearing assemblages (M1) by lower-pressure symplectitic coronas of orthopyroxene + cordierite (M2). In the inner aureole (<500 m from the contact), where M1 garnet is strongly resorbed, high-Y+HREE monazite (X_Y+HREE 0.14–0.18) occurs as small isolated grains and as discontinuous rims on partially resorbed pre-M2 monazites that were liberated from garnet. Xenotime also occurs as small isolated grains within M2 coronas. Ion-microprobe dating of thin, high-Y rims indicates that new monazite growth occurred during M2. Monazite–xenotime miscibility-gap temperatures are consistent with Al-solubility-in-orthopyroxene thermometry estimates, indicating that peak temperatures in the inner aureole are accurately recorded and preserved by monazite. M2 monazite records, therefore, the temperature and timing of M2 metamorphism. Two net-transfer reactions, modelled using singular value decomposition in the system P-Y-HREE-LREE, are proposed to account for the growth of M2 phosphates: (1) 38 Grt1 + 1 Mnz1 = 1.13 Mnz2 and (2) 737 Grt1 + 1 Ap = 1 Mnz2 + 3.4 Xno2. Reaction (1) conserves P and gave rise to locally coronitic high-Y overgrowths on partially resorbed pre-M2 monazite, whereas reaction (2) accounts for the growth of small new monazite and xenotime grains. Both reactions were highly localized within individual M2 coronas due to slow intergranular diffusion accompanying fluid-undersaturated metamorphism in the MLP aureole. Similar monazite-forming reactions are expected in other polymetamorphosed granulites.This revised version was published online in November 2004 with corrections to the reference Pyle JM et al. (2002).     

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

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

Conventional and in situ geochronology of the Teplá Crystalline unit, Bohemian Massif: implications for the processes involving monazite formation

The Teplá Crystalline unit (TCU), western Bohemian Massif, proves highly suitable for studying the effects of differential metamorphic reworking on the U–Th–Pb systematics in monazite, as the overprint of Variscan regional metamorphism onto high-grade Cadomian paragneisses intensifies progressively towards the northwest. Although variably hampered by scarcity, small size, and low uranium contents of monazite, isotope dilution–thermal ionisation mass spectrometry of monazite from paragneisses from the garnet, staurolite, and kyanite zones of the TCU gives a narrow ²⁰⁶Pb/²³⁸U age range from 387 to 382 Ma for Variscan peak metamorphism. These data are supported by 382–373 Ma monazite ages derived from electron microprobe analyses. Inheritance of older components in grains from the central TCU imply major “resetting” of pre-Variscan monazite around 380 Ma, possibly due to widespread garnet growth during Variscan metamorphism, which led to the consumption of pre-Variscan high-Y monazite and subsequent growth of new low-Y monazite. Concordant 498–494 Ma monazite ages in a migmatitic paragneiss close to the adjacent Mariánské Lázně Complex (MLC) grew in response to metagabbro emplacement in the MLC from 503 to 496 Ma and not during either Cadomian or Variscan regional metamorphism. Backscatter imaging and electron microprobe analyses reveal that discordant monazite of the migmatite comprises a mix of various age domains that range from ca. 540 to 380 Ma. Combined evidence presented here suggests that instead of Pb loss by volume diffusion, the apparent resetting of the U–Th–Pb systematics in monazite rather involves new crystal growth or regrowth by recrystallisation and dissolution/reprecipitation.     

Sequential, syndeformational porphyroblast growth during Hercynian low-pressure/high-temperature metamorphism in the Canigou massif, Pyrenees

The sequence of growth of garnet, staurolite and aluminosilicate in Fe-rich metapelitic rocks from the Canigou massif, Pyrenees, is established using evidence of inclusion, reaction and pseudomorphing textures between the different minerals, compositional zoning patterns in garnet and staurolite (that can be related to the KFMASH reaction grid), and the geometric relations between inclusion trails in the porphyroblasts and the matrix microstructures. The evidence indicates that garnet and staurolite commenced growth before aluminosilicate in all cases, even where all three are in textural equilibrium. Interpretation of the reaction textures between the porphyroblasts and of the compositional zoning in garnet and staurolite in terms of the KFMASH reaction grid indicates the importance of continuous reactions in the development of these phases. Some garnet and staurolite porphyroblasts underwent renewed growth during breakdown, producing rims enriched in Mn and Zn respectively. The presence of aluminosilicate in these assemblages (i.e. the absence of a clear andalusite-absent zone in the field) is attributed to a strong pressure-dependence for the aluminosilicate-producing reactions. Porphyroblast-matrix microstructural relations indicate that Hercynian metamorphism in the massif was synchronous with the development of the regional subhorizontal foliation (S3).     

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

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

Multi-stage pseudomorphic replacement of garnet during polymetamorphism: 1. Microstructures and their interpretation

Metapelites from the southern aureole of the Vedrette di Ries tonalite (eastern Alps) were variably overprinted by contact and earlier regional metamorphic events during pre-Alpine and Alpine metamorphic cycles. In these rocks, starting from a primary garnet mica-schist (garnet stage), a complex sequence of transformations, affecting the site of the garnet, has been recognized. In the outermost part of the aureole, the primary garnet sites are occupied by nodules of kyanite (kyanite stage). Closer to the tonalite, kyanite is replaced by staurolite (staurolite stage), which in turn is pseudomorphed by muscovite (muscovite stage). The aggregates of kyanite do not overgrow garnet directly; they post-date a stage (fibrolite stage) represented by the pseudomorphic alteration of garnet into fibrolitic sillimanite plus biotite. A further sericite stage is likely to have occurred between the fibrolite and kyanite stages. Preservation of the sub-spherical garnet shape during all these transformations and persistence of mineralogical and textural relicts from earlier stages were favoured by the very low strain experienced by the rocks since the garnet stage. The textural sequence is in agreement with the metamorphic history of this part of the Austroalpine basement of the Eastern Alps: the garnet and fibrolite stages, and the coeval main foliation of the samples, are referred to the high-grade Hercynian metamorphism; the kyanite stage to the Eo-Alpine metamorphism; the staurolite and muscovite stages to the Oligocene contact metamorphism. It is suggested that kyanite growth as microgranular aggregates took place in polymetamorphic rocks where static, high- P /low- T  metamorphism overprinted high- T  assemblages that contained sillimanite or andalusite.     

Three generations of monazite in Austroalpine basement rocks to the south of the Tauern Window: evidence for Variscan, Permian and Eo-Alpine metamorphic events

Three monazite generations were observed in garnet-bearing micaschists from the Schobergruppe in the basement to the south of the Tauern Window, Eastern Alps. Low-Y monazite of Variscan age (321?±?14?Ma) and high-Y monazite of Permian age (261?±?18?Ma) are abundant in the mica-rich rock matrix and in the outer domains of large garnet crystals. Pre-Alpine monazite commonly occurs as polyphase grains with low-Y Variscan cores and high-Y Permian rims. Monazite of Eo-Alpine age (112?±?22?Ma) is rarer and was observed as small, partly Y-enriched grains (3?wt. %?Y₂O₃) in the rock matrix and within garnet. Based on monazite-xenotime thermometry, Y?+?HREE values in monazite indicate minimum crystallization conditions of 500?°C during the Variscan and 650?°C for the Permian and Alpine events, respectively. Garnet zoning and thermobarometric calculations with THERMOCALC 3.21 record an amphibolite facies, high-pressure stage of ~600?°C/13?C16?kbar, followed by a thermal maximum at 650?C700?°C and 6?C9?kbar. The Eo-Alpine age for these two events is supported by inclusions of Cretaceous monazite in the garnet domains used for thermobarometric constraints and through the high growth temperatures of Eo-Alpine monazite, which is consistent with that of the thermal maximum (~700?°C). The age and growth conditions of a few Mn-rich garnet cores, sporadically present within Eo-Alpine garnet, are unclear because inclusions of monazite, plagioclase and biotite necessary for thermobarometric- and age constraints are absent. However, based on monazite thermometry, Permian and Variscan metamorphic conditions were high enough for the growth of pre-Alpine garnet. The formation of Variscan garnet and its later resorption, plus Y-release, would also explain the high Y in Permian monazite, which cannot originate from preexisting Variscan monazite only. Monazite of Variscan, Permian and/or Eo-Alpine ages were also observed in other garnet-bearing micaschists from the Schobergruppe. This suggests that the basement of the Schobergruppe was overprinted by three discrete metamorphic events at conditions of at least lower amphibolite facies. While the Variscan event affected all parts of this basement, the younger events are more pronounced in its structurally lower units.     

Regression Modelling of Metamorphic Reactions in Metapelites, Snow Peak, Northern Idaho

The second of two periods of regional metamorphism that affectedpelitic rocks near Snow Peak caused complete re-equilibrationof mineral assemblages and resulted in a consistent set of metamorphicisograds. Metamorphic chlorite and biotite occur in the lowestgrade rocks. With increasing grade, garnet, staurolite, andkyanite join the assemblage, resulting in a transition zonecontaining all the above phases. At higher grade, chlorite,and finally staurolite disappear. Mass balance relations at isograds and among minerals of low-varianceassemblages have been modelled by a non-linear least-squaresregression technique. The progressive sequence can be describedin terms of schematic T-X_H2O relations among chlorite, biotite,garnet, staurolite, and kyanite at P_total above the KFMASH invariantpoint involving those phases. The first appearance of garnetwas the result of an Fe-Mg-Mn continuous reaction. As temperaturerose, the garnet zone assemblage encountered the stauroliteisograd reaction, approximated by the model reaction: 3?0 chlorite + 1?5 garnet + 3?3 muscovite + 05 ilmenite = 1?0staurolite + 3?1 biotite + 1?5 plagioclase + 3?3 quartz + 10?3H₂O. The staurolite zone corresponds to buffering along this reactionto the intersection where chlorite, biotite, garnet, staurolite,and kyanite coexist. The transition zone assemblage formed byreaction at this T–X _H2O intersection which migrates towardmore H2O-rich fluid composition with progressive reaction. Thenet reaction at the intersection is approximated by the transitionzone reaction: 1?0 chlorite +1?1 muscovite + 0?2 ilmenite = 2?7 kyanite + 1?0biotite + 0?4 albite + 4?2 H₂O. Chlorite was commonly the first phase to have been exhaustedand the remaining assemblage was buffered along a staurolite-outreaction, represented by the model reaction: 1?0 staurolite + 3?4 quartz + 0?4 anorthite + 1?4 garnet + 0?1ilmenite + 7?9 kyanite + 2?0 H₂O. Consumption of staurolite by this reaction resulted in the highestgrade assemblage, which contains kyanite, garnet, biotite, muscovite,quartz, plagioclase, ilmenite, and graphite.     

Interplay between equilibrium and kinetics in prograde metamorphism of pelites: an example from the Nelson aureole, British Columbia

The distribution of metapelitic mineral assemblages in the Nelson aureole, British Columbia, generally conforms to what is predicted from phase equilibria. However, in detail, the sequence and spacing of isograds, mineral textures and mineral compositions and mineral chemical zoning do not. Two of the main disequilibrium features in the aureole are: (i) delay in the onset and progress of several reactions, i.e. overstepping in temperature; and (ii) unreactivity of staurolite and especially garnet porphyroblasts when they are reactants in prograde reactions. The thermal overstepping is ascribed to difficulty of nucleation of the product porphyroblasts and sluggishness of dissolution of porphyroblasts when they are reactants. The extent to which these kinetic barriers delay the onset of reaction is related to the reaction affinity of each reaction, defined herein as the Gibbs free‐energy difference between the thermodynamically stable, but not‐yet‐crystallized, products and the metastable reactants. For oversteps in temperature (ΔT), reaction affinity is, in turn, related to the difference in entropy (ΔS) between these two states through the relation A = ΔT * ΔS. Mineral reactions which release large quantities of H₂O, such as chlorite‐consuming reactions, have a higher entropy change per unit of temperature overstep, and therefore a higher reaction affinity, than those which release little or no H₂O, such as the chlorite‐free staurolite‐consuming reaction. Thermal overstepping is consequently expected to be less for the former than for the latter, as was estimated in the aureole where 0 to 30 °C overstepping was required for garnet, staurolite and andalusite growth from a muscovite + chlorite‐bearing precursor rock and ～70 °C overstepping was required for the growth of Al₂SiO₅ from a staurolite‐bearing, chlorite‐free precursor. In all cases, reaction progress was strongly influenced by the presence or absence of fluid, with presence of fluid lowering kinetic barriers to nucleation and growth and therefore the degree of thermal overstepping. Textural features of rocks from the nearly coincident garnet, staurolite and andalusite isograds are suggestive of a fluid‐catalysed ‘cascade effect’ in which reaction took place in a narrow temperature interval; several competing muscovite + chlorite‐consuming reactions, some metastable, appear to have occurred in parallel. Metamorphic reaction, fluid release and possibly fluid presence in general in the aureole were episodic rather than continuous, and in several cases well removed from equilibrium conditions. The extent to which these findings apply to regional metamorphism depends on several factors, a major one being enhanced deformation, which is expected to lower kinetic barriers to nucleation and growth.     

Growth, preservation of Paleoproterozoic-shear-zone-hosted monazite, north of the Western Dharwar Craton (India), and implications for Gondwanaland assembly

We examine the conditions and processes of growth and preservation of multiaged monazite in micaceous matrix and in garnet porphyroblasts in staurolite–kyanite mica schists hosted in a hitherto-undiscovered shear zone that limits the northern extent of the Western Dharwar Craton (WDC), India. Garnet in the footwall schists grew during mid-crustal (600 ± 40 °C, 7.3 ± 1.2 kbar) loading and cooling as a consequence of the northward transport of the WDC lithologies. U–Th–Pb (total) ages in monazites in the matrix and in post-tectonic garnets yield well-defined peaks at 2.5, 2.2 and 1.9 Ga. In garnet, 2.5 and 2.2 Ga monazite grains, and 2.2 Ga monazites with 2.5 Ga cores are commonly occluded, but monazites with 1.9 Ga mantles around older cores are rare. By contrast, in the matrix, 1.9 Ga monazite grains and monazite with 1.9 Ga mantles around older cores are prominent, but the peak age frequencies of the two older populations are significantly lower than for monazites hosted as inclusions in garnet. Both in the matrix and garnet, the low-Th, high-Y domains in monazites yield the two older peak ages, while the 1.9 Ga ages correspond to the high-Th, low-Y domains. The preponderance of older ages in monazite hosted as inclusions in garnet relative to matrix monazites is because garnets formed between 2.2 and 1.9 Ga shielded the older monazites from dissolution–precipitation at 1.9 Ga. A few 1.9 Ga monazites hosted as inclusions in the garnet rims suggest renewed garnet growth at post-1.9 Ga. Multiple Pb–Pb age populations (2.5, 2.25, 2.1 and 1.8 Ga) in detrital zircon in the Sahanataha Group north of the Paleoarchean Antongil-Masora block (NE Madagascar) are identical to the multiple monazites ages north of the WDC, inferred to share a similar history and to be contiguous with the Antongil-Masora block in pre-Jurassic reconstructions of the Gondwanaland. We suggest the newly discovered Paleoproterozoic tectonic zone continued westward into Madagascar north of the Antongil-Masora block and constituted the hitherto-unexplained basement for the multiaged detrital zircons in the Sahanataha quartzites (337).