Garnet Nucleation Research Articles

Metamorphic P–T paths of Archean granulite facies metasedimentary lithologies from the eastern Beartooth Mountains of the northern Wyoming Province, Montana, USA: constraints from quartz-in-garnet (QuiG) Raman elastic barometry, geothermobarometry, and thermodynamic modeling

Abstract. Metamorphic pressure and temperature (P–T) paths in late-Archean high-grade rocks of the eastern Beartooth Mountains of Montana (USA), a portion of the Wyoming Province, are established by a combination of imaging, analytical, and modeling approaches. Garnet inclusion mechanical and chemical thermobarometry, applied to several granulite-facies migmatites and an iron formation, is particularly useful in constraining the prograde P–T conditions. Quartz-in-garnet (QuiG) elastic Raman barometry was used on quartz inclusions in garnet for all samples studied. For a smaller subset of four representative samples, QuiG constraints were used in conjunction with Ti-in-quartz (TitaniQ) and Ti-in-biotite (TiB) thermometry to establish unique prograde inclusion entrapment P–T conditions. Ti measurements of garnet hosts and cathodoluminescence (CL) imagery of inclusion and matrix quartz grains were employed to check for Ti loss/diffusion. Lastly, inclusion studies were supplemented with thermodynamic modeling and matrix chemical thermobarometry to examine garnet nucleation temperatures and peak metamorphic conditions. Disagreement between the volume strain and elastic tensor methods used to calculate quartz inclusion pressures implies that quartz inclusions studied are under strong differential strain. Prograde entrapment results from the two inclusion thermobarometry pairs used are distinct: 0.55–0.70 GPa and 475–580 ∘C (QuiG–TitaniQ) versus 0.85–1.10 GPa and 665–780 ∘C (QuiG–TiB). Garnet modal isopleth modeling indicates that the majority of garnet growth occurred at ∼ 450–600 ∘C, implying that P–T conditions of garnet growth are interpreted to be most reliably represented by QuiG–TitaniQ inclusion thermobarometry. Normal distributions of calculated QuiG inclusion pressures and the concentration of mineral inclusions in garnet cores suggest that the majority of garnet inclusions were entrapped during a single stage of porphyroblast growth. A general lack of evidence from CL imagery for post-entrapment mechanical or chemical modifications to quartz inclusions suggests that quartz inclusions used to calculate entrapment P–T largely preserve their initial entrapment conditions. Biotite inclusions preserve higher temperatures than quartz inclusions in the same garnets, likely due to Fe–Mg exchange with garnet hosts that allowed Ti content of biotite to change after entrapment. Pseudosection modeling and matrix chemical thermobarometry of multiple, independent lithologies examined during inclusion studies suggest a range of peak granulite facies conditions of ∼ 0.50–0.70 GPa and 730–800 ∘C. Peak metamorphic P–T modeling work from this study, together with evidence of regional amphibolite facies overprinting in immediately adjacent samples, indicates an overall clockwise metamorphic P–T path with nearly isobaric prograde heating to peak temperatures. Interpreted P–T path reconstructions are consistent with metamorphism developed in a more modern-style continental arc subduction zone and are observed in portions of the northern Wyoming Province as exemplified by metasupracrustal lithologies of the eastern Beartooth Mountains.

Lu–Hf Geochronology on Single Garnets in a Micaschist from the North Qilian Orogenic Belt

The acquisition of the timing and duration of metamorphic mineral growth is the key to understanding the evolution of metamorphic belts. Garnet Lu–Hf geochronology is becoming increasingly powerful in this aspect. It is believed that garnet Lu–Hf radiometric systems are preserved during low-temperature metamorphism. However, this hypothesis has not been systematically tested. To examine the Lu–Hf systematics of garnets during low-temperature metamorphism, we conducted radiometric dating on individual garnet crystals of different sizes from a single micaschist in the North Qilian orogenic belt. The garnet Lu–Hf dates correlate well with the grain sizes and their core Mn concentrations. The positive correlation between the ages and grain sizes suggests that grain size is a proxy for preserving garnet nucleation and growth history. Small grains nucleated and grew later than large crystals. The Lu–Hf date of the individual garnet crystals faithfully recorded the total growth time span. The date difference of ~16 Myr is the minimum duration of the total garnet growth. The age discrepancy between the micaschist and the eclogite indicates they may not have experienced the same subduction and exhumation in the North Qilian orogenic belt.

Implications of garnet nucleation overstepping for the P–T evolution of the Lesser Himalayan Sequence of central Nepal

AbstractRecent studies have demonstrated that kinetic factors can significantly influence garnet nucleation, delaying its appearance with respect to the equilibrium predictions. Overstepping of garnet nucleation occurs in both contact and regional metamorphic settings, with extremely variable degrees: The factors controlling such highly variable degrees of overstepping are still unclear. This study focuses on garnet nucleation and growth in aluminous metapelites from the Barrovian inverted metamorphic pile of the upper portion of the Lesser Himalayan Sequence (Upper‐LHS; central Nepal), with the aim of (i) investigating if (and how) the bulk‐rock composition can influence overstepping of garnet nucleation and (ii) which are the implications of garnet nucleation overstepping for the P–T evolution of a Barrovian metamorphic sequence. Detailed petrographic, microstructural and compositional data are presented for six phyllitic schists, containing porphyroblasts of garnet, staurolite and/or kyanite. Their P–T evolution is constrained through thermodynamic forward modelling (i.e., isochemical phase diagrams combined with isopleth thermobarometry), assuming that equilibrium was attained at every stage of their metamorphic evolution. Comparison between the P–T conditions inferred for the growth of garnet core, the assemblages predicted to be stable at these P–T conditions and the modelled garnet‐in reaction boundary suggests that the studied samples have experienced different degrees of apparent thermal (ΔT) and/or baric (ΔP) overstepping of garnet nucleation. We suggest that the bulk‐rock MnO and CaO amounts might have influenced the apparent ΔT and ΔP overstepping of the garnet‐in reaction: more specifically, the higher the bulk‐rock MnO content, the more pronounced the apparent ΔT overstepping, whereas the lower the bulk‐rock CaO content, the greater the ΔP overstepping. However, rather than an effective delay of garnet appearance with respect to equilibrium predictions, the apparent ΔT overstepping of garnet nucleation could reflect the attainment of the critical 0.5% threshold of garnet abundance, below which garnet is not readily detected in thin section. Kinetic factors seem much less critical in controlling the growth of the garnet rim at peak P–T conditions, confirming that peak metamorphic conditions constrained through equilibrium approaches based on the composition of garnet rim and of the matrix assemblage can be considered as reliable. Overall, the P–T paths of the studied samples are characterized by prograde heating coupled with tectonic overload (peak‐P conditions of 9.5–10.5 kbar, 580–590°C), followed by heating during exhumation (peak‐T conditions of 8.2–8.9 kbar, 610–630°C), supporting those thermo‐mechanical models that predict a period of slowdown (or quiescence) of the Main Central Thrust activity.

Thermal pulse induced by emplacement of Ramba leucogranites in southern Tibet

AbstractThe thermal histories of Himalayan leucogranites provide critical information for unravelling the post‐collisional geodynamics of the Himalayas. The Ramba Dome is located at the intersection of the Tethyan Himalayan leucogranite belt with the Yadong–Gulu Rift and hosts several generations of granitic intrusions. Of these intrusions, the 8‐Ma two‐mica granites and garnet leucogranite dykes are the youngest of Himalayan leucogranites. In this study, we focus on the carbonaceous staurolite schist located ~1.3 km from the intrusion to constrain the thermal history of the aureole that marked the cessation of leucogranite magmatism. The schist contains euhedral garnet and staurolite porphyroblasts in a foliated matrix of muscovite + biotite + chlorite + plagioclase + quartz + graphite. The staurolite shows minor compositional variations from the inclusion‐free core to the inclusion‐rich rim. By contrast, the garnet features a distinctive bell‐shaped Mn profile and increasing Mg# from the garnet core to rims. In a graphite‐bearing equilibrium phase diagram for a modified bulk composition with garnet cores removed, the garnet rim composition suggests a peak temperature of ~550°C, consistent with an independent thermometer based on the Raman spectra of carbonaceous materials (RSCM; 548 ± 9°C). The P–T condition lies within the narrow low‐variance field bracketed by the staurolite‐in and chlorite‐out boundaries, indicating minimal overstepping of staurolite nucleation and growth. On the other hand, the garnet core composition indicates 520°C at 2.5 kbar, about 40°C higher than the predicted garnet‐in boundary (~480°C). This apparent temperature overstep corresponds to a small chemical affinity (<5 kJ/mol 12 O) for garnet nucleation, comparable to previous estimates. The sharp boundaries of the high‐Ca sector zoning in the core indicate limited diffusion modification (~1.5 Ma if at the peak temperature). The short thermal pulse involves advective heat transfer by leucogranite emplacement, followed by rapid cooling toward the end of Himalayan magmatism and rapid exhumation likely facilitated by the Yadong–Gulu Rift.

Testing the equilibrium model: An example from the Caledonian Kalak Nappe Complex (Finnmark, Arctic Norway)

AbstractThe equilibrium model has been tested using Barrovian garnet‐zone micaschists of the Kalak Nappe Complex. In our model, equilibrium in the MnNCKFMASHT system was established across the entire rock volume during prograde metamorphism except for garnet, which developed growth zoning preserved at levels controlled by the kinetics of intracrystalline diffusion. The preservation potential of the disequilibrium fluctuations required for nucleation of garnet has been considered in our simulations, using a moving boundary multicomponent diffusion and growth model. Results indicate that the core of garnet that crystallizes during regional metamorphism does not retain the major element composition of the nucleus but reflects the compositional signature of the immediate overgrowth. The differences between the metamorphic conditions of successive garnet growth steps of the samples indicate characteristic trends in their pressure‐temperature evolutions that can be predicted with the equilibrium model. There is some latitude with regard to the absolute metamorphic conditions due to the inherent uncertainty of the thermodynamic data and the approximation of the reactive volume composition. However, the slopes of the pressure‐temperature paths together with systematic trends in the lithological, geochemical, and Lu–Hf garnet whole‐rock isotopic properties of the rocks, as well as their garnet crystal size frequency distributions, enable the identification of the Veidnes, Bekkarfjord, and Kolvik Nappes involved in the Caledonian Orogeny and provide new insight into their metamorphic evolutions. According to our findings, the base of the Kalak Nappe Complex experienced wide‐spread Barrovian‐type metamorphism at c. 420 Ma with a gradient of ∼40 bar/°C and peak conditions of ∼560°C and 6.7 kbar in the Bekkarfjord and Veidnes Nappes, whereas the hinterland‐placed Kolvik Nappe was metamorphosed at peak conditions of ∼590°C and 7.5 kbar. This event was preceded by moderate‐pressure metamorphism at c. 423 Ma resulting in garnet crystallization exclusively in the Bekkarfjord Nappe, along a gradient of ∼20 bar/°C and peak conditions of ∼570°C and 6.0 kbar. We consider both of these metamorphic and deformational episodes to be different stages of the Scandian phase during the Caledonian Orogeny. An additional orogenic event at c. 464 Ma is preserved in the Veidnes Nappe, which was juxtaposed with the Scandian Nappes by repeated out‐of‐sequence thrusting associated with the final continent–continent collision. This nappe provides a unique insight into an earlier stage of the Caledonian Orogeny, indicating a low‐pressure Barrovian‐type metamorphic history with a gradient of ∼15 bar/°C and peak conditions of ∼560°C and 4.5 kbar.

Pronounced and rapid exhumation of the Connecticut Valley Trough revealed through quartz in garnet Raman barometry and diffusion modelling of garnet dissolution–reprecipitation reactions

AbstractMetamorphic rocks from the Connecticut Valley Trough (CVT), Vermont, and Massachusetts, have been examined using quartz‐in‐garnet (QuiG) and conventional thermobarometry, thermodynamic reaction modelling, diffusion modelling, and 40Ar/39Ar thermochronology to constrain their P–T–t paths during Acadian metamorphism and subsequent exhumation. Numerous samples, collected in the vicinity of the Acadian domes, contain garnet porphyroblasts that display cloudy zones characterized by numerous fluid inclusions and modified garnet compositions associated with the replacement of the original garnet by biotite±muscovite±plagioclase±quartz±low Xgrs/enriched Xsps. QuiG and conventional thermobarometry constrain both the conditions of garnet nucleation and peak P–T conditions to have occurred at ~0.85–1.05 GPa, ~550–600°C. Most notably, QuiG barometry was performed on inclusions adjacent to these reaction zones in conjunction with Gibbs method reaction modelling to reveal that these dissolution–reprecipitation reactions occurred during nearly isothermal decompression from the peak P–T conditions to around ~0.3 GPa, 550°C. Diffusion modelling reveals that the Mn zoning profiles created during garnet resorption that accompanied decompression formed in less than c. 3 Ma, which constrains the tectonic exhumation to have occurred at 8–10 mm/year. Subsequent cooling to 500°C occurred rapidly at a rate of 100°C/Ma, followed by slower cooling reaching 1.7°C /Ma by the mid Carboniferous. This is the first reported example of QuiG barometry revealing a multi‐stage metamorphic history and highlights the utility of this method for unravelling complex metamorphic terranes.

Preferred orientations of garnet porphyroblasts reveal previously cryptic templating during nucleation

Electron back scattered diffraction data of garnet crystals from the Nelson Aureole, British Columbia and the Mosher’s Island formation, Nova Scotia, reveals that 22 garnet crystals are all oriented with one of three crystal directions parallel to the trace of the foliation plane in thin section. Structural models suggest that these relationships are due to preferential garnet nucleation onto muscovite, with the alignment of repeating rows of Al octahedra and Si tetrahedra in each leading to inheritance of garnet orientation from the muscovite. These results highlight that epitaxial nucleation may be a prevalent process by which porphyroblast minerals nucleate during metamorphism and carry implications for the role that non-classic nucleation pathways play in the crystallization of metamorphic minerals, the distribution of porphyroblasts in metamorphic rocks, and, in cases in which nucleation is the rate limiting step for crystallization, the energetics of metamorphic reactions.

Garnet as a monitor for melt–rock interaction: Textural, mineralogical, and compositional evidence of partial melting and melt‐driven metasomatism

AbstractIn this study, we focus on a partially melted garnet‐bearing granulite from the Salvador–Esplanade Belt (Salvador da Bahia, Brazil), and examine the behaviour of major and trace elements during partial melting and melt‐driven metasomatism. Phase equilibria modelling and U–Th–Pb geochronology show that the sample underwent partial melting during the heating segment of the decompression path from ~1.2 GPa and 675–700°C to ~0.8 GPa and 790°C at c. 2.06 Ga. During the final stage of decompression, from 0.8 to ~0.5 GPa, physical segregation of melt resulted in the establishment of chemical potential gradients and mass transfer between the host granulite and the leucosome. Modelling shows that H2O, CaO, K2O, and Na2O diffused from the melt into the host residue, whereas SiO2 was transferred from the host granulite into the adjacent leucosome. Opposed senses of diffusional transfer resulted in the formation of a quartz‐rich anhydrous leucosome and a quartz‐depleted selvedge in the host granulite. Compositional maps show that garnet exhibit contrasting major and trace element distributions depending on their textural position. The largest garnet located in the quartz‐depleted selvedge preserve their original Ca and trace element growth zoning. A transition from bell‐shaped profiles for Y and heavy rare earth elements to bowl‐shaped profiles for light rare earth elements is consistent with a typical Rayleigh fractionation model, fast intergranular element mobility, and rock‐wide equilibrium during prograde partial melting. In contrast, smaller garnet away from the leucosome show prograde growth zoning modified by intragranular diffusion, as evidenced by a network of open channels and healed cracks that act as connecting pathways between the matrix and garnet core. This results in either subtle modification of both major and trace elements adjacent to the inner core inclusions, or a complete re‐equilibration. Recognition that the original Ca growth zoning was later modified by intragranular diffusion implies that misleading thermobarometric results and tectonic interpretations would be obtained if the core composition was used to fingerprint the early garnet nucleation stage. This study demonstrates that at high temperature (>750°C) and in the presence of melt, REE are not less vulnerable to diffusive resetting than divalent cations like Ca2+.

Petrochronology of Wadi Tayin Metamorphic Sole Metasediment, With Implications for the Thermal and Tectonic Evolution of the Samail Ophiolite (Oman/UAE)

AbstractOphiolite metamorphic soles preserve important records of ophiolite emplacement, but there have been few detailed investigations into their non‐mafic portions. We present new thermobarometric and petrochronologic data from a metasediment and mafic restite in the upper Wadi Tayin sole exposure in the Samail (Oman‐UAE) Ophiolite. Thermodynamic modeling suggests metasedimentary garnet nucleation at ~4 kb, ~550°C and final growth at 7.5 ± 1.2 kbar, 665 ± 32°C, occurring by 93.0 ± 0.5 Ma (Lu‐Hf isochron). Zircon U‐Pb dates of 106.9 ± 2.3 (detrital) and 98.7 ± 1.7 to 94.1 ± 1.6 Ma (metamorphic) bracket the initiation of metamorphism, and monazite U‐Pb dates from ~97–89 Ma suggest a lengthy period of growth or recrystallization. A mafic titanite U‐Pb age of 92.2 ± 1.8 Ma records the earliest possible juxtaposition of high‐ and lower‐grade sole rocks. These and other data suggest that (i) the Wadi Tayin sole preserves an inverted metamorphic, metasomatic, and age gradient,(ii) metasediment metamorphism occurred during, or soon after, crystallization of the overlying ophiolite (≤96.5 Ma); and (iii) sole metasediments define a thermal gradient continuous with hotter, higher‐P amphibolites. Some of these data conflict with existing models for sole formation, and we propose several hypotheses to explain them. Cooling of the sole below Ar closure by ~92 Ma suggests that strain rapidly partitioned away from the sole, leading to large‐scale, thin‐skinned thrust emplacement of the ophiolite >100 km across the continental margin and the late, cool underthrusting of the continental margin.

The (chemical) potential for understanding overstepped garnet nucleation and growth

AbstractOverstepping of porphyroblast-forming reactions has been shown to occur in regional, contact, and subduction zone metamorphism, calling into question the paradigm that metamorphic mineral reactions occur at or very close to thermodynamic equilibrium. These overstepped reactions result from the fact that nucleation and growth of new phases requires a thermodynamic driving force or a “departure” from equilibrium. We use phase equilibria modeling to elucidate the energetic consequences of over-stepped garnet nucleation and growth by comparing the chemical potentials of garnet-forming oxide components (MnO, CaO, FeO, MgO, Al2O3) in two sets of calculations: one in which Gibbs free energy is minimized and one in which the minimization proceeds under identical conditions but in the forced absence of garnet. We focus on 12 examples from the literature that have previously described garnet nucleation as minimally overstepped (garnet nucleation at the P-T of initial garnet stability) or garnet nucleation as more substantially overstepped (garnet nucleation at P-T conditions greater than initial garnet stability). For a small P-T interval above nominal garnet-in reactions, differences in the chemical potentials between the two calculations are commonly minimal. In all tested examples calculated using two versions of the THERMOCALC thermodynamic data set, the chemical potential of Al2O3 (μAl2O3) diverges between garnet-bearing and garnet-absent calculations at greater P-T conditions than that of MnO, CaO, FeO, and MgO. The P-T interval between thermodynamic garnet-in and the point at which μAl2O3 differs substantially between the two sets of calculations appears to be a function of bulk-rock MnO content, reemphasizing the role that small quantities of MnO play in the apparent stability of garnet in calculated phase equilibria. These results highlight the importance of considering multiple thermodynamic data sets, the location of the garnet-in curve, and the abundance of mineral phases in the discussion of overstepped metamorphic reactions. The results have implications for determining kinetic barriers to crystal nucleation and growth and considering the most appropriate way of defining “garnet-in” for samples that have experienced overstepping.

Thermodynamic modeling of high-grade metabasites: a case study using the Tso Morari UHP eclogite

Thermodynamic modeling is an important technique to simulate the evolution of metamorphic rocks, particularly the poorly preserved prograde metamorphic reactions. The development of new thermodynamic modeling techniques and availability of updated thermodynamic databases and activity–composition (a–X) relations, call for an evaluation of best practices for modeling pressure–temperature (P–T) paths of metabasites. In this paper, eclogite from the Tso Morari UHP terrane, NW India, is used as a representative metabasite to directly compare the outputs (pseudosections and P–T paths) generated from recent versions of the widely used THERMOCALC and Theriak-Domino programs. We also evaluate the impact of using the most updated thermodynamic database (ds 62, Holland and Powell in J Metamorph Geol 29(3):333–383, 10.1111/j.1525-1314.2010.00923.x, 2011) relative to an older version (ds 55, Holland and Powell in J Metamorph Geol 16(3):309–343, 10.1111/j.1525-1314.1998.00140.x, 1998), and the effect of the user’s choice of mineral a–X relations while considering the effect of garnet fractionation on the rock’s effective bulk composition. The following modeling protocols were assessed: (1) TC33; THERMOCALC version 3.33 with database ds 55 and garnet a–X relations of White et al. (J Metamorph Geol 25(5):511–527, 10.1111/j.1525-1314.2007.00711.x, 2007); (2) TC47; THERMOCALC version 3.47 with database ds 62 and garnet a–X relations of White et al. (J Metamorph Geol 32(3):261–286, 10.1111/jmg.12071, 2014a); (3) TDG; Theriak-Domino with database ds 62 and garnet a–X relations of White et al. (2014a), and (4) TDW; Theriak-Domino with database ds 62 and garnet a–X relations of White et al. (2007). TC47 and TDG modeling yield a similar peak metamorphic P–T of 34 ± 1.5 kbar at 544 ± 15 °C and 551 ± 12 °C, respectively. The results are 5–8 kbar higher in pressure than that determined from TC33 modeling (26 ± 1 kbar at 565 ± 8 °C), and TDW modeling (28.5 ± 1.5 kbar at 563 ± 13 °C). Results indicate that all four modeling protocols generally provide consistent metamorphic phase relations and thermodynamic simulations regarding fractionation of the bulk composition and prograde metamorphism within uncertainty. In all model calculations, the initial bulk composition measured by XRF does not represent the effective bulk composition at the time of garnet nucleation. The choice of garnet a–X relations can affect predictions of peak pressure, regardless of program choice. This study illustrates the importance of careful consideration of which a–X relations one chooses, as well as the need for comparison between modeling predictions and evidence from the geochemistry and petrography of the rock(s) themselves.

Regional Quartz Inclusion Barometry and Comparison with Conventional Thermobarometry and Intersecting Isopleths from the Connecticut Valley Trough, Vermont and Massachusetts, USA

AbstractA comparative analysis of Raman shifts of quartz inclusions in garnet was made along two traverses across the Connecticut Valley Trough (CVT) in western New England, USA, to examine the regional trends of quartz inclusion in garnet (QuiG) Raman barometry pressure results and to compare this method with conventional thermobarometry and the method of intersecting garnet core isopleths. Overall, Raman shifts of quartz inclusions ranged from 1·2 to 3·5 cm–1 over all field areas and displayed a south to north decrease, matching the overall decrease in mapped metamorphic grade. Raman shifts of quartz inclusions typically did not show systematic variation with respect to their radial position within a garnet crystal, and indicate that garnet probably grew at nearly isothermal and isobaric pressure–temperature (P–T) conditions. The P–T conditions inferred from conventional thermobarometry were in the range of ∼500–575 °C and ∼7·4–10·3 kbar over the sample suite and are in good agreement with previous published thermobarometry throughout the CVT. These P–T results are broadly consistent with QuiG barometry and also suggest that garnet grew isothermally and isobarically at near peak P–T conditions. However, P–T conditions and P–T paths inferred using either garnet core thermobarometry or garnet core intersecting isopleths yield results that are internally inconsistent and generally disagree with the pressure results from QuiG barometry. Garnet core isopleth intersections consistently plotted between the nominal garnet-in curve on mineral assemblage diagrams and the P–T conditions constrained by QuiG isomekes for the majority of the sample suite. Additionally, most samples’ P–T results from QuiG barometry and rim thermobarometry show marked disagreement from those derived from garnet core thermobarometry, compared with the minority that showed agreement within uncertainty. Pressures calculated from QuiG barometry ranged from 8·5 to 9·5 kbar along the traverses in western Massachusetts (MA) and central Vermont (VT) and from 6·5 to 7·5 kbar in northern VT indicating an increase in peak burial of 3–6 km from north to south. Along the western end of the central VT traverse, there are differences in measured Raman shifts and inferred peak pressures of up to 1 kbar across the Richardson Memorial Contact (RMC), indicating a possible fault contact with minor post-peak metamorphic shortening of up to ∼3 km. In contrast, along an east–west traverse in the vicinity of the Goshen Dome, MA, there was little observed variation in Raman shifts across the contact. By contrast, QuiG barometry clearly indicates significant discontinuities in peak pressure east of the Strafford Dome in central VT. This supports the interpretation that post-peak metamorphic shortening was necessary to juxtapose upper staurolite–kyanite zone rocks next to lower garnet zone pelites. Overall, it is concluded that garnet core thermobarometry and garnet core isopleths may provide unreliable results for the P–T conditions of garnet nucleation and inferred P–T paths during garnet growth unless independently verified. The consistency of QuiG results with rim thermobarometry indicates that peak metamorphic conditions previously reported for the CVT using garnet rim thermobarometry are robust and that variation in QuiG barometry results is a valuable tool to analyze structural features within a metamorphic terrane.

Evidence for ultrahigh-pressure metamorphism discovered in the Appalachian orogen

AbstractThe Appalachian orogen has long been enigmatic because, compared to other parts of the Paleozoic orogens that formed following the subduction of the Iapetus Ocean, direct evidence for ultrahigh-pressure (UHP) metamorphism has never been found. We report the first discovery of coesite in the Appalachian orogen in a metapelite from the mid-Ordovician (Taconic orogeny) Tillotson Peak Complex in Vermont (USA). Relict coesite occurs within a bimineralic SiO2 inclusion in garnet. In situ elastic barometry and trace-element thermometry allow reconstruction of the garnet growth history during prograde metamorphism. The data are interpreted to indicate garnet nucleation and crystallization during blueschist- to eclogite-facies subduction zone metamorphism, followed by garnet rim growth at UHP conditions of > 28 kbar and > 530 ° C. Results provide the first direct evidence that rocks of the Appalachian orogen underwent UHP metamorphism to depths of > 75 km and warrant future studies that constrain the extent of UHP metamorphism.

Nucleation and Initial Growth of Garnet in Low-Grade Metamorphic Rocks of the Sanbagawa Metamorphic Belt, Kanto Mountains, Japan

The beginning of the recrystallization of minerals within a subducting oceanic plate provides a valuable record of dehydration within the subduction zone. Pelitic schists of the Nagatoro area, Kanto Mountains, Japan, record the initial stages of garnet growth. Consequently, these rocks were studied to analyze garnet nucleation and growth during metamorphism of the Sanbagawa metamorphic belt, one of the world’s most comprehensively studied subduction complexes. The garnet grains are small, euhedral, and occur only within micaceous lamellae that define the schistosity. Crystal size distribution analyses revealed most of the garnet grains follow the log-normal size distribution, indicating that they formed in the same event. A few exceptionally large garnet grains exist that do not seem to follow the log-normal distribution. The latter garnet grains contain a rounded fragmental area with a different chemical composition inside the core. It is possible that detrital fragments of garnet contribute to the irregular crystal size distribution of garnet in the studied area. Many of the smaller (log-normal) garnet grains have relatively large, homogeneous Mn-rich cores. The lack of chemical zoning within the garnet cores suggests that they grew under constant pressure and temperature in response to overstepping of the garnet-in reaction. The chemical composition changes very sharply at the boundary between the core and the surrounding mantle. The size of the Mn-rich core is different from sample to sample, suggesting that the nucleation was controlled by the local chemical condition of each sample.

Post-entrapment modification of residual inclusion pressure and its implications for Raman elastic thermobarometry

Abstract. Residual pressure can be preserved in mineral inclusions, e.g. quartz-in-garnet, after exhumation due to differential expansion between inclusion and host crystals. Raman spectroscopy has been applied to infer the residual pressure and provides information on the entrapment temperature and pressure conditions. However, the amount of residual pressure relaxation cannot be directly measured. An underestimation or overestimation of residual pressure may lead to significant errors between calculated and actual entrapment pressure. This study focuses on three mechanisms responsible for the residual pressure modification: (1) viscous creep; (2) plastic yield; (3) proximity of inclusion to the thin-section surface. Criteria are provided to quantify how much of the expected residual pressure is modified due to these three mechanisms. An analytical solution is introduced to demonstrate the effect of inclusion depth on the residual pressure field when the inclusion is close to the thin-section surface. It is shown that for a quartz-in-garnet system, the distance between the thin-section surface and inclusion centre needs to be at least 3 times the inclusion radius to avoid pressure release. In terms of viscous creep, representative case studies on a quartz-in-garnet system show that viscous relaxation may occur from temperatures as low as 600–700 ∘C depending on the particular pressure–temperature (P–T) path and various garnet compositions. For quartz entrapped along the prograde P–T path and subject to viscous relaxation at peak T above 600–700 ∘C, its residual pressure after exhumation may be higher than predicted from its true entrapment conditions. Moreover, such a viscous resetting effect may introduce apparent overstepping of garnet nucleation that is not related to reaction affinity.

Implications of overstepping of garnet nucleation for geothermometry, geobarometry and P–T path calculations

The composition of a porphyroblast such as garnet is not governed by equilibrium relations when it nucleates and grows after considerable overstepping. The approach followed here is to assume the composition is controlled by the maximum driving force (MDF) or parallel tangent model and to examine the potential pitfalls for assuming equilibrium. The MDF model ensures Fe-Mg partitioning equilibrium between the matrix phases and growing porphyroblast, but the net transfer equilibrium controlling grossular content has a different stoichiometry from the equilibrium relations. An important result of this study is that garnet zoning profiles generated using this model and assuming chemical fractionation are nearly identical to zoning profiles grown assuming continuous equilibrium and it is impossible to discern whether garnet grew under near-equilibrium conditions or only after considerable overstepping by examination of the zoning profile alone.Pressure–temperature paths calculated using the method of intersecting isopleths from simulations in which garnet was grown after overstepping and assuming the MDF model typically display nearly isothermal loading or loading with minor heating paths, even when the garnet was grown under isothermal and isobaric conditions. Garnet-plagioclase barometry on the garnet core and rim also suggests loading during garnet growth, even for crystals grown isothermally and isobarically. Garnet-biotite Fe-Mg exchange thermometry does, however, recover the correct temperature to within 10 °C for both the garnet core and rim.Comparison of natural samples to the theoretical calculations suggests that the local effective bulk composition in the vicinity of garnet is depleted in Na2O, in addition to FeO, MnO and CaO, thus rendering calculation schemes that rely on knowledge of the bulk composition difficult to administer. To the extent that garnet has nucleated and grown after considerable overstepping, application of methods of extracting P–T paths from the chemical zoning in garnet will yield incorrect results if those methods assume continuous equilibration with the matrix. The major discrepancy comes from calculation of the P–T conditions of the garnet core. Simulations and natural samples suggest that under typical scenarios the garnet rim is in near equilibrium with the matrix assemblage so equilibrium approaches (thermobarometry or intersecting isopleths) should recover the peak metamorphic conditions with reasonable assuredness.

Magmatic Mn-rich garnets in volcanic settings: Age and longevity of the magmatic plumbing system of the Miocene Ramadas volcanism (NW Argentina)

The Miocene “Corte Blanco Tuff” rhyolite deposit is the product of a large volume and high intensity Plinian eruption from the solitary and monogenetic Ramadas Volcanic Centre (Central Andes, Province of Salta, NW Argentina). The “Corte Blanco Tuff” consists of vitreous tube pumices with rare euhedral sub-millimetric Mn-garnet phenocrysts, typically hosting inclusions of U-phases as zircon and monazite. Here, we present new textural, major and trace elemental analyses of garnet, zircon and glass that, combined with in situ U-(Th)-Pb zircon and monazite dating, are used to reconstruct the thermobaric environment of formation, age and longevity of the magmatic plumbing system of the Ramadas magma. The results indicate to a crystallization path of a peraluminous rhyolitic melt at shallow crustal levels (≤6 km), as sequentially tracked by the initial nucleation of zircon (780 °C at 9.16 Ma) and garnet (above or at ca. 700 °C), to the final monazite growth (660–670 °C, at 8.70 Ma) in a water-saturated (H2O = 3–5 wt%) environment, shortly before the eruption started. These data (1) define for the first time the primary magmatic origin of Mn-garnet in a rhyolitic volcanic setting; (2) provide new partition coefficients of rare earth elements (REE) between natural garnet, zircon and rhyolitic melts; and (3) permit reconstruction of the magmatic processes that resulted in the Ramadas eruption. On a wider scale, our results document the spatio-temporal (P-T conditions, timing and longevity) time scales involved in the petrogenesis of a shallow peraluminous water-saturated rhyolitic magmatic plumbing system that is able to generate the conditions for extremely explosive Plinian eruptions.

UHP metamorphism recorded by phengite eclogite from the Caledonides of northern Sweden: P–T path and tectonic implications

AbstractThe Seve Nappe Complex (SNC) of the Scandinavian Caledonides records a well‐documented history of high pressure (HP) and ultra‐high pressure (UHP) metamorphism. Eclogites of the SNC occur in two areas in Sweden, namely Jämtland and Norrbotten. The Jämtland eclogites and associated rocks are well‐studied and provide evidence for late Ordovician UHP metamorphism, whereas the Norrbotten eclogites, formed during the late Cambrian (Furongian)/Early Ordovician, have not been studied in such detail, especially in terms of the P–T conditions of their formation. Within the studied eclogite, clinopyroxene contains a high‐Na core and two rims: inner, medium‐Na and outer, low‐Na. Garnet consists of a high‐Ca euhedral core, low‐Ca inner rim and medium‐Ca outer rim. A similar pattern occurs within phengite, where high‐Si cores are enveloped by medium and low‐Si rims. The compositions of the mineral cores, inner rims and outer rims reflect three stages in the metamorphic evolution of the eclogite. Applied Quartz‐in‐Garnet geobarometry, coupled with Zr‐in‐rutile geothermometry reveal that garnet nucleation (E0 stage) took place at 1.5–1.6 GPa and 620–660°C. The eclogite peak‐pressure assemblage developed during the E1 stage, it consists of garnet+omphacite+phengite+rutile+coesite? and yields P–T conditions of 2.8–3.1 GPa and 660–780°C as constrained by conventional geothermobarometry and thermodynamic modelling in the NCKFMMnASHT system. Later, lower‐pressure stages E2 and E3 record conditions of 2.2–2.8 GPa, 680–780°C and 2.1 GPa, 735°C, respectively. The prograde metamorphic evolution of the eclogite is inferred from inclusions of epidote, amphibole and clinopyroxene within garnet. The presence of amphibole–quartz–plagioclase symplectites, secondary epidote/zoisite and titanite replacing rutile record the later retrograde changes taking place at <1.5 GPa (referred as E4 stage). The obtained P–T conditions indicate that the Norrbotten eclogites underwent a metamorphic evolution characterized by a clockwise P–T path with peak metamorphism reaching up to coesite stability field within a relatively cold subduction regime (7.8°C/km). The obtained results provide the first evidence for UHP metamorphism in the SNC above the Arctic Circle and document cold subduction regime and multistage exhumation of the deeply subducted Baltican margin at early stage of the Caledonian Orogeny.

Determining the amount of overstepping required to nucleate garnet during Barrovian regional metamorphism, Connecticut Valley Synclinorium

AbstractThe novel method of inclusion barometry coupled with the calculation of the required affinity for garnet nucleation is applied to three samples from the previously well‐characterized Connecticut Valley Synclinorium in central Vermont. Raman shifts for quartz inclusions record a range of maximum peak shifts of the quartz 464 cm−1 peak from 2.4 to 3.0 cm−1. Temperature of garnet nucleation was constrained by calculating mineral assemblage diagrams in the MnNCKFMASHT system and plotting the intersection of quartz inclusion in garnet barometry (QuiG, quartz‐in‐garnet) with Zr‐in‐rutile thermometry. Utilizing the intersection of Zr‐in‐rutile thermometry with QuiG barometry, garnet nucleation is inferred to have occurred within a P–T range of ~8.6–9.5 kbar and ~560–575°C. These P–T conditions for garnet nucleation are significantly higher than calculated equilibrium garnet‐in isograds for the three samples. Affinities for garnet nucleation were calculated as the difference between the free energy of a fictive garnet composition based on the matrix assemblage and the free energy of the nucleated garnet. The calculated nucleation affinity varied from 300 to 600 kJ/mol O for St–Ky grade samples. These results suggest that the assumption that metamorphism proceeds as a sequence of near‐equilibrium conditions cannot, in general, be made for regional metamorphic terranes. This body of work agrees with numerous recent studies showing that garnet‐producing reactions must be overstepped in order to for garnet to nucleate.

Garnet growth after overstepping

Overstepping of metamorphic reactions is required to provide the driving force necessary for porphyroblast nucleation and growth. Forward models of garnet nucleation and growth are presented assuming an affinity for nucleation that corresponds to several tens of degrees or several kilobars of overstepping. The composition of garnet that nucleates and grows is assumed to be that which provides the largest decrease in free energy. With these assumptions, the zoning predicted for a garnet grown under isothermal, isobaric conditions is revealed to be sufficiently similar to the zoning predicted for garnet grown under continuous near-equilibrium conditions that distinction based on the shape of zoning profiles alone does not appear to be possible. Furthermore, the growth of an initial garnet crystal following nucleation after overstepping depletes the affinity for subsequent nucleation by sequestration of Mn into the existing garnet crystal. Progressive nucleation, therefore, requires additional energy input, most likely through changes in pressure and temperature, to replenish the affinity. The predicted zoning in garnets nucleated following substantial growth of the first nucleated garnet does not match the typical bell-shaped profile of Rayleigh fraction but rather displays distinctly diagnostic zoning depending on the assumed rate limiting step for growth. Diffusion controlled growth results in relatively broader and flattened profiles whereas interface controlled growth results in later garnets having a peaked core Mn profile. This latter profile would relax by diffusion in relatively short times (less than 1Ma) and would thus not likely to be preserved, but appears to have been observed in garnet zone samples from central Vermont. Calculations for this sample also indicate that garnet ceased growing when chlorite was exhausted, at which point considerable affinity remained indicating that equilibrium was not achieved. Tectonic interpretations based on P–T paths calculated assuming near-equilibrium nucleation and growth of garnet are thus likely to require reevaluation in light of the magnitude of overstepping needed for nucleation and the possibility that equilibrium was not attained at the metamorphic peak.