Evaluating chemical equilibrium in metamorphic rocks using major element and Sm–Nd isotopic age zoning in garnet, Townshend Dam, Vermont, USA

Gatewood, Matthew P.; Dragovic, Besim; Stowell, Harold H.; Baxter, Ethan F.; Hirsch, David M.; Bloom, Rose

doi:10.1016/j.chemgeo.2015.02.017

Cited by 46 publications

(48 citation statements)

References 54 publications

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“…Resorption and recrystallization of the core during retrograde metamorphism, akin to formation of atoll garnets, would explain the age profile but is not reflected in thin section textures or in keeping with the prograde zonation generally preserved in garnet crystals. A recent study by Gatewood, et al (2015) also reported garnet cores apparently younger than rim ages in certain porphyroblasts from the same rock and attributed the age difference to local matrix heterogeneity, possibly including open system matrix evolution during/after garnet growth. A shift in the whole rock 143 Nd/ 144 Nd or 147 Sm/ 144 Nd that occurred between growth of the core and the outer zones could explain an anomalously young core age since pairing the current volumeaveraged whole rock with the garnet core would violate isochron assumptions.…”

Section: Interpretation Of the Late Cretaceous (Uhp) Metamorphic Agesmentioning

confidence: 90%

Late Cretaceous UHP metamorphism recorded in kyanite–garnet schists from the Central Rhodope Mountains, Bulgaria

Collings

Savov

Maneiro

et al. 2016

Lithos

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In this study, we report the first discovery of microdiamond inclusions in kyanite-garnet schists from the Central Rhodope Mountains in Bulgaria. These inclusions occur in garnets from metapelites that are part of a metaigneous and meta-sedimentary mélange hosted by Variscan (Hercynian) orthogneiss. Ultra-high-pressure (UHP) conditions are further supported by the presence of exsolved needles of quartz and rutile in the garnet and by geothermobarometry estimates that suggest peak metamorphic temperatures of 750-800°C and pressures in excess of 4 GPa. The discovery of UHP conditions in the Central Rhodopes of Bulgaria compliments the well-documented evidence for such conditions in the southernmost (Greek) part of the Rhodope Massif. Dating of garnets from these UHP metapelites (Chepelare Shear Zone) using Sm-Nd geochronology indicates a Late Cretaceous age (70.5-92.7 Ma) for the UHP metamorphic event. This is significantly younger than previously reported ages and suggests that the UHP conditions are associated with the Late Mesozoic subduction of the Vardar Ocean northward beneath the Moesian platform (Europe). The present-day structure of the RM is the result of a series of subduction-exhumation events that span the Cenozoic, alongside subsequent post-orogenic extension and metamorphic core complex formation.

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Section: Interpretation Of the Late Cretaceous (Uhp) Metamorphic Agesmentioning

confidence: 90%

Late Cretaceous UHP metamorphism recorded in kyanite–garnet schists from the Central Rhodope Mountains, Bulgaria

Collings

Savov

Maneiro

et al. 2016

Lithos

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“…In any case, diffusion rates are temperature-dependent, so equilibration volume is expected to increase during heating (and drastically decrease upon cooling, as rocks tend to "dry up"), and as shown in Figure 1c, it may differ substantially for different chemical species (Carlson 1989;Spear and Daniel 2001). This is particularly critical for some of the trace elements relevant to petrochronology (e.g., Gatewood et al 2015). An essential parameter controlling the extent of local (dis)equilibrium is the rate at which local reactions proceed towards equilibrium compared to the rate of change in physical parameters (P-T-X, e.g., the heating rate).…”

Section: Open Questions On Bulk Composition Effects and The Size Of Ementioning

confidence: 99%

Local Bulk Composition Effects on Metamorphic Mineral Assemblages

Lanari

Engi

2017

Reviews in Mineralogy and Geochemistry

157

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“…A straightforward approach to testing whether the sections of the mega garnet core (M1) grew in an isotopic heterogeneity and/or disequilibrium within the rock system, is to observe the correlation between Mn concentration in garnet and garnet growth age (e.g. Baxter & DePaolo, 2002;Gatewood et al, 2015). This approach is not practical due to the flat Mn profiles in M1 garnet.…”

Section: Growth Of Mega Garnetmentioning

confidence: 99%

Micro‐sampling Lu–Hf geochronology reveals episodic garnet growth and multiple high‐P metamorphic events

Hao

Liu

Vervoort

et al. 2016

Journal Metamorphic Geology

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We present Lu–Hf dates from multiple growth zones within two large garnet porphyroblasts by a micro‐sampling method. A single eclogite sample, taken from the Huwan shear zone in the Hong'an orogen, was investigated with Lu–Hf geochronology by micro‐sampling and traditional bulk separation methods. The sample contains a few large garnet porphyroblasts up to several centimetres in diameter and a second major population of smaller (0.1–2.0 mm) garnet porphyroblasts, comprised of dark cores and pale rims. Elemental compositions and mineral inclusions in the garnet appear consistent with two garnet generations. Lu–Hf dates (c. 400–264 Ma) were determined from twelve micro‐sawed garnet sections from two large garnet porphyroblasts. These Lu–Hf dates overlap with age peaks defined by 115 SIMS zircon U–Pb analyses from the same eclogite sample. Bulk analyses of smaller garnet separates define a minimum date of c. 306 Ma and a maximum date of c. 252 Ma for the dark cores and pale rims respectively. These Lu–Hf dates were interpreted to bracket the period of garnet growth and the broad interval from c. 400 to 264 Ma is best explained by protracted and episodic garnet growth, which may require that these rocks experienced two subduction cycles that were initiated during the Devonian and terminated in the Triassic.

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Evaluating chemical equilibrium in metamorphic rocks using major element and Sm–Nd isotopic age zoning in garnet, Townshend Dam, Vermont, USA

Cited by 46 publications

References 54 publications

Late Cretaceous UHP metamorphism recorded in kyanite–garnet schists from the Central Rhodope Mountains, Bulgaria

Late Cretaceous UHP metamorphism recorded in kyanite–garnet schists from the Central Rhodope Mountains, Bulgaria

Local Bulk Composition Effects on Metamorphic Mineral Assemblages

Micro‐sampling Lu–Hf geochronology reveals episodic garnet growth and multiple high‐P metamorphic events

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