Garnet crystallization mechanisms and localized polymetamorphism in the southwestern Meguma Terrane, Nova Scotia, Canada

Nagurney, Alexandra B.; Caddick, Mark J.; White, Chris E.

doi:10.1111/jmg.12678

Cited by 3 publications

(1 citation statement)

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“…Although largely employed, this approach does not account for the kinetics of garnet crystallization, which is related to the energy barriers of interface processes and diffusion across the system (e.g., Carlson, 2011; Gaidies et al, 2017). The information required to recognize and better understand the kinetics of fundamental rock‐forming processes that affect garnet porphyroblast crystallization may be obtained from the systematic compositional and microstructural characterization of garnet populations (e.g., Carlson, 1989; Carlson et al, 1995; Daniel & Spear, 1998, 1999; Denison et al, 1997; Gaidies et al, 2011, 2015; Gaidies & George, 2021; George & Gaidies, 2017; Hirsch, 2008; Kretz, 1973, 1993; Nagurney et al, 2022; Spear et al, 1998).…”

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confidence: 99%

Mechanisms and durations of metamorphic garnet crystallization in the lower nappes of the Caledonian Kalak Nappe Complex, Arctic Norway

Yogi,

Gaidies,

Heldwein

et al. 2024

Journal Metamorphic Geology

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The 3D microstructure and compositional zoning of garnet populations in micaschists from the Kolvik and Bekkarfjord nappes indicate the quasi‐equilibration of their major components across the rock matrices during interface‐controlled, size‐independent garnet growth. There is microstructural evidence for foliation‐parallel, small‐scale resorption of garnet rims in the Kolvik Nappe, influencing the metamorphic peak conditions obtained from thermodynamic modelling. The local chemical compositions of rims less affected by resorption indicate a peak temperature of ~630°C, which is ~40°C higher than the temperature obtained from resorbed rims of the largest garnet crystal. Using a diffusion geospeedometry approach that considers the geometry of the compositional zoning of the garnet population, as well as the higher, more realistic peak temperature, a duration of 1 to 4.9 Myr is obtained for garnet growth in the Kolvik Nappe. This is approximately 1 order of magnitude faster than duration estimates obtained when using the apparent, lower temperature estimated from the resorbed garnet rims. Concomitantly to garnet growth in the Kolvik Nappe, garnet overgrowths formed in the Bekkarfjord Nappe for circa 2.5 Myr at metamorphic peak temperatures of ~560°C. The garnet growth durations obtained here are comparable with the uncertainty on the Lu–Hf garnet–whole rock isochron ages of 419.9 ± 2.4 Ma and 423.0 ± 1.9 Ma, previously obtained for these rocks. These results provide new insight into the timescales of repeated Barrovian‐type metamorphic events experienced by the lower nappes of the Kalak Nappe Complex during the Caledonian Orogeny in Arctic Norway. This study emphasizes the importance of microstructural and chemical characterization of garnet populations in assessing metamorphic crystallization mechanisms and the extent of equilibration of garnet‐forming components during prograde metamorphism. Moreover, our results provide means for reducing the uncertainty on metamorphic durations obtained via diffusion geospeedometry and, so, contributing to our understanding of geological timescales and processes.

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Section: Introductionmentioning

confidence: 99%

Mechanisms and durations of metamorphic garnet crystallization in the lower nappes of the Caledonian Kalak Nappe Complex, Arctic Norway

Yogi,

Gaidies,

Heldwein

et al. 2024

Journal Metamorphic Geology

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Conjugate shear model for emplacement of mid-crustal plutons during dextral transpression, southern Nova Scotia, Canada: tectonic and petrological consequences

Collins,

Murphy

2024

Can. J. Earth Sci.

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The South Mountain Batholith (SMB) is a syntectonic composite batholith emplaced in the upper crust within the Meguma terrane between 380 Ma and 370 Ma during the later stages of the Neoacadian orogeny. Coeval plutons in southern Nova Scotia are surrounded by mid-crustal (~4 kbar) andalusite-staurolite aureoles and are discordant to NE-trending, regional Neoacadian folds. Detailed field studies, combined with published results, indicate emplacement within a dextral transpressional regime during the transition from distributed (D1) to focused heterogeneous strain (D2), which provided vertical conduits that facilitated magma ascent. The Port Mouton Pluton (PMP) intruded along P-orientated crustal-scale fractures as a series of subvertical granitic sheets, which were progressively rotated and folded with ongoing dextral shearing. By contrast, the Barrington Passage pluton (BPP) intruded between crustal-scale, antithetic (sinistral) P-shear fractures and spread laterally between them as pulsed increments to form a layered, subhorizontal, sill-like complex resembling the laccolithic structure of the SMB. The SMB was emplaced below the Meguma Supergroup, with magma derived from underthrust Avalon terrane and Silurian Rockville Notch Group. The lack of mantle components in the SMB suggests transpressional orogenesis facilitated conductive crustal heating without significant mantle addition, consistent with low p-wave velocities of Meguma lower crust. 400-355 Ma zircons, recorded either as inherited grains in granites or in felsic granulite xenoliths, imply the Neoacadian thermal anomaly extended for 45 Ma, but magmatism represented only ~40% of that perturbation.

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Late Devonian syntaxis in the Northern Appalachian orogen

Piette-Lauzière,

Larson,

Kellett

et al. 2024

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The pre-accretionary shapes of cratonic margins form successions of promontories and re-entrants inherited from the rifting of supercontinents. In accretionary orogens, the extent of deformation related to a collision with a continent characterized by an irregular margin is obfuscated through the partitioning of deformation along pre-existing structures. In the Northern Appalachians, the extent of the deformation related to the oblique collision of the Meguma terrane with the composite Laurentian margin is disputed. Herein, we use a framework based on modern collisional settings to investigate the Late Devonian to Mississippian deformation inboard of the Avalonia - Meguma boundary and evaluate the regional tectonic setting. We combine published shear zone kinematic interpretations, deformation ages, and regional 40 Ar/ 39 Ar cooling ages with structural interpretation of aeromagnetic and gravimetric depth slices covering the Northern Appalachians. We find that the deformation related to the collision of the Meguma terrane, attributed to the Neoacadian orogeny, has a larger structural footprint than previously documented. While this deformation is partitioned in multiple structures in the Canadian Appalachians, northern New England is characterized by rapid crustal deformation, high paleo-elevation, and fast erosional exhumation, similar to modern syntaxis structures.

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Garnet crystallization mechanisms and localized polymetamorphism in the southwestern Meguma Terrane, Nova Scotia, Canada

Cited by 3 publications

References 103 publications

Mechanisms and durations of metamorphic garnet crystallization in the lower nappes of the Caledonian Kalak Nappe Complex, Arctic Norway

Mechanisms and durations of metamorphic garnet crystallization in the lower nappes of the Caledonian Kalak Nappe Complex, Arctic Norway

Conjugate shear model for emplacement of mid-crustal plutons during dextral transpression, southern Nova Scotia, Canada: tectonic and petrological consequences

Late Devonian syntaxis in the Northern Appalachian orogen

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