Kinematics of subduction in the Ibero-Armorican arc constrained by 3D microstructural analysis of garnet and pseudomorphed lawsonite porphyroblasts from Île de Groix (Variscan belt)

Abstract. The small island of Groix in southern Brittany, France, is well known for exceptionally well-preserved outcrops of Variscan blueschists, eclogites, and garnetiferous mica schists that mark a Late Devonian suture between Gondwana and Armorica. The kinematics of polyphase deformation in these rocks is reconstructed based on 3D microstructural analysis of inclusion trails within garnet and pseudomorphed lawsonite porphyroblasts using differently oriented thin sections and X-ray tomography. Three sets of inclusion trails striking NE–SW, NNW–SSE, and WNW–ESE are recognized and interpreted to witness a succession of different crustal shortening directions orthogonal to these strikes. The curvature sense of sigmoidal and spiral-shaped inclusion trails of the youngest set is shown to be consistent with northwest and northward subduction of Gondwana under Armorica, provided that these microstructures developed by overgrowth of actively forming crenulations without much porphyroblast rotation. Strongly non-cylindrical folds locally found on the island are reinterpreted as fold-interference structures instead of having formed by progressive shearing and fold-axis reorientation. Six samples of a lower-grade footwall unit of the Groix ophiolitic nappe (Pouldu schists) were also studied. Inclusion trails in these rocks strike E–W, similar to the youngest set recognized on Groix island. They record Carboniferous N–S shortening during continental collision. These new microstructural data from southern Brittany bear a strong resemblance to earlier measured in inclusion-trail orientations in the northwestern Iberia Massif. A best fit between both regions suggests not more than about 15∘ anticlockwise rotation of Iberia during the Cretaceous opening of the Gulf of Biscay.

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Kinematics of subduction in the Ibero-Armorican arc constrained by 3D microstructural analysis of garnet and pseudomorphed lawsonite porphyroblasts from Ile de Groix (Variscan belt)

10.5194/se-2020-175 ◽

2020 ◽

Author(s):

Domingo Aerden ◽

Mohammad Sayab ◽

Aidan Forde ◽

Alejandro Ruiz-Fuentes

Keyword(s):

Microstructural Analysis ◽

Small Island ◽

Lower Grade ◽

Thin Sections ◽

The North ◽

Anticlockwise Rotation ◽

Strong Resemblance ◽

Inclusion Trails ◽

Shaped Inclusion ◽

Cylindrical Folds

Abstract. The small island of Groix in southern Brittany, France, is well known for its excellent outcrops of Variscan blueschists, eclogites and garnetiferous micaschists that define a Late-Devonian suture between Gondwana and Armorica. The kinematics of polyphase deformation in these rocks is reconstructed based on 3D microstructural analysis of inclusion trails in garnet- and pseudomorphed lawsonite porphyroblasts using multiple, differently oriented thin sections of single samples and X-ray tomography. Three sets of inclusion trails striking NE-SW, NNW-SSE and WNW-ESE are interpreted to witness a succession of different crustal shortening directions orthogonal to these trends. The curvature sense of sigmoidal- and spiral-shaped inclusion trails of the youngest set is shown to be consistent with southward thrusting or northward subduction of Gondwana under Armorica, provided that these microstructures developed by overgrowth of actively forming crenulations instead of the previously envisaged 'snowball' mechanism. The latter predicts an opposite thrusting direction which is at odds with the regional tectono-metamorphic zonation in the Ibero-Armorican Arc. Strongly non-cylindrical folds locally found on Ile de Groix are reinterpreted as fold-interference structures instead of having formed by progressive shearing. Six additional samples of lower-grade footwall units of the Groix ophiolite were also studied. The oldest inclusion trails in these rocks have similar trends as the youngest one in Ile de Groix. Our new inclusion-trail data for southern Brittany bear a strong resemblance with those documented previously in the north-western Iberian Massif and suggest about 20° anticlockwise rotation of Iberia during the early Cretaceous opening of the Gulf of Biscay.

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Micro-Fabric Analyzer (MFA): A New Semiautomated ArcGIS-Based Edge Detector for Quantitative Microstructural Analysis of Rock Thin-Sections

ISPRS International Journal of Geo-Information ◽

10.3390/ijgi10020051 ◽

2021 ◽

Vol 10 (2) ◽

pp. 51 ◽

Cited By ~ 1

Author(s):

Roberto Visalli ◽

Gaetano Ortolano ◽

Gaston Godard ◽

Rosolino Cirrincione

Keyword(s):

Physical Properties ◽

Electron Backscatter Diffraction ◽

Microstructural Analysis ◽

Image Features ◽

Elemental Distribution ◽

Edge Detector ◽

Sandstone Sample ◽

Thin Sections ◽

X Ray ◽

Distribution Maps

Micro-Fabric Analyzer (MFA) is a new GIS-based tool for the quantitative extrapolation of rock microstructural features that takes advantage both of the characteristics of the X-ray images and the optical image features. Most of the previously developed edge mineral grain detectors are uniquely based on the physical properties of the X-ray-, electron-, or optical-derived images; not permitting the exploitation of the specific physical properties of each image type at the same time. More advanced techniques, such as 3D microtomography, permit the reconstruction of tridimensional models of mineral fabric arrays, even though adjacent mineral grain boundaries with the same atomic density are often not detectable. Only electron backscatter diffraction (EBSD) allows providing high-performing grain boundary detection that is crystallographically differentiated per mineral phase, even though it is relatively expensive and can be executed only in duly equipped microanalytical laboratories by suitably trained users. Instead, the MFA toolbox allows quantifying fabric parameters subdivided per mineral type starting from a crossed-polarizers high-resolution RGB image, which is useful for identifying the edges of the individual grains characterizing rock fabrics. Then, this image is integrated with a set of micro-X-ray maps, which are useful for the quantitative extrapolation of elemental distribution maps. In addition, all this is achieved by means of low-cost and easy-to-use equipment. We applied the tool on amphibolite, mylonitic-paragneiss, and -tonalite samples to extrapolate the particle fabric on different metamorphic rock types, as well as on the same sandstone sample used for another edge detector, which is useful for comparing the obtained results.

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On the geometry of S-shaped inclusion trails in garnet porphyroblasts

Mineralogical Magazine and Journal of the Mineralogical Society ◽

10.1180/minmag.1967.036.279.19 ◽

1967 ◽

Vol 36 (279) ◽

pp. 453-456 ◽

Cited By ~ 34

Author(s):

Derek Powell ◽

J. E. Treagus

Keyword(s):

Garnet Porphyroblasts ◽

Inclusion Trails ◽

Shaped Inclusion

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Effect of cementation on ultrasonic velocities in sandstones

Geophysics ◽

10.1190/1.2431327 ◽

2007 ◽

Vol 72 (2) ◽

pp. E53-E58 ◽

Cited By ~ 4

Author(s):

Ashraf M. Al-Tahini ◽

Carl H. Sondergeld ◽

Chandra S. Rai

Keyword(s):

Saudi Arabia ◽

Microstructural Analysis ◽

Acoustic Velocity ◽

Minor Role ◽

Point Counting ◽

Ultrasonic Velocities ◽

Thin Sections ◽

Acoustic Velocities ◽

A Minor

We determine the acoustic velocities for samples cored from the Jauf and Unayzah sandstone formations of Saudi Arabia. We use microstructural analysis including thin sections and point counting to quantify cementation. Velocities in these formations are strongly controlled by the combination of porosity and cementation. Consequently, rocks of similar porosity but with different cementation materials display different velocities. The objective of this study is to understand the effect of cementation on the acoustic velocity. Cements in these two formations have variable compositions and properties. Pure quartz overgrowth plays a major role in increasing velocities while clay and clay coatings play a minor role. We found that clay coatings inhibit the quartz overgrowth cement leading to a decrease in velocities. Understanding the influence of various cementation types on velocity, and thus elastic properties in sandstone cores, enables an understanding of the variation of sonic velocities and moduli across these formations. The uniqueness of this study is that we emphasize the quantification of the role of cement and not just mineral volume.

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Comment and Reply on "Do smoothly curved, spiral-shaped inclusion trails signify porphyroblast rotation?"

Geology ◽

10.1130/0091-7613(1992)020<1054:carods>2.3.co;2 ◽

1992 ◽

Vol 20 (11) ◽

pp. 1054 ◽

Cited By ~ 9

Author(s):

Simon Wallis ◽

T. H. Bell ◽

A. Forde ◽

N. Hayward

Keyword(s):

Inclusion Trails ◽

Shaped Inclusion

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Do smoothly curving, spiral-shaped inclusion trails signify porphyroblast rotation?

Geology ◽

10.1130/0091-7613(1992)020<0059:dscssi>2.3.co;2 ◽

1992 ◽

Vol 20 (1) ◽

pp. 59 ◽

Cited By ~ 42

Author(s):

T. H. Bell ◽

A. Forde ◽

N. Hayward

Keyword(s):

Inclusion Trails ◽

Shaped Inclusion

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Microstructural shear bands in mylonites dated using muscovite sub-spectra from high-definition ultra-high-vacuum (UHV) argon diffusion experiments with phengitic white mica

10.5194/egusphere-egu2020-1813 ◽

2020 ◽

Author(s):

Marnie Forster ◽

Ruoran Nie ◽

Sonia Yeung ◽

Gordon Lister

Keyword(s):

High Vacuum ◽

Microstructural Analysis ◽

Shear Bands ◽

Shear Zones ◽

White Mica ◽

Ultra High Vacuum ◽

High Definition ◽

Thin Sections ◽

History Of ◽

Ar Geochronology

With excellent outcrop, the eclogite-blueschist belt exposed in the Cycladic archipelago in the Aegean Sea, Greece, offers a spectacular natural laboratory in which to decipher the structural geology of a highly extended orogenic belt and to ascertain the history of the different fabrics and microstructures that can be observed. Using phengitic white mica we demonstrate a robust correlation of age with microstructure, once again dispelling the myth that 40Ar/39Ar geochronology using this mineral, produces cooling ages alone.Further, we show that high-definition ultra-high-vacuum (UHV) 39Ar diffusion experiments using phengitic white mica routinely allow the extraction of muscovite sub-spectra in the first 10-30% of 39Ar gas release during 40Ar/39Ar geochronology. The muscovite sub-spectrum is distinct and separate to the main spectrum which is dominated by mixing of gas released from phengite as well as muscovite. The muscovite sub-spectra allow consistent estimates of the timing of the formation of microstructural shear bands in various mylonites, as well as allowing quantitative estimates of temperature variation with time during the cooling history of the eclogite-blueschist belt. Our new data reveals hitherto unsuspected variation in the timing of exhumation of individual slices of the eclogite-blueschist belt, caused by Eocene and Miocene detachment-related shear zones.This study thus illustrates a new method for the quantitative determination of the timing of movement in mylonites and/or in strongly stretched metamorphic tectonites. Shear bands formed in such structures are rarely coarsely crystalline enough to allow mineral grains that can be individually dated using laser spot analysis. Where phengitic white mica is involved, interlaying is usually so fine as to preclude the application of laser methods. In any case, laser methods do not have the capability of extracting exact and detailed age-temperature spectra, and can never achieve the definition of the multitudinous steps of the age spectrum evident from our high-definition UHV diffusion experiments.Previous work in the Cycladic eclogite-blueschist belt has incorrectly assumed that the diffusion parameters for phengitic white mica were the same as for muscovite. Arrhenius data suggest this is not the case, and that phengitic white mica is considerably more retentive of argon than muscovite. Previous workers have also erred in dismissing microstructural variation in age as an artefact, supposedly as the result of the incorporation of excess argon. This has led to inconsistencies in interpretation, because phengite is able to retain argon at temperatures that exceed those estimated using metamorphic mineral parageneses. In consequence, we discover a robust correlation between microstructure and age, even down to the detail present in complex tectonic sequence diagrams produced during fabric and microstructural analysis of individual thin-sections.A critical factor is that the recognition of muscovite sub-spectra requires Arrhenius data in order to recognise the steps dominated by release of 39Ar from muscovite. In turn this requires precise measurement of temperature during each heating step. To apply percentage-release formula for the estimation of diffusivity, there must be a sharp rise to the temperature in question, then that temperature must be maintained at a constant value, then dropped sharply to relatively low values.

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