Marble mylonites in the Bancroft shear zone, Ontario, Canada: microstructures and deformation mechanisms

Mylonitization of medium-grade marbles in the Bancroft shear zone, Ontario, Canada, is characterized by decreasing grain-size of both calcite and graphite, and a variety of textures. Calcite grain-sizes vary from several millimeters in the protolith, to 50–200 μm in mylonite, to <30 μm in ultramylonite. Corresponding calcite grain shapes are equant in the protolith, elongate in protomylonite (first-developed dimensional preferred orientation), equant in coarse mylonite, elongate in fine mylonite (second-developed dimensional preferred orientation) and generally equant in ultramylonite, which suggests that external energy (applied stress) that tends to elongate grains competed with internal energy sources (e.g. distortional strain) that favor equant shapes. Graphite grain-size changes from several millimeters to centimeters in the protolith to submicroscopic in ultramylonite. In the mylonitic stages, graphite is present as dark bands, while in the ultramylonitic stage it is preserved as a fine coating on calcite grains.Based on textural evidence, twinning (exponential creep; regime I), dynamic recrystallization (power law creep; regime II) and possibly grain boundary sliding superplasticity (regime III) are considered the dominant deformation mechanisms with increasing intensity of mylonitization; their activity is largely controlled by calcite grain-size. Calcite grain-size reduction occurred predominantly by the process of rotation recrystallization during the early stages of mylonitization, as indicated by the occurrence of core and mantle or mortar structures, and by the grain-size of subgrains and recrystallized grains. Grain elongation in S-C structures indicates the activity of migration recrystallization; these structures are not the result of flattening of originally equant grains. Differential stress estimates in coarse mylonites and ultramylonites, based on recrystallized grain-size, are 2–5 and 14–38 MPa, respectively. Initial grain-size reduction of graphite occurred by progressive separation along basal planes, analogous to mica fish formation in quartzo-feldspathic mylonites.Calcite-graphite thermometry on mylonitic and ultramylonitic samples shows that the metamorphic conditions during mylonitization were 475 ± 50°C, which, combined with a differential stress value of 26 MPa, gives a strain rate of 1.2 x 10⁻¹⁰s⁻¹ based on constitutive equations; corresponding displacement rates are <38 mmyr⁻¹.     

Mica beards in three slates: Morphology,formation and bearing on the strain history

Beards are intergrowths of mica, chlorite and quartz or feldspar on detrital grains in weakly metamorphosed sedimentary rocks. The three samples studied contain beards of mica and quartz (or feldspar). Their internal morphology shows a variation in mica orientation related to the external foliation (matrix). Formation occurred during foliation development with a syntaxial growth history. In three dimensions, the beards have oblate shapes, so that they are present in all planes cut parallel to the shortening direction. This indicates flattening strain conditions around the detrital grains during the beard formation.Bulk inhomogeneous shortening, resulting in an anastomosing foliation, can explain the local extension within the plane containing the fold axis and the shortening direction. Concurrently, an approximately constant lenght in the direction of the fold axis is maintained on a regional scale (plane strain). Volume loss probably plays an important role during the formation history.

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Zusammenfassung Glimmerbärte in schwach metamorphen Sedimenten sind syntektonische Verwachsungen von Glimmer, Chlorit, Quarz oder Feldspat auf detritischen Körnern. Die drei hier untersuchten Proben enthalten Bärte aus Hellglimmer und Quarz (oder Feldspat). Die Glimmerorientierung zeigt Variationen in Relation zum umgebenden Schieferungsgefüge (Matrix). Die Bildung der Bärte fand mit syntaxialem Wachstum während der Schieferung statt. Dreidimensional haben die Bärte eine abgeplattete Gestalt, so daß sie in allen Schnittlagen parallel zur Verkürzungsrichtung zu beobachten sind. Dies weist auf plättende Verformung um detritische Körner während der Bildung der Bärte hin.Bulk inhomogeneous shortening führt zu einem linsigen Schieferungsgefüge und kann bei gleichzeitiger Annahme einer nicht volumenkonstanten Deformation als Erklärung für die im Korngefüge beobachtete Dehnung parallel zur Faltenachse herangezogen werden. Im regionalen Maßstab wird aber eine konstante Länge in Richtung der Faltenachse und damit ebene Deformation angenommen.

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Résumé Les barbes de mica sont des intercroissances de quartz (ou feldspath) et de mica sur des grains détritiques dans les roches sédimentaires peu métamorphisées. Elles ont été étudiées dans trois échantillons. Leur morphologie interne montre une variation dans l'orientation du mica en relation avec la schistosité externe (matrice). Leur formation s'est produite pendant le développement de la schistosité, avec une histoire de croissance «syntaxiale». Les barbes ont une forme oblongue en trois dimensions, de sorte qu'elles sont visibles dans toutes les sections parallèles à la direction de raccourcissement. Cela indique, pendant la formation des barbes, l'existence d'un aplatissement autour des grains détritiques.Le raccourcissement non homogène, qui entraîne une foliation anastomosée peut être invoqué, conjointement à une déformation avec perte de volume, pour expliquer l'allongement que présente localement la microstructure parallèlement à l'axe des plis. A l'échelle régionale, toutefois, on admet la constance de la longuer selon la direction des axes et, partant, le caractère plan de la déformation.

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Comparison of garnet-biotite, calcite-graphite, and calcite-dolomite thermometry in the Grenville Orogen; Ontario, Canada

The Elzevir Terrane of the Grenville Orogen in southern Ontario contains metapelites and abundant graphitic marbles that were regionally metamorphosed from the upper greenschist to upper amphibolite facies. Comparative thermometry was undertaken with widely used calibrations for the systems garnet-biotite, calcite-dolomite, and calcite-graphite. Temperatures that are obtained from matrix biotites paired with prograde garnet near-rim analyses are usually consistent with those determined using calcite-graphite thermometry. However, calcite-graphite thermometry occasionally yields low temperatures due to lack of equilibration of anomalously light graphite. Application of calcite-graphite and garnet-biotite systems may yield temperatures up to 70 °C higher than calcite-dolomite in amphibolite facies rocks. Calcite-dolomite temperatures most closely approach those from calcite-graphite and garnet-biotite when the samples contain a single generation of dolomite and calcite grains contain no visible dolomite exsolution lamellae. However, some of these samples yield temperatures considerably lower than temperatures calculated from calcite-graphite and garnet-biotite thermometry, indicating that the calcite-dolomite thermometer may have been partially reset during retrogression. Estimated peak metamorphic temperatures of regional metamorphism between Madoc (upper greenschist facies) and Bancroft (upper amphibolite facies) range from 500 to 650 °C. These results place the chlorite-staurolite isograd at 540 °C, the kyanite-sillimanite isograd at 590 °C, and the sillimanite-K-feldspar isograd at 650 °C. Although each thermometer may have an absolute uncertainty of as much as ±50 °C, the 50 to 60 °C temperature differences between the isograds are probably accurate to 10 to 20 °C. An incomplete picture of the thermal gradients can result from the application of only one thermometer in a given area. Simultaneous application of several systems allows one to recognize and overcome the inherent limitations of each thermometer. Received: 26 March 1997 / Accepted: 15 April 1998     

Variation of illite/muscovite <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar age spectra during progressive low-grade metamorphism: an example from the US Cordillera

⁴⁰Ar/³⁹Ar step-heating data were collected from micron to submicron grain-sizes of correlative illite- and muscovite-rich Cambrian pelitic rocks from the western United States that range in metamorphic grade from the shallow diagenetic zone (zeolite facies) to the epizone (greenschist facies). With increasing metamorphic grade, maximum ages from ⁴⁰Ar/³⁹Ar release spectra decrease, as do total gas ages and retention ages. Previous studies have explained similar results as arising dominantly or entirely from the dissolution of detrital muscovite and precipitation/recrystallization of neo-formed illite. While recognizing the importance of these processes in evaluating our results, we suggest that the inverse correlation between apparent age and metamorphic grade is controlled, primarily, by thermally activated volume diffusion, analogous to the decrease in apparent ages with depth observed for many thermochronometers in borehole experiments. Our results suggest that complete resetting of the illite/muscovite Ar thermochronometer occurs between the high anchizone and epizone, or at roughly 300 °C. This empirical result is in agreement with previous calculations based on muscovite diffusion parameters, which indicate that muscovite grains with radii of 0.05–2 μm should have closure temperatures between 250 and 350 °C. At high anchizone conditions, we observe a reversal in the age/grain-size relationship (the finest grain-size produces the oldest apparent age), which may mark the stage in prograde subgreenschist facies metamorphism of pelitic rocks at which neo-formed illite/muscovite crystallites typically surpass the size of detrital muscovite grains. It is also approximately the stage at which neo-formed illite/muscovite crystallites develop sufficient Ar retentivity to produce geologically meaningful ⁴⁰Ar/³⁹Ar ages. Results from our sampling transect of Cambrian strata establish a framework for interpreting illite/muscovite ⁴⁰Ar/³⁹Ar age spectra at different stages of low-grade metamorphism and also illuminate the transformation of illite to muscovite. At Frenchman Mtn., NV, where the Cambrian Bright Angel Formation is at zeolite facies conditions, illite/muscovite ⁴⁰Ar/³⁹Ar data suggest a detrital muscovite component with an apparent age ≥967 Ma. The correlative Carrara Fm. is at anchizone conditions in the Panamint and Resting Spring Ranges of eastern California, and in these locations, illite/muscovite ⁴⁰Ar/³⁹Ar data suggest an early Permian episode of subgreenschist facies metamorphism. The same type of data from equivalent strata at epizone conditions (greenschist facies) in the footwall of the Bullfrog/Fluorspar Canyon detachment in southern Nevada reveals a period of slow-to-moderate Late Cretaceous cooling.     

Variation in fold geometry in the Yuso basin, northern Spain: implications for the deformation regime

First generation structures in greywackes of the Yuso Group from the Cantabrian Mountains of northern Spain show a distinct variation in geometry with depth in a regional synclinal structure (Curavacas and Lechada synclines); they are easily distinguished from other deformation events. In the structurally uppermost level we find ‘flap folds’. Flap folds are recumbent structures with the inverted limb preserved. Below this level ‘cascade folds’ are found. These structures have a vergence opposite to that of parasitic folds. The nomenclature adopted is from Harrison and Falcon. Characteristically, these structures have shallowly dipping axial surfaces, in agreement with the shallow dip of the axial plane (regional) cleavage. In the lowermost structural level, upright parasitic folds with a steep cleavage are present. The variation in fold geometry is accompanied by a general steepening of the regional cleavage with increasing depth. In the absence of overprinting relationships the F₁ fold geometries are included in a single deformation event.The steepening of the cleavage with depth reflects the change in orientation of the maximum shortening direction from sub-vertical in the upper part of the syncline to sub-horizontal in the lower part. With increasing depth the deformation regime during F₁ changed from bending to buckling. The deformation regime on the regional scale, however, is associated with basement subsidence and passive formation of the regional synclinal structure. Furthermore, the absence of a distinct microfabric for the different F₁ folds indicates that on a small scale a similar deformation regime was present. We conclude, therefore, that the scale at which we study a structure only reflects the deformation regime at that particular scale. Consequently, the overall deformation regime cannot be determined from single outcrops or microstructural analysis alone.     

Chlorite-mica aggregates: morphology,orientation, development and bearing on cleavage formation in very-low-grade rocks

Chlorite-mica aggregates in slates from northern Spain have been investigated in very-thin thin sections. Specimens from various parts of a fold show different morphologies that are dependent on the operating deformation mechanism. From the limb to the hinge rigid-body rotation becomes less important and folding and intragranular kinking are more common. This is associated with an increasing aspect ratio (width/length) for the aggregates.Chlorite-mica aggregates were, in part, present before cleavage development and are statistically parallel to the bedding enveloping surface. Variation in degree of distortion of chlorite suggests continuous solution and growth during deformation. Muscovite, on the other hand, is generally more highly deformed, suggesting that it was all early. Splitting of micas along (001)-planes occurs, creating extension sites where chlorite growth can take place. Thus, an early diagenetic/metamorphic formation with mimetic growth of chlorite on detrital micas is favoured for the origin of the aggregates.The behaviour of the aggregates during deformation provides information on a mechanism for cleavage development. As a result of intragranular kinking, high angle boundaries are created, with parts of the deformed grains at a low angle to the cleavage plane. Mechanical rotation appears to be the important mechanism for cleavage development in these rocks. Modification by recrystallization, solution and growth processes can account for the observed microstructure.     

An unusual ‘crack-seal’ vein geometry

Inclusion bands in quartz that fills extensional veins are considered to be the result of a mechanism named ‘crack-seal’ by Ramsay. A microstructure is decribed here in which chlorite grains, occurring as inclusions arranged in inclusion bands, are oriented with their long axes oblique to both the quartz high-angle grain boundaries and inclusion-band traces. Syntaxial growth of chlorite on wall-rock phyllosilicates may have caused this microstructure. Consequences of this process in the determination of the strain history of extensional veins are discussed.