scholarly journals Fuchsite and chrome-epidote from Fiskenæsset

1976 ◽  
Vol 73 ◽  
pp. 67-69
Author(s):  
M Ghisler
Keyword(s):  
Shear Zones ◽  
White Mica ◽  
Hydrothermal Solutions ◽  
Secondary Minerals

In many places in the Fiskenæsset region the anorthositic rocks show a characteristic emerald green staining. The occurrences are connected with faults and shear zones, along which hydrothermal solutions have circulated, resulting in a bleaching and alteration of the anorthositic rocks (Ghisler, 1970). The plagioclase became saussuritized to white mica and epidote, and occassionally calcite is also found. Mafic minerals were altered to chlorite and more rarely to tale. In some places such as the Majorqap qava area a green rock consisting of epidote and an emerald green mineral is found as 10-50 cm thick lenses in anorthosite. The emerald green mineral mainly occurs.in the vicinity of chromite horizons or where accessory chromite is present in the anorthosite. Chromium was extracted from these chromite grains and entered the lattice of some of the minerals formed as aresult of the breakdown of plagioclase. Some mineralogical data on these two chromium bearing secondary minerals, fuchsite and chrome-epidote, are given below.

scholarly journals Timing of Paleozoic Exhumation and Deformation of the High-Pressure Vestgӧtabreen Complex at the Motalafjella Nunatak, Svalbard

Minerals ◽  
2020 ◽  
Vol 10 (2) ◽  
pp. 125 ◽  
Author(s):  
Christopher J. Barnes ◽  
Katarzyna Walczak ◽  
Emilie Janots ◽  
David Schneider ◽  
Jarosław Majka
Keyword(s):  
High Pressure ◽  
Tectonic Evolution ◽  
Shear Zones ◽  
White Mica ◽  
Strike Slip ◽  
Structural Studies ◽  
Metamorphic Overprint ◽  
Subaerial Exposure ◽  
Facies Metamorphism

The Vestgӧtabreen Complex exposed in the Southwestern Caledonian Basement Province of Svalbard comprises two Caledonian high-pressure units. In situ white mica 40Ar/39Ar and monazite Th-U-total Pb geochronology has resolved the timing of the tectonic evolution of the complex. Cooling of the Upper Unit during exhumation occurred at 476 ± 2 Ma, shortly after eclogite-facies metamorphism. The two units were juxtaposed at 454 ± 6 Ma. This was followed by subaerial exposure and deposition of Bullbreen Group sediments. A 430–400 Ma late Caledonian phase of thrusting associated with major sinistral shearing throughout Svalbard deformed both the complex and the overlying sediments. This phase of thrusting is prominently recorded in the Lower Unit, and is associated with a pervasive greenschist-facies metamorphic overprint of high-pressure lithologies. A c. 365–344 Ma geochronological record may represent an Ellesmerian tectonothermal overprint. Altogether, the geochronological evolution of the Vestgӧtabreen Complex, with previous petrological and structural studies, suggests that it may be a correlative to the high-pressure Tsäkkok Lens in the Scandinavian Caledonides. It is suggested that the Vestgӧtabreen Complex escaped to the periphery of the orogen along the sinistral strike-slip shear zones prior to, or during the initial stages of continental collision between Baltica and Laurentia.

scholarly journals In Situ and Step‐Heating 40 Ar/ 39 Ar Dating of White Mica in Low‐Temperature Shear Zones (Tenda Massif, Alpine Corsica, France)

Tectonics ◽  
2020 ◽  
Vol 39 (12) ◽  
Author(s):  
Alexandre Beaudoin ◽  
Stéphane Scaillet ◽  
Nicolas Mora ◽  
Laurent Jolivet ◽  
Romain Augier
Keyword(s):  
Low Temperature ◽  
Shear Zones ◽  
White Mica ◽  
Step Heating ◽  
Temperature Shear

Cyclic brittle-ductile behaviour recorded in exhuming high-pressure continental units of the Northern Apennines.

2020 ◽  
Author(s):  
Francesco Giuntoli ◽  
Giulio Viola
Keyword(s):  
High Pressure ◽  
High Strain ◽  
Shear Zones ◽  
White Mica ◽  
Northern Apennines ◽  
Strain Partitioning ◽  
Coseismic Slip ◽  
Quartz Veins ◽  
Mylonitic Foliation ◽  
History Of

<p>Exhumation of subducted high-pressure units is favoured by relatively narrow, high-strain shear zones, where most metamorphic and deformational processes occur. Unfortunately, these are commonly overprinted and/or partly or fully obliterated along the exhumation path by younger fabrics or by metamorphic re-equilibration. Their identification and characterization are, therefore, of primary importance when aiming at reconstructing the deepest (and thus earliest) tectonometamorphic history of high-pressure crustal units.</p><p>The Northern Apennines (Italy) offer the opportunity to study a unique setting where continental units (Tuscan Metamorphic Units) were subducted to high-pressure conditions and then exhumed and juxtaposed against non-metamorphic units (Tuscan Nappe). We have studied a well exposed section in the Monticiano-Roccastrada Unit of the Mid Tuscan Ridge (MTR), where a mesoscopic (~20 m length and 5 m high) compressional duplex deforms the Palaeozoic-Triassic quartz-rich metasandstones, metaconglomerates and minor metapelites of the Monte Quoio - Montagnola Senese Unit with a top-to-the-NE sense of shear (Arenarie di Poggio al Carpino Formation; Casini et al., 2007).</p><p>Our approach is based on detailed fieldwork, microstructural and petrological investigations. Field observations reveal severe strain partitioning within the duplex between metapelite levels, corresponding to 10-50 cm thick high-strain zones, and metasandstone levels, which form relatively strain-free metric horses. Early generations of quartz veins are highly transposed (sheath folds occur) parallel to the metapelitic high-strain shear zones. Veins are composed of iso-oriented quartz, forming up to several centimetre long single-grain ribbons, Mg-carpholite (XMg~ 0.65) needles and K-white mica marking the stretching lineation. Carpholite in the transposed veins invariably defines the stretching direction of shear zones. These high-P veins coexist with a later generation of less deformed, oblique quartz veins. The mylonitic foliation in the metapelites is defined by quartz, chloritoid, pyrophyllite and K-white mica forming a stretching lineation coherent with the one visible in the veins. Geometrical, cross-cutting and petrographic relations suggest that there has occurred cyclic deformation between brittle and viscous conditions, with the veins forming broadly syn-mylonitic shearing. Thermodynamic modeling results suggest >0.8 GPa and ~350°C for the formation of both the high-pressure veins and the mylonitic foliation.</p><p>Shear zones were subsequently folded about the NNW-SSE axis of the regional antiform associated with the MTR. Later brittle overprinting is represented by quart-filled tension gashes and localized C’ planes, mostly within the more competent metasandstone levels, indicating top-to-the-SW reactivation. In summary, our results suggest a cyclic brittle-ductile behaviour occurring at high pressure conditions. This could potentially reflect the repeated alternation between aseismic creep (viscous) and coseismic slip (brittle) during the first stages of the exhumation history of this portion of the northern Apennines, from lower to middle crustal levels in a compressional top-to-the-NE setting. Dating of K-white mica is ongoing to constrain the geodynamic scenario of such shear zone.</p><p> </p><p>Casini, G., Decandia, F.A., Tavarnelli, E., 2007. Analysis of a mesoscopic duplex in SW Tuscany, Italy: implications for thrust system development during positive tectonic inversion. Geol. Soc. London, Spec. Publ. 272, 437–446.</p><p> </p>

Jurassic to Early Cretaceous postaccretional sinistral transpression in north-central Chile (latitudes 31–32°S)

2011 ◽  
Vol 149 (2) ◽  
pp. 208-220 ◽  
Author(s):  
UWE RING ◽  
ARNE P. WILLNER ◽  
PAUL W. LAYER ◽  
PETER P. RICHTER
Keyword(s):  
Early Cretaceous ◽  
Shear Zones ◽  
White Mica ◽  
Strike Slip ◽  
Fault System ◽  
Low Grade ◽  
North Central ◽  
Central Chile ◽  
Metamorphic Basement ◽  
Atacama Fault System

AbstractWe describe the geometry and kinematics of a Jurassic to Early Cretaceous transpressive sinistral strike-slip system within a metamorphic basement inlier of the Mesozoic magmatic arc near Bahia Agua Dulce at latitudes 31–32°S in north-central Chile and discuss possible relations with the Atacama Fault System further north. Sinistral transpression overprints structures of an accretionary system that is represented by the metamorphic basement. Sub-vertical semi-ductile NNW-striking strike-slip shear zones are the most conspicuous structures. Chlorite and sericite grew, and white mica and quartz dynamically recrystallized, suggesting low-grade metamorphic conditions during semi-ductile deformation. Folds at the 10–100 metre scale developed before and during strike-slip shearing. The folds are deforming a former sub-horizontal transposition foliation that originated during prior accretion processes. The folds have axes sub-parallel to the strike-slip shear zones and sub-vertical axial surfaces indicating a component of shortening parallel to the shear-zone boundaries, suggesting an overall transpressive deformation regime. Transpressive strike-slip deformation also affects Middle Triassic (Anisian) basal breccias of the El Quereo Formation.40Ar–39Ar laser ablation ages of synkinematically recrystallized white mica in one of the shear zones provide an age of 174–165 Ma for the waning stages of semi-ductile strike-slip shearing. The semi-ductile shear zones are cut by mafic and rhyolite dykes. Two rhyolite dykes yield40Ar–39Ar ages of 160.5 ± 1.7 Ma and 131.9 ± 1.7 Ma, respectively. The latter dyke has been affected by brittle faulting. Fault-slip analysis shows that the kinematics of the faulting event is similar to the one of the semi-ductile shearing event, suggesting that sinistral transpression continued after ~130 Ma. Timing, kinematics and geographic position suggest that the shear zones at Bahia Agua Dulce represent a southern continuation of the prominent Atacama Fault System that affected the Jurassic/Early Cretaceous arc over its ~1400 km length.

Geochronological evidence for repeated brittle reactivations of a pre-existing plastic shear zone: The Himdalen–Ørje deformation zone, Southern Norway

2021 ◽  
Author(s):  
Espen Torgersen ◽  
Roy Gabrielsen ◽  
Johan Petter Nystuen ◽  
Roelant van der Lelij ◽  
Morgan Ganerød ◽  
...  
Keyword(s):  
Shear Zone ◽  
Deformation Zone ◽  
Shear Zones ◽  
White Mica ◽  
Fault Gouge ◽  
Plastic Shear ◽  
Brittle Deformation ◽  
Sveconorwegian Orogen ◽  
Fault Gouges ◽  
Southern Norway

<p>It is well known that faults, once formed, become permanent weaknesses in the crust, localizing subsequent brittle strain increments. The case of repeated brittle reactivations localized along pre-existing plastic shear zones is less recognized, although this situation is frequently observed in many geologically old terranes.</p><p>We have studied the prolonged deformation history of the Himdalen–Ørje Deformation Zone (HØDZ) in SE Norway by combining K–Ar and <sup>40</sup>Ar–<sup>39</sup>Ar geochronology with structural analysis. The HØDZ consists of a large variation of deformation products from mylonites and cataclasites to pseudotachylites and fault gouge. Several generations of mylonites make up the ductile part of HØDZ, called the Ørje shear zone, a km-think SW-dipping shear zone within the late Mesoproterozoic Sveconorwegian orogen. <sup>40</sup>Ar–<sup>39</sup>Ar dating of white mica from one of these mylonites give a plateau age of c. 908 Ma, interpreted to constrain the timing of late-Sveconorwegian extensionial reactivation of the Ørje shear zone.</p><p>This mylonitic fabric is extensively reworked in a brittle fashion along the SW-dipping Himdalen fault, a 10–25 m thick fault zone of cataclasite, breccia, fault gouge and, in places, abundant pseduotachylite veins. <sup>40</sup>Ar–<sup>39</sup>Ar dating of pseduotachylite material gives several small plateaus between c. 375 and 300 Ma, whereas K-feldspar clasts from the cataclasitically deformed host rock carry a Caledonian signal (plateau at c. 435 Ma). K–Ar dating of three fault gouges constrain the timing of gouge development at c. 270 and 200 Ma. Two of the fault gouges also contain protolithic K-bearing mineral phases that overlap in age with the c. 375 Ma pseudotachylite <sup>40</sup>Ar–<sup>39</sup>Ar plateau age, consistent with field observations of the former reworking the latter.</p><p>In sum, the HØDZ records multiple Paleozoic and Mesozoic brittle reactivations of the early Neoproterozoic (and older) mylonitic Ørje shear zone. Most of the brittle deformation is interpreted to have accumulated during development of the Permian Oslo rift and its subsequent latest Triassic evolution. The suggested late Devonian (c. 375 Ma) initiation of brittle deformation does not have a clear tectonic association, but we speculate that it relates to strike-slip displacements caused by the Variscan orogen, as also suggested for the sub-parallel Tornquist zone to the south.</p>

Complex kinematics in a major ductile shear zone, NW Shetland: Evidence of ductile extrusion during Caledonian transpression

2021 ◽  
Author(s):  
Timothy Armitage ◽  
Robert Holdsworth ◽  
Robin Strachan ◽  
Thomas Zach ◽  
Diana Alvarez-Ruiz ◽  
...  
Keyword(s):  
Shear Zone ◽  
Hanging Wall ◽  
Regional Scale ◽  
Shear Zones ◽  
White Mica ◽  
Strike Slip ◽  
Amphibolite Facies ◽  
Lateral Extrusion ◽  
Shear Sense ◽  
Ductile Shear

<p>Ductile shear zones are heterogeneous areas of strain localisation which often display variation in strain geometry and combinations of coaxial and non-coaxial deformation. One such heterogeneous shear zone is the c. 2 km thick Uyea Shear Zone (USZ) in northwest Mainland Shetland (UK), which separates variably deformed Neoarchaean orthogneisses in its footwall from Neoproterozoic metasediments in its hanging wall (Fig. a). The USZ is characterised by decimetre-scale layers of dip-slip thrusting and extension, strike-slip sinistral and dextral shear senses and interleaved ultramylonitic coaxially deformed horizons. Within the zones of transition between shear sense layers, mineral lineations swing from foliation down-dip to foliation-parallel in kinematically compatible, anticlockwise/clockwise-rotations on a local and regional scale (Fig. b). Rb-Sr dating of white mica grains via laser ablation indicates a c. 440-425 Ma Caledonian age for dip-slip and strike-slip layers and an 800 Ma Neoproterozoic age for coaxial layers. Quartz opening angles and microstructures suggest an upper-greenschist to lower-amphibolite facies temperature for deformation. We propose that a Neoproterozoic, coaxial event is overprinted by Caledonian sinistral transpression under upper greenschist/lower amphibolite facies conditions. Interleaved kinematics and mineral lineation swings are attributed to result from differential flow rates resulting in vertical and lateral extrusion and indicate regional-scale sinistral transpression during the Caledonian orogeny in NW Shetland. This study highlights the importance of linking geochronology to microstructures in a poly-deformed terrane and is a rare example of a highly heterogeneous shear zone in which both vertical and lateral extrusion occurred during transpression.</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gepj.0cf6ef44e5ff57820599061/sdaolpUECMynit/12UGE&app=m&a=0&c=d96bb6db75eed0739f2a6ee90c9ad8fd&ct=x&pn=gepj.elif&d=1" alt=""></p>

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

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

<p>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 <sup>40</sup>Ar/<sup>39</sup>Ar geochronology using this mineral, produces cooling ages alone.</p><p>Further, we show that high-definition ultra-high-vacuum (UHV) <sup>39</sup>Ar diffusion experiments using phengitic white mica routinely allow the extraction of muscovite sub-spectra in the first 10-30% of <sup>39</sup>Ar gas release during <sup>40</sup>Ar/<sup>39</sup>Ar 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.</p><p>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.</p><p>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.</p><p>A critical factor is that the recognition of muscovite sub-spectra requires Arrhenius data in order to recognise the steps dominated by release of <sup>39</sup>Ar 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.</p>

scholarly journals Origin of sediments during Cretaceous continent—continent collision in the Romanian Southern Carpathians: preliminary constraints from 40Ar/39Ar single-grain dating of detrital white mica

2013 ◽  
Vol 64 (5) ◽  
pp. 375-382 ◽  
Author(s):  
Franz Neubauer ◽  
Ana-Voica Bojar
Keyword(s):  
Late Jurassic ◽  
Upper Cretaceous ◽  
Shear Zones ◽  
White Mica ◽  
Low Grade ◽  
Late Permian ◽  
Ductile Shear Zones ◽  
Single Grain ◽  
Ductile Shear ◽  
Southern Carpathians

Abstract Single grains of detrital white mica from the lowermost Upper Cretaceous Sinaia Flysch have been dated using the 40Ar/39Ar technique. The Sinaia Flysch was deposited in a trench between the Danubian and Getic microcontinental pieces after the closure of the Severin oceanic tract. The Danubian basement is largely composed of a Panafrican/Cadomian basement in contrast to the Getic/Supragetic units with a Variscan-aged basement, allowing the distinction between these two blocks. Dating of detrital mica from the Sinaia Flysch resulted in predominantly Variscan ages (329 ± 3 and 288 ± 4 Ma), which prove the Getic/Supragetic source of the infill of the Sinaia Trench. Subordinate Late Permian (263 ± 8 and 255 ±10 Ma), Early Jurassic (185 ± 4 and 183 ± 3 Ma) and Late Jurassic/Early Cretaceous (149 ± 3 and 140 ± 3 Ma) ages as well as a single Cretaceous age (98 ± 4 Ma) are interpreted as representing the exposure of likely retrogressive low-grade metamorphic ductile shear zones of various ages. Ductile shear zones with similar 40Ar/39Ar white mica ages are known in the Getic/Supragetic units. The Cretaceous ages also show that Cretaceous metamorphic units were already subject to erosion during the deposition of the Sinaia Flysch.

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