scholarly journals Exhumation of the High‐Pressure Tsäkkok Lens, Swedish Caledonides: Insights From the Structural and White Mica 40 Ar/ 39 Ar Geochronological Record

Tectonics ◽  
2020 ◽  
Vol 39 (7) ◽  
Author(s):  
C. J. Barnes ◽  
P. Jeanneret ◽  
K. Kullerud ◽  
J. Majka ◽  
D. A. Schneider ◽  
...  
Keyword(s):  
High Pressure ◽  
White Mica

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.

Metamorphic evolution of Raspas complex (Ecuador) and its relation with a J-K belt of melanges in NW of the South American plate.

2020 ◽  
Author(s):  
Mayda Arrieta-Prieto ◽  
Carlos Zuluaga-Castrillón ◽  
Oscar Castellanos-Alarcón ◽  
Carlos Ríos-Reyes
Keyword(s):  
High Pressure ◽  
Subduction Zones ◽  
Mineral Chemistry ◽  
White Mica ◽  
Petrographic Analysis ◽  
South American ◽  
The South ◽  
South American Plate ◽  
Subduction Channel ◽  
Southern Ecuador

<p>High-pressure complexes along the Earth's surface provide evidence of the processes involved in both the crystallization of rocks in the subduction channel and its exhumation. Such processes are key to understand the dynamics and evolution of subduction zones and to try to reconstruct P-T trajectories for these complexes.</p><p>Previous studies on the Raspas complex (southern Ecuador) agree to state that it is composed of metamorphic rocks, mainly blueschists and eclogites, containing the mineral assemblage: glaucophane + garnet + epidote + omphacite + white mica + rutile ± quartz ± apatite ± pyrite ± calcite; which stabilized in metamorphic conditions of high pressure and low temperature. Additionally, the Raspas Complex has been genetically related to accretion and subduction processes of seamounts, which occurred in South America during the Late Jurassic - Early Cretaceous interval; and the exhumation of the complex was related to subduction channels. However, the evidence presented in the existing literature makes little emphasis on the reconstruction of thermobarometric models for the rocks of this complex.</p><p>By combining petrographic observations, whole-rock chemistry, and mineral chemistry in this work; it was possible to determine that pressure values of 10 ± 3 Kbar and temperature values of 630 ± 30 ° C, (obtained by simulations with THERMOCALC®) correspond to an event of retrograde metamorphism, suffered by the complex during its exhumation. This theory is complemented by the specific textures (that suggest this retrograde process) observed during petrographic analysis, such as amphibole replacing pyroxene, garnet chloritization, plagioclase crystallization and rutile replacement by titanite.</p><p>The results obtained, together with the thermobarometry data published for the Arquía complex in Colombia, allow us to establish a P-T trajectory, that may suggest a genetic relationship between these two complexes as a result of the tectonic processes associated with an active subduction margin that affected the NW margin of the South American plate at the end of the Jurassic.</p><p> </p>

How to disclose local equilibrium in subducted metapelites from the Cima Lunga Unit (Central Alps)

2020 ◽  
Author(s):  
Francesca Piccoli ◽  
Pierre Lanari ◽  
Jörg Hermann ◽  
Thomas Pettke
Keyword(s):  
High Pressure ◽  
Early Stage ◽  
Local Equilibrium ◽  
White Mica ◽  
Geochemical Data ◽  
Ultramafic Rocks ◽  
Central Alps ◽  
Large White ◽  
Icp Ms ◽  
Polyphase Inclusions

<p>Subducted metapelites are more prone to re-equilibrate during exhumation than mafic or ultramafic rocks to the point that recognizing high-pressure (HP) relicts is often very challenging. Geologic evidence from the Cima Lunga Unit (Central Alps) show this apparent discrepancy between high to ultra-high pressure metamorphism (28 kbar and 780 °C) recorded in mafic/ultramafic lenses, and Barrovian metamorphism (<10 kbar, 650°C) in the adjacent metapelitic rocks. We collected a white mica – garnet – biotite – plagioclase – kyanite (+ quartz, + zircon, + rutile) bearing metapelite adjacent to the garnet metaperidotite lens that displays an apparently well equilibrated Barrovian mineral assemblage (garnet + plagioclase + biotite), with no macroscopic or microtextural indication of a HP and/or HT metamorphic event (e.g. omphacite crystals; migmatitic texture; polyphase inclusions). Nevertheless, microstructures like atoll-like garnet or large white mica flakes surrounded by biotite and ilmenite replacing rutile suggest incomplete re-equilibration. We investigated garnet and phengite crystals by electron probe and laser ablation-ICP-MS mapping. Major and trace element mapping reveals very complex mineral zoning in both minerals. In particular, high Ti content in phengite and increasing P and Zr contents in pyrope-rich garnet indicate that the studied rock underwent a HP-HT event. This is also supported by Zr in rutile thermometry that indicates temperatures well above the Barrovian metamorphism (T > 700 °C). We combined detailed textural analysis with petrological-geochemical data and thermodynamic modelling to reconstruct the metamorphic evolution of the studied rock. We show that, thank to incomplete re-equilibration, the rock documents an evolution from prograde to UHP-HT peak (27 kbar and 800 °C) to retrograde (Barrovian) conditions (10 kbar and 620 °C). Noteworthy, peak metamorphic conditions of metapelite coincide with peak metamorphic conditions of the garnet metaperidotite. Lastly, geochemical evidence for minor wet melting of the studied metapelite at HP-HT conditions was recognized and is likely linked to the dehydration of chlorite to form garnet peridotite in the adjacent ultramafic body. We propose that metapelites and ultramafic rocks were coupled before subduction or at least in its early stage. This finding opens new scenarios for the geodynamic interpretation of the Cima Lunga unit. We propose that the ultramafic lenses at Cima di Gagnone were parts of the exhumed and serpentinised mantle emplaced at the hyper-extended European continental margin of the Piemont-Ligurian ocean. Slices of the margin were detached and tectonically mixed in the subduction channel. These new constraints call for re-evaluation of the paleogeographic position of the Adula-Cima Lunga nappe.</p>

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>

Squeezed Under the Sheet: White Mica Records High Tectonic Stresses Within a Decollement Thrust

2020 ◽  
Author(s):  
Miisa Häkkinen ◽  
Samuel Angiboust ◽  
Benoit Dubacq ◽  
Martine Simoes
Keyword(s):  
High Pressure ◽  
Stress Distribution ◽  
White Mica ◽  
Tectonic Stresses ◽  
Paleozoic Basement ◽  
Thrust Zone ◽  
High Silica ◽  
Basement High ◽  
Orogenic Belts ◽  
Se France

<p>Tectonic stresses at the base of decollement thrusts are generally expected to be low due to the presence of mechanically weak evaporites. Yet, the presence of abundant micro-seismicity in the region expected to correspond to the evaporitic layer remains paradoxical. We study here a fossil thrust zone from the base of the Digne nappe (SE France) where exotic thrust slices formed by brecciated Paleozoic basement micaschists are observed within the Mio-Pliocene decollement. Petrographic investigations reveal the presence of highly-substituted phengitic rims (up to Si=3.43 apfu) around pre-alpine muscovitic cores. Similar micaschists sampled in a basement high further North do not exhibit these phengitic rims around muscovite, thus suggesting that white mica zoning relates to a younger overprint. Such high-Silica phengites are commonly found in high-pressure terranes (i.e. 7-15 kbars depending on the buffering assemblage) but are not expected in foreland regions, such as in the Digne area where the overburden has never been thicker than c.5km (i.e. approximately 1.3 kbar). We propose that the mica zoning observed reflects the former presence of non-lithostatic stresses (possibly on the order of several kilobars) related to the elastic charging of a thrust slice “squeezed” at the base of the moving nappe. This finding sheds light on stress distribution as well as on the origin of micro-seismicity along active decollement thrusts in orogenic belts.</p>

scholarly journals Syn-deformational melt percolation through a high-pressure orthogneiss and the exhumation of a subducted continental wedge (Orlica-Śnieżnik Dome, NE Bohemian Massif)

2021 ◽  
Author(s):  
Carmen Aguilar ◽  
Pavla Štípská ◽  
Francis Chopin ◽  
Karel Schulmann ◽  
Pavel Pitra ◽  
...  
Keyword(s):  
High Pressure ◽  
Bohemian Massif ◽  
White Mica ◽  
Gradual Transition ◽  
Fine Grained ◽  
Rock Types ◽  
Melt Percolation ◽  
T Path ◽  
Textural Changes ◽  
Wide Zone

<h3>High-pressure granitic orthogneiss of the south-eastern Orlica–Śnieżnik Dome (NE Bohemian Massif) shows relics of a shallow-dipping S1 foliation, reworked by upright F2 folds and a mostly pervasive N-S trending subvertical axial planar S2 foliation. Based on macroscopic observations, a gradual transition perpendicular to the subvertical S2 foliation from banded to schlieren and nebulitic orthogneiss was distinguished. All rock types comprise plagioclase, K-feldspar, quartz, white mica, biotite and garnet. The transition is characterized by increasing presence of interstitial phases along like-like grain boundaries and by progressive replacement of recrystallized K-feldspar grains by fine-grained myrmekite. These textural changes are characteristic for syn-deformational grain-scale melt percolation, which is in line with the observed enrichment of the rocks in incompatible elements such as REEs, Ba, Sr, and K, suggesting open-system behaviour with melt passing through the rocks. The P–T path deduced from the thermodynamic modelling indicates decompression from ~15−16 kbar and ~650–740 ºC to ~6 kbar and ~640 ºC. Melt was already present at the P–T peak conditions as indicated by the albitic composition of plagioclase in films, interstitial grains and in myrmekite. The variably re-equilibrated garnet suggests that melt content may have varied along the decompression path, involving successively both melt gain and loss. The 6–8 km wide zone of vertical foliation and migmatite textural gradients is interpreted as vertical crustal-scale channel where the grain-scale melt percolation was associated with horizontal shortening and vertical flow of partially molten crustal wedge en masse.</h3>

Mélanges in southern Mexico: geochemistry and metamorphism of Las Ollas complex (Guerrero terrane)

2000 ◽  
Vol 37 (9) ◽  
pp. 1309-1320 ◽  
Author(s):  
Oscar Talavera Mendoza
Keyword(s):  
High Pressure ◽  
Low Temperature ◽  
Early Cretaceous ◽  
Mineral Chemistry ◽  
White Mica ◽  
Island Arc ◽  
Southern Mexico ◽  
Calcic Amphibole ◽  
Guerrero Terrane

Las Ollas complex (LOC) is a subduction complex spatially associated with the early Cretaceous Zihuatanejo-Huetamo subterrane (Guerrero terrane) in southern Mexico. LOC tectonic mélanges compose of a stack of east-dipping, west-vergent tectonic sheets containing blocks of metabasalt, metadolerite, metagabbro, ultramafics, volcaniclastics, quartz-rich sandstone, and chert enveloped in a highly sheared clastic or serpentinitic matrix. Most igneous and igneous-derived metamorphic blocks show geochemical and isotopic features typical of island-arc tholeiitic suites: (i) low TiO2 (0.13 to 0.91%) and Zr (5 to 57 ppm) contents; (ii) high (LFSE/HFSE)N ratios; low LaN/YbN (0.5 to 4) values; and, high εNd(T) (+7.9 to +8.0) ratios. Petrographical and mineral chemistry evidence indicates that blocks underwent early recrystallization under high pressure and low temperature (HP-LT), blueschist facies conditions during subduction. Typical assemblages include blue (sodic through calco-sodic to Na-rich calcic) amphibole + lawsonite ± tremolite ± Mg-chlorite ± white mica ± albite ± quartz. Phase relations and chlorite thermometry suggest temperatures of about 200°-330o C and pressures of 5-7 kbar. It is proposed that sedimentary blocks were generated by in situ remobilization and mixing, whereas igneous blocks most probably derived from the chemically and isotopically identical Zihuatanejo island-arc suite. Our data suggest that LOC represents part of a subduction complex formed by eastward-directed subduction related with the evolution of the early Cretaceous Zihuatanejo island arc.

Metamorphic rocks of an intermediate facies series juxtaposed at the Start boundary, southwest England

1981 ◽  
Vol 118 (3) ◽  
pp. 297-301 ◽  
Author(s):  
D. Robinson
Keyword(s):  
High Pressure ◽  
Diffraction Analysis ◽  
Mineral Assemblage ◽  
White Mica ◽  
Lower Pressure ◽  
Metamorphic Rocks ◽  
X Ray Diffraction ◽  
X Ray ◽  
High Pressure Metamorphism ◽  
The North

SummaryX-ray diffraction analysis of metamorphic rocks from the Start peninsula shows that the dominant mineral assemblage in the Start schists is quartz-muscovite-chlorite ± paragonite ± albite. The assemblage in the Devonian phyllites to the north of the Start boundary is quartz-muscovite-chlorite and the crystallinity of the white mica there is indicative of greenschist fades. The b0 parameter of white micas indicates that the metamorphism was of an intermediate facies series (Barrovian type), but of a lower pressure variety to the north of the Start boundary juxtaposed against a higher pressure variety to the south. No evidence exists of the high pressure metamorphism needed to support tectonic models in which the Start boundary is identified as a suture representing a Variscan subduction margin.

Pressure–temperature–deformation paths of closely associated ultra-high-pressure (diamond-bearing) crustal and mantle rocks of the Kimi complex: implications for the tectonic history of the Rhodope Mountains, northern Greece

2006 ◽  
Vol 43 (12) ◽  
pp. 1755-1776 ◽  
Author(s):  
E Mposkos ◽  
A Krohe
Keyword(s):  
High Pressure ◽  
White Mica ◽  
Temperature Deformation ◽  
Low Grade ◽  
Uhp Metamorphism ◽  
Northern Greece ◽  
Crustal Rocks ◽  
Ultra High Pressure ◽  
T Path ◽  
Static Annealing

The ultra-high-pressure (UHP) Kimi complex (uppermost eastern Rhodope Mountains) is a tectonic mixture of crustal and mantle derived associations. Pressure–temperature (P–T) paths and microtextural and geochronological data reveal that crustal and mantle parts juxtaposed against each other at a depth corresponding to ~15 kbar (1 kbar = 100 MPa) had separate ascend histories. The crustal rocks comprise amphibolitised eclogites, orthogneisses, marbles, and migmatitic pelitic gneisses. The latter document UHP metamorphism within the dehydration-melting range of pelitic gneisses, with maximum P–T conditions of >45 kbar at ~1000 °C, as determined by diamond inclusions in garnet and rutile needle exsolutions in Na-bearing garnet. Decompression was combined with only little cooling before 15 kbar, followed by more significant cooling between 15 and 10 kbar. This P–T path probably reflects ascent of UHP rocks within a subduction channel, followed by accretion in the lower crust of a thickened wedge. Although the first ascend phase was probably rapid, the overall time span for UHP metamorphism and final exhumation may have extended over more than 70 Ma. A U–Pb sensitive high-resolution ion microprobe (SHRIMP) age on zircons of ±149 Ma was suggested to date the UHP metamorphism, whereas Rb–Sr white mica and U–Pb zircon ages from syn-shearing pegmatites of ±65 Ma constrain medium- to low-grade shearing and final exhumation of UHP rocks. Mantle parts consisting of spinel–garnet metaperidotites and garnet pyroxenites reached maximum P–T conditions in the garnet-peridotite field at T > 1200 °C and P > 25 kbar. This was associated with plastic flow and followed by severe near isothermal cooling to T < 800 °C at 15 kbar and static annealing. A garnet–clinopyroxene whole-rock Sm–Nd age from a garnet pyroxenite of ±119 Ma probably reflects the age of metamorphic mantle processes (static annealing following the high P/high T strain episode), rather than constraining the age of UHP metamorphism.

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