Retrogression of eclogite-facies shear zones by short-lived fluid infiltration during the Caledonian orogeny, Lofoten islands, Norway

2013 ◽  
Vol 390 (1) ◽  
pp. 443-466 ◽  
Author(s):  
H. W. Fournier ◽  
J. K. W. Lee ◽  
A. Camacho ◽  
R. A. Creaser
Keyword(s):  
Shear Zones ◽  
Fluid Infiltration ◽  
Lofoten Islands ◽  
Caledonian Orogeny ◽  
Eclogite Facies

The distribution of the H2O content in nominally anhydrous minerals and its effect on shear zone widening (Holsnøy, West-Norway)

2020 ◽  
Author(s):  
Lisa Kaatz ◽  
Julien Reynes ◽  
Timm John ◽  
Stefan Schmalholz ◽  
Jörg Hermann ◽  
...  
Keyword(s):  
Shear Zone ◽  
Effective Diffusion ◽  
Shear Zones ◽  
Fluid Composition ◽  
Strong Impact ◽  
Rock Interaction ◽  
Nominally Anhydrous Minerals ◽  
The Core ◽  
Fluid Infiltration ◽  
Eclogite Facies

<p>High-grade anorthositic granulites from Holsnøy in the Bergen Arcs (western Norway) were subducted and underwent high pressure (HP) eclogite-facies metamorphism during Caledonian orogeny. They indicate that local eclogitization is linked to an interplay between deformation, fluid infiltration and subsequent fluid-rock interaction. The final result is an interconnected network of hydrous eclogite-facies shear zones surrounded by pristine unreacted granulites. This local transient eclogitization process temporarily weakens the subducting plate and therefore, might have had a strong impact on its deformation.  </p><p>In a first quantitative study we combined detailed field-mapping with numerical modelling to investigate the evolution of hydrous eclogite-facies shear zones with respect to the regional far-field stress and we discuss the strain partitioning. Although it is supposed that strain localises within the shear zones, we were able to show that widening overcomes the effect of stretching because of the fluid-rock interaction during deformation. The availability of a free fluid phase, which is continuously infiltrating the system, has a strong effect on shear zone widening. The most appropriate effective diffusion coefficient to emulate nature-like structures and hydration front widths by simple, hydro-mechanical numerical models was 10<sup>-12 </sup>m<sup>2</sup>.s<sup>-1</sup>. Our first conclusions suggest that a continuous fluid infiltration seems to be required to reproduce the observed structures. However, a complex model is necessary to understand how the fluid infiltrates and consequently, transforms the granulite adjacent to the shear zone widening.</p><p>Mass balance considerations reveal that the eclogitization of the granulite did not result in significant compositional changes, hence the fluid composition was quickly rock buffered. In order to better understand the link between enhancing deformation and fluid-infiltration fronts, we aim to determine the H<sub>2</sub>O content stored in minerals (including nominally anhydrous minerals, NAMs) perpendicular to the deformation structure from the core of the eclogite-facies shear zone to the macroscopically unaffected granulite. Hydrogen in garnet, pyroxene, plagioclase can significantly weaken the mineral structure, especially when substituting for silica. Additionally, it is crucial to constrain the amount of H<sub>2</sub>O needed for the transition from nominally anhydrous to hydrous assemblages. The H<sub>2</sub>O content was measured using transmission Fourier transform infrared spectroscopy using single points and maps to investigate potential zoning. An entire 20 cm wide transect was investigated, between unaltered granulite and the core of the eclogite-facies shear zone. This study will provide new constraints on the dynamic weakening processes affecting metastable dry and rigid crustal rocks.</p>

scholarly journals Subduction interface processes recorded by eclogite-facies shear zones (Monviso, W. Alps)

Lithos ◽  
2011 ◽  
Vol 127 (1-2) ◽  
pp. 222-238 ◽  
Author(s):  
S. Angiboust ◽  
P. Agard ◽  
H. Raimbourg ◽  
P. Yamato ◽  
B. Huet
Keyword(s):  
Shear Zones ◽  
Eclogite Facies

Late Caledonian transpression and the structural controls on pluton construction; new insights from the Omey Pluton, western Ireland

2015 ◽  
Vol 106 (1) ◽  
pp. 11-28 ◽  
Author(s):  
William McCarthy ◽  
R. John Reavy ◽  
Carl T. Stevenson ◽  
Michael S. Petronis
Keyword(s):  
Spatial Distribution ◽  
Host Rock ◽  
Shear Zones ◽  
Magma Ascent ◽  
Structural Controls ◽  
Rock Structure ◽  
Granite Complex ◽  
Caledonian Orogeny ◽  
Western Ireland

ABSTRACTThe Galway Granite Complex is unique among the British and Irish Caledonian granitoid terranes, as it records punctuated phases of magmatism from ∼425–380 Ma throughout the latest phase of the Caledonian Orogeny. Remapping of the Omey Pluton, the oldest member of this suite, has constrained the spatial distribution and contact relationships of the pluton's three main facies relative to the nature of the host rock structure. The external contacts of the pluton are mostly concordant to the limbs and hinge of the Connemara Antiform. New AMS data show that a subtle concentric outward dipping foliation is present, and this is interpreted to reflect pluton inflation during continued magma ingress. Combined field, petrographic and AMS data show that two sets of shear zones (NNW–SSE and ENE–WSW) cross-cut the concentric foliation, and that these structures were active during the construction of the pluton. We show that regional sinistral transpression at ∼420 Ma would have caused dilation along the intersection of these two fault sets, and suggest that this facilitated centralised magma ascent. Lateral emplacement was controlled by the symmetry of the Connemara Antiform to ultimately produce a discordant phacolith. We propose that regional sinistral transpression at ∼420 Ma influenced the siting of smaller intrusions over NNW–SSE faults, and that the later onset of regional transtension caused larger volumes of magma to intrude along the E–W Skird Rocks Fault at ∼400 Ma.

Rapid fluid-driven transformation of lower continental crust associated with thrust-induced shear heating

2020 ◽  
Author(s):  
Bjørn Jamtveit ◽  
Kristina G. Dunkel ◽  
Arianne Petley-Ragan ◽  
Fernando Corfu ◽  
Dani W. Schmid
Keyword(s):  
Regional Metamorphism ◽  
Shear Zones ◽  
Amphibolite Facies ◽  
Source Rocks ◽  
Granitic Rocks ◽  
Time Interval ◽  
Fluid Infiltration ◽  
Facies Metamorphism ◽  
Shear Heating ◽  
Host Rocks

<p>Caledonian eclogite- and amphibolite-facies metamorphism of initially dry Proterozoic granulites in the Lindås Nappe of the Bergen Arcs, Western Norway, is driven by fluid infiltration along faults and shear zones. The granulites are also cut by numerous dykes and pegmatites that are spatially associated with metamorphosed host rocks. U-Pb geochronology was performed to constrain the age of fluid infiltration and metamorphism. The ages obtained demonstrate that eclogite- and amphibolite-facies metamorphism were synchronous within the uncertainties of our results and occurred within a maximum time interval of 5 Myr, with a mean age of ca. 426 Ma.  Caledonian dykes and pegmatites are granitic rocks characterised by a high Na/K-ration, low REE-abundance and positive anomalies of Eu, Ba, Pb, and Sr. The most REE-poor compositions show HREE-enrichment. Melt compositions are consistent with wet melting of plagioclase- and garnet-bearing source rocks. The most likely fluid source is dehydration of Paleozoic metapelites, located immediately below the Lindås part of the Jotun-Lindås microcontinent, during eastward thrusting over the extended margin of Baltica. Melt compositions and thermal modelling suggest that short-lived fluid-driven metamorphism of the Lindås Nappe granulites was related to shear heating at lithostatic pressures in the range 1.0-1.5 GPa. High-P (≈2 GPa) metamorphism within the Nappe was related to weakening-induced pressure perturbations, not to deep burial. Our results emphasize that both prograde and retrograde metamorphism may proceed rapidly during regional metamorphism and that their time-scales may be coupled through local production and consumption of fluids.</p>

Evolution of brittle structures in plagioclase-rich rocks at high-grade metamorphic conditions – Linking laboratory results to field observations

2020 ◽  
Author(s):  
Sarah Incel ◽  
Jörg Renner ◽  
Bjørn Jamtveit
Keyword(s):  
Structural Evolution ◽  
Confining Pressure ◽  
Shear Zones ◽  
Ductile Shear Zones ◽  
Laboratory Results ◽  
Experimental Laboratory ◽  
Eclogite Facies ◽  
Ductile Shear ◽  
Host Rocks ◽  
Lower Crustal

<p>Plagioclase-rich lower crustal granulites exposed on the Lofoten archipelago, N Norway, display pseudotachylytes, reflecting brittle deformation, as well as ductile shear zones, highlighting plastic deformation. Pristine pseudotachylytes often show no or very little difference in mineral assemblage to their host-rocks that exhibit limited, if any, metamorphic alteration. In contrast, host-rock volumes that developed ductile shear zones exhibit significant hydration towards amphibolite or eclogite-facies assemblages within and near the shear zones. We combine experimental laboratory results and observations from the field to characterize the structural evolution of brittle faults in plagioclase-rich rocks at lower crustal conditions. We performed a series of deformation experiments on intact granulite samples at 2.5 GPa confining pressure,  a strain rate of 5×10<sup>-5</sup> s<sup>-1</sup>,  temperatures of 700 and 900 °C, and total strains of either ~7-8 % or ~33-36 %. Samples were either deformed ‘as-is’, i.e. natural samples without any treatment, or with ~2.5 wt.% H<sub>2</sub>O added. Striking similarities between the experimental and natural microstructures suggest that the transformation of precursory brittle structures into ductile shear zones at eclogite-facies conditions is most effective when hydrous fluids are available in excess.</p>

scholarly journals Deeply subducted continental fragments – Part 1: Fracturing, dissolution–precipitation, and diffusion processes recorded by garnet textures of the central Sesia Zone (western Italian Alps)

Solid Earth ◽  
2018 ◽  
Vol 9 (1) ◽  
pp. 167-189 ◽  
Author(s):  
Francesco Giuntoli ◽  
Pierre Lanari ◽  
Martin Engi
Keyword(s):  
High Pressure ◽  
Diffusion Processes ◽  
Shear Zones ◽  
Granulite Facies ◽  
Thermodynamic Modelling ◽  
Italian Alps ◽  
Eclogite Facies ◽  
Compositional Patterns ◽  
And Diffusion ◽  
Insight Into

Abstract. Contiguous continental high-pressure terranes in orogens offer insight into deep recycling and transformation processes that occur in subduction zones. These remain poorly understood, and currently debated ideas need testing. The approach we chose is to investigate, in detail, the record in suitable rock samples that preserve textures and robust mineral assemblages that withstood overprinting during exhumation. We document complex garnet zoning in eclogitic mica schists from the Sesia Zone (western Italian Alps). These retain evidence of two orogenic cycles and provide detailed insight into resorption, growth, and diffusion processes induced by fluid pulses in high-pressure conditions. We analysed local textures and garnet compositional patterns, which turned out remarkably complex. By combining these with thermodynamic modelling, we could unravel and quantify repeated fluid–rock interaction processes. Garnet shows low-Ca porphyroclastic cores that were stable under (Permian) granulite facies conditions. The series of rims that surround these cores provide insight into the subsequent evolution: the first garnet rim that surrounds the pre-Alpine granulite facies core in one sample indicates that pre-Alpine amphibolite facies metamorphism followed the granulite facies event. In all samples documented, cores show lobate edges and preserve inner fractures, which are sealed by high-Ca garnet that reflects high-pressure Alpine conditions. These observations suggest that during early stages of subduction, before hydration of the granulites, brittle failure of garnet occurred, indicating high strain rates that may be due to seismic failure. Several Alpine rims show conspicuous textures indicative of interaction with hydrous fluid: (a) resorption-dominated textures produced lobate edges, at the expense of the outer part of the granulite core; (b) peninsulas and atoll garnet are the result of replacement reactions; and (c) spatially limited resorption and enhanced transport of elements due to the fluid phase are evident along brittle fractures and in their immediate proximity. Thermodynamic modelling shows that all of these Alpine rims formed under eclogite facies conditions. Structurally controlled samples allow these fluid–garnet interaction phenomena to be traced across a portion of the Sesia Zone, with a general decrease in fluid–garnet interaction observed towards the external, structurally lower parts of the terrane. Replacement of the Permian HT assemblages by hydrate-rich Alpine assemblages can reach nearly 100 % of the rock volume. Since we found no clear relationship between discrete deformation structures (e.g. shear zones) observed in the field and the fluid pulses that triggered the transformation to eclogite facies assemblages, we conclude that disperse fluid flow was responsible for the hydration.

Timing and exhumation of eclogite facies shear zones, Musgrave Block, central Australia

1997 ◽  
Vol 15 (6) ◽  
pp. 735-751 ◽  
Author(s):  
A. CAMACHO ( [email protected]. ◽  
W. COMPSTON ◽  
M. McCULLOCH ◽  
I. McDOUGALL
Keyword(s):  
Shear Zones ◽  
Central Australia ◽  
Eclogite Facies
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