Shear deformation and eclogite formation within granulite-facies anorthosites of the Bergen Arcs, western Norway

Austrheim, Håkon; Griffin, William L.

doi:10.1016/0009-2541(85)90124-x

Cited by 226 publications

(201 citation statements)

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“…They consist of lherzolites interlayered with websterites and garnet pyroxenites. Calculated equilibrium P-T conditions and age data of these lenses (Kühn et al 2000) give 1 GPa, 743°C to 977°C, and 842 ± 12 Ma, which are consistent with those of the Grenvillian granulite-facies metamorphism recorded in surrounding anorthositic rocks (Austrheim and Griffin 1985). As with the surrounding anorthositic rocks, the ultramafic rocks were also affected by high-pressure metamorphism during the Caledonian orogeny and transformed from spinel lherzolite to garnet lherzolite with hydrous and carbonaceous phases (Kühn et al 2000).…”

Section: Geological Setting and Sample Descriptionsupporting

confidence: 75%

“…The Lindås Nappe anorthosite complex has undergone two main phases of metamorphism and deformation -the Grenvillian orogeny and the Caledonian orogeny. During the Grenvillian orogeny, the complex developed a dry granulite-facies mineralogy (800°C to 900°C, <1 GPa) (Austrheim and Griffin 1985) and a granulite-facies foliation. Typically, this foliation is characterized by disc-shaped coronas that are oriented in the same direction and by alternating plagioclase-rich and mafic layers that produce a strong banding.…”

Section: Geological Setting and Sample Descriptionmentioning

confidence: 99%

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Characterization of olivine fabrics and mylonite in the presence of fluid and implications for seismic anisotropy and shear localization

Jung

Austrheim

2014

Earth Planet Sp

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The Lindås Nappe, Bergen Arc, is located in western Norway and displays two high-grade metamorphic structures. A Precambrian granulite facies foliation is transected by Caledonian fluid-induced eclogite-facies shear zones and pseudotachylytes. To understand how a superimposed tectonic event may influence olivine fabric and change seismic anisotropy, two lenses of spinel lherzolite were studied by scanning electron microscope (SEM) and electron back-scattered diffraction (EBSD) techniques. The granulite foliation of the surrounding anorthosite complex is displayed in ultramafic lenses as a modal variation in olivine, pyroxenes, and spinel, and the Caledonian eclogite-facies structure in the surrounding anorthosite gabbro is represented by thin (<1 cm) garnet-bearing ultramylonite zones. The olivine fabrics in the spinel bearing assemblage were E-type and B-type and a combination of A-and B-type lattice preferred orientations (LPOs). There was a change in olivine fabric from a combination of A-and B-type LPOs in the spinel bearing assemblage to B-and E-type LPOs in the garnet lherzolite mylonite zones. Fourier transform infrared (FTIR) spectroscopy analyses reveal that the water content of olivine in mylonite is much higher (approximately 600 ppm H/Si) than that in spinel lherzolite (approximately 350 ppm H/Si), indicating that water caused the difference in olivine fabric. Fabric strength of olivine gets weaker as the grain size reduced, and as a result, calculated seismic properties for the two deformation stages reveal that P-and S-velocity anisotropies are significantly weaker in the mylonite. Microtextures and LPO data indicate that the deformation mechanism changed from dominant dislocation creep in spinel lherzolite to dislocation creep accompanied by grain-boundary sliding in mylonite. Shear localization in the mylonite appears to be originated from the grain size reduction through (1) enhanced dynamic recrystallization of olivine in the presence of water and (2) Zener pinning of clinopyroxene or (3) by ultracomminution during the pseudotachylyte stage.

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Section: Geological Setting and Sample Descriptionsupporting

confidence: 75%

Section: Geological Setting and Sample Descriptionmentioning

confidence: 99%

Characterization of olivine fabrics and mylonite in the presence of fluid and implications for seismic anisotropy and shear localization

Jung

Austrheim

2014

Earth Planet Sp

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“…In the Bergen Arcs, mylonitic eclogitefacies shear zones transect coarser, apparently unaltered, granulite-facies rocks, suggesting that deformation, and arguably fluid, were necessary for triggering the eclogiteforming reactions (Austrheim and Griffin, 1985). What is still unclear is whether deformation triggered the metamorphic transformation or whether fluid triggered the metamorphic reactions, thereby forming a weak zone and hence allowing further deformation and grain size reduction to proceed.…”

Section: Grain Size Reductionmentioning

confidence: 99%

Exhumation of (ultra-)high-pressure terranes: concepts and mechanisms

Warren

2013

Solid Earth

113

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Abstract. The formation and exhumation of high and ultrahigh-pressure, (U)HP, rocks of crustal origin appears to be ubiquitous during Phanerozoic plate subduction and continental collision events. Exhumation of (U)HP material has been shown in some orogens to have occurred only once, during a single short-lived event; in other cases exhumation appears to have occurred multiple discrete times or during a single, long-lived, protracted event. It is becoming increasingly clear that no single exhumation mechanism dominates in any particular tectonic environment, and the mechanism may change in time and space within the same subduction zone. Subduction zone style and internal force balance change in both time and space, responding to changes in width, steepness, composition of subducting material and velocity of subduction. In order for continental crust, which is relatively buoyant compared to the mantle even when metamorphosed to (U)HP assemblages, to be subducted to (U)HP conditions, it must remain attached to a stronger and denser substrate. Buoyancy and external tectonic forces drive exhumation, although the changing spatial and temporal dominance of different driving forces still remains unclear. Exhumation may involve whole-scale detachment of the terrane from the subducting slab followed by exhumation within a subduction channel (perhaps during continued subduction) or a reversal in motion of the entire plate (eduction) following the removal of a lower part of the subducting slab. Weakening mechanisms that may be responsible for the detachment of deeply subducted crust from its stronger, denser substrate include strain weakening, hydration, melting, grain size reduction and the development of foliation. These may act locally to form narrow high-strain shear zones separating stronger, less-strained crust or may act on the bulk of the subducted material, allowing whole-scale flow. Metamorphic reactions, metastability and the composition of the subducted crust all affect buoyancy and overall strength. Future research directions include identifying temporal and spatial changes in exhumation mechanisms within different tectonic environments, and determining the factors that influence those changes.

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“…In the Bergen Arcs, Proterozoic anorthositic granulites are transformed into kyanite eclogites in shear zones and along microshears (Austrheim and Griffin, 1985). This work, like those of Mork, demonstrates the importance of deformation and fluid access in promoting eclogite-forming reactions, and emphasizes the possible metastable persistence of dry mineral assemblages through high-P metamorphism.…”

Section: Low-p Protoliths Of External Eclogitesmentioning

confidence: 54%

‘On the eclogites of Norway’—65 years later

Griffin

1987

Mineral. mag.

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The Western Gneiss Region (WGR) of Norway consists largely of Proterozoic orthogneisses, but also contains paragneisses, peridotites, anorthosites, gabbros and coarse-grained intermediate-acid 'rapakivi granites'. All of these lithologies enclose eclogites. Structural and isotopic data suggest that many of these rocks were juxtaposed by early Caledonian thrusting prior to eclogite formation at ca. 425 Ma.Low-P protoliths can be demonstrated for many eclogites. Prograde metamorphism to eclogite facies is demonstrated by inclusion suites within garnet grains and zoning of eclogite minerals. The regional distribution of Kn (gnt/cpx) and X~ x shows a decrease in Tmax, and in the corresponding P, away from the present coastline. The lowest values (500 ~ 10 kbar) are found in the Sunnfjord area and the highest (~ 800 ~ along the coast of Sunnmore and Nordmore.Maximum pressures were reached at temperatures 100-200 ~ < Tmax-This P-T-t path is consistent with the preservation of jadeite-rich cpx (and possible coesite) in the coastal regions. Earlier overestimates of P .... based on partitioning of A1 between opx and gnt, resulted from combination of early low-T (low-A1) opx and T values derived from cpx/gnt equilibrated at Tmax. Despite pervasive later amphibolitization, high-P assemblages (phengite + kyanite, omphacite + quartz) are locally preserved within gneisses near the coast. The high-P metamorphism can be explained by westward subduction of the Baltic continental plate beneath the Greenland plate, during the Caledonian orogeny.At least some of the Mg-Cr garnet peridotites of the WGR were derived from low-P protoliths (spinel + chlorite peridotites, enclosing high-A1 pyroxenites). While Sm-Nd mineral ages of most eclogites cluster around 425 Ma, garnet peridotites and their enclosed garnet pyroxenites give Proterozoic Sm-Nd mineral ages (1700-1000 Ma). The tectonic position of the Mg-Cr garnet peridotites, relative to the Caledonian high-pressure metamorphism, remains to be resolved.

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Shear deformation and eclogite formation within granulite-facies anorthosites of the Bergen Arcs, western Norway

Cited by 226 publications

References 10 publications

Characterization of olivine fabrics and mylonite in the presence of fluid and implications for seismic anisotropy and shear localization

Characterization of olivine fabrics and mylonite in the presence of fluid and implications for seismic anisotropy and shear localization

Exhumation of (ultra-)high-pressure terranes: concepts and mechanisms

‘On the eclogites of Norway’—65 years later

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