Amphibolization of the Tso Morari UHP eclogites: a record of fluid infiltration at amphibolite-facies during uplift in the subduction channel

Caledonian eclogite- and amphibolite-facies metamorphism of initially dry Proterozoic granulites in the Lind&#229;s 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. &#160;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 Lind&#229;s part of the Jotun-Lind&#229;s microcontinent, during eastward thrusting over the extended margin of Baltica. Melt compositions and thermal modelling suggest that short-lived fluid-driven metamorphism of the Lind&#229;s Nappe granulites was related to shear heating at lithostatic pressures in the range 1.0-1.5 GPa. High-P (&#8776;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.

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Metamorphic Differentiation via Enhanced Dissolution along High Permeability Zones

Journal of Petrology ◽

10.1093/petrology/egaa096 ◽

2020 ◽

Author(s):

Jo Moore ◽

Andreas Beinlich ◽

Sandra Piazolo ◽

Håkon Austrheim ◽

Andrew Putnis

Keyword(s):

Mass Transfer ◽

Shear Zone ◽

Mineral Chemistry ◽

Amphibolite Facies ◽

Metamorphic Rocks ◽

High Permeability ◽

Enhanced Dissolution ◽

Fluid Infiltration ◽

Mass Redistribution ◽

Compositional Variations

Abstract Metamorphic differentiation, resulting in segregated mineral bands, is commonly recorded in metamorphic rocks. Despite the ubiquitous nature of compositionally layered metamorphic rocks, the processes that are responsible for metamorphic differentiation receive very little attention. Here, detailed petrography, quantitative mineral chemistry and bulk rock analyses are applied to investigate compositional variations and assemblage microstructure. Furthermore, thermodynamic modelling is applied to provide additional constraints on the P–T–XH2O conditions of assemblage formation and mass transfer. The studied outcrop, located within the Bergen arcs of southwestern Norway, preserves the hydration of anorthositic granulite at amphibolite-facies conditions. The amphibolite-facies hydration is expressed as both a statically hydrated amphibolite and a shear zone lithology, defined by the interlayering of amphibolite with leucocratic domains. Within the granulite, quartz-lined fractures surrounded by amphibolite-facies alteration haloes represent relics of initial fluid infiltration associated with brittle failure. The fracture assemblage (quartz + plagioclase + zoisite + kyanite ± muscovite ± biotite) is identical to that occurring within leucocratic domains of the shear zone. Consequently, the compositional layering of the shear zone lithology is linked to fluid infiltration along localized zones of high permeability that result from fracturing. Mass-balance calculations indicate that quartz-lined fractures and compositional differentiation of the shear zone resulted from mass redistribution internal to the shear zone rather than partial melting or precipitation of minerals from externally derived fluid. The process of internal fractionation within the shear zone is driven by enhanced dissolution along highly permeable fracture planes resulting in the loss of MgO, Fetot and K2O from the leucocratic domains. Elements dissolved in the fluid are then transported and ultimately either precipitated in comparatively impermeable amphibolite domains or removed from the system resulting in an overall mass loss. The mass transfer causing metamorphic differentiation of the shear zone is the result of coupled reaction and diffusion under differential stress. The mechanisms of mass redistribution observed within this shear zone provides further insight into the processes that facilitate mass transfer in the Earth’s crust.

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From granulite hydration to metamorphic differentiation: Evolution of a shear zone.

10.5194/egusphere-egu2020-12862 ◽

2020 ◽

Author(s):

Andrew Putnis ◽

Jo Moore ◽

Andreas Beinlich ◽

Sandra Piazolo ◽

Håkon Austrheim

Keyword(s):

Mass Transfer ◽

Partial Melting ◽

Shear Zone ◽

Strain Distribution ◽

Amphibolite Facies ◽

Fluid Flux ◽

Transfer Processes ◽

Fluid Infiltration ◽

Mass Transfer Processes ◽

Local Dissolution

The studied outcrop, located within the Bergen arcs of southwestern Norway, preserves the hydration of an anorthositic granulite at amphibolite-facies conditions. The amphibolite-facies hydration is expressed as both a statically hydrated amphibolite and a shear zone rock, defined by the interlayering of amphibolite with leucocratic domains. Within the outcrop, quartz-filled fractures and their associated amphibolite alteration haloes crosscut the granulite. These fractures are relicts of the initial fluid infiltration event. The fracture assemblage (quartz + plagioclase + zoisite + kyanite &#177; muscovite &#177; biotite) is equivalent to that occurring locally within leucocratic domains of the shear zone. Due to the textural and compositional similarities between quartz-filled fractures and leucocratic domains, the compositional layering of the shear zone rock may be directly linked to fracturing during initial fluid infiltration. Mass-balance calculations indicate quartz-filled fractures and compositional differentiation of the shear zone form by internal fractionation rather than partial melting or precipitation of minerals from an eternally derived fluid. This inferred fluid connectivity combined with the enhanced local dissolution indicates the presence of a continuously replenished fluid along fracture pathways. The overall conclusion is that the mass transfer processes that result in metamorphic differentiation of the shear zone lithologies are dependent on both continuous fluid flux and heterogeneous strain distribution.

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Metamorphic differentiation via enhanced dissolution along high strain pathways

10.5194/egusphere-egu2020-12939 ◽

2020 ◽

Author(s):

Jo Moore ◽

Andreas Beinlich ◽

Sandra Piazolo ◽

Håkon Austrheim ◽

Andrew Putnis

Keyword(s):

Shear Zone ◽

Mineral Chemistry ◽

Amphibolite Facies ◽

Metamorphic Rocks ◽

Compositional Variation ◽

Transfer Processes ◽

Enhanced Dissolution ◽

Fluid Infiltration ◽

Mass Transfer Processes ◽

Local Dissolution

Metamorphic differentiation, resulting in the segregation of minerals into compositional bands, is a common feature of metamorphic rocks. Considering the ubiquitous nature of compositionally layered metamorphic rocks, the processes that are responsible for metamorphic differentiation have received very little attention. The studied outcrop, located within the Bergen arcs of southwestern Norway, preserves the hydration of an anorthositic granulite at amphibolite-facies conditions. The amphibolite-facies hydration is expressed as both a statically hydrated amphibolite and a shear zone rock, defined by the interlayering of amphibolite with leucocratic domains. Detailed petrography, quantitative mineral chemistry and bulk rock analyses are applied to investigate compositional variation with assemblage microstructure. Within the outcrop, quartz-filled fractures and their associated amphibolite alteration haloes, are observed crosscutting the granulite. These fractures are demonstrated to be relict of the initial fluid infiltration event. The fracture assemblage (quartz + plagioclase + zoisite + kyanite &#177; muscovite &#177; biotite) is equivalent to that occurring locally within leucocratic domains of the shear zone. Due to the textural and compositional similarities between quartz-filled fractures and leucocratic domains, the compositional layering of the shear zone rock may be directly linked to fracturing during initial fluid infiltration. &#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; Mass-balance and thermodynamic calculations indicate quartz-filled fractures and compositional differentiation of the shear zone form by internal fractionation rather than partial melting or precipitation of minerals from an eternally derived fluid. The process of internal fractionation within the shear zone is attributed to enhanced dissolution along fracture pathways, resulting in the loss of MgO, Fe2O3 and K2O within leucocratic domains. These elements, being more mobile in the fluid, are then transported and ultimately either precipitated in amphibolite lithologies or escape with the fluid, resulting in an overall volume loss in the shear zone. This inferred fluid connectivity combined with the enhanced local dissolution indicates the presence of a continuously replenished fluid along fracture pathways, leading to the overall conclusion that the mass transfer processes that result in metamorphic differentiation of the shear zone lithologies are dependent on both continuous fluid flux and heterogeneous strain distribution.

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Formation of Amphibole and Clinozoisite-Epidote in Eclogite owing to Fluid Infiltration during Exhumation in a Subduction Channel

Journal of Petrology ◽

10.1093/petrology/egs040 ◽

2012 ◽

Vol 53 (10) ◽

pp. 1969-1998 ◽

Cited By ~ 75

Author(s):

H.- J. Massonne

Keyword(s):

Subduction Channel ◽

Fluid Infiltration

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Taconic Metamorphism Preserved in the Baie Verte Peninsula, Newfoundland Appalachians: Geochronological Evidence for Ophiolite Obduction and Subduction and Exhumation of the Leading Edge of the Laurentian (Humber) Margin During Closure of the Taconic Seaway

Geoscience Canada ◽

10.12789/geocanj.2014.41.055 ◽

2014 ◽

Vol 41 (4) ◽

pp. 459 ◽

Cited By ~ 9

Author(s):

Sébastien Castonguay ◽

Cees R. Van Staal ◽

Nancy Joyce ◽

Thomas Skulski ◽

James P. Hibbard

Keyword(s):

Leading Edge ◽

Amphibolite Facies ◽

Subduction Channel ◽

Zircon Age ◽

Ophiolitic Rocks ◽

Eclogite Facies ◽

Radiometric Ages ◽

Geochronological Evidence ◽

Structural Base

The Baie Verte Peninsula, western Newfoundland Appalachians, preserves evidence for Early to Mid Ordovician closure of the Taconic seaway, which led to obduction of the Baie Verte oceanic tract (BVOT) ophiolites onto the Laurentian (Humber) margin and Taconic orogenesis. The scarcity of Taconic radiometric ages (and predominance of Silurian (Salinic) data) from the Humber margin rocks (down-going plate) has been problematic, calling into question the intensity and existence of Taconic collisional orogenesis. 40Ar/39Ar and in situ U–Pb geochronology was undertaken on metamorphosed units from the Laurentian basement (Mesoproterozoic East Pond Metamorphic Suite), from the ca. 560 Ma Birchy Complex forming the leading edge of the Humber margin, and from the ca. 490 Ma ophiolitic rocks of the BVOT (Advocate Complex) in order to address this question. Our results confirm evidence of Taconic metamorphism along the Humber margin and at the base of the ophiolites. Ages obtained from the structural base of the Advocate Complex (481–465 Ma) are interpreted to reflect the timing of accretion and internal thickening of the ophiolite, whereas data from the underlying Birchy Complex (467–461 Ma) record the underthrusting and exhumation of the leading edge of the Humber margin along a subduction channel, penecontemporaneously with final obduction of the BVOT. A concordant ca. 465 Ma zircon age and REE data obtained from retrogressed eclogite of the East Pond Metamorphic Suite suggest that the parautochthonous Humber margin was locally subducted to eclogite-facies conditions during the Taconic collision and partly exhumed to amphibolite-facies conditions prior to a strong Silurian (Salinic) tectonometamorphic overprint.SOMMAIRELa péninsule de Baie Verte dans les Appalaches de l’ouest de Terre-Neuve a conservé des indices de la fermeture du bras de mer taconique, qui a mené à l’obduction des ophiolites de la bande océanique de Baie Verte (BOBV) sur la marge laurentienne (Humber) et à l’orogénèse taconique. La rareté des âges radiométriques taconiques (et la prédominance des données siluriennes (saliniques)) provenant des roches de la marge de Humber (i.e. la plaque subductée) a été problématique, mettant en question l’intensité et l’existence de la collision orogénique taconique. De la géochronologie 40Ar/39Ar et U–Pb in situ a été réalisée sur des unités métamorphisés provenant du socle laurentien (la Suite Métamorphique d’East Pond d’âge Mésoprotérozoïque), du Complexe de Birchy daté à ca. 560 Ma formant la partie frontale de la marge de Humber, and des roches ophiolitiques de la BOBV (Complexe d’Advocate) datée à ca. 490 Ma afin de confronter ce questionnement. Nos résultats confirment les indices de métamorphisme taconique le long de la marge de Humber et à la base des ophiolites. Les âges obtenus à la base structurale du Complexe d’Advocate (481–465 Ma) sont interprétés comme reflétant la période d’accrétion et d’épaississement interne de l’ophiolite, tandis que les données du Complexe de Birchy sous-jacent (467–461 Ma) enregistrent le sous-charriage et l’exhumation de la partie frontale de la marge de Humber au sein d’un chenal de subduction, de façon pénécontemporaine à l’obduction finale de la BOBV. Un âge concordant de ca. 465 Ma d’un zircon et les données de terres rares provenant d’une éclogite rétromorphosée de la suite métamorphique d’East Pond suggèrent que la marge de Humber parautochtone a été localement subductée à des conditions du faciès éclogitique durant la collision taconique et partiellement exhumée à des conditions du faciès des amphibolites précédant la forte surimposition tectonométamorphique silurienne (salinique).

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