Retrograde metasomatic effects on phase assemblages in an interlayered blueschist–greenschist sequence (Coastal Cordillera, Chile)

Halama, Ralf; Konrad‐Schmolke, Matthias

doi:10.1016/j.lithos.2014.12.004

Cited by 12 publications

(14 citation statements)

References 92 publications

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“…The mineral modal difference between fabrics A and B (in particular the absence of blue amphibole) does not necessarily indicate a pressure drop. The apparent greenschist facies overprint visible within fabric B may reflect a moderate temperature increase (50–100°C; see the P‐T path in Figure b) due to the warming of the Arolla slice after underplating or it may also be related to the Ca metasomatic overprint affecting foliated cataclasites [ Evans , ; see also Halama and Konrad‐Schmolke , ] (Figure b). The P‐T‐time path obtained on overprinted, foliated cataclasites (sample #60b; 40–43 Ma; 460–520°C; 0.7–0.9 GPa; Figure b) [ Angiboust et al ., ] indicates that cataclasis leading to the genesis of fabric B took place before 42 Ma at depths larger than 20 km (Figure ).…”

Section: Discussionmentioning

confidence: 99%

Probing the transition between seismically coupled and decoupled segments along an ancient subduction interface

Angiboust

Kirsch

Oncken

et al. 2015

Geochem Geophys Geosyst

100

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The transition zone at the downdip end of seismic coupling along subduction interfaces is often the site of megathrust earthquake nucleation and concentrated postseismic afterslip, as well as the focus site of episodic tremor and slip features. Exhumed remnants of the former Alpine subduction zone found in the Swiss Alps allow analyzing fluid and deformation processes near the transition zone region (30-40 km paleodepth). The Dent Blanche Thrust (DBT) is a lower blueschist-facies shear zone interpreted as a fossilized subduction interface where granitic mylonites overlie a metamorphosed accretionary wedge. We report field observations from the DBT region where multiple, several tens of meters thick foliated cataclastic networks are interlayered within the basal DBT mylonites. Petrological results and microstructural observations indicate that the various cataclasis events took place at near-peak metamorphic conditions (400-5008C, 1.1-1.3 GPa) during subduction of the Tethyan seafloor in Eocene times (42-48 Ma). Some of these networks exhibit mutual crosscutting relationships between mylonites, foliated cataclasites, and vein systems indicating mutual overprinting between brittle deformation and ductile creep. Whole-rock chemical compositions, in situ 40 Ar age data of recrystallized phengite, and Sr isotopic signatures reveal that DBT rocks also underwent multiple hydrofracturing and metasomatic events via the infiltration of fluids mainly derived from the oceanic metasediments underneath the DBT. From the rock fabrics, we infer strain rate fluctuations of several orders of magnitude beyond subduction strain rates (10 212 s 21) accompanied by fluctuation of supralithostatic and quasi-lithostatic fluid pressures (1 k > 0.95). DBT brittle-plastic deformation switches highlight the diversity of deformation processes and fluid-rock interactions in the transition zone region of the subduction interface.

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Section: Discussionmentioning

confidence: 99%

Probing the transition between seismically coupled and decoupled segments along an ancient subduction interface

Angiboust

Kirsch

Oncken

et al. 2015

Geochem Geophys Geosyst

100

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“…Fluid-mediated recrystallization and re-equilibration is to be expected during greenschist-facies metamorphism because fluid ingress during exhumation through the greenschist facies is a very common phenomenon (Proyer, 2003;Villa, 2010). Moreover, water saturation during decompression from blueschist to greenschist facies can be attained without external water influx, as decompression along the retrograde P-T path may result in dehydration because the capability of the rock to retain water diminishes (Halama and Konrad-Schmolke, 2015). Calc-alkaline dikes of andesitic and basaltic composition, which are probably related to emplacement of Periadriatic plutons at ca.…”

Section: Greenschist-facies Mylonitesmentioning

confidence: 99%

Rb-Sr and in situ 40Ar/39Ar dating of exhumation-related shearing and fluid-induced recrystallization in the Sesia zone (Western Alps, Italy)

et al. 2018

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“…Another consequence of fluid-rock interaction at depth is the variable metasomatic imprint that obliterates the original geochemical signature of the affected lithologies (e.g., Harlow and Sorensen, 2005;Vitale Brovarone et al, 2014;Bebout and Penniston-Dorland, 2016). Selective retrogression due to exhumation-related fluid circulation may enhance, in specific cases, the formation of blueschists and greenschists interlayered at centimeter to meter scale (e.g., Bröcker, 1990;Barrientos and Selverstone, 1993;Halama and Konrad-Schmolke, 2015). However, many studies have proposed that bulk-rock geochemical differences may lead to the equilibration of cofacial greenschists and blueschists in the subducted material (Dungan et al, 1983;Owen, 1989;Baziotis and Mposkos, 2011;Hyppolito, 2014).…”

Section: ■ Introductionmentioning

confidence: 99%

“…Here we study a fully exposed, exhumed transitional greenschist-blueschist facies segment of a late Paleozoic paleo-accretionary wedge from central Chile (Pichilemu region) that provides a great opportunity for linking fossil subduction-related markers of fluid mobility such as vein systems together with structures and deformation patterns. The first part of this work aims at determining the controlling factors that result in the occurrence of layered blueschists and greenschists in this region because no consensus exists regarding the origin of this layering, interpreted either as a retrogressive fluid imprint or as a consequence of centimeter-scale bulk-rock composition variations (Hyppolito, 2014;Halama and Konrad-Schmolke, 2015). Resolving this issue is critical for the second part of this study, namely the design of a prograde fluid-production model that aims at understanding the chronology of vein-formation events, which will ultimately refine our vision of fluid mobility in active subduction margins.…”

Section: ■ Introductionmentioning

confidence: 99%

Multiple veining in a paleo–accretionary wedge: The metamorphic rock record of prograde dehydration and transient high pore-fluid pressures along the subduction interface (Western Series, central Chile)

et al. 2020

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High pressure–low temperature metamorphic rocks from the late Paleozoic accretionary wedge exposed in central Chile (Pichilemu region) are characterized by a greenschist-blueschist lithological association with interbedded metasediments that reached peak burial conditions of ∼400 °C and 0.8 GPa during late Carboniferous times. We herein combine new extensive field observations, structural measurements, and geochemical and petrological data on vein and matrix material from Pichilemu transitional greenschist-blueschist facies rocks. The studied veins were first filled by albite, followed by quartz and calcite as well as glaucophane and winchite. Field, structural, and microscopic zoning patterns show that these rocks underwent a protracted sequence of prograde vein-opening events, which have been largely transposed to the main foliation before and during underplating in the basal accretion site near 25–30 km depth. While some of the earliest albite-filled vein sets may have formed after prograde breakdown of sub–greenschist facies minerals (<250 °C), our thermodynamic modeling shows that relatively minor amounts of fluid are produced in the subducted pile by dehydration reactions between 250 and 400 °C along the estimated geothermal gradient. It also confirms that the formation of interlayered blueschist and greenschist layers in Pichilemu metavolcanics is a consequence of local bulk composition variations, and that greenschists are generally not formed due to selective exhumation-related retrogression of blueschists. The early vein sets are a consequence of prograde internal fluid production followed by sets of hydrofractures formed at near-peak burial that are interpreted as a record of external fluid influx. We postulate that such a fractured sequence represents a close analogue to the high-Vp/Vs regions documented by seismological studies within the base of the seismogenic zone in active subduction settings.

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Retrograde metasomatic effects on phase assemblages in an interlayered blueschist–greenschist sequence (Coastal Cordillera, Chile)

Cited by 12 publications

References 92 publications

Probing the transition between seismically coupled and decoupled segments along an ancient subduction interface

Probing the transition between seismically coupled and decoupled segments along an ancient subduction interface

Rb-Sr and in situ 40Ar/39Ar dating of exhumation-related shearing and fluid-induced recrystallization in the Sesia zone (Western Alps, Italy)

Multiple veining in a paleo–accretionary wedge: The metamorphic rock record of prograde dehydration and transient high pore-fluid pressures along the subduction interface (Western Series, central Chile)

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