UHP Ti-chondrodite in the Zermatt-Saas serpentinite: Constraints on a new tectonic scenario

Luoni, Pietro; Rebay, Gisella; Spalla, Maria Iole; Zanoni, Davide

doi:10.2138/am-2018-6460

Cited by 29 publications

(22 citation statements)

References 19 publications

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“…3b). Pieces comprising only serpentinites, equilibrated at similar depths, have also recently been reported (Luoni et al, 2018). Rocks recovered from greater depths in other compilations are all continental.…”

Section: Trustable Natural Probes For the Plate Interfacementioning

confidence: 68%

The subduction plate interface: rock record and mechanical coupling (from long to short timescales)

Agard

Plunder

Angiboust

et al. 2018

Lithos

208

294

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Short-and long-term processes at or close to the subduction plate interface (e.g.,mineral transformations, fluid release, seismicity and more generally deformation) might be more closely related than previously thought. Increasing evidence from the fossil rock record suggests that some episodes of their long geological evolution match or are close to timescales of the seismic cycle. This contribution uses rocks recovered (episodically) from subduction zones, together with insights from thermomechanical modelling, to provide a new dynamic vision of the nature, structure and properties of the plate interface and to bridge the gap between the mechanical behavior of active subduction zones (e.g.,coupling inferred from geophysical monitoring) and fossil ones (e.g., coupling required to detach and recover subducted slab fragments). Based on critical observations and an exhaustive compilation of worldwide subducted oceanic units (for which the presence near the plate interface, rock types, pressure, temperature, T/P gradients, thickness and timing of detachment can be assessed), the present study demonstrates how long-term mechanical coupling exerts a key control on detachment from the slab and potential rock recovery. Critical assessment of rock T/P characteristics indicates that these fragments can indeed be used as natural probes and provide reliable information on subduction interface dynamics down to~2.8 GPa. Rock clusters are identified at depths of 30, 55-60 and 80 km, with some differences between rock types. Data also reveal a first-order evolution with subduction cooling (in the first~5 Myr), which is interpreted as reflecting a systematic trend from strong to weak mechanical coupling, after which subduction is lubricated and mostly inhibits rock recovery. This contribution places bounds on the plate interface constitution, regular thickness (b300 m; i.e. where/when there is no detachment), changing geometry and effective viscosity. The concept of 'coupled thickness' is used here to capture subduction interface dynamics, notably during episodes of strong mechanical coupling, and to link long-and short-term deformation. Mechanical coupling depends on mantle wedge rheology, viscosity contrasts and initial structures (e.g.,heterogeneous lithosphere, existence of décollement horizons, extent of hydration, asperities) but also on boundary conditions (convergence rates, kinematics), and therefore differs for warm and cold subduction settings. Although most present-day subduction zone segments (both along strike and downdip) are likely below the detachment threshold, we propose that the most favorable location for detachment corresponds to the spatial transition between coupled and decoupled areas. Effective strain localization involves dissolution-precipitation and dislocation creep but also possibly brittle fractures and earthquakes, even at intermediate depths.

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Section: Trustable Natural Probes For the Plate Interfacementioning

confidence: 68%

The subduction plate interface: rock record and mechanical coupling (from long to short timescales)

Agard

Plunder

Angiboust

et al. 2018

Lithos

208

294

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“…The Zermatt-Saas Zone, which can be correlated with the metaophiolites in our study area, consists of typical units of Atlantic-type (slow-spreading) oceanic crust, including ultramafic rocks, metagabbros, and metabasalts with locally preserved sedimentary cover sequences [43,93]. Most lithologies of the Zermatt-Saas Zone display high-pressure (HP) to ultra-high (UHP) mineral assemblages [44][45][46]48,52,94].…”

Section: Regional Geological Backgroundmentioning

confidence: 99%

“…The well-known metaophiolites of the NW Alps include the Zermatt Saas and the Combin Zones [42,43]. The Zermatt-Saas Zone comprises dominant serpentinized peridotites with metagabbros and rodingitic dikes, metabasalts, associated with a metasedimentary cover consisting of metaquartzites, marble, and calcschists, all showing effects of high pressure (HP) to ultra high pressure ( UHP) subduction-related Alpine metamorphism [44][45][46][47][48][49][50][51][52][53][54]. The Combin Zone consists of carbonate and terrigeneous flysch-type calcschists hosting tectonic sheets of metamafic and ultramafic rocks.…”

mentioning

confidence: 99%

Superposed Sedimentary and Tectonic Block-In-Matrix Fabrics in a Subducted Serpentinite Mélange (High-Pressure Zermatt Saas Ophiolite, Western Alps)

et al. 2019

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The primary stratigraphic fabric of a chaotic rock unit in the Zermatt Saas ophiolite of the Western Alps was reworked by a polyphase Alpine tectonic deformation. Multiscalar structural criteria demonstrate that this unit was deformed by two ductile subduction-related phases followed by brittle-ductile then brittle deformation. Deformation partitioning operated at various scales, leaving relatively unstrained rock domains preserving internal texture, organization, and composition. During subduction, ductile deformation involved stretching, boudinage, and simultaneous folding of the primary stratigraphic succession. This deformation is particularly well-documented in alternating layers showing contrasting deformation style, such as carbonate-rich rocks and turbiditic serpentinite metasandstones. During collision and exhumation, deformation enhanced the boudinaged horizons and blocks, giving rise to spherical to lozenge-shaped blocks embedded in a carbonate-rich matrix. Structural criteria allow the recognition of two main domains within the chaotic rock unit, one attributable to original broken formations reflecting turbiditic sedimentation, the other ascribable to an original sedimentary mélange. The envisaged geodynamic setting for the formation of the protoliths is the Jurassic Ligurian-Piedmont ocean basin floored by mostly serpentinized peridotites, intensely tectonized by extensional faults that triggered mass transport processes and turbiditic sedimentation.

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“…This is consistent with the lack of Ti-chondrodite, which also belongs to the humite group minerals, in the Ulamertoq samples. Ti-chondrodite is stable at higher pressure conditions (2 GPa) than Ti-clinohumite in a peridotite system [31,47,48]. Partial substitution of fluorine for OH in titanian clinohumite and substitution of Cr for Al in chlorite stabilize these hydrous minerals at higher temperature conditions compared to F-free clinohumite and Cr-free chlorite [20,29,35,[49][50][51].…”

Section: Pressure-temperature Conditions For the Titanian Clinohumitementioning

confidence: 99%

“…Another difference is the presence of titanian chondrodite in the Isua peridotite [12,24]. Experimental results and studies on natural samples suggest that the titanian chondrodite-bearing Isua peridotite is stable at higher pressure conditions than the studied titanian clinohumite-bearing dunite from Ulamertoq, and that the Isua rocks likely suffered ultra-high pressure conditions [31,47,48].…”

Section: Comparison With Titanian Clinohumite-bearing Ultramafic Rockmentioning

confidence: 99%

Titanian Clinohumite-Bearing Peridotite from the Ulamertoq Ultramafic Body in the 3.0 Ga Akia Terrane of Southern West Greenland

et al. 2019

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A titanian clinohumite-bearing dunite was recently found in the Ulamertoq ultramafic body within the 3.0 Ga Akia Terrane of southern West Greenland. Titanian clinohumite occurs as disseminated and discrete grains. Titanian clinohumite contains relatively high amounts of fluorine, reaching up to 2.4 wt.%. The high-Fo content of olivine (Fo93) coupled with low Cr/(Cr + Al) ratio of orthopyroxene implies that the dunite host is not of residual origin after melt extraction by partial melting of the primitive mantle. Olivine grains are classified into two types based on abundances of opaque mineral inclusions: (1) dusty inclusion-rich and (2) clear inclusion-free olivines. Opaque inclusions in coarse-grained olivines are mainly magnetite. Small amounts of ilmenite are also present around titanian clinohumite grains. The observed mineral association indicates partial replacement of titanian clinohumite to ilmenite (+magnetite) and olivine following the reaction: titanian clinohumite = ilmenite + olivine + hydrous fluid. The coexistence of F-bearing titanian clinohumite, olivine, and chromian chlorite indicates equilibration at around 800–900 °C under garnet-free conditions (<2 GPa). Petrological and mineralogical characteristics of the studied titanian clinohumite-bearing dunite are comparable to deserpentinized peridotites derived from former serpentinites. This study demonstrates the importance of considering the effects of hydration/dehydration processes for the origin of ultramafic bodies found in polymetamorphic Archaean terranes.

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UHP Ti-chondrodite in the Zermatt-Saas serpentinite: Constraints on a new tectonic scenario

Cited by 29 publications

References 19 publications

The subduction plate interface: rock record and mechanical coupling (from long to short timescales)

The subduction plate interface: rock record and mechanical coupling (from long to short timescales)

Superposed Sedimentary and Tectonic Block-In-Matrix Fabrics in a Subducted Serpentinite Mélange (High-Pressure Zermatt Saas Ophiolite, Western Alps)

Titanian Clinohumite-Bearing Peridotite from the Ulamertoq Ultramafic Body in the 3.0 Ga Akia Terrane of Southern West Greenland

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