Brittle Deformation During Eclogitization of Early Paleozoic Blueschist
Metadatos
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FRONTIERS MEDIA SA
Materia
Eclogitization High-pressure brittle deformation Slab dehydration Pore-fluid overpressure Seismometamorphism
Fecha
2020Referencia bibliográfica
Bukała M, Barnes CJ, Jeanneret P, Hidas K, Mazur S, Almqvist BSG, Ko ´smi ´nska K, Klonowska I, Surka J ˇ and Majka J (2020) Brittle Deformation During Eclogitization of Early Paleozoic Blueschist. Front. Earth Sci. 8:594453. doi: 10.3389/feart.2020.594453
Patrocinador
National Science Center, Poland National Science Centre, Poland 2014/14/E/ST10/00321 2019/33/N/ST10/01479; Polish National Agency for Academic Exchange scholarship PPN/IWA/2018/1/00046/U/0001 PPN/IWA/2018/1/00030/U/00001Resumen
The Tsäkkok Lens of the Scandinavian Caledonides represents the outermost Baltican
margin that was subducted in late Cambrian/Early Ordovician time during closure of the
Iapetus Ocean. The lens predominantly consists of metasedimentary rocks hosting
eclogite bodies that preserve brittle deformation on the μm-to-m scale. Here, we
present a multidisciplinary approach that reveals fracturing related to dehydration and
eclogitization of blueschists. Evidence for dehydration is provided by relic glaucophane
and polyphase inclusions in garnet consisting of clinozoisite + quartz ± kyanite ±
paragonite that are interpreted as lawsonite pseudomorphs. X-Ray chemical mapping
of garnet shows a network of microchannels that propagate outward from polyphase
inclusions. These microchannels are healed by garnet with elevated Mg relative to the
surrounding garnet. Electron backscatter diffraction mapping revealed that Mg-rich
microchannels are also delimited by low angle (<3°) boundaries. X-ray computed
microtomography demonstrates that some garnet is transected by up to 300 μm wide
microfractures that are sealed by omphacite ± quartz ± phengite. Locally, mesofractures
sealed either by garnet- or omphacite-dominated veins transect through the eclogites. The
interstices within the garnet veins are filled with omphacite + quartz + rutile +
glaucophane ± phengite. In contrast, omphacite veins are predominantly composed of
omphacite with minor apatite + quartz. Omphacite grains are elongated along [001] crystal
axis and are preferably oriented orthogonal to the vein walls, indicating crystallization
during fracture dilation. Conventional geothermobarometry using omphacite, phengite and
garnet adjacent to fractures, provides pressure-temperature conditions of 2.47 ±
0.32 GPa and 620 ± 60°C for eclogites. The same method applied to a mesoscale
garnet vein yields 2.42 ± 0.32 GPa at 635 ± 60°C. Zirconium-in-rutile thermometry applied
to the same garnet vein provides a temperature of ∼620°C. Altogether, the microchannels,
microfractures and mesofractures represent migration pathways for fluids that were
produced during glaucophane and lawsonite breakdown. The microfractures are likely
precursors of the mesoscale fractures. These dehydration reactions indicate that high pore-fluid pressure was a crucial factor for fracturing. Brittle failure of the eclogites thus
represents a mechanism for fluid-escape in high-pressure conditions. These features may
be directly associated with seismic events in a cold subduction regime.