Brittle Deformation During Eclogitization of Early Paleozoic Blueschist

Abstract

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.

Diachronous collision in the Seve Nappe Complex: Evidence from Lu–Hf geochronology of eclogites (Norrbotten, North Sweden)

“…This estimate has been somewhat confirmed by Bukała, Barnes, et al. (2020) reporting a brittle deformation of Tsäkkok eclogites in alike temperature, but pressure up to 25 kbar. Mørk et al.…”

Detrital zircon U-Pb geochronology of a metasomatic calc-silicate in the Tsäkkok Lens, Scandinavian Caledonides

“…Fluid activity related to dehydration of eclogite bodies within the Tsäkkok Lens at high pressure conditions was recently presented by Bukała et al (2020a). Furthermore, documentation of the influx of Ca-rich fluids is recorded in the Vaimok Lens during exhumation by both eclogites (Bukała et al 2018) and the metasedimentary rocks (Barnes et al 2019).…”

“…Provided the irregular boundaries of these zircon rims with the cores, they likely represent dissolution-reprecipitation or growth due to subsequent magmatic or metamorphic events, representing recycling of the zircon grains through multiple tectonic episodes. It is possible that the temperature conditions of the metasomatic event(s) during Caledonian orogenesis influenced how the fluids reacted with zircon, as exhumation of the Vaimok Lens proceeded through hotter temperatures (up to ~750°C; Bukała et al 2018) than compared to the maximum temperature of the Tsäkkok Lens (~620°C; Bukała et al 2020a).…”

“…The timing of eclogite-facies metamorphism is similar for both lenses at c. 482 Ma (Root & Corfu 2012, Fassmer et al 2021) but the metamorphic conditions for the Tsäkkok Lens (~2.5 GPa and ~620°C; Bukała et al 2020a, Fassmer et al 2021 are lower than the conditions for the Vaimok Lens (~3.0 GPa and ~720°C; Bukała et al 2018). Kullerud et al 1990, Barnes et al 2020b.…”

“…Influx of Ca-rich fluids in the Vaimok Lens was recorded during exhumation of the lens by both eclogites (Bukała et al 2018) and metasedimentary rocks (Barnes et al 2019). Dehydration and fluid migration within the Tsäkkok Lens eclogites at high pressure conditions was recently documented by Bukała et al (2020a), but a similar instance of Ca-rich fluid influx in the lens has not been documented during exhumation. Nevertheless, the timing and conditions of eclogite-facies metamorphism suggest common history of subduction and exhumation during Caledonian orogenesis for the two lenses (Andréasson 2020).…”