UHP metamorphism recorded by phengite eclogite from the Caledonides of northern Sweden: P–T path and tectonic implications

Abstract

The Seve Nappe Complex (SNC) of the Scandinavian Caledonides records a well‐documented history of high pressure (HP) and ultra‐high pressure (UHP) metamorphism. Eclogites of the SNC occur in two areas in Sweden, namely Jämtland and Norrbotten. The Jämtland eclogites and associated rocks are well‐studied and provide evidence for late Ordovician UHP metamorphism, whereas the Norrbotten eclogites, formed during the late Cambrian (Furongian)/Early Ordovician, have not been studied in such detail, especially in terms of the P–T conditions of their formation. Within the studied eclogite, clinopyroxene contains a high‐Na core and two rims: inner, medium‐Na and outer, low‐Na. Garnet consists of a high‐Ca euhedral core, low‐Ca inner rim and medium‐Ca outer rim. A similar pattern occurs within phengite, where high‐Si cores are enveloped by medium and low‐Si rims. The compositions of the mineral cores, inner rims and outer rims reflect three stages in the metamorphic evolution of the eclogite. Applied Quartz‐in‐Garnet geobarometry, coupled with Zr‐in‐rutile geothermometry reveal that garnet nucleation (E0 stage) took place at 1.5–1.6 GPa and 620–660°C. The eclogite peak‐pressure assemblage developed during the E1 stage, it consists of garnet+omphacite+phengite+rutile+coesite? and yields P–T conditions of 2.8–3.1 GPa and 660–780°C as constrained by conventional geothermobarometry and thermodynamic modelling in the NCKFMMnASHT system. Later, lower‐pressure stages E2 and E3 record conditions of 2.2–2.8 GPa, 680–780°C and 2.1 GPa, 735°C, respectively. The prograde metamorphic evolution of the eclogite is inferred from inclusions of epidote, amphibole and clinopyroxene within garnet. The presence of amphibole–quartz–plagioclase symplectites, secondary epidote/zoisite and titanite replacing rutile record the later retrograde changes taking place at <1.5 GPa (referred as E4 stage). The obtained P–T conditions indicate that the Norrbotten eclogites underwent a metamorphic evolution characterized by a clockwise P–T path with peak metamorphism reaching up to coesite stability field within a relatively cold subduction regime (7.8°C/km). The obtained results provide the first evidence for UHP metamorphism in the SNC above the Arctic Circle and document cold subduction regime and multistage exhumation of the deeply subducted Baltican margin at early stage of the Caledonian Orogeny.

High-spatial resolution dating of monazite and zircon reveals the timing of subduction–exhumation of the Vaimok Lens in the Seve Nappe Complex (Scandinavian Caledonides)

“…These conditions are comparable to peak temperatures of ~ 735 °C (at 2.1 GPa) experienced by the Vaimok Lens eclogite assemblages during decompression (i.e. the E3 stage of Bukała et al 2018). The E3 stage of Bukała et al (2018) records an influx of Carich fluid into the Vaimok Lens, which could catalyze monazite breakdown to apatite, allanite and clinozoisite (Budzyń et al 2011).…”

“…The Seve Nappe Complex (SNC) comprises rocks of the Baltica rifted passive margin that were subsequently subducted to (U)HP conditions (e.g. Albrecht 2000;Bukała et al 2018;Gilio et al 2015;Janák et al 2013;Klonowska et al 2014Klonowska et al , 2016Klonowska et al , 2017Kullerud et al 1990;Majka et al 2014;Santallier 1988;Stephens and van Roermund 1984) and constitutes the upper part of the Middle Allochthon of the Scandinavian Caledonides (e.g. Gee et al 2013).…”

“…Our study area, the Vaimok Lens (Fig. 2), comprises Neoproterozoic metasediments and felsic metavolcanic rocks hosting metamafic (eclogite) bodies (Albrecht 2000;Andréasson et al 1985;Bukała et al 2018;Santallier 1988). The record of prograde to peak metamorphism is nearly obliterated within the metasedimentary rocks, as they are strongly overprinted by exhumation-related deformation and retrogressive metamorphism (Albrecht 2000), the latter also affected a large portion of the eclogitic bodies.…”

“…As a result, most studies investigating the metamorphic history have relied on extracting information from the few eclogitic bodies that were not strongly overprinted by retrogressive metamorphism. Recent investigation of well-preserved eclogites hosted in the metasediments yielded peak-pressure of 2.8-3.1 GPa (at ~ 700 °C) and peak-temperature of ~ 735 °C (at ~ 2.1 GPa), demonstrating that these rocks reached UHP conditions and experienced nearly isothermal decompression during exhumation (Bukała et al 2018). Our understanding of the timing of subduction and exhumation, however, remains equivocal.…”

“…Analyses were conducted at AGH University of Science and Technology (Kraków, Poland), using a JEOL JXA8230 electron microprobe. The analytical procedure followed the protocol of Bukała et al (2018) and utilized a 1 µm spot was utilized for WDS with operating conditions of 15 kV and 120 nA.…”