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Volume 32 Issue 6
Dec.  2021
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Botao Li, Hans-Joachim Massonne, Xiaoping Yuan. Pressure-Temperature Evolution of a Mylonitic Gneiss from the Lower Seve Nappe in the Handöl Area, Central Sweden. Journal of Earth Science, 2021, 32(6): 1496-1511. doi: 10.1007/s12583-021-1413-3
Citation: Botao Li, Hans-Joachim Massonne, Xiaoping Yuan. Pressure-Temperature Evolution of a Mylonitic Gneiss from the Lower Seve Nappe in the Handöl Area, Central Sweden. Journal of Earth Science, 2021, 32(6): 1496-1511. doi: 10.1007/s12583-021-1413-3

Pressure-Temperature Evolution of a Mylonitic Gneiss from the Lower Seve Nappe in the Handöl Area, Central Sweden

doi: 10.1007/s12583-021-1413-3
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  • Ultrahigh-pressure metamorphism has recently been reported from various crustal rocks in the Seve Nappe Complex (SNC) in which microdiamonds were found. However, in gneiss from the Lower Seve Nappe (LSN), neither any direct petrographic indication for UHP was reported nor the metamorphic evolution was well constrained. We studied a mylonitic gneiss from the Handöl area of the LSN and applied phase-diagram modeling and Ti-in-biotite thermometry. Based on the compositions of garnet and biotite and observed mineral assemblages, a path was reconstructed passing through about 8 kbar and 730℃ at prograde metamorphism. Peak-pressure and initial retrograde stages occurred at 9.0-10.2 kbar at 745-775℃, and 7-9 kbar at < 750℃, respectively. No ultrahigh-pressure evidence was recognized compatible with medium-pressure metamorphism deduced in earlier studies of gneiss from the SNC. As higher peak pressures were reported recently for metamorphic rocks of the LSN, a possible interpretation is that slices or erased blocks were subducted, metamorphosed at different depths, and exhumed in a subduction channel. However, the dominant gneiss of the SNC experienced only a medium-pressure metamorphism in the upper part of the downgoing Baltica Plate. Rocks from different depth levels were brought together in an exhumation channel located between Baltica and the overlying plate.
  • Electronic Supplementary Materials: Supplementary materials containing Figs. S1 and S2 are available in the online version of this article at https://doi.org/10.1007/s12583-021-1413-3.
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Pressure-Temperature Evolution of a Mylonitic Gneiss from the Lower Seve Nappe in the Handöl Area, Central Sweden

doi: 10.1007/s12583-021-1413-3

Abstract: Ultrahigh-pressure metamorphism has recently been reported from various crustal rocks in the Seve Nappe Complex (SNC) in which microdiamonds were found. However, in gneiss from the Lower Seve Nappe (LSN), neither any direct petrographic indication for UHP was reported nor the metamorphic evolution was well constrained. We studied a mylonitic gneiss from the Handöl area of the LSN and applied phase-diagram modeling and Ti-in-biotite thermometry. Based on the compositions of garnet and biotite and observed mineral assemblages, a path was reconstructed passing through about 8 kbar and 730℃ at prograde metamorphism. Peak-pressure and initial retrograde stages occurred at 9.0-10.2 kbar at 745-775℃, and 7-9 kbar at < 750℃, respectively. No ultrahigh-pressure evidence was recognized compatible with medium-pressure metamorphism deduced in earlier studies of gneiss from the SNC. As higher peak pressures were reported recently for metamorphic rocks of the LSN, a possible interpretation is that slices or erased blocks were subducted, metamorphosed at different depths, and exhumed in a subduction channel. However, the dominant gneiss of the SNC experienced only a medium-pressure metamorphism in the upper part of the downgoing Baltica Plate. Rocks from different depth levels were brought together in an exhumation channel located between Baltica and the overlying plate.

Electronic Supplementary Materials: Supplementary materials containing Figs. S1 and S2 are available in the online version of this article at https://doi.org/10.1007/s12583-021-1413-3.
Botao Li, Hans-Joachim Massonne, Xiaoping Yuan. Pressure-Temperature Evolution of a Mylonitic Gneiss from the Lower Seve Nappe in the Handöl Area, Central Sweden. Journal of Earth Science, 2021, 32(6): 1496-1511. doi: 10.1007/s12583-021-1413-3
Citation: Botao Li, Hans-Joachim Massonne, Xiaoping Yuan. Pressure-Temperature Evolution of a Mylonitic Gneiss from the Lower Seve Nappe in the Handöl Area, Central Sweden. Journal of Earth Science, 2021, 32(6): 1496-1511. doi: 10.1007/s12583-021-1413-3
  • The Scandinavian Caledonides are known for abundant occurrences of high-pressure (HP) and ultrahigh-pressure (UHP) rocks (Gee, 2020; Wain et al., 2000). This orogen is the result of the Early Paleozoic collision of Baltica (Norway-Sweden) and Laurentia (Greenland-northeastern North America) after closure of the Iapetus Ocean (Hacker and Gans, 2005). This collision must have been complicated because multiple subduction events of the Baltica margin beneath Laurentia resulted in three or four orogenic HP-UHP terranes of different ages (e.g., Brueckner and van Roermund, 2004). Among these terranes, UHP metamorphism was firstly identified in the Western Gneiss Region of Norway by preserved coesite in eclogite (Butler et al., 2013; Carswell et al., 2003; Cuthbert et al., 2000; Terry et al., 2000; Wain et al., 2000; Wain, 1997; Smith, 1984). Reports of coesite pseudomorphs and microdiamonds in host gneisses followed (Wain, 1997; Dobrzhinetskaya et al., 1995) leading to the view that deep subduction of both continental and oceanic crust shaped the Scandinavian Caledonides (e.g., Kylander-Clark et al., 2008).

    Recently, another UHP terrane in the Scandinavian Caledonides, located in the Seve Nappe Complex (SNC) in Sweden, was also firstly detected on eclogite using geothermobarometry (Bukała et al., 2018; Klonowska et al., 2016; Majka et al., 2014a; Janák et al., 2013) followed by the discovery of microdiamond in gneiss (Klonowska et al., 2017; Majka et al., 2014b). Such gneiss from the Scandinavian Caledonides was carefully studied again resulting in peak pressures clearly below the stability field of diamond (SNC < 10 kbar, Li et al., 2020; Western Gneiss Region < 15 kbar, Liu and Massonne, 2019). In addition, Massonne and Li (2020) proposed that eclogites of the Caledonides, similar to other orogenic belts, were already exhumed in a subduction channel before the (micro)continent- continent collision started (see also Massonne, 2012) and not formed by continental subduction implying that eclogite bodies and surrounding gneiss experienced different metamorphic evolutions. The common pressure-temperature (P-T) evolution of both rock types began with the development of an exhumation channel between the colliding continental plates in which HP-UHP rocks were incorporated. Such a geodynamic scenario was suggested by Li et al. (2020) to explain the occurrence of HP-UHP rock bodies embedded in medium-pressure (MP) gneiss of the SNC. The new findings and views motivated us to re-investigate further gneisses from the SNC reported to potentially contain microdiamond.

    The selected gneiss occurs in the Handöl area of the Lower Seve Nappe (LSN). Another reason to choose this area was the lack of any precise P-T history deduced for the outcropping metamorphic rocks. The only existing P-T conditions of 10–11.5 kbar at 480–600 ℃ (Bergman, 1992) are based on garnet-biotite thermometry and garnet-biotite-muscovite barometry, which not necessarily define peak-pressure conditions. In other areas of the LSN, also only single P-T conditions were reported (Fassmer et al., 2017; Klonowska et al., 2016; Majka et al., 2014a). The lack of a determined P-T path in a wider terrane would block the establishment of the comprehensive geodynamic history (e.g., Rahimi and Massonne, 2020; Zhang et al., 2019; Sun et al., 2018). In order to constrain a more precise P-T evolution for a gneiss of the LSN in the Handöl area and to find further evidence for UHP, we applied phase- diagram modeling using PERPLE_X. In addition, we compare our results with published P-T conditions for rocks of the same nappe also to better understand the relevant geodynamic process causing a particular metamorphic path.

  • The Scandinavian Caledonides, an archetype orogenic belt composed of thin, laterally extensive, far-traveled nappes or thrust sheets (Törnebohm, 1888), are the result of a continent- continent collision starting with a Mid-Silurian (Scandian) collision of western Baltica and eastern Laurentia after closure of the Iapetus Ocean (Hacker and Gans, 2005; Fossen, 2000; Andersen and Jamtveit, 1990; Stephens and Gee, 1985). During the collision, the passive margin of Baltica was underthrusted beneath Laurentia resulting in the emplacement of nappes onto the Baltica Craton, accompanied by crustal thickening, and, ultimately, collapse of the mountain belt in Early Devonian times (Fossen, 2000; Andersen and Jamtveit, 1990; Stephens and Gee, 1985). By virtue of the deep erosional level, the internal parts of the Scandinavian Caledonides expose middle and lower crustal sections involved in a subduction-exhumation process and nappe stacking (Giuntoli et al., 2018). The nappes are traditionally described as a stack of parautochthonous to allochthonous sheets containing more exotic units at structurally higher levels. According to the tectonostratigraphic position, the nappes are commonly subdivided into the Lower, Middle, Upper and Uppermost Allochthons (Fig. 1a) (Gee et al., 1985). The SNC belongs to the lower part of the Upper Allochthon (Garfunkel and Greiling, 1998) or the upper part of the Middle Allochthon (Gee et al., 2010), represents the outermost part of Baltica including the ocean-continent transition zone (Andréasson et al., 1998), and is sandwiched between lower-grade nappes (Bender et al., 2018; Andréasson and Gorbatschev, 1980) with peak P-T conditions decreasing both structurally up- (Köli Nappes) and downwards (Särv Nappes).

    Figure 1.  (a) Tectonostratigraphic map of the Scandinavian Caledonides slightly modified after Gee et al. (2010) and Klonowska et al. (2017); (b) simplified geological map of the Jämtland region, Sweden (modified after Klonowska et al., 2017 and references therein); (c) field image of the studied mylonitic gneiss.

    Subdivision of the SNC into the Upper, Middle and Lower Seve Nappes is based on the presence of internal thrust sheets (Sjöström, 1983). The entire SNC was originally thought to be a MP to HP area (Bergman, 1992; van Roermund, 1989, 1985; Santallier, 1988; Nicholson, 1984; Sjöström, 1983; van Roermund and Bakker 1983; Arnbom, 1980). Recently, more precise HP conditions of 15 kbar and 690 ℃ were reconstructed for metasediment in the Upper Seve Nappe (Grimmer et al., 2015). However, the Middle Seve Nappe is more complicated, because MP in paragneiss (Li et al., 2020), HP in micaschist (Grimmer et al., 2015) and UHP in various rocks (Bukala et al., 2018; Klonowska et al., 2017, 2014; Majka et al., 2014b; Janák et al., 2013) were reported. Microdiamond in gneiss was formerly suggested to be direct petrographic evidence for UHP metamorphism in the entire Middle Seve Nappe (Klonowska et al., 2017; Majka et al., 2014b). For the LSN, both HP and UHP metamorphism were proposed as a result of pseudosection modeling, but no such evidence (coesite, microdiamond) of UHP was found in rocks.

    In general, isotopic dating demonstrates that the ages of peak-pressure metamorphism become progressively younger from north to south in the SNC within the range of 500–450 Ma (e.g., Barnes et al., 2019; Petrík et al., 2019; Fassmer et al., 2017; Majka et al., 2012; Brueckner and van Roermund, 2007). The age tendency may represent independent episodes, separated by intervals of quiescence (Brueckner and van Roermund, 2007), or may be due to a continuous process of contraction and partial subduction (Gee et al., 2013). However according to the summary by Bender et al. (2018, Fig. 6 therein), the peak metamorphic ages in the SNC scatter only around ca. 460 Ma for all nappes of the SNC.

  • The LSN is mainly composed of micaschist, quartzite, and metapsammite with subordinate gneiss, diverse metabasic rocks, and rare peridotite and serpentinite (Giuntoli et al., 2018). So far, regarding the crustal rocks, only metabasic rocks (including eclogite and pyroxenite) and gneisses in northern Jämtland have been investigated for their metamorphic evolution. Eclogites were found in the LSN of this area only (Klonowska et al., 2016; Majka et al., 2014a). Their HP-UHP peak metamorphism was published with ca. 14 kbar at 550 ℃ (van Roermund, 1985), 25–26 kbar at 650–700 ℃ (Majka et al., 2014a) and 28–40 kbar at 750–900 ℃ (Klonowska et al., 2016). The associated gneisses yielded HP peak conditions of ca. 16.5 kbar at 650–680 ℃ (Litjens, 2002) and 25–27 kbar at 680–760 ℃ (Fassmer et al., 2017). In the LSN, a pervasive amphibolite- facies foliation either overprinted the (U)HP fabric or represents the main metamorphic fabric (Giuntoli et al., 2018). This foliation is due to mylonitization constrained at conditions of 7.5–9.7 kbar and ca. 600 ℃ in amphibolite located in central Jämtland between the town of Åre and the village of Storlien (Fig. 1b; Giuntoli et al., 2018). According to dating of rocks from northern Jämtland (Fassmer et al., 2017), the HP-UHP rocks were suggested to have experienced an Ordovician subduction.

    Ultra-high pressure metamorphism was reported for the Middle Seve Nappe by microdiamonds in paragneiss at Tväråklumparna Mt. in the Handöl area (Fig. 1b) of western/central Jämtland (Majka et al., 2014b). The studied nappe, called Blåhammarfjället Nappe, from this area belongs to the LSN (Bergman, 1992; Sjöström, 1983). Amphibolite, micaschist and calc-silicate gneiss are the main rock types in this nappe. According to conventional thermobarometry, the metamorphic conditions were estimated to have been 10–11.5 kbar at 480–600 ℃ (Bergman, 1992). Silurian (427–437 Ma) metamorphic ages were obtained from calcsilicates and amphibolites of the LSN in the area between the villages of Handöl and Storlien (Gromet et al., 1996) (Fig. 1b).

    The studied gneiss from the LSN (Blåhammarfjället Nappe) in the Handöl area occurs along the Handölan River (Fig. 1b). This gneiss shows a strict foliation oriented in NW-SE direction (Fig. 1c). No leucosome was noted in the field where this gneiss is outcropping around WGS coordinates 63°14'48.7"N, 12°26'15.4"E. Reddish garnet grains (ca. 0.4 cm) can be seen in some rocks with the naked eye there. We selected one of the sampled gneisses (SWE3-2c) with relatively large garnet for constraining the P-T path.

  • The bulk-rock composition (Table 1) of the selected gneiss was determined with a PHILIPS PW2400 X-ray fluorescence (XRF) spectrometer. Mineral compositions (Table 2), backscattered electron (BSE) images (Figs. 2, 3) and X-ray compositional maps for garnet (Fig. 4) were obtained with a CAMECA SX100 electron microprobe (EMP). Both analytical instruments were hosted at the Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Germany. The analytical methods followed the protocol reported in Li and Massonne (2016) and Li et al. (2017). Mineral formulae and the molar fractions of components were calculated from EMP analyses with the program CALCMIN (Brandelik, 2009).

    Original XRFa XRFb XRFc Average of garnet core analyses EBC
    For prograde stage (stage Ⅰ) For peak and retrograde stages (Ⅱ+Ⅲ)
    SiO2 75.25 76.26 75.88 75.50 38.12 78.37
    TiO2 0.51 0.51 0.51 0.51 0.02 0.54
    Al2O3 12.15 12.31 12.25 12.19 21.70 11.82
    FeO 3.19 3.24 3.22 3.20 31.95 1.70
    MnO 0.06 0.06 0.06 0.06 1.25
    MgO 1.06 1.08 1.07 1.07 6.98 0.76
    CaO 1.01 1.03 1.02 1.02 1.15 1.02
    Na2O 1.34 1.36 1.36 1.35 0.01 1.44
    K2O 3.60 3.65 3.63 3.61 n.a. 3.85
    P2O5 0.22 n.a.
    H2O 0.50 1.00 1.50 n.a. 0.50
    SUM 98.39 100.00 100.00 100.00 101.18 100.00
    Fig. S1 Fig. 6 Fig. S2 Fig. 7
    XRFa-c". Bulk-rock composition (wt.%) modified from "original" by adding different H2O amounts; "EBC" was obtained by subtracting 5 vol.% (modal content) of garnet core (see "Average of garnet core analyses", n.a. not analyzed) from the "original". The detailed procedures for such modifications are explained in the text. The figure, in which the pseudosection for a specific bulk-rock composition is exhibited, is given in the last row.

    Table 1.  Bulk-rock compositions for the studied gneiss SWE3-2c determined by X-ray fluorescence (XRF) spectrometry (original) and modified (XRFa-c, EBC) for the calculation of P-T pseudosections

    Grt Bt Pl Kf
    Comment Core Rim In Grt core In Grt rim Around Grt In matrix In Grt core Around Grt In matrix In Grt rim Around Grt In matrix
    SiO2 38.17 38.43 35.50 35.18 35.00 34.96 66.53 62.37 59.41 61.68 62.57 62.62
    TiO2 0.02 0.01 4.83 5.13 3.74 3.14 0.00 0.00 0.00 0.01 0.00 0.02
    Al2O3 21.48 21.65 18.93 18.15 18.70 18.96 20.71 22.81 24.80 18.83 18.83 19.29
    Cr2O3 0.02 0.04 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    (FeO)tot 31.65 31.39 12.50 14.34 15.42 15.02 0.18 0.00 0.03 0.86 0.06 0.00
    MnO 1.29 1.07 0.00 0.03 0.01 0.01 0.00 0.00 0.02 0.02 0.04 0.00
    MgO 6.90 7.07 12.17 11.11 10.61 11.03 0.00 0.00 0.00 0.01 0.03 0.00
    CaO 1.10 1.22 0.03 0.00 0.02 0.00 0.91 3.65 5.80 0.06 0.00 0.15
    Na2O 0.00 0.00 0.43 0.28 0.12 0.07 11.09 9.56 8.11 0.82 1.18 1.95
    K2O n.a. n.a. 9.73 9.95 9.85 10.15 0.12 0.17 0.16 16.16 15.87 14.56
    BaO n.a. n.a. 0.29 0.27 0.21 0.16 0.00 0.00 0.00 0.11 0.16 0.28
    H2Ocalc n.c. n.c. 4.01 3.97 3.92 3.92 n.c. n.c. n.c. n.c. n.c. n.c.
    Total 100.63 100.89 98.43 98.41 97.60 97.42 99.55 98.56 98.34 98.56 98.74 98.89
    Si 5.938 5.959 2.655 2.660 2.675 2.675 2.930 2.798 2.687 2.924 2.946 2.933
    Ti 0.002 0.001 0.272 0.292 0.215 0.181 0.000 0.000 0.000 0.000 0.000 0.000
    Al 3.939 3.956 1.668 1.617 1.685 1.710 1.075 1.206 1.322 1.052 1.045 1.065
    Cr 0.003 0.005 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
    Fe3+ 0.058 0.039 n.c. n.c. n.c. n.c. 0.005 0.000 0.001 0.028 0.002 0.000
    Fe2+ 4.046 4.022 0.782 0.907 0.986 0.961 n.c. n.c. n.c. n.c. n.c. n.c.
    Mn 0.170 0.141 0.000 0.002 0.001 0.001 0.000 0.000 0.001 0.001 0.001 0.000
    Mg 1.601 1.634 1.357 1.252 1.209 1.258 n.c. n.c. n.c. n.c. n.c. n.c.
    Ca 0.183 0.203 0.002 0.000 0.001 0.000 0.043 0.176 0.281 0.003 0.000 0.008
    Na 0.000 0.000 0.062 0.042 0.018 0.010 0.947 0.831 0.711 0.076 0.107 0.177
    K n.c. n.c. 0.928 0.960 0.960 0.991 0.007 0.010 0.009 0.977 0.953 0.870
    Ba n.c. n.c. 0.009 0.008 0.006 0.005 0.000 0.000 0.000 0.002 0.003 0.005
    H n.c. n.c. 2.00 2.00 2.00 2.00 n.c. n.c. n.c. n.c. n.c. n.c.
    Structural formulae were calculated as follows: garnet: Al+Cr+Fe3+=4, Fe2++Mn+Mg+Ca+Na=6; Bt: O=11; feldspar: O=8; mineral abbreviations as in the text; "n.a." not analyzed; "n.c." not calculated.

    Table 2.  Representative EMP analyses (in wt.%) of minerals in sample SWE3-2c

    Figure 2.  Photomicrographs of the studied sample SWE3-2c. (a) Part of a thin-section prepared from the sample showing mylonitic foliation and an earlier orientation of biotite (Bt) stored in the garnet (Grt) rim (upper left); (b), (c) bent and oriented sillimanite (Sil), K-feldspar (Kfs) and plagioclase (Pl) along alternating quartz (Qtz) and biotite layers seen under plane-polarized light (b) and under crossed nicols (c); (d) back-scattered electron (BSE) image of the mylonitic fabric; Mnz. monazite; (e) pseudomorphs of sillimanite after kyanite under plane-polarized light.

    Figure 3.  BSE images showing inclusions in garnet (Grt), minerals around garnet, and textural positions of TiO2 minerals ('Rt') and ilmenite (Ilm). (a) Biotite (Bt)+plagioclase (Pl)+quartz (Qtz)+Ilm in the garnet core; (b) 'Rt' in the garnet rim; (c) intergrowth of Bt+Pl replacing a part of the garnet rim; (d) K-feldspar (Kfs) in the garnet rim and around garnet; (e) small Bt grains enclosed in the garnet rim with no connection to the matrix by cracks; Zrn. zircon; (f) 'Rt' and Ilm in the matrix.

    Figure 4.  Element concentration maps of large garnet grains in sample SWE3-2c. (a) A single very large grain, probably a special atoll garnet (see text); (b) a cluster of large grains. EMP full analyses of the exhibited garnet grains were plotted in terms of molar fractions. Abbreviations as in the text.

  • Sample SWE3-2c shows a very strict foliation due to a late mylonitization (Fig. 2a). K-feldspar (Kfs, 19%–22%, determined by point-counting on thin sections), plagioclase (Pl, 12%), garnet (Grt, 8%), and sillimanite (Sil, 2%–3%) are wrapped by biotite (Bt, 7%–10%) and recrystallized quartz (Qtz, 45%) ribbons (Fig. 2a). Accessory phases are rutile (Rt), ilmenite (Ilm), apatite (Ap), monazite (Mnz) and zircon (Zrn). According to the large contents of quartz, feldspar and biotite (Fig. 2a) and the corresponding bulk-rock composition, the protolith of SWE3-2c was an immature psammite. The above given abbreviations as well as those in figures and tables follow Kretz (1983). In addition, Amp was used for amphibole, Ph for muscovite-phengite, M for melt and V for H2O (hydrous fluid). 'Rt' marks tiny TiO2 minerals that could not be unequivocally identified as rutile by polarizing microscopy.

    Garnet is inequigranular with grain sizes from 0.05 to 4 mm in diameter (Fig. 2a). The garnet grains are either euhedral or flattened perpendicular to the direction of the main mylonitic foliation (Figs. 2b, 2c). The flattening is consistent with the orientation of biotite included in the garnet rim (Fig. 2a). In general, garnet is chemically zoned characterized by the increase of Ca and decrease of Mn from core to rim (Fig. 4b). The change of the Mg and Fe contents are minor. Garnet core and rim compositions, given in terms of molar fractions of end-member components, are within the range Grs0.028–0.033Sps0.025–0.028 Py0.25–0.27Alm0.66–0.68 and Grs0.034–0.040Sps0.021–0.025Py0.27–0.29 Alm0.65–0.67, respectively. According to the Mn maps, all garnet grains contain a thin outer rim (Figs. 4a, 4b) characterized by enrichment of Mn, which is commonly interpreted to be due to garnet resorption and reprecipitation. One garnet grain was found with an inner core which has the same composition as the garnet rim (Fig. 4a). This could be a special atoll garnet as reported by Faryad (2012), who suggested that rim and replaced core formed synchronously by infiltration of hydrous fluids. Quartz, biotite, plagioclase and ilmenite are enclosed in the garnet core (Fig. 3a), whereas biotite, rutile and K-feldspar are included in the garnet rim (Figs. 3b, 3d, 3e). The garnet rim was partially replaced by biotite+plagioclase intergrowths (Fig. 3c), which give evidence for melt crystallization during early retrogression (Holness et al., 2011; Kriegsman and Álvarez-Valero, 2010).

    Biotite occurs in four textural positions. Its composition varies depending on them (Fig. 5a, Table 2). Biotite (Bt4) in the matrix with sizes of up to 3×1 mm2 is characterized by XMg= Mg/(Mg+Fe2+)=0.52–0.60 and Ti=0.13–0.23 per formula unit (pfu) (Fig. 2). Biotite (Bt3) replacing garnet shows XMg=0.48– 0.55 and Ti=0.16–0.25 pfu (Fig. 3c). Biotite enclosed in the garnet rim (Bt2: XMg=0.58–0.65, Ti=0.23–0.35 pfu) (Fig. 3e) and core (Bt1: XMg=0.61–0.68, Ti=0.18–0.32 pfu) also differ in composition (Figs. 3a, 5a). According to the results shown in Fig. 5 there are two generations of biotite included in the garnet core differing by XMg and Ti content (Bt1-1: XMg=0.63–0.68, Ti=0.18– 0.22 pfu; Bt1-2: XMg=0.61–0.65, Ti=0.28–0.32 pfu).

    Figure 5.  (a) Compositions of biotite (Bt) in different textural positions given in terms of XMg, XTi (see text), and Ti per formula unit (pfu); (b), (c) results of Ti-in biotite geothermometry based on the geothermometer by Wu and Chen (2015). Grt. Garnet.

    Plagioclase (An0.26–0.36Ab0.62–0.72Or0.006–0.017) in the matrix, being up to 1.5 mm in length, is anhedral and oriented along the mylonitic foliation (Figs. 2a, 2d). Plagioclase included in the garnet core (Fig. 3a) and replacing the garnet rim (Fig. 3c) is characterized by An0.03–0.045Ab0.95–0.96Or0.005–0.007 and An0.13–0.17 Ab0.82–0.85Or0.005–0.008, respectively.

    K-feldspar occurs in three textural positions with different compositions (Table 2). (1) In the matrix it appears as porphyroclast with XK=K/(Na+K+Ca)=0.82–0.88 (Fig. 2d). There, it is anhedral with grain sizes up to 3 mm in length and oriented along the mylonitic foliation (Figs. 2a, 2d). Around garnet, K-feldspar shows XK=0.86–0.90 (Fig. 3d). Enclosed in the garnet rim, K-feldspar is characterized by XK=0.92–0.95 (Fig. 3d).

    Matrix quartz recrystallized as ribbons along the mylonitic foliation (Figs. 2b, 2c). Bent and oriented pseudomorphs of sillimanite after kyanite exist in the matrix with sizes up to 2 mm in length (Fig. 2e). Ilmenite frequently occurs enclosed in the garnet core (Fig. 3a). Some ilmenite also was found in the matrix replacing rutile (Fig. 3f). TiO2 minerals ('Rt'), containing almost no Zr (< 100 ppm) as analyzed with the EMP, are enclosed in the outer garnet rim and occur in the matrix (Figs. 3b, 3f).

  • P-T pseudosections were constructed with PERPLE_X (Connolly, 2005; version 6.8.0) and an internally consistent thermodynamic data set for minerals (hp62ver.dat; Holland and Powell, 2011) in the system Na2O-K2O-CaO-FeO-MnO-MgO- Al2O3-SiO2-TiO2-H2O for the bulk-rock composition (Table 1: XRFa-c) that was assumed to be the one of the early metamorphic stage Ⅰ characterized by the growth of the garnet core. For late metamorphic stages (Ⅱ: growth of the garnet rim, Ⅲ: retrograde metamorphism), Mn was excluded from the chemical system due to fractionation of this element in the garnet core (see the low MnO contents in the garnet rim: Fig. 4; Table 2). The corresponding effective bulk-rock composition (Table 1: EBC) was obtained by subtracting the garnet core (see Li and Massonne, 2016). For this purpose, we used the following strategy: the percentage of the volume (ca. 60%, corresponding to the area in a thin section) of the garnet core with respect to the whole garnet was estimated from the recorded garnet X-ray maps (see Fig. 4). Considering this percentage and the modal content of garnet in the rock (ca. 8 vol.%), the core volume with respect to the bulk rock (ca. 5 vol.% for SWE3-2c) was determined. The mean of the chemical composition of the garnet core (Table 1) was calculated based on the achieved EMP analyses. This composition (considering the garnet core volume) was subtracted from the bulk-rock composition of SWE3-2c.

    Both bulk-rock and effective bulk-rock compositions (Table 1) were normalized to 100 wt.% for the calculations of the P-T pseudosections for metamorphic stages Ⅰ and Ⅱ+Ⅲ. Trivalent iron was neglected because magnetite is absent and the amounts of ferric iron in minerals are low (Table 2). A content of 0.5 wt.% H2O was estimated, according to the amount of hydrous minerals (mainly biotite) in the rock, for the calculation of the late metamorphic stages Ⅱ+Ⅲ. (Table 1: EBC). The H2O contents of 1 wt.% and 1.5 wt.% (Table1: XRFb-c), higher than those estimated for stages Ⅱ+Ⅲ, were additionally used for stage Ⅰ as prograde metamorphism commonly leads to release of water that can leave the system.

    The following thermodynamic solution models were selected from the applied PERPLE_X package: Bio(HP) for biotite (Powell and Holland, 1999), Chl(HP) for chlorite (Holland et al., 1998), feldspar_B for plagioclase and K-feldspar (Benisek et al., 2010), Gt(WPH) for garnet (White et al., 2007), GlTrTsPg for amphibole (e.g., Wei et al., 2003), melt(HP) for haplogranitic melt (Holland and Powell, 2011; White et al., 2001), Mica(M) for paragonite with maximal 50% muscovite component (Massonne, 2010), Omph(HP) for clinopyroxene (Zeh et al., 2005; Holland and Powell, 1996), and Opx(HP) for orthopyroxene (Powell and Holland, 1999). Other models were taken from "THERMOCALC" (written comm. by J. A. D. Connolly): Pheng(HP) for potassic white mica with maximal 50% paragonite component, St(HP) for staurolite, T for talc, and Ctd(HP) for chloritoid. The models hCrd and IlGkPy (maximum 30 mol.% geikilite component) are ideal solutions used for cordierite and ilmenite, respectively, based on the thermodynamic data for corresponding end-members given by Holland and Powell (2011). Quartz, rutile, Al-silicates and zoisite were considered as pure phases. We excluded mica (tip, tbi, ann1), feldspar (mic), amphibole (rieb, mrb, cumm, grun), and the O2 buffers qfm and mthm (see Massonne et al., 2018).

  • The pseudosection calculations for the above mentioned bulk-rock and effective bulk-rock compositions were undertaken for an extended P-T range (5–50 kbar, 500–1 000 ℃) to also explore the P-T range of stable diamond (Day, 2012). After checking the four obtained pseudosections (3 for stage Ⅰ and 1 for stages Ⅱ+Ⅲ) the P-T range was reduced to 4–14 kbar and 600–850 ℃ because other P-T conditions are incompatible with the observed mineral compositions and assemblages.

    Regarding to prograde metamorphism, the compositional isopleths (Grs0.031Sps0.027Py0.28) of the garnet core were applied; they intersect in the P-T ranges 6.2–9.0 kbar and 655–725 ℃ (Fig. S1), 7.2–8.8 kbar and 705–755 ℃ (Fig. 6b), and 6.8–7.8 kbar and 705–735 ℃ (Fig. S2) for the corresponding bulk-rock compositions with H2O=0.5 wt.%, 1 wt.%, and 1.5 wt.%, respectively (Table 1), considering errors as amplified by Li et al. (2020). The calculated garnet modal contents (3 vol.%–6 vol.%) in these three P-T ranges (Table 1; Figs, 6c, S1a, S2b) are consistent with the observed garnet core content (ca. 5 vol.%). However, the calculated melt modal content (approaching to 10 vol.%–15 vol.% in Fig. S2c) for XRFc (Table 1) exceeds the limit for melt loss at 7 vol.% (Brown, 2007; Rosenberg and Handy, 2005). This conflicts with the observation of lacking leucosomes in the field. But the calculated melt modal content (< 7 vol.%) (Fig. 6c) for XRFb (Table 1) is compatible with retention of melt during this stage. Therefore and as discussed later, the core of garnet was probably formed in the P-T range of 7.2–8.8 kbar and 705–755 ℃ (see yellow area in Fig. 6c) from the bulk-rock composition XRFb. The calculated assemblage of M+Ph+Kfs+Pl+Grt+Bt+Qtz±Ilm±Rt (Figs. 6a, 6b) for this range matches the observed inclusions of Ilm+Bt1+Pl (An=0.038 in average)+Qtz in the garnet core (Table 3).

    Figure 6.  (a) P-T pseudosection for bulk-rock composition XRFb (Table 1). The yellow line marks the solidus; (b) isopleths for Grs0.031Sps0.027Py0.28 (garnet core), XMg (0.63, 0.66) in biotite, and An=0.038 in plagioclase included in the garnet core; (c) isopleths for modal contents of garnet and melt. Specific isopleths in (b) and (c) were used to define the P-T conditions of metamorphic stage Ⅰ (red ellipsis). The yellow portion within the red ellipsis refers to melt contents less than 7 vol.%. Mineral abbreviations as in the text.

    Observed mineral assemblage Petrographic evidence Calculated mineral assemblage
    Stage Ⅰ GrtcorePlBt
    IlmQtz±Ky±Kfs
    Ilm+Bt1+Pl (An=0.038)+Qtz in Grt core;
    Ky pseudomorph; Kfs porphyroclast
    MPhKfsPlGrtBtQtz±Ilm±Rt (Fig. 6)
    or PhKfsPlGrtBtQtz±Ky±Sil±Ilm±Rt (Fig. S1)
    Stage Ⅱ GrtrimKfsBt
    Rt±Ky±Pl
    'Rt'+Bt2+Kfs in Grt rim;
    Ky pseudomorph; Pl porphyroclast
    KfsPlGrtBtRtKy±Ph±M (Fig. 7)
    Stage Ⅲ KfsPlBtSil±Rt±Ilm Kf+Pl+Bt around Grt; Ilm replaced Rt in matrix; Sil in Ky pseudomorph
    In addition, the petrographic evidences for the observed mineral assemblages are given. A calculated mineral assemblage for stage Ⅲ cannot be fixed because of local equilibria that do not allow us to define an effective bulk-rock composition necessary for a realistic pseudosection calculation.

    Table 3.  Comparison between the observed and calculated mineral assemblages

    Regarding to the peak metamorphism, the P-T range of 9.0–10.2 kbar and 745–775 ℃, calculated with the effective bulk-rock composition (EBC of Table 3), was constrained from the averaged isopleths of Grs0.037Py0.28 in the garnet rim and the mineral assemblage of Kfs+Pl+Grt+Bt+Rt+Ky±Ph±M in the P-T pseudosection (Fig. 7). This calculated mineral assemblage is in agreement with the observed inclusions of 'Rt'+Bt2+Kfs in the garnet rim (Figs. 3b, 3d, 3e; Table 3). In addition, the pseudomorphs of sillimanite after kyanite (Figs. 2b, 2e) verify the existence of kyanite at peak metamorphism. The calculated garnet modal contents (ca. 2 vol.%–3 vol.% for the rim in the rock) (Fig. 7b) in the given P-T range is reasonable. The calculated melt content (≤7 vol.%) (Fig. 7a) is also consistent with lacking leucosomes in the field (Fig. 1c).

    Figure 7.  (a) P-T pseudosection for effective bulk-rock compositions EBC (Table 1) to estimate the conditions for the formation of the garnet rim (stage Ⅱ) and retrograde metamorphism (stage Ⅲ). In addition, isopleths for melt volume (yellow lines) and P-T limits of specific minerals (colored lines) are shown; (b) isopleths for garnet and melt volumes and the intersections for Grs0.037Py0.28 (garnet rim) and XMg (0.62) in biotite included in the garnet rim. These intersections define the P-T conditions of stage Ⅱ (blue ellipsis). The red line shows a likely path from stage Ⅱ to stage Ⅲ. Mineral abbreviations as in the text.

    According to the observed TiO2 minerals ('Rt') in the matrix, the pseudomorph of sillimanite after kyanite (Figs. 2c, 2e), Pl+Bt+Kfs replacing the garnet rim (Fig. 3c), and the calculated melt content (< 7 vol.%) (Fig. 7b), initial retrograde P-T conditions in the range of 7.0–9.5 kbar and 750–800 ℃ are compatible with the calculated assemblage of M+Kfs+Pl+Grt+Bt+Sil+ Rt+Qtz in the P-T pseudosection (blue area in Fig. 7). In addition, Pl+Bt3+Kfs replacing the garnet rim point to recrystallized melt during early retrogression (Holness et al., 2011; Kriegsman and Álvarez-Valero, 2010). On the basis of the P-T pseudosection (Fig. 7), Ph was absent during the early retrogression (blue area in Fig. 7), which is consistent with the absence of Ph in the thin section. Ilmenite around Rt in the matrix (Fig. 2f) grew at a late retrograde stage (Fig. 7a).

  • Biotite (Bt1-1 with XMg=0.66 and Bt1-2 with XMg=0.63) included in the garnet core and that one (Bt2) in the rim with XMg=0.62 are in agreement with the prograde (Fig. 6b) and peak metamorphism (Fig. 7b), respectively. Thus, they are suitable to check the corresponding temperatures applying Ti-in-biotite geothermometry. We used the calibration by Wu and Chen (2015) for this thermometry which considers the P-T range of 450–840 ℃ at 1–19 kbar and XTi (Ti/(Fe+Mg+AlVI+Ti)= 0.02–0.14 in biotite. Both P-T and compositional ranges match biotite in the studied rock as the Ti contents of biotite do not exceed XTi=0.13 (XTi=0.07–0.12 for Bt1 included in the garnet core, 0.09–0.13 for Bt2 in the garnet rim, and 0.05–0.08 in Bt4).

    For the thermometry, all iron was considered divalent, based on the lack of Fe3+-rich phases in the rock, such as magnetite. The input pressures were set at 8 and 9.5 kbar which are in the ranges of pressures for the formation of the garnet core and rim, respectively (see Section 4.2, Figs. 6, 7). Temperature estimates are 634–699 ℃ with an average of 670±16 ℃ (Bt1-1 with XTi=0.07–0.08) and 728–766 ℃ with an average of 746±16 ℃ (Bt1-2 with XTi=0.10–0.12) for biotite enclosed in the garnet core at 8.0 kbar (Fig. 5b), 711–795 ℃ (average: 766±25 ℃) at 9.5 kbar for Bt2 included in the garnet rim, and 642–723 ℃ (average: 690±27 ℃) at 9.5 kbar for Bt4 flakes in the matrix (Fig. 5c).

  • According to the zoning of garnet and the observed mineral assemblages related to different metamorphic stages (Table 3), a clockwise P-T path was unraveled. The reconstructed prograde P-T path at medium-pressure conditions unequivocally excludes HP-UHP metamorphic conditions for the studied sample (Fig. 8). The assemblage of garnet (core)+biotite+ilmenite+plagioclase+ quartz points to prograde conditions of 6.2–8.8 kbar and 655–750 ℃ (Figs. 6, S1, S2). A known H2O content in the bulk-rock composition would reduce this P-T range significantly. Our calculations with different H2O contents in this composition demonstrate that the result obtained for 1 wt.% in the bulk-rock composition (XRFb, Table 1) would fit best for the following reasons: (1) the composition with 1.5 wt.% (XRFc) would yield so much melt during prograde metamorphism from stage Ⅰ to stage Ⅱ so that leucosomes should have formed, which were not observed, and (2) the relevant isopleths (garnet core etc.) hardly intersect when using the bulk-rock composition with 0.5 wt.% (XRFa). According to the inclusion of Bt1-2 (mean XMg=0.63), the anorthite content in plagioclase (An0.03–0.045, mean An=0.038), and the relevant garnet isopleths (Fig. 6b), the aforementioned P-T conditions (655–750 ℃) can be restricted to those around 8 kbar and 730 ℃ (stage Ⅰ). This implies that the garnet core started to grow within its P-T field. This implication is strengthened by Spear (2017) who argued for kinetic factors (energetic barriers) being responsible for a significant overstepping of the P-T limit of garnet. Examples for this phenomenon in higher grade metasediments leading to nearly chemically homogeneous porphyroblastic garnet were recently presented by Li et al.(2021, 2020). Indeed, the common garnet core in gneiss SWE3-2c is relatively chemically homogeneous. However at the deduced high peak temperatures (≥750 ℃), intracrystalline cation diffusion could have homogenized the garnet core. The effect of this diffusion can be estimated using the special atoll garnet of Fig. 4a. In contrast to the aforementioned example of such an atoll garnet published by Faryad (2012), the contact is not sharp anymore due to cation diffusion. Nevertheless, the remaining part of the original core of the garnet in Fig. 4a shows the typical garnet core composition if analyzed in a certain distance to the contact of old and replaced garnet. Thus, diffusion can be excluded here as explanation for the chemically homogeneous garnet core. Whereas Bt1-2, included in this core, probably coexisted with it, Bt1-1, which gave lower temperatures around 670 ℃, preceded the formation of garnet. Unfortunately, this biotite cannot be used alone for the determination of P-T conditions as the Si content, a potential geobarometer (Massonne, 2021), is too poorly calibrated in regard of the P-T dependence.

    Figure 8.  Comparison of the here obtained peak P-T conditions (stage Ⅱ) and published conditions for rocks from the Lower Seve Nappe in the SNC. The red line shows the retrograde P-T path from Fig. 7b; the dashed red line is the inferred part of the retrograde P-T path. Mineral abbreviations as in the text.

    Peak metamorphic conditions of 9.0–10.2 kbar and 745–775 ℃ (stage Ⅱ) in the granulite facies were deduced from garnet (rim)+rutile+Bt2+K-feldspar+kyanite pseudomorph (Fig. 8a). This P-T range is compatible with Ti-in-biotite thermometry applied to Bt2 (766 ℃ at 9.5 kbar, Figs. 5c). The very low Zr contents in rutile enclosed in the garnet rim are incompatible with temperatures above 700 ℃ at about 9 kbar following the calibration by Tomkins et al. (2007). We assume that at stage Ⅱ, at which melting occurred, no zircon was present due to the solubility of this accessory mineral in (granitic) melt. In such a case, Zr contents in rutile would appear that are lower compared to rutile saturated with Zr by the presence of zircon. However, such a saturation can be expected for matrix rutile as we found tiny zircon grains in the matrix. This implies that the temperature of stage Ⅲ (750–800 ℃) determined by pseudosection modeling was overestimated. Much more likely is a temperature around 690 ℃ (or even somewhat lower under the assumption of a lower pressure than 9.5 kbar) obtained by Ti-in-biotite thermometry for this stage. A reason for the temperature overestimation using the modeling approach could be the fact that matrix minerals can also be part of stage Ⅱ. Thus, we must consider that the stable minerals at stage Ⅲ were rather sillimanite and ilmenite, which had replaced kyanite and rutile, respectively, and suggest rather pressures of 6–7 kbar at this stage. During the transition from stage Ⅱ to Ⅲ, melt, indicated by plagioclase+ Bt3+K-feldspar replacing the garnet rim (see Holness et al., 2011; Kriegsman and Álvarez-Valero, 2010), crystallized.

    The here derived overall metamorphic P-T range (655–775 ℃, 6–10 kbar) is in agreement with the SNC considered as a MP-HP metamorphic area (Bergman, 1992; van Roermund, 1989, 1985; Santallier, 1988; Nicholson, 1984; Sjöström, 1983; van Roermund and Bakker 1983; Arnbom, 1980). But gneisses of the LSN from northern Jämtland (Fig. 1a) also experienced higher pressures of 16.5 kbar at 650–680 ℃ (Litjens, 2002) and 25–27 kbar at 680–760 ℃ (Fassmer et al., 2017; see Fig. 8). Regarding to metabasic rocks (mainly eclogites) enveloped in such gneiss, both HP and UHP peak conditions were published (25–26 kbar at 650–700℃ by Majka et al., 2014a; 28–40 kbar at 750–900 ℃ by Klonowska et al., 2016). In addition, similar temperatures (700–800 ℃), but lower pressures (13–20 kbar) were reported by van Roermund (1989) for garnet peridotite located near the base of the LSN. Thus, quite different peak pressures (Fig. 8) were determined for gneiss of the LSN which can be compatible with those derived for eclogite but also differ considerably among both rock types. Subsequently, we discuss two principle geodynamic scenarios that can explain this pressure accordance and contrast for eclogite and gneiss.

  • Various authors (e.g., Giuntoli et al., 2018; Bergman, 1992; Arnbom et al., 1980) argued for a (nearly) complete resetting of previously metamorphosed rocks at P-T conditions of the amphibolite and granulite facies in units of the Scandinavian Caledonides characterized by preserved eclogite and diamondiferous gneiss. The formation of such HP-UHP terranes is believed to be due to deep subduction of Baltica (e.g., Bender et al., 2018; Klonowska et al., 2016; Majka et al., 2014b) with eclogites that were part of this crust (Griffin and Brueckner, 1980). Resetting preferably of gneiss erased the HP-UHP record of these rocks mainly by transformation of coesite to quartz and Na-rich clinopyroxene (+quartz) to plagioclase. According to published geodynamic models (Gee, 2020; Bender et al., 2018; Klonowska et al., 2016; Corfu et al., 2014; Majka et al., 2014b), rocks of the SNC also experienced this single subduction-exhumation event in the Scandian period. The suggested resetting in the amphibolite facies was accompanied by mylonitization in the LSN (Giuntoli et al., 2018). The corresponding mylonitic foliation was suggested to have formed at 7.5–9.7 kbar and ca. 600 ℃ (Fig. 8). Also partial melting has the potential of erasing indicators of previous HP-UHP events in rocks (Faryad and Cuthbert, 2020). As migmatitic gneiss is a common rock type in HP-UHP terranes of the Scandinavian Caledonides, this resetting factor could be relevant.

    The studied rock disproves the above geodynamic scenario. In fact, the rocks in the Handöl area are significantly affected by mylonitization, although at pressures in the P-T field of stable sillimanite and, thus, lower than suggested by Giuntoli et al. (2018), but the selected rock contains relics of the pre-mylonitic stage especially large garnet grains. Their chemical composition as well as inclusions in garnet allowed us to reconstruct the pre-mylonitic history of the studied gneiss. The strategy, successfully applied to such a reconstruction, was also presented in previous works (e.g., Li et al., 2019; Zhou et al., 2019; Chen et al., 2018; Xiang et al., 2018; Waizenhöfer and Massonne, 2017; Yin et al., 2015). The results obtained here demonstrate that the mylonitic rocks at Handöl never experienced P-T conditions of the eclogite facies. Peak pressures were below 11 kbar.

  • A geodynamic scenario to explain diverse peak metamorphisms in the MP, HP, and UHP ranges was suggested for the Middle Seve Nappe by Li et al. (2020, see Fig. 12 therein). Massonne and Li (2020) generalized such a scenario for subduction of an oceanic plate and subsequent continent-continent collision being relevant for the Baltica-Laurentia collision after subduction of the Iapetus as well. The geodynamic scenario is based on earlier observations and geodynamic explanations of significant peak-pressure contrasts between country-rock gneisses and lenses of eclogite (Liu and Massonne, 2019; Li and Massonne, 2016; Massonne, 2012) and different peak pressures of eclogites in a single extended body (Li et al., 2017). The HP-UHP rocks of different type (mainly metasediments and metabasites) were subducted as slices or blocks to great depths and exhumed in a subduction channel (Massonne and Li, 2020).

    The derived MP metamorphism with peak-pressure conditions of 9.0–10.2 kbar at 745–775 ℃ for sample SWE3-2c from the LSN is comparable with the gneiss in the Middle Seve Nappe (ca. 8.5 kbar at 700 ℃) studied by Li et al. (2020). Both MP rocks were (1) part of a Late Neoproterozoic (Gee et al., 2013; Andréasson, 1994) to Cambrian (Li et al., 2020) sedimentary succession deposited at the margin of Baltica or on top of the tip of this plate and (2) not subducted, but more or less contemporaneously thrust under a ca. 30–35 km thick island arc (Majka et al., 2014b) or microcontinent (Brueckner and van Roermund, 2004; Roberts, 2003) compatible with the determined peak pressure around 9.5 kbar. When (or soon after) both plates (Baltica and island arc/microcontinent) came in touch (1) a slab-breakoff process (Davies and von Blanckenburg, 1995) occurred leading to further subduction only of the Iapetus crust previously adhered to Baltica and (2) the entrapment of HP-UHP rocks that had reached the top of the subduction channel. Both events might have taken place in the age interval 460–465 Ma considering the mean age of 460 Ma for peak metamorphism given in the summary by Bender et al. (2018), and the age of 472 Ma (Petrík et al., 2019) related to the formation of UHP rocks that was earlier because of their exhumation in the subduction channel ahead of downgoing Baltica. Continuous underthrusting of Baltica led to the evolution of an exhumation channel (see Massonne and Li, 2020; Massonne, 2016) between Baltica and the overlying plate. In this environment mixing of slices and bodies of MP, HP, and UHP rocks and possibly the aforementioned mylonitization at amphibolite-facies conditions took place whereas the nappe formation of the SNC might have occurred significantly later in the evolution of the Scandinavian Caledonides.

    So far, no HP-UHP lenses were found in the studied LSN of the Handöl area, which is similar to the Middle Seve Nappe at Åreskutan Mountain mentioned in Li et al. (2020). These authors interpreted the lack of HP-UHP bodies in these areas that they were introduced into the sedimentary pile at the tip of the lower continental plate (Baltoscandian margin) when the continent-continent collision started. The eclogite-facies rocks should have been preferentially included in the lower portion of this pile which experienced a higher peak pressure than the metasediments at the top (the studied rock with peak pressure of 9–10.2 kbar). This would explain why, for instance, no eclogite was found in the Handöl area of the LSN, but in metasediments in north Jämtland of the LSN (Klonowska et al., 2016; Majka et al., 2014a; van Roermund and Bakker, 1983) with possible peak pressures higher than 11 kbar.

  • Our petrological study of a mylonitic gneiss from the LSN resulted in a clockwise prograde P-T path. This path starts at P-T conditions around 8 kbar and 730 ℃ (stage Ⅰ in Fig. 8), passes through the range 9–10.2 kbar and 745–775 ℃ (stage Ⅱ), and ends in the range of 7.0–9.0 kbar and < 750 ℃. The mylonitic stage occurred in the P-T field of stable sillimanite probably at 6–7 kbar.

    The view that the SNC represents MP metamorphic rocks (Bergman, 1992; van Roermund and Bakker, 1983; Arnbom, 1980) is supported by our study. We relate these rocks to the upper portion of the downgoing Baltica Plate. In contrast, the HP-UHP rocks of the SNC recently studied by Fassmer et al. (2017), Klonowska et al. (2016), Litjens (2002), and Majka et al. (2014a) represent deeply subducted rocks, possibly after tectonic erosion (Massonne and Li, 2020). These rocks were exhumed in a subduction channel and became part of the SNC rocks in an exhumation channel located between Baltica and the overlying plate. Mylonitization of the MP rocks either occurred in this exhumation channel or later when crystalline nappes were formed and stacked.

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