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

Barnes Cristopher J., Majka Jarosław, Bukała Michał, Nääs Erika, Rousku Sabine

Geology, Geophysics and Environment

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2021

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Vol. 47, no. 1

21--31

EN

The Tsäkkok Lens of the Seve Nappe Complex in the Scandinavian Caledonides comprises eclogite bodies hosted within metasedimentary rocks. These rocks are thought to be derived from the outermost margin of Baltica along the periphery of the Iapetus Ocean, but detrital records from the sedimentary rocks are lacking. Many metasedimentary outcrops within the lens expose both well-foliated metapelitic rocks and massive calc-silicates. The contacts between these two lithologies are irregular and are observed to trend at all angles to the high-pressure foliation in the metapelites. Where folding is present in the metapelites, the calc-silicate rocks are also locally folded. These relationships suggest metasomatism of the metapelites during the Caledonian orogenesis. Zircon U-Pb geochronology was conducted on sixty-one zircon grains from a calc-silicate sample to investigate if they recorded the metasomatic event and to assess the detrital zircon populations. Zircon grains predominantly show oscillatory zoning, sometimes with thin, homogeneous rims that have embayed contacts with the oscillatory-zoned cores. The zircon cores yielded prominent early Stenian, Calymmian, and Statherian populations with a subordinate number of Tonian grains. The zircon rims exhibit dissolution-reprecipitation of the cores or new growth and provide ages that span similar time frames, indicating overprinting of successive tectonic events. Altogether, the zircon record of the calc-silicate suggests that the Tsäkkok Lens may be correlated to Neoproterozoic basins that are preserved in allochthonous positions within the northern extents of the Caledonian Orogen.

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Ba- and Ti-enriched dark mica from the UHP metasediments of the Seve Nappe Complex, Swedish Caledonides

Majka J., Kruszewski Ł., Rosen A., Klonowska I.

Mineralogia

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2015

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Vol. 46, iss. 1/2

41--50

EN

We report on the occurrence of peculiar Ba- and Ti-enriched dark mica in metasedimentary rocks that underwent high-pressure metamorphism in the diamond stability field followed by decompression to granulite facies conditions. The mica occurs as well-developed preserved laths in a quartzofeldspathic matrix. The mean concentrations of BaO and TiO2 in the mica are 11.54 and 7.80wt%, respectively. The maximum amounts of these components are 13.38wt% BaO and 8.45wt% TiO2. The mean crystallochemical formula can be expressed as (K0.54Ba0.39Na0.02Ca0.01)Σ0.96(Fe1.37Mg0.85Ti0.50Al0.29Mn0.01Cr0.01)Σ3.03(Si2.59Al1.41)Σ4.00O10(OH1.30O0.66F0.02S0.01)Σ1.99, with oxyannite, oxy-ferrokinoshitalite and siderophyllite as dominating end-members. Based on the petrographical observations, it is proposed that the dark mica was formed at a rather late stage in the evolution of the parental rock, i.e. under granulite facies conditions.

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Microdiamonds in paragneisses from the allochthonous unit of the Swedish Caledonides: precious testimony of the UHP metamorphism

Klonowska I.

Geology, Geophysics and Environment

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2015

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Vol. 41, no. 1

94--95

EN

The highest metamorphic grade allochthonous unit in the Scandinavian Caledonides is represented by the Seve Nappe Complex (SNC) that belongs to the Middle Allochthon. This unit is interpreted to be derived from the outermost margin of Baltica that is dominated by metasediments and metamafics. Eclogites, peridotites and migmatites occur within the SNC only locally. Nonetheless, detailed field studies followed by comprehensive analytical investigations of high grade rocks within the studied unit led to the discovery of the ultra-high pressure metamorphism (UHPM) within eclogites, garnet pyroxenites and paragneisses from central and northern Jämtland in Sweden (e.g. Janák et al. 2013, Klonowska et al. 2014, 2015, Majka et al. 2014). Diamond, in addition to coesite, is an index mineral for the UHPM and the first microdiamonds in the SNC of the Swedish Caledonides were recently discovered in paragneisses from the Snasahögarna Mt. (Majka et al. 2014). After this discovery, reinvestigation of gneiss samples from the Åreskutan Mt., the classical area, where thrusting was defined for the first time in the Scandes (Törnebohm 1888) and where the UHPM has been lately indicated by the thermodynamic modelling (Klonowska et al. 2014), resulted in findings of microdiamond inclusions in garnets. Microdiamonds were identified by the Raman microspectroscopy in garnets from two gneiss samples. Single diamond grains range in size between 1 and 3 μm. Partial transformation of diamond to the disordered graphite is a common feature observed in these inclusions. Microdiamonds are found as single grains as well as multi-phase inclusions together with disordered graphite and carbonates. Aside from microdiamond inclusions, single grain of moissanite, a natural form of SiC, has been also identified. Moissanite inclusion in garnet is 3μm across and exhibits negative crystal shape. In-situ moissanite has been found only in few places so far, including rocks from the ultra-high pressure terranes, eclogite and serpentinite from the Dabie-Sulu orogenic belt in China (Qi et al. 2007, Xu et al. 2008), dunite from Luobusa ophiolite in Tibet (Liang et al. 2014) and felsic granulites from Bohemian Massif (Perraki & Faryad 2014). Findings of the microdiamond inclusions in the allochthonous rocks, that represent continental crust, indicate the UHP metamorphism and subduction to mantle depths (>100 km), whereas the occurrence of moissanite points to the highly-reducing conditions during this deep subduction. Increased number of discoveries of the UHPM in the SNC shed a new light on the subduction-exhumation processes involved in a formation of the Caledonian mountain chain in Scandinavia. A new tectonic model for this orogenic belt has been recently proposed by Majka et al. (2014), where authors introduce a ‘vacuum cleaner mechanism’ being responsible for an exhumation of the deeply subducted rocks.

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Metamorphic evolution of the Seve Nappe complex in the Snasahogarna area, Swedish Caledonides

Rosen A., Majka J., Klonowska I.

Geology, Geophysics and Environment

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2012

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Vol. 38, no. 4

521--522

EN

The Middle Seve Nappe in the Snasahogarna mountains, western Jamtland, Sweden, is composed of high grade metamorphic rocks emplaced in far-travelled nappes. The investigation of these paragneisses, derived from the Baltica margin, can contribute information about the subduction and exhumation processes, which controlled the formation of the Seve Nappe Complex (SNC) in the Scandinavian Caledonides. Recent studies in other parts of the orogen have shown that the rocks of the SNC likely have experienced pressures higher than what was previously described (Janak et al. 2012, Klonowska et al. in press). These latest PT studies along with geochronological dates available suggest that the collision between Baltica and Laurentia has commenced c. 30 m.y. earlier (at ca. 450 Ma) than generally accepted. Samples of kyanite- and garnet-bearing meta-sediments were collected along a profile at Tvaraklumparna (Snasahogarna area) and investigated in thin sections using light microscopy followed by BSE imaging, WDS analysis and Raman spectroscopy. Preliminary results show that dominant garnet reaches up to 7 mol. % of Grs content. The garnet commonly contains inclusions of quartz surrounded by radial cracks and grains of polycrystalline quartz. More rare are inclusions of kyanite and white mica with a Si-contents reaching 3.34 a.p.f.u. Detailed studies of garnet revealed dense areas of micrometer size inclusions exhibiting negative crystal shape present in the core regions. Raman studies suggest these inclusions are at least partly formed by microdiamond. Textural evidence of peak metamorphic conditions reaching the stability field of coesite together with microdiamonds preserved in garnet cores confirms that the Seve Nappe crustal rocks of the Snasahogarna area has undergone ultrahigh pressure metamorphism.