| 1. |
- Barnes, C. J., et al.
(författare)
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Using Th-U-Pb geochronology to extract crystallization ages of Paleozoic metamorphic monazite contaminated by initial Pb
- 2021
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Ingår i: Chemical Geology. - : Elsevier. - 0009-2541 .- 1872-6836. ; 582
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Tidskriftsartikel (refereegranskat)abstract
- Geochronology of Th-rich minerals is advantageous as it allows use of three isotopic systems (i.e., Pb-206/U-238, Pb-207/U-235, and Pb-208/Th-232) for accurate data assessment. The Pb-208/Th-232 system is especially advantageous in cases where the dated mineral includes an initial Pb component, as Pb-208/Th-232 is the least sensitive to the effects of initial Pb amongst the three systems. This benefit is demonstrated with monazite from a white mica schist of the Tsakkok Lens, Scandinavian Caledonides, where three distinct generations of Paleozoic monazite (Mnzsingle bondI, Mnz-II, Mnz-III) are recognized and dated using laser ablation inductively coupled mass spectrometry. The generations are interpreted to represent monazite crystallization in high-pressure conditions (Mnzsingle bondI), followed by lower-pressure monazite growth (Mnz-II), and likely dissolution-reprecipitation of the pre-existing monazite (Mnz-III). The results are compared in Tera-Wasserburg, Wetherill, and Th-U-Pb concordia space for each monazite generation. In both Tera-Wasserburg and Wetherill space, the data are all discordant and indicate an initial Pb component in the monazite. The trend and magnitude of discordance due to initial Pb in Mnz-I and Mnz-II is generally controlled by UO2 content of the monazite, with higher UO2 equating to greater radiogenic Pb and a dampening of the initial Pb effect, which is most prominent in the Pb-207/U-235 system. For the same generations, initial Pb discordance of Pb-206/U-238 versus Pb-208/Th-232 is less apparent due to the insensitivity of Pb-208/Th-232. Mnz-III does not follow the initial Pb trends, likely due to disturbance of the chemical and isotopic systems during recrystallization. Additional discordance in Mnz-I and Mnz-II, which is not related to initial Pb, is recognized and increases with actinide content. The additional discordance may be due to Pb-mobilization in Mnz-I and Mnz-II domains and is revealed when utilizing the( 208)Pb/Th-232 system due to its insensitivity to initial Pb effects. Consequently, relying only on the U-Pb systems can lead to significant initial Pb overcorrections in Tera-Wasserburg or Wetherill concordia space and to calculations of erroneously young concordia dates. The Th-U-Pb concordia method, incorporating all three systems, does not require an initial Pb correction and, therefore, can account for the additional discordance. The Th-U-Pb concordia dates are interpretated as accurate crystallization ages for Mnz-I (484.7 +/- 1.1 Ma, MSWD: 1.4) and Mnz-II (474.7 +/- 1.2 Ma, MSWD: 1.9). The timing for Mnz-III formation is not well-resolved as it formed via result of dissolution-reprecipitation of the pre-existing monazite, likely under lower amphibolite- to greenschist-facies conditions.
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| 2. |
- Jeanneret, Pauline, et al.
(författare)
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Deciphering the tectonometamorphic history of subducted metapelites using quartz-in-garnet and Ti-in-quartz (QuiG-TiQ) geothermobarometry-A key for understanding burial in the Scandinavian Caledonides
- 2022
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Ingår i: Journal of Metamorphic Geology. - : Wiley. - 0263-4929 .- 1525-1314. ; 41:2, s. 235-70
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Tidskriftsartikel (refereegranskat)abstract
- The Seve Nappe Complex is a subduction-related high-grade metamorphic unit that was emplaced onto the margin of Baltica during Caledonian orogenesis. In this paper, the tectonometamorphic evolution of the Lower Seve Nappe in the Scandinavian Caledonides was characterized with the help of the continuous Collisional Orogeny in the Scandinavian Caledonides (COSC-1) drill core, using a combination of various P-T estimation techniques based on garnet-quartz mineral pairs (quartz-in-garnet and Ti-in-quartz [QuiG-TiQ]), conventional thermobarometry and thermodynamic modelling of phase equilibria. This multi-method approach yields complementary results and delivers critical data to constrain a comprehensive pressure-temperature-deformation-time (P-T-D-t) evolutionary path for the metasedimentary rocks of the Lower Seve Nappe. In the garnetiferous metasedimentary rocks, quartz inclusions in garnet preserve the P-T conditions of three distinct garnet growth stages corresponding to three metamorphic stages Ms1 to Ms3, including prograde and peak metamorphic conditions. Ms1 and Ms2 stages were constrained via quartz inclusions in garnet core and mantle. They are relatively close in the P-T space and could be considered as one single continuous prograde event occurring at epidote-amphibolite facies conditions of 460-520 degrees C and 0.6-0.85 GPa. The growth of the garnet outermost rim defines the Ms3 stage at amphibolite facies conditions of 590-610 degrees C and 1.13-1.18 GPa and corresponds to the peak metamorphic conditions. The microstructural analysis shows that the finite ductile strain pattern of the Lower Seve Nappe results from the superposition of four deformation phases. The initial phase D1 is defined by the Si foliation that is still preserved as a curved inclusion trail in the garnet core. The D2 phase initiated contemporaneously with garnet core growth and the development of muscovite-biotite-plagioclase S2 foliation. Garnet outermost rim growth marks the end of the prograde path and peak metamorphic conditions. This stage is overprinted by the D3 phase and Ms4 stage associated with the development of the main regional metamorphic and mylonitic fabric S3 associated with C'-type shear bands along the retrograde path. Ms4 stage, which was constrained using traditional thermobarometric techniques, corresponds to the chemical re-equilibration of the metasedimentary minerals and occurred under amphibolite facies conditions at similar to 570-610 degrees C and 0.78-1.00 GPa. The D3 phase is then generally weakly to strongly overprinted by later lower grade deformation D4 phase at greenschist facies conditions (Ms5). Ar-40/Ar-39 ages of syn-kinematic white mica and biotite indicate that the final stage of the thrusting of the Lower Seve Nappe and thus the timing of its emplacement onto the Offerdal Nappe occurred at c. 423 Ma. Collectively, these results are consistent with previous estimates of the timing and conditions of metamorphism derived from the Lower Seve Nappe especially in west-central Jamtland. However, application of QuiG-TiQ thermobarometry demonstrated that quartz inclusions in garnet can preserve different aspects of garnet growth, which are not accessible by traditional methods especially in complex terranes, and therefore provided new significant insights into the Lower Seve prograde evolution.
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| 3. |
- Klonowska, Iwona, et al.
(författare)
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Eclogite and garnet pyroxenite from Stor Jougdan, Seve Nappe Complex, Sweden : implications for UHP metamorphism of allochthons in the Scandinavian Caledonides
- 2016
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Ingår i: Journal of Methamorphic Geology. - : Wiley. - 0263-4929. ; 34:2, s. 103-119
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Tidskriftsartikel (refereegranskat)abstract
- Ultrahigh-pressure metamorphism (UHPM) has recently been discovered in far-travelled allochthons of the Scandinavian Caledonides, including finding of diamond in the Seve Nappe Complex. This UHPM of Late Ordovician age is older and less recognized than that in the Western Gneiss Region of southwestern Norway, which was related to terminal collision between Baltica and Laurentia. Here we report new evidence of UHPM in the Lower Seve Nappe, recorded by eclogite and garnet pyroxenite from the area of Stor Jougdan in northern Jämtland, central Sweden. Peak-metamorphic assemblage of eclogite, garnet + omphacite + phengite + rutile + coesite? yields P–Tconditions of 2.8–4.0 GPa and 750–900 °C, constrained by conventional geothermobarometry and thermodynamic modelling in the NCKFMTASH system. The prograde metamorphic evolution of the eclogite is inferred from inclusions of zoisite and amphibole in garnet, which are stable at lower pressure, whereas the retrograde evolution is recorded by formation of diopsidic clinopyroxene + plagioclase symplectites after omphacite, growth of amphibole replacing these symplectites, and of titanite around rutile. In garnet pyroxenite the peak-metamorphic assemblage consists of garnet + orthopyroxene + clinopyroxene + olivine. P–T conditions of 2.3–3.8 GPa and 810–960 °C have been derived based on the conventional geothermobarometry and thermodynamic modelling in the CFMASH and CFMAS systems. Retrograde evolution has been recognized from replacement of pyroxene and garnet by amphibole. The results show that eclogite was metamorphosed during deep subduction of continental crust, most probably derived from the continental margin of Baltica, whereas the origin and tectonic setting of the garnet pyroxenite is ambiguous. The studied pyroxenite/peridotite of Baltican subcontinental affinity could have been metamorphosed as a part of the subducting plate and exhumed due to the downward extraction of a forearc lithospheric block.
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| 4. |
- Kosminska, Karolina, et al.
(författare)
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Deciphering late Devonian-early CarboniferousP-T-tpath of mylonitized garnet-mica schists from Prins Karls Forland, Svalbard
- 2020
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Ingår i: Journal of Metamorphic Geology. - : WILEY. - 0263-4929 .- 1525-1314. ; 38:5, s. 471-493
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Tidskriftsartikel (refereegranskat)abstract
- Quartz-in-garnet inclusion barometry integrated with trace element thermometry and calculated phase relations is applied to mylonitized schists of the Pinkie unit cropping out on the island of Prins Karls Forland, western part of the Svalbard Archipelago. This approach combines conventional and novel techniques and allows deciphering of the pressure-temperature (P-T) evolution of mylonitic rocks, for which theP-Tconditions could not have been easily deciphered using traditional methods. The results obtained suggest that rocks of the Pinkie unit were metamorphosed under amphibolite facies conditions at 8-10 kbar and 560-630 degrees C and mylonitized at ~500 to 550 degrees C and 9-11 kbar. TheP-Tresults are coupled with in-situ Th-U-total Pb monazite dating, which records amphibolite facies metamorphism atc.359-355 Ma. This is the very first evidence of late Devonian-early Carboniferous metamorphism in Svalbard and it implies that the Ellesmerian Orogeny on Svalbard was associated with metamorphism up to amphibolite facies conditions. Thus, it can be concluded that the Ellesmerian collision between the Franklinian margin of Laurentia and Pearya and Svalbard caused not only commonly accepted brittle deformation and weak greenschist facies metamorphism, but also a burial and deformation of rock complexes at much greater depths at elevated temperatures.
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