| 1. |
- Barnes, C. J., et al.
(författare)
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Exhumation of the High-Pressure Tsakkok Lens, Swedish Caledonides : Insights From the Structural and White Mica(40)Ar/Ar-39 Geochronological Record
- 2020
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Ingår i: Tectonics. - : American Geophysical Union (AGU). - 0278-7407 .- 1944-9194. ; 39:7
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Tidskriftsartikel (refereegranskat)abstract
- Integrated structural, geochemical, and geochronological investigations were conducted on metasedimentary rocks in the eclogite-bearing Tsakkok Lens of the Seve Nappe Complex (Scandinavian Caledonides) to resolve its exhumation history. Three deformation events are defined. D1 is likely related to the prograde to peak-metamorphic stages, represented by a locally preserved S1. D2 resulted in vertical shortening and is defined by a pervasive S2 and cm-/m-scale F2 closed folds. D2 terminated with Scandian thrusting, which emplaced the overlying Koli Nappe Complex. D3 records NE-SW shortening and constitutes m-/km-scale F3 open folds that deformed the Tsakkok Lens and Koli Nappe Complex together. In situ white mica(40)Ar/Ar-39 geochronology was conducted on select metasedimentary samples possessing S1 or S2 to resolve the timing of exhumation. Postdecompression cooling of the Tsakkok Lens is best recorded by samples containing S1 or S2 that yield homogeneous white mica chemistry and(40)Ar/Ar-39 dates. The timing of cooling is resolved to 477.2 +/- 4.1 Ma (S1) and 475.3 +/- 3.5 Ma (S2). Vertical shortening of the lens during exhumation may have proceeded until 458.1 +/- 9.0 Ma. Later-stage deformation during Scandian thrusting penetrated the Tsakkok Lens at 429.9 +/- 9.0 Ma, or younger. This resulted in noncoaxial deformation of the metasedimentary rocks, producing heterogeneous white mica chemistry and partially reset the older(40)Ar/Ar-39 cooling record. Temperatures for deformation are resolved to the upper greenschist-lower amphibolite facies. Altogether, the Tsakkok Lens records rapid exhumation from eclogite-facies conditions to midcrustal depths or shallower, followed by emplacement of the overlying Koli Nappe Complex.
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| 2. |
- Petrik, I, et al.
(författare)
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Monazite Behaviour during Metamorphic Evolution of a Diamond-bearing Gneiss : a Case Study from the Seve Nappe Complex, Scandinavian Caledonides
- 2019
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Ingår i: Journal of Petrology. - : OXFORD UNIV PRESS. - 0022-3530 .- 1460-2415. ; 60:9, s. 1773-1796
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Tidskriftsartikel (refereegranskat)abstract
- Monazite is a common mineral in metapelitic rocks including those that underwent ultrahigh-pressure (UHP) metamorphism. During metamorphic evolution monazite adapts its composition to the changing mineral assemblage, especially in its heavy rare earth element contents. We studied this process in diamond-bearing gneiss containing monazite, from Saxnas in the Seve Nappe Complex of the Scandinavian Caledonides. Although the rock has been re-equilibrated under granulite-facies and partial melting conditions, it still preserves minerals from the UHP stage: garnet, kyanite, rutile, and especially diamond. Microdiamonds occur in situ as inclusions in garnet, kyanite and zircon, either as single crystals or as polyphase inclusions with Fe-Mg carbonates, rutile and CO2. Both monazite and diamond occur in the rims of garnet showing the highest pyrope content and a secondary peak of yttrium. Such a position indicates thermally activated diffusion under high temperature at the end of prograde metamorphism. Monazite compositions show negative Eu anomalies, which we interpret to be inherited from the source rock, not reflecting the coexistence with plagioclase and/or K-feldspar, which are unstable at UHP conditions. Our results suggest that the effect of whole-rock composition may be more important than that of coexisting phases. The UHP monazite was most probably formed from allanite during subduction and prograde metamorphism. The monazites included in garnet and kyanite are mostly unaltered, whereas those in the matrix show breakdown coronas consisting of apatite, REE-epidote/allanite and REE-carbonate, probably formed as a result of pressure decrease and cooling. U-Th-Pb chemical age dating of monazites yields an isochron centroid age of 472 +/- 3 Ma. We interpret this age as monazite growth under UHP conditions related to subduction of the Baltica continental margin in Early Ordovician time.
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