| 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. |
- 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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| 3. |
- Bukala, Michal, et al.
(författare)
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Brittle deformation during eclogitization of early Paleozoic blueschist
- 2020
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Ingår i: Frontiers in Earth Science. - LAUSANNE SWITZERLAND : Frontiers Media SA. - 2296-6463. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- The Tsakkok Lens of the Scandinavian Caledonides represents the outermost Baltican margin that was subducted in late Cambrian/Early Ordovician time during closure of the Iapetus Ocean. The lens predominantly consists of metasedimentary rocks hosting eclogite bodies that preserve brittle deformation on the mu m-to-m scale. Here, we present a multidisciplinary approach that reveals fracturing related to dehydration and eclogitization of blueschists. Evidence for dehydration is provided by relic glaucophane and polyphase inclusions in garnet consisting of clinozoisite + quartz +/- kyanite +/- paragonite that are interpreted as lawsonite pseudomorphs. X-Ray chemical mapping of garnet shows a network of microchannels that propagate outward from polyphase inclusions. These microchannels are healed by garnet with elevated Mg relative to the surrounding garnet. Electron backscatter diffraction mapping revealed that Mg-rich microchannels are also delimited by low angle (<3 degrees) boundaries. X-ray computed microtomography demonstrates that some garnet is transected by up to 300 mu m wide microfractures that are sealed by omphacite +/- quartz +/- phengite. Locally, mesofractures sealed either by garnet- or omphacite-dominated veins transect through the eclogites. The interstices within the garnet veins are filled with omphacite + quartz + rutile + glaucophane +/- phengite. In contrast, omphacite veins are predominantly composed of omphacite with minor apatite + quartz. Omphacite grains are elongated along [001] crystal axis and are preferably oriented orthogonal to the vein walls, indicating crystallization during fracture dilation. Conventional geothermobarometry using omphacite, phengite and garnet adjacent to fractures, provides pressure-temperature conditions of 2.47 +/- 0.32 GPa and 620 +/- 60 degrees C for eclogites. The same method applied to a mesoscale garnet vein yields 2.42 +/- 0.32 GPa at 635 +/- 60 degrees C. Zirconium-in-rutile thermometry applied to the same garnet vein provides a temperature of similar to 620 degrees C. Altogether, the microchannels, microfractures and mesofractures represent migration pathways for fluids that were produced during glaucophane and lawsonite breakdown. The microfractures are likely precursors of the mesoscale fractures. These dehydration reactions indicate that high pore-fluid pressure was a crucial factor for fracturing. Brittle failure of the eclogites thus represents a mechanism for fluid-escape in high-pressure conditions. These features may be directly associated with seismic events in a cold subduction regime.
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| 4. |
- Cai, Pengjie, et al.
(författare)
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Two stages of crust-mantle interaction during oceanic subduction to continental collision : Insights from mafic-ultramafic complexes in the North Qaidam orogen
- 2021
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Ingår i: Gondwana Research. - : Elsevier. - 1342-937X .- 1878-0571. ; 89, s. 247-264
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Tidskriftsartikel (refereegranskat)abstract
- Subducted sedimentary rocks contribute to different reservoirs in the mantle, which results in crust-mantle interaction; however, the metasomatic processes involved in this sedimentary contribution to the mantle still remain poorly understood. Chromite is a robust mineral and together with its inclusions, occurring in orogenic peridotite, it is an important candidate for decoding both the petrogenetic and tectonic settings as well as crust-mantle interaction. This study presents an integrated petrological and geochemical study of the chromitite-bearing Luofengpo mafic-ultramafic complex (CLMC) in the North Qaidam orogen, China. The CLMC consists of amphibole pyroxenite, serpentinized peridotite, and serpentinized pyroxenite. Electron probe micro-analysis and laser ablation inductively coupled plasma mass spectrometry analysis demonstrated that the studied chromite grains display compositional zoning with the cores preserving primary igneous compositions. Chromitites with high Cr#(100*Cr/(Cr+Al)) signatures are enriched in IPGE (Os, Ir, and Ru) and depleted in PPGE (Rh, Pt, and Pd) indicating that they formed in the supra-subduction zone mantle wedge. Core compositions of chromite grains indicate that the parental melts of the chromitites were similar to typical boninitic melts, formed in a forearc setting related to subduction initiation. Petrographic, geochronological, and mineralogical analyses suggest that the CLMC underwent at least two stages of crust-mantle interaction. The first stage (metasomatic agent) occurred at 483.1 ± 3.5 Ma. The metasomatic agent was a hydrous melt derived from a mixed source of carbonates and sediments, subducted with the south Qilian oceanic slab. The second stage (metasomatic fluids) occurred at 434.2 ± 2.1 Ma and the metasomatic fluids originated from subduction and exhumation of the deeply subducted continental crust. These results suggest that the CLMC represents a supra-subduction zone mantle wedge slice, metasomatized by at least two stages of subduction crustal fluids during the transition from oceanic subduction to continental collision in the North Qaidam ultrahigh pressure metamorphic belt.
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| 5. |
- Holmberg, Johanna, et al.
(författare)
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Decompressional equilibration of the Midsund granulite from Otrøy, Western Gneiss Region, Norway
- 2019
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Ingår i: Geologica Carpathica. - Bratislava : Slovak Acdemy of Sciences. - 1335-0552 .- 1336-8052. ; 70:6, s. 471-482
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Tidskriftsartikel (refereegranskat)abstract
- The Western Gneiss Region (WGR) of the Scandinavian Caledonides is an archetypal terrain for high-pressure(HP) and ultrahigh-pressure (UHP) metamorphism. However, the vast majority of lithologies occurring there bear no,or only limited, evidence for HP or UHP metamorphism. The studied Midsund HP granulite occurs on the island of Otrøy,a locality known for the occurrence of the UHP eclogites and mantle-derived, garnet-bearing ultramafics. The Midsundgranulite consists of plagioclase, garnet, clinopyroxene, relict phengitic mica, biotite, rutile, quartz, amphibole, ilmeniteand titanite, among the most prominent phases. Applied thermodynamic modelling in the NCKFMMnASHT systemresulted in a pressure–temperature (P–T) pseudosection that provides an intersection of compositional isopleths ofXMg (Mg/Mg+Fe) in garnet, albite in plagioclase and XNa (Na/Na+Ca) in clinopyroxene in the stability field of melt +plagioclase + garnet + clinopyroxene + amphibole + ilmenite. The obtained thermodynamic model yields P–T conditions of1.32–1.45 GPa and 875–970 °C. The relatively high P–T recorded by the Midsund granulite may be explained as an effectof equilibration due to exhumation from HP (presumably UHP) conditions followed by a period of stagnation under HTat lower-to-medium crustal level. The latter seems to be a more widespread phenomenon in the WGR than previouslythought and may well explain commonly calculated pressure contrasts between neighboring lithologies in the WGR andother HP–UHP terranes worldwide.
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| 6. |
- 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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| 7. |
- Jeanneret, Pauline, et al.
(författare)
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Geochronological constraints on the trans-Hudsonian tectono-metamorphic evolution of the pre-Athabasca basement within the Wollaston-Mudjatik Transition Zone, Saskatchewan
- 2017
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Ingår i: Precambrian Research. - : ELSEVIER SCIENCE BV. - 0301-9268 .- 1872-7433. ; 301, s. 152-178
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Tidskriftsartikel (refereegranskat)abstract
- The Hudsonian Pressure-Temperature-Deformation-time-(P-T-D-t) evolution of the pre-Athabasca crystalline basement (>similar to 1700 Ma) of the Wollaston-Mudjatik Transition Zone (WMTZ) highlights two major tectono-metamorphic events Ml-Dl and M2-D2. The ages of these two event have been obtained by in-situ LA-ICPMS U-Th-Pb dating of monazite from Grt-Crd bearing pelitic gneiss and U-Pb analyses performed on zircons from pegmatites, using both exposed basement and drill cores from the Wolly-McClean exploration drilling project. The M1-D1 event, interpreted as the burial of the thinned Hearne margin via southward thrusting to pressures varying from 10 to 6 kbar, occurred between ca.1840 and 1813 Ma. The M2-D2 event, producing the northeast structural trend of the WMTZ, was formed in a sinistral transpressional tectonic regime during the late stage of the Hudsonian oblique collision between ca. 1813 and 1770 Ma. Thermobarometric estimates on the M2-D2 assemblages show that the entire studied area was reequilibrated at about 5 kbar and 750-825 degrees C. trans-Hudsonian pegmatites are viewed as the main proto-ore of the uranium-rich Athabasca unconformity-type deposits. Formation, transfer and differentiation of these trans-Hudsonian pegmatites are replaced in this P-T-D-t evolution. During the 1840-1813 Ma Ml-Dl partial melting event, the first batches of melt produced in the deep crust are the most likely enriched in uranium. Then, these melts were transferred upwards to the upper crustal levels owing to the development of crustal scale steeply-dipping D2 shear zones, and finally were differentiated to form uranium-enriched pegmatites between 1813 and 1770 Ma. Some monazite and zircon grains within retrogressed migmatites recorded a later event at ca. 1720 Ma, interpreted as the terminal cooling event down to 300-400 degrees C responsible for partial retrogression of metamorphic assemblages. This age provides new insights into the timing of the onset of the Athabasca sedimentation that should therefore be at least 1710 Ma old or younger.
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