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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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- Bukała, Michał, et al.
(författare)
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UHP metamorphism recorded by phengite eclogite from the Caledonides of northern Sweden : P-T path and tectonic implications
- 2018
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Ingår i: Journal of Metamorphic Geology. - : Wiley. - 0263-4929 .- 1525-1314. ; 36:5, s. 547-566
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Tidskriftsartikel (refereegranskat)abstract
- The Seve Nappe Complex (SNC) of the Scandinavian Caledonides records a well-documented history of high pressure (HP) and ultra-high pressure (UHP) metamorphism. Eclogites of the SNC occur in two areas in Sweden, namely Jamtland and Norrbotten. The Jamtland eclogites and associated rocks are well-studied and provide evidence for late Ordovician UHP metamorphism, whereas the Norrbotten eclogites, formed during the late Cambrian (Furongian)/Early Ordovician, have not been studied in such detail, especially in terms of the P-T conditions of their formation. Within the studied eclogite, clinopyroxene contains a high-Na core and two rims: inner, medium-Na and outer, low-Na. Garnet consists of a high-Ca euhedral core, low-Ca inner rim and medium-Ca outer rim. A similar pattern occurs within phengite, where high-Si cores are enveloped by medium and low-Si rims. The compositions of the mineral cores, inner rims and outer rims reflect three stages in the metamorphic evolution of the eclogite. Applied Quartz-in-Garnet geobarometry, coupled with Zr-in-rutile geothermometry reveal that garnet nucleation (E0 stage) took place at 1.5-1.6GPa and 620-660 degrees C. The eclogite peak-pressure assemblage developed during the E1 stage, it consists of garnet+omphacite+phengite+rutile+coesite? and yields P-T conditions of 2.8-3.1GPa and 660-780 degrees C as constrained by conventional geothermobarometry and thermodynamic modelling in the NCKFMMnASHT system. Later, lower-pressure stages E2 and E3 record conditions of 2.2-2.8GPa, 680-780 degrees C and 2.1GPa, 735 degrees C, respectively. The prograde metamorphic evolution of the eclogite is inferred from inclusions of epidote, amphibole and clinopyroxene within garnet. The presence of amphibole-quartz-plagioclase symplectites, secondary epidote/zoisite and titanite replacing rutile record the later retrograde changes taking place at <1.5GPa (referred as E4 stage). The obtained P-T conditions indicate that the Norrbotten eclogites underwent a metamorphic evolution characterized by a clockwise P-T path with peak metamorphism reaching up to coesite stability field within a relatively cold subduction regime (7.8 degrees C/km). The obtained results provide the first evidence for UHP metamorphism in the SNC above the Arctic Circle and document cold subduction regime and multistage exhumation of the deeply subducted Baltican margin at early stage of the Caledonian Orogeny.
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- 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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- Callegari, Riccardo, et al.
(författare)
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Early Neoproterozoic magmatism and Caledonian metamorphism recorded by the Mårma terrane, Seve Nappe Complex, northern Swedish Caledonides
- 2023
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Ingår i: Journal of the Geological Society. - : Geological Society of London. - 0016-7649 .- 2041-479X. ; 180:5
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Tidskriftsartikel (refereegranskat)abstract
- Petrology, geochronology and bulk-rock chemistry are combined to investigate the early Neoproterozoic magmatismand Cambrian–Ordovician metamorphism in the northern Swedish Caledonides. This work includes several lithologies of theMårma terrane in the Seve Nappe Complex exposed in the Kebnekaise region. U–Pb zircon geochronology yielded crystallizationages of 835 ± 8 Ma for a mylonitic orthogneiss, 864 ± 3 Ma for the Vistas Granite and 840 ± 7 Ma for an intruded granitic dyke,whereas a gabbro and a granodiorite intrusion gave U–Pb zircon crystallization ages of 856 ± 3 Ma and 850 ± 1 Ma, respectively.U–Pb monazite dating of the mylonitic orthogneiss gave an upper intercept age of 841 ± 7 Ma and a lower intercept age of443 ± 20 Ma. Pressure–temperature estimates derived for the mylonitic orthogneiss reveal metamorphic peak-pressure and peaktemperatureof 10.5–11.75 kbar and 550–610°C and 7.4–8.1 kbar at 615–675°C, respectively.Metamorphic pressure–temperatureestimates for the Vistas Granite yield 6.5–7.5 kbar at 600–625°C. Whole-rock chemistry coupled with U–Pb geochronologyindicates that bimodal magmatism was related to attempted rifting of Rodinia between 870 and 840 Ma.
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| 7. |
- Ducharme, T. A., et al.
(författare)
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Stretched Thin : Oligocene Extrusion and Ductile Thinning of the Basal Unit Along the Evia Shear Zone, NW Cyclades, Greece
- 2022
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Ingår i: Tectonics. - : American Geophysical Union (AGU). - 0278-7407 .- 1944-9194. ; 41:12
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Tidskriftsartikel (refereegranskat)abstract
- We present field-based structural, geochronological, and petrological evidence for a major new Oligocene extensional shear zone, the Evia Shear Zone (ESZ), on the island of Evia in the NW Aegean Sea. Strain related to this structure occurs in a diffuse zone structurally underlying the nappe contact between the Cycladic Blueschist Unit and Basal Unit, within rock previously attributed to the Tsakei Unit or Basal Unit metaflysch. Structures within the ESZ record stretching in both the X and Y directions under ductile and brittle-ductile conditions, indicative of a component of oblate coaxial strain during overall top-to-E general shear. New white mica Ar-40/Ar-39 geochronology from the ESZ shows that the structure was accommodating ductile deformation in the late Oligocene. Thermodynamic modeling shows that rock within the ESZ records greenschist facies retrogressive conditions of 310 +/- 15 degrees C and 7 +/- 1 kbar. Zircon (U-Th)/He thermochronology demonstrates that both the CBU and Basal Unit had exhumed into the brittle crust by the middle Miocene, slightly earlier than indicated by low-temperature thermochronology for the HP-LT units throughout the Aegean. We interpret these data as evidence that the Basal Unit underwent rapid exhumation via syn-orogenic processes persisting until at least the early Miocene, facilitated in part by normal-sense displacement along the ESZ. The occurrence of structures representative of coaxial deformation in the shear zone alludes to a major role for ductile thinning in controlling exhumation of HP rocks in the Cyclades, perhaps influencing the relative rates of exhumation observed across the region.
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| 8. |
- Fassmer, K., et al.
(författare)
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Middle Ordovician subduction of continental crust in the Scandinavian Caledonides - an example from Tjeliken, Seve Nappe Complex, Sweden.
- 2017
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 172
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Tidskriftsartikel (refereegranskat)abstract
- The Seve Nappe Complex of the Scandinavian Caledonides is thought to be derived from the distal passive margin of Baltica which collided with Laurentia in the Scandian Phase of the Caledonian Orogeny at 430–400 Ma. Parts of the Seve Nappe Complex were affected by pre-Scandian high- and ultrahigh-pressure metamorphism, in a tectonic framework that is still unclear, partly due to uncertainties about the exact timing. Previous age determinations yielded between ~ 505 and ~ 446 Ma, with a general trend of older ages in the North (Norrbotten) than in the South (Jämtland). New age determinations were performed on eclogite and garnet–phengite gneiss at Tjeliken in northern Jämtland. Thermodynamic modelling yielded peak metamorphic conditions of 25–27 kbar/680–760 °C for the garnet–phengite gneiss, similar to published peak metamorphic conditions of the eclogite (25–26 kbar/650–700 °C). Metamorphic rims of zircons from the garnet–phengite gneiss were dated using secondary ion mass spectrometry and yielded a concordia age of 458.9 ± 2.5 Ma. Lu–Hf garnet-whole rock dating yielded 458 ± 1.0 Ma for the eclogite. Garnet in the eclogite shows prograde major-element zoning and concentration of Lu in the cores, indicating that this age is related to garnet growth during pressure increase, i.e. subduction. The identical ages from both rock types, coinciding with published Sm–Nd ages from the eclogite, confirm subduction of the Seve Nappe Complex in Northern Jämtland during the Middle Ordovician in a fast subduction–exhumation cycle.
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| 9. |
- Gee, David G., et al.
(författare)
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Middle thrust sheets in the Caledonide orogen, Sweden: the outer margin of Baltica, the continent–ocean transition zone and late Cambrian–Ordovician subduction–accretion
- 2020
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Ingår i: Sweden. - : Geological Society of London. - 9781786204608 ; 50:1, s. 517-548
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Nappes of continental outer and outermost margin affinities (Middle Allochthon) were transported from locations west of the present Norwegian coast and thrust eastwards onto the Baltoscandian foreland basin and platform. They are of higher metamorphic grade than underlying thrust sheets and most are more penetratively deformed. These allochthons are treated here in three groups. The lower thrust sheets comprise Paleoproterozoic crystalline basement (e.g. Tännäs Augen Gneiss Nappe) and greenschist facies, Neoproterozoic, siliciclastic metasedimentary rocks (e.g. Offerdal Nappe). These are overthrust by a Cryogenian−Ediacaran succession intruded by c. 600 Ma dolerites (Baltoscandian Dyke Swarm) with an affinity to mid-ocean ridge basalt containing normal to enriched incompatible element contents (Särv Nappes). The upper sheets are dominated by higher-grade allochthons (Seve Nappe Complex) with similar, mainly siliciclastic sedimentary protoliths, more mafic magmatism and some solitary ultramafic bodies. Within this early Ediacaran continent−ocean transition zone (COT) assemblage, generally metamorphosed in amphibolite facies, some nappes experienced migmatization, and eclogites are present. Evidence of ultrahigh-pressure metamorphism has been obtained from garnet peridotites and eclogites; recently, microdiamonds have been discovered in paragneisses. Subduction of the COT started by the late Cambrian and accretion continued through the Ordovician, prior to the Baltica–Laurentia collision. Thrusting of all these Middle allochthons onto the foreland basin exceeds a distance of 400 km.
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| 10. |
- 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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