The Experts below are selected from a list of 327 Experts worldwide ranked by ideXlab platform
Michael B Stephens - One of the best experts on this subject based on the ideXlab platform.
-
middle thrust sheets in the caledonide orogen sweden the outer margin of baltica the continent ocean transition zone and late cambrian ordovician subduction accretion
Geological Society London Memoirs, 2020Co-Authors: David G Gee, Iwona Klonowska, Pergunnar Andreasson, Michael B StephensAbstract: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. Tannas 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 (Sarv 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. (Less)
-
chapter 21 middle thrust sheets in the caledonide orogen sweden the outer margin of baltica the continent ocean transition zone and late cambrian ordovician subduction accretion
Geological Society London Memoirs, 2020Co-Authors: Iwona Klonowska, Pergunnar Andreasson, Michael B StephensAbstract: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. Tannas 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 (Sarv 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.
Jiři Konopasek - One of the best experts on this subject based on the ideXlab platform.
-
anticlockwise metamorphic pressure temperature paths and Nappe stacking in the reisa Nappe complex in the scandinavian caledonides northern norway evidence for weakening of lower continental crust before and during continental collision
Solid Earth, 2019Co-Authors: Carly Faber, Erling J. K. Ravna, Holger Stunitz, Deta Gasser, Petr Jeřabek, Katrin Kraus, Fernando Corfu, Jiři KonopasekAbstract:This study investigates the tectonostratigraphy and metamorphic and tectonic evolution of the Caledonian Reisa Nappe Complex (RNC; from bottom to top: Vaddas, Kafjord, and Nordmannvik Nappes) in northern Troms, Nor-way. Structural data, phase equilibrium modelling, and U-Pb zircon and titanite geochronology are used to constrain the timing and pressure-temperature (P-T) conditions of deformation and metamorphism during Nappe stacking that facilitated crustal thickening during continental collision. Five samples taken from different parts of the RNC reveal an anticlockwise P-T path attributed to the effects of early Silurian heating (D 1) followed by thrusting (D 2). At ca. 439 Ma during D 1 the Nordmannvik Nappe reached the highest meta-morphic conditions at ca. 780 • C and ∼ 9-11 kbar inducing kyanite-grade partial melting. At the same time the Kafjord Nappe was at higher, colder, levels of the crust ca. 600 • C, 6-7 kbar and the Vaddas Nappe was intruded by gabbro at > 650 • C and ca. 6-9 kbar. The subsequent D 2 shearing occurred at increasing pressure and decreasing temperatures ca. 700 • C and 9-11 kbar in the partially molten Nordman-nvik Nappe, ca. 600 • C and 9-10 kbar in the Kafjord Nappe, and ca. 640 • C and 12-13 kbar in the Vaddas Nappe. Multi-stage titanite growth in the Nordmannvik Nappe records this evolution through D 1 and D 2 between ca. 440 and 427 Ma, while titanite growth along the lower RNC boundary records D 2 shearing at 432±6 Ma. It emerges that early Silurian heating (ca. 440 Ma) probably resulted from large-scale magma underplating and initiated partial melting that weakened the lower crust, which facilitated dismembering of the crust into individual thrust slices (Nappe units). This tectonic style contrasts with subduction of mechanically strong continental crust to great depths as seen in, for example, the Western Gneiss Region further south.
-
Anticlockwise metamorphic pressure–temperature paths and Nappe stacking in the Reisa Nappe Complex in the Scandinavian Caledonides, northern Norway: evidence for weakening of lower continental crust before and during continental collision
Solid Earth, 2019Co-Authors: Carly Faber, Erling J. K. Ravna, Holger Stunitz, Deta Gasser, Petr Jeřabek, Katrin Kraus, Fernando Corfu, Jiři KonopasekAbstract:This study investigates the tectonostratigraphy and metamorphic and tectonic evolution of the Caledonian Reisa Nappe Complex (RNC; from bottom to top: Vaddas, Kafjord, and Nordmannvik Nappes) in northern Troms, Nor-way. Structural data, phase equilibrium modelling, and U-Pb zircon and titanite geochronology are used to constrain the timing and pressure-temperature (P-T) conditions of deformation and metamorphism during Nappe stacking that facilitated crustal thickening during continental collision. Five samples taken from different parts of the RNC reveal an anticlockwise P-T path attributed to the effects of early Silurian heating (D 1) followed by thrusting (D 2). At ca. 439 Ma during D 1 the Nordmannvik Nappe reached the highest meta-morphic conditions at ca. 780 • C and ∼ 9-11 kbar inducing kyanite-grade partial melting. At the same time the Kafjord Nappe was at higher, colder, levels of the crust ca. 600 • C, 6-7 kbar and the Vaddas Nappe was intruded by gabbro at > 650 • C and ca. 6-9 kbar. The subsequent D 2 shearing occurred at increasing pressure and decreasing temperatures ca. 700 • C and 9-11 kbar in the partially molten Nordman-nvik Nappe, ca. 600 • C and 9-10 kbar in the Kafjord Nappe, and ca. 640 • C and 12-13 kbar in the Vaddas Nappe. Multi-stage titanite growth in the Nordmannvik Nappe records this evolution through D 1 and D 2 between ca. 440 and 427 Ma, while titanite growth along the lower RNC boundary records D 2 shearing at 432±6 Ma. It emerges that early Silurian heating (ca. 440 Ma) probably resulted from large-scale magma underplating and initiated partial melting that weakened the lower crust, which facilitated dismembering of the crust into individual thrust slices (Nappe units). This tectonic style contrasts with subduction of mechanically strong continental crust to great depths as seen in, for example, the Western Gneiss Region further south.
Albrecht Steck - One of the best experts on this subject based on the ideXlab platform.
-
The Maggia Nappe: an extruding sheath fold basement Nappe in the Lepontine gneiss dome of the Central Alps
International Journal of Earth Sciences, 2019Co-Authors: Albrecht Steck, Jean-luc Epard, Henri MassonAbstract:The Lepontine gneiss dome represents a unique region of the Central Alps where Oligocene–Miocene amphibolite facies grade rocks and fold Nappes of the deepest tectonic level of the Alpine orogenic belt are exposed in a tectonic window. The Cenozoic structures of the Maggia Nappe reveals a giant tens of kilometre-scale tubular fold structure that cross-cut through the surrounding lower Penninic Nappes from its root situated in the southern steep-belt of the Alps near Bellinzona. The Mesozoic sedimentary cover of the Maggia Nappe is typical for the Helvetic stratigraphic domain. The age of formation of the lower Penninic fold-Nappes by ductile detachment of the upper European crust during its underthrusting below the higher Penninic and Austroalpine orogenic lid and Adriatic indenter was estimated between 40 and 30 Ma. Maximal pressures of 8–9 kbars and temperatures of 600–700 °C were attended during and after the Nappe emplacement some 30–22 Ma ago. The Maggia and surrounding Nappes are crosscut by the isograds of the Barrovian regional metamorphism.
-
Structure, geometry and kinematics of the northern Adula Nappe (Central Alps)
Swiss Journal of Geosciences, 2014Co-Authors: Mattia Cavargna-sani, Jean-luc Epard, Albrecht SteckAbstract:The eclogitic Adula Nappe of the Central Alps (cantons Graubünden and Ticino, Switzerland) displays an exceptionally complex internal structure with the particularity of enclosing numerous slices of Mesozoic cover rocks (Internal Mesozoic) within the Palaeozoic gneiss basement. This study is principally based on detailed lithological and structural mapping of selected areas of the northern Adula Nappe. Specific focus was placed on the Mesozoic slivers embedded in pre-Mesozoic basement (Internal Mesozoic). The most pervasive structures are related to the Zapport deformation phase that is responsible for the development of a fold-Nappe and ubiquitous north-directed shear. Locally, the structures in the upper and frontal part of the Nappe can be assigned to the older ductile Ursprung phase. These earlier structures are only compatible with top-to-S shear movement. The superposition of the Ursprung and Zapport phases is responsible for the north-dipping internal duplex-like structure and the sliced aspect of the Northern and Central Adula Nappe. We conclude that the Adula Nappe represents a major shear zone involving the entire Nappe and responsible for the emplacement of the Lower Penninic sediments and the Middle Penninic Nappes in the eastern part of the Lepontine Dome.
-
structural development of the tso morari ultra high pressure Nappe of the ladakh himalaya
Tectonophysics, 2008Co-Authors: Jean-luc Epard, Albrecht SteckAbstract:Abstract A continental subduction-related and multistage exhumation process for the Tso Morari ultra-high pressure Nappe is proposed. The model is constrained by published thermo-barometry and age data, combined with new geological and tectonic maps. Additionally, observations on the structural and metamorphic evolution of the Tso Morari area and the North Himalayan Nappes are presented. The northern margin of the Indian continental crust was subducted to a depth of > 90 km below Asia after continental collision some 55 Ma ago. The underthrusting was accompanied by the detachment and accretion of Late Proterozoic to Early Eocene sediments, creating the North Himalayan accretionary wedge, in front of the active Asian margin and the 103–50 Ma Ladakh arc batholith. The basic dikes in the Ordovician Tso Morari granite were transformed to eclogites with crystallization of coesite, some 53 Ma ago at a depth of > 90 km (> 27 kbar) and temperatures of 500 to 600 °C. The detachment and extrusion of the low density Tso Morari Nappe, composed of 70% of the Tso Morari granite and 30% of graywackes with some eclogitic dikes, occurred by ductile pure and simple shear deformation. It was pushed by buoyancy forces and by squeezing between the underthrusted Indian lithosphere and the Asian mantle wedge. The extruding Tso Morari Nappe reached a depth of 35 km at the base of the North Himalayan accretionary wedge some 48 Ma ago. There the whole Nappe stack recrystallized under amphibolite facies conditions of a Barrovian regional metamorphism with a metamorphic field gradient of 20 °C/km. An intense schistosity with a W–E oriented stretching lineation L1 and top-to-the E shear criteria and crystallization of oriented sillimanite needles after kyanite, testify to the Tso Morari Nappe extrusion and pressure drop. The whole Nappe stack, comprising from the base to top the Tso Morari, Tetraogal, Karzok and Mata–Nyimaling-Tsarap Nappes, was overprinted by new schistosities with a first N-directed and a second NE-directed stretching lineation L2 and L3 reaching the base of the North Himalayan accretionary wedge. They are characterized by top-to-the S and SW shear criteria. This structural overprint was related to an early N- and a younger NE-directed underthrusting of the Indian plate below Asia that was accompanied by anticlockwise rotation of India. The warping of the Tso Morari dome started already some 48 Ma ago with the formation of an extruding Nappe at depth. The Tso Morari dome reached a depth of 15 km about 40 Ma ago in the eastern Kiagar La region and 30 Ma ago in the western Nuruchan region. The extrusion rate was of about 3 cm/yr between 53 and 48 Ma, followed by an uplift rate of 1.2 mm/yr between 48 and 30 Ma and of only 0.5 mm/yr after 30 Ma. Geomorphology observations show that the Tso Morari dome is still affected by faults, open regional dome, and basin and pull-apart structures, in a zone of active dextral transpression parallel to the Indus Suture zone.
Christopher L Kirkland - One of the best experts on this subject based on the ideXlab platform.
-
provenance of neoproterozoic sediments in the sarv Nappes middle allochthon of the scandinavian caledonides la icp ms and sims u pb dating of detrital zircons
Precambrian Research, 2011Co-Authors: Yaron Beerishlevin, David G Gee, S Claesson, Anna Ladenberger, Jaroslaw Majka, Christopher L Kirkland, Peter Robinson, Dirk FreiAbstract:We present U-Pb age data for detrital zircons from dike-intruded Neoproterozoic sedimentary rocks of the Caledonian Middle Allochthon in central Sweden and Norway. Detrital zircons from 11 samples from the Sarv, Saetra and upper Leksdal Nappes (informally referred to as the Sarv Nappes) are clustered within ca. 0.9-1.75 Ga, but display a bimodal distribution with major ca. 1.45-1.75 Ga and ca. 0.9-1.2 Ga components. An apparent increase of younger (0.9-1.2 Ga) components to the northwest reflects varying source terranes. Detrital zircons from an additional sample from the lower part of the Leksdal Nappe, of uncertain affiliation to the Sarv has a prominent 1.75-1.85 Ga component supporting previous suggestions that this part of the Nappe belonged to a more proximal basin. Comparison of the Sarv age probability patterns with data from basement windows and basement slices within the Middle Allochthon in central Sweden and Norway supports the derivation of the sediments from the attenuated Baltican continental crust on which they were presumably deposited. Similar comparisons suggest that derivation from the southern segment of the Fennoscandian Shield or from eastern segments of Laurentia is less likely, mostly because they include also older components. We infer that the ca. 200 km wide belt of attenuated Baltican continental crust included northern extensions of Mesoproterozoic to early Neoproterozoic terranes exposed in the southern part of the Fennoscandian Shield and the easternmost part of Laurentia, which at ca. 900 Ma were still adjacent. Pre-1.75 Ga terranes of the Fennoscandian Shield were probably isolated from the Sarv distal basin(s) by intracratonic basins and uplifted margins associated with early development of this extended continental crust. The significantly older ages in the lower part of the Leksdal Nappe and its inferred more proximal position support this model. The proposed northern extension of Mesoproterozoic-early Neoproterozoic terranes can explain in a simpler way the occurrence of such detritus in many Caledonide-Appalachian allochthons exposed at the margins of the North Atlantic, with no need to infer large displacement along the axis of the Caledonide Orogen or to postulate selective transport of Grenville-age material from the south over large distances.One of our Sarv samples located at the Norwegian coast revealed Caledonian reworking at ca. 395 Ma. This age agrees with ages of late-tectonic amphibolite-facies metamorphism and pegmatite intrusion recorded in this part of the Caledonides.
-
provenance and terrane evolution of the kalak Nappe complex norwegian caledonides implications for neoproterozoic paleogeography and tectonics
The Journal of Geology, 2007Co-Authors: Christopher L Kirkland, Stephen J Daly, Martin J WhitehouseAbstract:Abstract The Kalak Nappe Complex (KNC) within the northernmost Arctic Norwegian Caledonides has traditionally been interpreted as representing the tectonically shortened margin of Baltica, consisting of a Precambrian basement and a late Precambrian to Cambrian cover deposited on the margin of the Iapetus Ocean. However, new geochronology indicates a distinctly different scenario. Detrital zircon U‐Pb dating, together with the magmatic and deformation history, shows that the KNC metasediments, previously considered as a single stratigraphic sequence, belong to at least two distinct successions. Metasediments of the Svaerholt Succession, within the lower (Kolvik and Olderfjord) Nappes of the KNC and affected by the late Grenvillian Rigolet deformation phase, were deposited between ca. 980 and 1030 Ma, constrained by intrusive granites and the youngest detrital zircons. The Soroy Succession occurs within the Soroy‐Seiland Nappe, the Havvatnet Imbricate Stack, and Veines Nappe (the upper Nappes of the KNC). I...
-
granitic magmatism of grenvillian and late neoproterozoic age in finnmark arctic norway constraining pre scandian deformation in the kalak Nappe complex
Precambrian Research, 2006Co-Authors: Christopher L Kirkland, J. S. Daly, Martin J WhitehouseAbstract:Abstract The Caledonian Orogen in Arctic Norway is characterized by a variety of Nappes thrust from west to east onto the Baltic Shield. Traditionally, this has been regarded as the product of two orogenic events: an earlier Finnmarkian (540–490 Ma) and a later Scandian event (400–425 Ma). However, ion microprobe U–Pb zircon dating of discordant plutonic rocks within the lowermost Nappes demonstrates that some of the deformation must have taken place in a Grenville (Sveconorwegian) event. This view is supported by the 981 ± 7 Ma, 978 ± 9 Ma and 973 ± 4 Ma ages of the Repvag, Harvika and Siedgoaivi adamellite bodies. These bodies cut the Soroy Succession, apparently after an earlier deformation event. On these grounds a Grenville (Sveconorwegian) event is responsible for the D2 deformation within the Olderfjord and Kolvik Nappes. Within the overlying Havvatnet Imbricate Stack, early deformation took place in the Neoproterozoic, “Porsanger Orogeny”. Evidence for this event is provided by the Litlefjord and Revsneshamn adamellite bodies dated at 841 ± 6 Ma and 839 ± 10 Ma and pegmatitic intrusions, dated at 826 ± 6 Ma and 833 ± 9 Ma, which show clear discordant structural relationships cutting F2 fold structures that affect the Klubben Psammite, the oldest unit of the Soroy Succession. Within the uppermost Nappe (Soroy-Seiland Nappe) syn-deformational migmatitic leucosomes in the Eidvageid Paragneiss yield crystallization ages of 709 ± 4 Ma. This age is indistinguishable from zircon overgrowths within the underlying Havvatnet Imbricate Stack. Hence, juxtaposition of these Nappes predates Scandian tectonism and occurred during the Snofjord event at c. 710 Ma. The component Nappes of the KNC show decreasing ages of anatexis on moving up the Nappe pile. Such temporal and spatial patterns are consistent with episodic terrane amalgamation from Grenvillian times. The KNC provides evidence for punctuated crustal anatexis and episodic orogenic deformation of c. 980 Ma (Grenvillian-Sveconorwegian), c. 840 Ma (Porsanger) and c. 710 Ma (Snofjord) age, overprinted by intense Scandian deformation. These data support the notion of a Grenville segment extending between Greenland and Baltica and require a radical revision to the tectonometamorphic evolution of the KNC. The KNC represents a collage of exotic, diachronously accreted, terranes overthrust by Llandovery flysch of Laurentian affinity.
Erling J. K. Ravna - One of the best experts on this subject based on the ideXlab platform.
-
Anticlockwise metamorphic pressure–temperature paths and Nappe stacking in the Reisa Nappe Complex in the Scandinavian Caledonides, northern Norway: evidence for weakening of lower continental crust before and during continental collision
Solid Earth, 2019Co-Authors: Carly Faber, Erling J. K. Ravna, Holger Stunitz, Deta Gasser, Petr Jeřabek, Katrin Kraus, Fernando Corfu, Jiři KonopasekAbstract:This study investigates the tectonostratigraphy and metamorphic and tectonic evolution of the Caledonian Reisa Nappe Complex (RNC; from bottom to top: Vaddas, Kafjord, and Nordmannvik Nappes) in northern Troms, Nor-way. Structural data, phase equilibrium modelling, and U-Pb zircon and titanite geochronology are used to constrain the timing and pressure-temperature (P-T) conditions of deformation and metamorphism during Nappe stacking that facilitated crustal thickening during continental collision. Five samples taken from different parts of the RNC reveal an anticlockwise P-T path attributed to the effects of early Silurian heating (D 1) followed by thrusting (D 2). At ca. 439 Ma during D 1 the Nordmannvik Nappe reached the highest meta-morphic conditions at ca. 780 • C and ∼ 9-11 kbar inducing kyanite-grade partial melting. At the same time the Kafjord Nappe was at higher, colder, levels of the crust ca. 600 • C, 6-7 kbar and the Vaddas Nappe was intruded by gabbro at > 650 • C and ca. 6-9 kbar. The subsequent D 2 shearing occurred at increasing pressure and decreasing temperatures ca. 700 • C and 9-11 kbar in the partially molten Nordman-nvik Nappe, ca. 600 • C and 9-10 kbar in the Kafjord Nappe, and ca. 640 • C and 12-13 kbar in the Vaddas Nappe. Multi-stage titanite growth in the Nordmannvik Nappe records this evolution through D 1 and D 2 between ca. 440 and 427 Ma, while titanite growth along the lower RNC boundary records D 2 shearing at 432±6 Ma. It emerges that early Silurian heating (ca. 440 Ma) probably resulted from large-scale magma underplating and initiated partial melting that weakened the lower crust, which facilitated dismembering of the crust into individual thrust slices (Nappe units). This tectonic style contrasts with subduction of mechanically strong continental crust to great depths as seen in, for example, the Western Gneiss Region further south.
-
anticlockwise metamorphic pressure temperature paths and Nappe stacking in the reisa Nappe complex in the scandinavian caledonides northern norway evidence for weakening of lower continental crust before and during continental collision
Solid Earth, 2019Co-Authors: Carly Faber, Erling J. K. Ravna, Holger Stunitz, Deta Gasser, Petr Jeřabek, Katrin Kraus, Fernando Corfu, Jiři KonopasekAbstract:This study investigates the tectonostratigraphy and metamorphic and tectonic evolution of the Caledonian Reisa Nappe Complex (RNC; from bottom to top: Vaddas, Kafjord, and Nordmannvik Nappes) in northern Troms, Nor-way. Structural data, phase equilibrium modelling, and U-Pb zircon and titanite geochronology are used to constrain the timing and pressure-temperature (P-T) conditions of deformation and metamorphism during Nappe stacking that facilitated crustal thickening during continental collision. Five samples taken from different parts of the RNC reveal an anticlockwise P-T path attributed to the effects of early Silurian heating (D 1) followed by thrusting (D 2). At ca. 439 Ma during D 1 the Nordmannvik Nappe reached the highest meta-morphic conditions at ca. 780 • C and ∼ 9-11 kbar inducing kyanite-grade partial melting. At the same time the Kafjord Nappe was at higher, colder, levels of the crust ca. 600 • C, 6-7 kbar and the Vaddas Nappe was intruded by gabbro at > 650 • C and ca. 6-9 kbar. The subsequent D 2 shearing occurred at increasing pressure and decreasing temperatures ca. 700 • C and 9-11 kbar in the partially molten Nordman-nvik Nappe, ca. 600 • C and 9-10 kbar in the Kafjord Nappe, and ca. 640 • C and 12-13 kbar in the Vaddas Nappe. Multi-stage titanite growth in the Nordmannvik Nappe records this evolution through D 1 and D 2 between ca. 440 and 427 Ma, while titanite growth along the lower RNC boundary records D 2 shearing at 432±6 Ma. It emerges that early Silurian heating (ca. 440 Ma) probably resulted from large-scale magma underplating and initiated partial melting that weakened the lower crust, which facilitated dismembering of the crust into individual thrust slices (Nappe units). This tectonic style contrasts with subduction of mechanically strong continental crust to great depths as seen in, for example, the Western Gneiss Region further south.
-
constraining peak p t conditions in uhp eclogites calculated phase equilibria in kyanite and phengite bearing eclogite of the tromso Nappe norway
Journal of Metamorphic Geology, 2012Co-Authors: Marian Janák, Erling J. K. Ravna, Kåre KullerudAbstract:Kyanite- and phengite-bearing eclogites have better potential to constrain the peak metamorphic P–T conditions from phase equilibria between garnet + omphacite + kyanite + phengite + quartz/coesite than common, mostly bimineralic (garnet + omphacite) eclogites, as exemplified by this study. Textural relationships, conventional geothermobarometry and thermodynamic modelling have been used to constrain the metamorphic evolution of the Tromsdalstind eclogite from the Tromso Nappe, one of the biggest exposures of eclogite in the Scandinavian Caledonides. The phase relationships demonstrate that the rock progressively dehydrated, resulting in breakdown of amphibole and zoisite at increasing pressure. The peak-pressure mineral assemblage was garnet + omphacite + kyanite + phengite + coesite, inferred from polycrystalline quartz included in radially fractured omphacite. This omphacite, with up to 37 mol.% of jadeite and 3% of the Ca-Eskola component, contains oriented rods of silica composition. Garnet shows higher grossular (XGrs = 0.25–0.29), but lower pyrope-content (XPrp = 0. 37–0.39) in the core than the rim, while phengite contains up to 3.5 Si pfu. The compositional isopleths for garnet core, phengite and omphacite constrain the P–T conditions to 3.2–3.5 GPa and 720–800 °C, in good agreement with the results obtained from conventional geothermobarometry (3.2–3.5 GPa & 730–780 °C). Peak-pressure assemblage is variably overprinted by symplectites of diopside + plagioclase after omphacite, biotite and plagioclase after phengite, and sapphirine + spinel + corundum + plagioclase after kyanite. Exhumation from ultrahigh-pressure (UHP) conditions to 1.3–1.5 GPa at 740–770 °C is constrained by the garnet rim (XCaGrt = 0.18–0.21) and symplectite clinopyroxene (XNaCpx = 0.13–0.21), and to 0.5–0.7 GPa at 700–800 °C by sapphirine (XMg = 0.86–0.87) and spinel (XMg = 0.60–0.62) compositional isopleths. UHP metamorphism in the Tromso Nappe is more widespread than previously known. Available data suggest that UHP eclogites were uplifted to lower crustal levels rapidly, within a short time interval (452–449 Ma) prior to the Scandian collision between Laurentia and Baltica. The Tromso Nappe as the highest tectonic unit of the North Norwegian Caledonides is considered to be of Laurentian origin and UHP metamorphism could have resulted from subduction along the Laurentian continental margin. An alternative is that the Tromso Nappe belonged to a continental margin of Baltica, which had already been subducted before the terminal Scandian collision, and was emplaced as an out-of-sequence thrust during the Scandian lateral transport of Nappes.
