P-T evolotion and High-temperature deformation of Precambrian eclogite, Sveconorwegian orogen

The 1.1-0.9 Ga Sveconorwegian orogen is one of several Grenvillian-aged orogenic belts that mark the amalgamation of supercontinent Rodinia. The highest-pressure rocks in the Sveconorwegian orogen are eclogites in the Eastern Segment (SW Sweden). The eclogites occur in a nappe in the high-grade metamorphic level of the Eastern Segment that represents a window into the deepest part of this Precambrian mountain belt. The aim of this thesis is to reconstruct the metamorphic history of the eclogite-bearing nappe by characterizating the deformation associated with exhumation (Paper I) and by reconstructing the P–T evolution (pressure and temperature; papers II and III).Paper I focuses on the deformation structures in the basal shear zone of the eclogite-bearing nappe. These structures developed during exhumation at high-temperature conditions. Top-to-the-east shear and east-directed flow produced intense folding, interpreted as formed by a combination of simple and pure shear. The interplay of shearing, folding, and melt localization lead to localized shear, high-temperature brittle fracturing, and the formation of high-temperature chevron folds in high-strain zones.Paper II retraces the metamorphic evolution of the eclogite-bearing nappe by thermodynamic modelling (THERMOCALC©) and construction of P–T pseudosections for two different types of eclogite. One of the samples gave information on both the prograde and the retrograde paths, and an estimate of peak metamorphic conditions of 850–900 °C and ~18 kbar. The first stage of the prograde path, representing a medium P/T gradient, is recorded in the core of garnet grains. The second part of the prograde path and the retrograde path are both steep. The chemical growth zoning of garnet is preserved which, together with the shape of the P–T path, reflects short residence time at high temperatures.Paper III reports the results of two independent trace element thermometers, which are based on the Zr-contents in rutile and Ti-contents in quartz. The combination of these two methods confirmed the P–T evolution calculated in Paper II. In particular, Ti-in-quartz thermometry are in agreement with the pseudosection estimates at high temperatures, and the minerals appear unaffected by diffusional resetting. A pseudosection model, showing the changes in modal abundance of different phases along the P–T path, demonstrates that rutile grains in the matrix recrystallized from smaller-sized rutile grains, and that this process was simultaneous with the main dehydration reaction in the rock (continuous breakdown of hornblende and formation of clinopyroxene). This study illustrates that Zr-in-rutile and Ti-in-quartz thermometry cannot only robustly constrain a prograde evolution, but when combined with a pseudosection model can also yield information on recrystallization processes. In fact, the combination of these methods provides an unrivalled tool for petrologic interpretation.The data presented in this thesis testifies to westward tectonic burial of continental crust at ~65 km depth and 890 °C at a late stage of the Sveconorwegian orogenesis. The following foreland-directed tectonic exhumation of the eclogite-bearing nappe was associated with partial melting, ductile flow folding and shearing. The character of both prograde and retrograde P–T paths suggests rapid tectonic burial and exhumation consistent with collision at the end of the Sveconorwegian orogeny

TRACKING THE P–T PATH OF PRECAMBRIAN ECLOGITE USING PSEUDOSECTION, Ti-IN-QUARTZ AND Zr-IN-RUTILE THERMOBAROMETRY

International audienceSveconorwegian eclogite occur as a nappe within the high-grade metamorphic region in southernSweden, which constitutes a window into the deepest part of this Precambrian mountain belt. Distinctmicrostructural domains (i.e., garnet core, garnet rim, and matrix) in a Fe-Ti-rich eclogite variety containabundant quartz, rutile and zircon.A pseudosection approach was first applied and compared to results from a combination of Zr-in-rutileand Ti-in-quartz. The pressure input used for both thermometers was first deduced for each microstructuraldomain from the pseudosection. For the garnet core, Zr-in-rutile yields temperatures of 700-715°C and Ti-inquartz~ 635°C at 7 kbar. For the garnet rim, temperatures of 760-790°C (Zr-in-rutile) and 740-890°C (Ti-inquartz)at 12-18 kbar were calculated. Matrix rutile recorded temperatures of ~ 810°C, while quartz recordedtemperatures up to ~ 890°C. Additionally, direct combination of Ti content in quartz and Zr content in rutileisopleths (i.e., independent from the pseudosection) yield a prograde path in nearly perfect agreement withthe one deduced from the pseudosection.The pseudosection shows that rutile was produced by continuous breakdown of ilmenite during the earlystages of prograde metamorphism, a reaction that ran to completion at ~ 730°C. Most rutile grains in garnetrim and matrix are interpreted to subsequently form by recrystallization of smaller matrix grains. However,they generally do not record the peak-P temperatures and instead range mostly between 775 and 815°C,interpreted as a result of more efficient recrystallization during a dehydration reaction (progressivereplacement of hornblende by clinopyroxene).This study illustrates that both Zr-in-rutile and Ti-in-quartz thermometry cannot only robustly constrain aprograde evolution, but when combined with a pseudosection model can also yield information onrecrystallization processes. In fact, the combination of these three methods provides an unrivalled tool forpetrologic interpretation.The variation in Ti concentration in quartz is small regardless of crystal size. This P–T path reach veryhigh temperatures (up to 875°C) with a high dP/dT ratio, both during prograde and retrograde histories. Thesteep P–T path, together with preservation of garnet growth zoning, symplectitic textures and the lack ofsignificant Ti diffusion in quartz is consistent with a short residence time at high-temperature, implyingunusual fast burial and exhumation of the eclogite-bearing nappe

Zircon U–Pb-Hf isotope data in eclogite and metagabbro from southern Sweden reveal a common long-lived evolution and enriched source

Several orogenies have shaped the bedrock of southern Sweden. While mafic intrusions represent significant sources of information for reconstructing geodynamics and crustal evolution, the characterization of the various generations of such intrusions in Sweden remains limited. We report in situ zircon U–Pb ages and Hf isotope data from a Fe-Ti eclogite and a coronitic metagabbro from the Eastern Segment in southern Sweden. Crystallisation ages at 1683 ± 17 Ma of the eclogite suggest affiliation with the surrounding 1730–1660 Ma Transscandinavian Igneous Belt intrusions that dominate the Eastern Segment. Secondary zircon growth and Pb-loss in the eclogite sample at 1459 ± 44 Ma and the crystallisation of the metagabbro at 1431 ± 26 Ma overlap and are related to magmatic activity during the Hallandian orogeny. Zircon Hf isotope signatures with chondritic and sub-chondritic values at ~1683 Ma and ~1431 Ma, respectively, correspond to an enriched (or mildly depleted) source in line with a “Mixed Svecofennian Crustal Reservoir”. These isotope signatures are more enriched than those in the surrounding gneisses. Zircon isotope data from the herein analysed zircon grains indicate that the eclogite and metagabbro had an enriched mafic source in the mid to lower crust, or within the subcontinental lithospheric mantle below Fennoscandia

P–T evolution of Precambrian eclogite in the Sveconorwegian orogen, SW Sweden

International audienceConditions of the prograde, peak-pressure and part of the decompressional P–T path of two Precambrian eclogites in the eastern Sveconorwegian orogen have been determined using the pseudosection approach. Cores of garnet from a Fe-Ti-rich eclogite sample record a first prograde and syn-deformational stage along a Barrovian geothermal gradient from ~670 °C and 7 kbar to 710 °C and 8.5 kbar. Garnet rims grew during further burial to 16.5-19 kbar at ~850-900 °C, along a steep dP/dT gradient. The pseudosection model of a kyanite-bearing eclogite sample of more magnesian bulk composition confirms the peak conditions. Matrix reequilibration associated with subsequent near-isothermal decompression and partial exhumation produced plagioclase-bearing symplectites replacing kyanite and clinopyroxene and is estimated at 850-870 °C and 10-11 kbar. The validity of the pseudosections is discussed in detail. It is shown that in pseudosection modelling the fractionation of FeO in accessory sulphides may cause a significant shift of field boundaries (here displaced by up to 1.5 kbar and 70 °C) and must not be neglected. Fast burial, exhumation and subsequent cooling are supported by the steepness of both the prograde and the decompressional P–T paths as well as the preservation of garnet growth zoning and the symplectitic reaction textures. These features are compatible with deep tectonic burial of the eclogite-bearing continental crust as part of the underthrusting plate (Eastern Segment, continent Baltica) in a collisional setting that led to an effectively doubled crustal thickness and subsequent exhumation of the eclogites through tectonic extrusion. Our results are in accordance with regional structural and petrological relationships, which demonstrate foreland-vergent partial exhumation of the eclogite-bearing nappe along a basal thrust zone and support a major collisional stage at c. 1 Ga. We argue that the similarities between Sveconorwegian and Himalayan eclogite occurrences emphasize the modern style of Grenvillian-aged tectonics

High-temperature deformation in the basal shear zone of an eclogite-bearing fold nappe, Sveconorwegian orogen, Sweden

Ductile shear zones associated with emplacement of high-pressure nappes are key features to resolve exhumation mechanisms. The Eastern Segment of the Sveconorwegian orogen hosts an eclogite-bearing fold nappe, whose basal shear zone shows structures associated with emplacement of the eclogite-bearing nappe and decompression under high to intermediate pressure granulite and upper amphibolite fades conditions. Based on detailed structural mapping of a 4 km well-exposed section of the basal shear zone, we describe two major phases of deformation. An early deformation stage (D1) formed a penetrative gneissic foliation and tectonic layering, including isoclinal folds (F1). The sequence was subsequently affected by up to km-scale tight south-vergent folds (F2) with sheared out limbs. At the outcrop scale, asymmetric F2 folds are commonly S-vergent, but symmetric folds with different degrees of tightness are also present. Melt was present at all stages of deformation and the structural relations demonstrate mutual feedback between melt localization and fold formation. F2 folds have shallowly E-plunging fold axis parallel to a stretching lineation defined by high-grade mineral aggregates. Both constitute prominent structures of the basal shear zone. F2-folds are associated with an axial planar fabric (S2), defined by upper-amphibolite- and locally granulite-facies mineral assemblages. D2 shear structures are associated with top-to-the-east kinematic indicators throughout the section. The D1 episode was responsible for emplacement of the eclogite-nappe into its present structural position. The subsequently developed lineation-parallel folds are interpreted to form by general shear, where the structures reflect the regional E-directed flow of the entire eclogite-nappe. (C) 2015 Elsevier B.V. All rights reserved

Unravelling a HP-HT progrademetamorphic history from quartzand rutile inclusions in garnet

International audienceTrace-element compositions of mineral inclusions ingarnet (e.g., quartz, zircon and rutile) are commonly used toprovide thermo(baro)metric estimates. However, thesystematics of the thermometric equilibria in these minerals athigh pressure and temperature are not fully understood. Toinvestigate this, we analyzed in-situ quartz and rutileinclusions in a Fe-Ti-rich eclogite from the Sveconorwegianorogen in southern Sweden by SIMS. Abundant quartz, rutileand zircon in distinct microstructural sites (garnet core, garnetrims and matrix) make this sample particularly suitable forthis study.A P–T path, peaking at 16.5–19 kbar and 850–900 °C hasbeen deduced for this rock by pseudosection modelling; inputpressures from this model were used for trace elementthermometry of each garnet microtextural domain. For garnetcores, Zr-in-rutile yields 700–715 °C and Ti-in-quartz 620–640 °C at 7 kbar. For garnet rims, temperature estimates are760–790 °C (Zr-in-rutile) and 740–890 °C (Ti-in-quartz) at12–18 kbar. Finally, matrix rutile records 775–800 °C andlocally ~900 °C, and quartz records temperatures up to ~890°C. Alongside, direct combination of Ti in quartz and Zr inrutile isopleths yields a prograde path that is nearly identicalto that deduced from the pseudosection.The pseudosection shows that rutile was produced bycontinuous breakdown of ilmenite during the early stages ofprograde metamorphism – a reaction that was completed at~730 °C. Thereafter, rutile grains in the garnet rim and thematrix grew larger by recrystallization. However, these rutilegrains generally do not record the peak-P temperatures, butinstead yield 775–815 °C. This is interpreted to reflectrecrystallization associated with a major progradedehydration reaction in the rock, involving continuousbreakdown of hornblende and production of clinopyroxene.This study illustrates that Zr-in-rutile and Ti-in-quartzthermobarometry can robustly constrain prograde P–Tconditions and also yield important insights onrecrystallization processes at high temperatures. Thecombination of these methods and integration of the resultswith pseudosection modelling provides a versatile tool forinvestigating the petrologic history of high-grade rocks

Tracking the P–T path of Precambrian eclogite using pseudosection modelling, Ti-in-quartz and Zr-in-rutile thermometry

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Rapid, paced metamorphism of blueschists (Syros, Greece) from laser-based zoned Lu-Hf garnet chronology and LA-ICPMS trace element mapping

Unravelling the timing and rate of subduction-zone metamorphism and H2O release requires linking time to the compositional changes of petrogenetic indicator minerals in blueschists and eclogites. Garnet is a key mineral in this regard, as it forms by, and records, H2O-releasing mineral reactions that can be linked to pressure and temperature conditions. These can be placed in time using chronology. Bulk-or multi-grain ages are the norm in garnet chronology. While these ages provide important and precise time constraints on reactions across both facies, the duration, rates and style of growth???crucial parameters in the petrological and geochemical evolution of subducting oceanic crust???remain unconstrained. Domain dating, i.e., dating of individual growth zones, is necessary to investigate these aspects, but is generally impeded in ???common???-sized garnet grains due to sample size requirements. To overcome these limitations, we use a low-loss micro-sampling by laser cutting combined with Lu-Hf chronology. We used this approach to precisely date multiple growth zones in a 1-cm garnet grain from glaucophane-bearing micaceous schist of the Cycladic Blueschist Unit (CBU), Syros Island, Greece. The analysis was combined with major- and trace-element mapping (EPMA, LA-ICPMS) to investigate garnet petrogenesis. Garnet core and mantle zones are chemically comparable and identical in age within 0.1 Myr precision (2??), indicating an initial growth pulse at 51.8 ?? 0.1 Ma (MSWD = 1.13). The two rim zones, which are chemically distinct, were resolvably younger at 51.3 ?? 0.2 Ma (MSWD = 0.67). Integrated with compositional data, these age constraints provide a time-resolved record of extremely fast, pulsed garnet growth and fluidrelease reactions, such as chloritoid breakdown. These reactions can be considered near-instantaneous on subduction time scales, indicating that re-equilibration far outpaces changes in pressure and temperature conditions. The analysed sample is associated with the upper-subunit of the CBU with oceanic affinities, which does not share the last part of the prograde history with the rest of the CBU, i.e., the margin sub-unit. We interpret our sample as a sliver that became attached to the base of the ophiolitic sub-unit and was thrusted onto the lower margin unit at high-pressure. Beyond providing insights into the timing of subduction-zone processes, our new protocol for zoned garnet Lu-Hf geochronology of ???common-sized??? garnet opens new possibilities for constraining the causes and rates of garnet growth and the pace of tectonic processes in general

The value of open-source clinical science in pandemic response: lessons from ISARIC

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