scholarly journals Blueschist mylonitic zones accommodating syn-subduction exhumation of deeply buried continental crust: the example of the Rocca Canavese Thrust Sheets Unit (Sesia–Lanzo Zone, Italian Western Alps)

2021 ◽  
Vol 114 (1) ◽  
Author(s):  
Manuel Roda ◽  
Michele Zucali ◽  
Luca Corti ◽  
Roberto Visalli ◽  
Gaetano Ortolano ◽  
...  
Keyword(s):  
Microstructural Analysis ◽  
Oceanic Lithosphere ◽  
Western Alps ◽  
White Mica ◽  
X Ray ◽  
Metamorphic History ◽  
Thrust Sheets ◽  
History Of ◽  
Tectonic Contact ◽  
Elemental Maps

AbstractThe Rocca Canavese Thrust Sheets Unit (RCTU) is a subduction-related mélange that represents the eastern-most complex of the Sesia–Lanzo Zone (SLZ), bounded by the Periadriatic (Canavese) Lineament that separates the Alpine subduction complex from the Southalpine domain. The RCTU is limited to the south by the Lanzo Massif (LM) and to the east by the Eclogitic Micaschists Complex (EMC). Particularly the tectonic contact area of the RCTU, adjacent to the neighbouring SLZ and the LM is characterised by a 100–200-m-thick mylonitic to ultra-mylonitic zone (MZ) that was active under blueschist-to greenschist-facies conditions. Despite the dominant mylonitic structure, some rocks (garnet-bearing gneiss, garnet-free gneiss and orthogneiss) still preserve pre-mylonitic parageneses in meter-sized domains. The scarcity of superposed structures and the small size of relicts impose a detailed microstructural analysis supported by chemical investigation to reconstruct the tectono-metamorphic history of the MZ. Therefore, we integrated the classical meso- and microstructural analysis approach with a novel quantitative technique based on the Quantitative X-Ray Map Analyzer (Q-XRMA), used to classify rock-forming minerals starting from an array of X-ray elemental maps, both at whole thin section and micro-domain scale, as well as to calibrate the maps for pixel-based chemical analysis and end-member component maps, relevant for a more robust conventional geothermobarometer application as well for calculating reliable PT pseudosections. Pre-Alpine relicts are garnet and white mica porphyroclasts in the garnet-bearing gneiss and biotite and K-feldspar porphyroclasts in garnet-free gneiss and orthogneiss, respectively, providing no PT constraints. The Alpine evolution of the MZ rocks, has been subdivided in three deformation and metamorphic stages. The first Alpine structural and metamorphic equilibration stage (D1 event) occurred at a pressure of ca. 1.25–1.4 GPa and at a temperature of ca. 420–510 °C, i.e. under blueschist-facies conditions. The D2 event, characterised by a mylonitic foliation that is pervasive in the MZ, occurred at ca. 0.95–1.1 GPa and ca. 380–500 °C, i.e. under epidote-blueschist-facies conditions. The D2 PT conditions in the MZ rocks are similar to those predicted for the blocks that constitute the RCTU mélange, and they overlap with the exhumation paths of the EMC and LM units. Therefore, the RCTU, EMC and LM rocks became coupled together during the D2 event. This coupling occurred during the exhumation of the different tectono-metamorphic units belonging to both continental and oceanic lithosphere and under a relatively cold thermal regime, typical for an active oceanic subduction zone, pre-dating Alpine continental collision.

Formation and evolution of a subduction-related mélange: The example of the Rocca Canavese Thrust Sheets (Western Alps)

2019 ◽  
Vol 132 (3-4) ◽  
pp. 884-896 ◽  
Author(s):  
Manuel Roda ◽  
Michele Zucali ◽  
Alessandro Regorda ◽  
Maria Iole Spalla
Keyword(s):  
Numerical Model ◽  
Mantle Wedge ◽  
Western Alps ◽  
Metamorphic History ◽  
Orogenic Wedge ◽  
Thrust Sheets ◽  
History Of ◽  
Natural Data ◽  
Formation And Evolution ◽  
Subduction Wedge

Abstract In the Sesia-Lanzo Zone, Western Alps, the Rocca Canavese Thrust Sheets (RCT) subunit is characterized by a mixture of mantle- and crust-derived lithologies, such as metapelites, metagranitoids, metabasics, and serpentinized mantle slices with sizes ranging from meters to hundreds of meters. Structural and metamorphic history suggests that the RCT rocks experienced a complex evolution. In particular, two different peak conditions were obtained for the metabasics, representing different tectono-metamorphic units (TMUs), namely, D1a under eclogite facies conditions and D1b under lawsonite-blueschist-facies conditions. The two TMUs were coupled during the syn-D2 exhumation stage under epidote-blueschist-facies conditions. The different rocks and metamorphic evolutions and the abundance of serpentinites in the tectonic mixture suggest a possible subduction-related mélange origin for the RCT. To verify whether a subduction-related mélange can record tectono-metamorphic histories similar to that inferred for the RCT, we compare the pressure-temperature evolutions with the results of a 2-D numerical model of ocean-continent subduction with mantle wedge serpentinization. The predictions of the numerical model fully reproduce the two peak conditions (D1a and D1b) and the successive exhumation history of the two TMUs within the subduction wedge. The degree of mixing estimated from field data is consistent with that predicted by the numerical simulation. Finally, the present-day location of the RCT, which marks the boundary between the orogenic wedge (Penninic and Austroalpine domains) and the southern hinterland (Southalpine domain) of the Alpine chain, is reproduced by the model at the end of the exhumation in the subduction wedge. Therefore, the comparison between natural data and the model results confirms the interpretation of the RCT as a subduction-related mélange that occurred during exhumation within a serpentinized mantle wedge.

Amphibolite associated with the Thetford Mines Ophiolite Complex at Belmina Ridge, Quebec

1981 ◽  
Vol 18 (12) ◽  
pp. 1878-1892 ◽  
Author(s):  
Tomas Feininger
Keyword(s):  
High Temperature ◽  
Dynamic Environment ◽  
Oceanic Lithosphere ◽  
The West ◽  
Ophiolite Complex ◽  
Fine Grained ◽  
Metamorphic History ◽  
History Of ◽  
Tectonic Contact ◽  
Coarse To Fine

Amphibolite in the Quebec Appalachians at Belmina Ridge (45°58′N; 71°28′W) constitutes a steeply dipping, allochthonous sheet as much as 800 m thick, between the obducted Thetford Mines Ophiolite Complex to the east and rocks of the allochthonous Caldwell Group to the west. The amphibolite, which structurally and chemically is unlike amphibolite in the adjacent Caldwell, is fresh, coarse to fine grained, and massive to layered. It is composed chiefly of edenitic to pargasitic amphibole, pyrope- and grossular-rich almandine, salite, and epidote, with small amounts of quartz, plagioclase, and accessory minerals. Metamorphism ranged from [Formula: see text] at the base of the sheet to T ~ 780 °C at the top, near the tectonic contact with rocks of the superjacent ophiolite complex, under a Barrovian facies series with Ptotal between 5 and 7 kbars (500–700 MPa).The amphibolite is interpreted to have been derived from ocean-floor basalt that was metamorphosed under a 20 km thick, westward-directed, obducted allochthon of oceanic lithosphere. With a thermal gradient of 40 °C∙km−1, the temperature at the base of the allochthon would have been 800 °C, whereas the temperature at the base of the Thetford Mines Ophiolite Complex (the uppermost third of the allochthon) would have been 312 °C or less, in agreement with the serpentinization history of the complex. The dynamic environment during obduction could continually furnish "fresh" (hot) lithosphere to override the site of metamorphism of the amphibolite. At the site, the temperature of the base of the allochthon rose progressively, and a relatively high temperature was maintained for ample time, of the order of 1 Ma, to have continuously heated the underlying rocks to raise them to a high temperature without the allochthon ever having been unrealistically hot. The allochthon was dismembered tectonically toward the close of obduction. The amphibolite was thrust westward and upward, to be emplaced at relatively little depth on rocks of the Caldwell Group, and under the ophiolite complex. The petrogenesis proposed for the amphibolite at Belmina Ridge is consistent with the apparently simple metamorphic history of the rock.

The metamorphic history of the concealed Caledonides of eastern England and their foreland

1993 ◽  
Vol 130 (5) ◽  
pp. 613-620 ◽  
Author(s):  
R. J. Merriman ◽  
T. C. Pharaoh ◽  
N. H. Woodcock ◽  
P. Daly
Keyword(s):  
Lower Devonian ◽  
Regional Metamorphism ◽  
White Mica ◽  
Fold Belt ◽  
Illite Crystallinity ◽  
Metamorphic History ◽  
Diagenetic Alteration ◽  
History Of ◽  
Volcaniclastic Rocks ◽  
Early Palaeozoic

AbstractWhite mica (illite) crystallinity data, derived mostly from borehole samples, have been used to generate a contoured metamorphic map of the concealed Caledonide fold belt of eastern England and the foreland formed by the Midlands Microcraton. The northern subcrop of the fold belt is characterized by epizonal phyllites and quartzites of possible Cambrian age, whereas anchizonal grades characterize Silurian to Lower Devonian strata of the Anglian Basin in the southern subcrop of the fold belt. Regional metamorphism in the Anglian Basin resulted from deep burial and Acadian deformation beneath a possible overburden of 7 km, assuming a metamorphic field gradient of 36 °C km-1. Late Proterozoic volcaniclastic rocks forming the basement of the microcraton show anchizonal to epizonal grades that probably developed during late Avalonian metamorphism. Cambrian to Tremadoc strata, showing late diagenetic alteration, rest on the basement with varying degrees of metamorphic discordance. During early Palaeozoic times, much of the microcraton was a region of slow subsidence with overburden thicknesses of 3.3–5.5 km. However, concealed Tremadoc strata in the northeast of the microcraton reach anchizonal grades and may have been buried to depths of 7 km beneath an overburden of uncertain age.

scholarly journals Correlation of Elastic Moduli and Serpentine Content in Ultramafic Rocks

2019 ◽  
Author(s):  
Aida Farough ◽  
Alexander Karrasch
Keyword(s):  
Physical Properties ◽  
Lower Crust ◽  
Elastic Moduli ◽  
Oceanic Lithosphere ◽  
Ultramafic Rocks ◽  
Seismic Velocities ◽  
Rock Composition ◽  
Metamorphic History ◽  
History Of ◽  
Thin Crust

Understanding the physical properties of ultramafic rocks is important for evaluating awide variety of petrologic models of the oceanic lithosphere, particularly upper mantle and lower crust. Hydration of oceanic peridotites results in increasing serpentine content, which affects lithospheric physical properties and the global bio/geochemical cycles of various elements. In understanding tectonic, magmatic and metamorphic history of the oceanic crust, interpreting seismic velocities, rock composition and elastic moduli are of fundamental importance. In this study we show that as serpentine content increases, density decreases linearly with a slope of 7.85. We also correlate increase in serpentine content with a linear decline in shear, bulk and Young’s moduli with slopes of 0.48, 0.77, 0.45 respectively. Our results show that increase in serpentine content of lower crust and forearc mantle could decrease elasticity of lithospehere and result in break-offs. Therefore tectonic processes at peridotite rich slow spreading ridges may be strongly affected by serpentine content, particularly serpentinization may be responsible for discontinuities in thin crust, and formation of weak fault zones.

XII.—Petrology and Petrogenesis of some Garnetiferous Peridotites

1963 ◽  
Vol 65 (12) ◽  
pp. 251-314 ◽  
Author(s):  
M. J. O'Hara ◽  
F. H. Stewart
Keyword(s):  
South Africa ◽  
Crystalline Rocks ◽  
Chemical Analyses ◽  
X Ray ◽  
Metamorphic History ◽  
Igneous Origin ◽  
Mineral Assemblages ◽  
History Of ◽  
Reliable Interpretation ◽  
Recent Experimental Work

SynopsisGarnetiferous peridotite masses which occur among gneisses in the Tafjord district of Norway are cold intrusions emplaced as crystalline rocks late in the metamorphic history of the region. No relationship other than similarity of mineral facies can be established between the peridotites and the eclogite bodies in the surrounding gneiss. Chemical analyses of twelve olivines, fourteen orthopyroxenes, nine chrome diopsides, nine garnets, three amphiboles and two rocks are presented, representing material from the Tafjord and Almklovdalen districts of Norway, a garnetiferous peridotite mass near Bellinzona, Switzerland, and the garnet-peridotite inclusions in the kimberlite pipes of South Africa. Optical and X-ray data for the analyzed and some unanalyzed assemblages are also presented. The mineral assemblages of these rocks are compared with each other, and with data from the peridotites of layered tholeiitic intrusions, alpine-type peridotites and the peridotite nodules in basalts. The garnet-peridotites of Norway, Switzerland and South Africa are believed to be little altered fragments of the mantle, whereas the peridotite nodules in basalts and the alpine-type peridotites are believed to be of igneous origin. Examination of the distribution of cations between the coexisting phases suggests that there are too many variables to permit a reliable interpretation of the results.The orthopyroxenes from the garnetiferous peridotites are not rich in A1203, contrary to expectations based upon recent experimental work.

Microstructural shear bands in mylonites dated using muscovite sub-spectra from high-definition ultra-high-vacuum (UHV) argon diffusion experiments with phengitic white mica

2020 ◽  
Author(s):  
Marnie Forster ◽  
Ruoran Nie ◽  
Sonia Yeung ◽  
Gordon Lister
Keyword(s):  
High Vacuum ◽  
Microstructural Analysis ◽  
Shear Bands ◽  
Shear Zones ◽  
White Mica ◽  
Ultra High Vacuum ◽  
High Definition ◽  
Thin Sections ◽  
History Of ◽  
Ar Geochronology

<p>With excellent outcrop, the eclogite-blueschist belt exposed in the Cycladic archipelago in the Aegean Sea, Greece, offers a spectacular natural laboratory in which to decipher the structural geology of a highly extended orogenic belt and to ascertain the history of the different fabrics and microstructures that can be observed. Using phengitic white mica we demonstrate a robust correlation of age with microstructure, once again dispelling the myth that <sup>40</sup>Ar/<sup>39</sup>Ar geochronology using this mineral, produces cooling ages alone.</p><p>Further, we show that high-definition ultra-high-vacuum (UHV) <sup>39</sup>Ar diffusion experiments using phengitic white mica routinely allow the extraction of muscovite sub-spectra in the first 10-30% of <sup>39</sup>Ar gas release during <sup>40</sup>Ar/<sup>39</sup>Ar geochronology. The muscovite sub-spectrum is distinct and separate to the main spectrum which is dominated by mixing of gas released from phengite as well as muscovite. The muscovite sub-spectra allow consistent estimates of the timing of the formation of microstructural shear bands in various mylonites, as well as allowing quantitative estimates of temperature variation with time during the cooling history of the eclogite-blueschist belt. Our new data reveals hitherto unsuspected variation in the timing of exhumation of individual slices of the eclogite-blueschist belt, caused by Eocene and Miocene detachment-related shear zones.</p><p>This study thus illustrates a new method for the quantitative determination of the timing of movement in mylonites and/or in strongly stretched metamorphic tectonites. Shear bands formed in such structures are rarely coarsely crystalline enough to allow mineral grains that can be individually dated using laser spot analysis. Where phengitic white mica is involved, interlaying is usually so fine as to preclude the application of laser methods. In any case, laser methods do not have the capability of extracting exact and detailed age-temperature spectra, and can never achieve the definition of the multitudinous steps of the age spectrum evident from our high-definition UHV diffusion experiments.</p><p>Previous work in the Cycladic eclogite-blueschist belt has incorrectly assumed that the diffusion parameters for phengitic white mica were the same as for muscovite. Arrhenius data suggest this is not the case, and that phengitic white mica is considerably more retentive of argon than muscovite. Previous workers have also erred in dismissing microstructural variation in age as an artefact, supposedly as the result of the incorporation of excess argon. This has led to inconsistencies in interpretation, because phengite is able to retain argon at temperatures that exceed those estimated using metamorphic mineral parageneses. In consequence, we discover a robust correlation between microstructure and age, even down to the detail present in complex tectonic sequence diagrams produced during fabric and microstructural analysis of individual thin-sections.</p><p>A critical factor is that the recognition of muscovite sub-spectra requires Arrhenius data in order to recognise the steps dominated by release of <sup>39</sup>Ar from muscovite. In turn this requires precise measurement of temperature during each heating step. To apply percentage-release formula for the estimation of diffusivity, there must be a sharp rise to the temperature in question, then that temperature must be maintained at a constant value, then dropped sharply to relatively low values.</p>

Geochronologic and thermobarometric constraints on the metamorphic history of the Fairbanks Mining District, western Yukon-Tanana terrane, Alaska

2002 ◽  
Vol 39 (7) ◽  
pp. 1107-1126 ◽  
Author(s):  
Thomas A Douglas ◽  
Paul W Layer ◽  
Rainer J Newberry ◽  
Mary J Keskinen
Keyword(s):  
Thrust Fault ◽  
White Mica ◽  
Mining District ◽  
East Central ◽  
Diamond Drill ◽  
Metamorphic History ◽  
Structural History ◽  
History Of ◽  
Radiogenic Argon ◽  
Single Grain

This study presents new petrologic and thermochronologic information from the Fairbanks district of east central Alaska that indicate a complex metamorphic and structural history for the western Yukon–Tanana terrane. Garnet–biotite and garnet–pyroxene thermometry and jadeite barometry yield prograde temperatures and pressures for the Chatanika eclogite (523°C, 14–15 kbar (1 kbar = 100 MPa)). Cooling from peak eclogitization is estimated from 40Ar/39Ar single grain geochronology at ~210–180 Ma. Secondary white mica ages of 140–115 Ma along the fault contact between eclogite and underlying lower amphibolite-facies rocks constrain the age of the event that placed the Chatanika eclogite over the Fairbanks schist. Based on observations from field mapping and diamond drill samples, we interpret this structural contact as a thrust fault. Garnet–biotite mineral pairs are reset by as much as 200°C within this fault zone. Biotite and white mica ages of ~100–110 Ma, combined with Jurassic amphibole ages in Fairbanks schist samples, indicate the Fairbanks schist and Chatanika eclogite cooled through biotite and white mica argon closure temperatures in the early Cretaceous. Intrusion of mid-Cretaceous, calc-alkalic, gold-related granitic plutons in the Fairbanks district are evidenced by loss of radiogenic argon in many of the 40Ar/39Ar age fractions. Eocene basalt is visible in six widely separated localities within the eastern part of the Fairbanks district. However, the pervasiveness of a 50 Ma resetting event in samples as far as 30 km from present day basalt localities indicates the Eocene flows were either deposited throughout the Fairbanks area or are associated with large plutons at depth.

scholarly journals Use of thermal modeling to assess the tectono-metamorphic history of the Lugo and Sanabria gneiss domes, Northwest Iberia

2009 ◽  
Vol 180 (3) ◽  
pp. 179-197 ◽  
Author(s):  
James E. Alcock ◽  
José R. Martínez Catalán ◽  
Ricardo Arenas ◽  
Alejandro Díez Montes
Keyword(s):  
Thermal Relaxation ◽  
Thermal Response ◽  
Crustal Thickening ◽  
Gneiss Domes ◽  
Metamorphic History ◽  
Thrust Sheets ◽  
History Of ◽  
Granite Magmatism ◽  
Geochronological Evidence ◽  
Northwest Iberia

Abstract The Lugo and Sanabria domes in Northwest Iberia have well constrained metamorphic and structural histories. Both occur in the Iberian autochthon and resulted from late-Variscan extensional collapse following crustal thickening related to the Variscan collision. The two domes developed beneath large thrust sheets, are cored by sillimanite-orthoclase anatectic gneiss, preserve evidence of a steep thermal gradient (≈ 1 °C MPa−1), and exhibit a distinct decrease in metamorphic grade to the east in the direction of nappe movement. Geochronological evidence indicates that the lower crust melted within ≈ 30 Ma of initial crustal thickening and that dome formation occurred within 50 Ma. The histories of the two domes are considered as the basis for one-dimensional finite-difference models of thermal response to changes in crustal thickness. Results from thermal models suggest that thickening was limited to the crust, provide a numeric explanation for timing and nature of granite magmatism, and indicate that high-temperature metamorphism and crustal anatexis may result directly from thermal relaxation, eliminating the need for significant mantle thermal contribution. Also, the models show that small differences in thickness of large, wedge-shaped thrust sheets can explain distinct P-T paths experienced by different limbs of the domes.

scholarly journals Quantitative X-ray Maps Analaysis of Composition and Microstructure of Permian High-Temperature Relicts in Acidic Rocks from the Sesia-Lanzo Zone Eclogitic Continental Crust, Western Alps

Minerals ◽  
2021 ◽  
Vol 11 (12) ◽  
pp. 1421
Author(s):  
Michele Zucali ◽  
Luca Corti ◽  
Manuel Roda ◽  
Gaetano Ortolano ◽  
Roberto Visalli ◽  
...  
Keyword(s):  
Continental Crust ◽  
Reactive Systems ◽  
Western Alps ◽  
Multivariate Statistical ◽  
X Ray ◽  
Phase Classification ◽  
Three Samples ◽  
Different Types ◽  
Elemental Maps ◽  
Sequential Phase

Three samples of meta-acidic rocks with pre-Alpine metamorphic relicts from the Sesia-Lanzo Zone eclogitic continental crust were investigated using stepwise controlled elemental maps by means of the Quantitative X-ray Maps Analyzer (Q-XRMA). Samples were chosen with the aim of analysing the reacting zones along the boundaries between the pre-Alpine and Alpine mineral phases, which developed in low chemically reactive systems. The quantitative data treatment of the X-ray images was based on a former multivariate statistical analytical stage followed by a sequential phase and sub-phase classification and permitted to isolate and to quantitatively investigate the local paragenetic equilibria. The parageneses thus observed were interpreted as related to the pre-Alpine metamorphic or magmatic stages as well as to local Alpine re-equilibrations. On the basis of electron microprobe analysis, specific compositional ranges were defined in micro-domains of the relict and new paragenetic equilibria. In this way calibrated compositional maps were obtained and used to contour different types of reacting boundaries between adjacent solid solution phases. The pre-Alpine and Alpine mineral parageneses thus obtained allowed to perform geothermobarometry on a statistically meaningful and reliable dataset. In general, metamorphic temperatures cluster at 600–700 ∘C and 450–550 ∘C, with lower temperatures referred to a retrograde metamorphic re-equilibration. In all the cases described, pre-Alpine parageneses were overprinted by an Alpine metamorphic mineral assemblage. Pressure-temperature estimates of the Alpine stage averagely range between 420 to 550 ∘C and 12 to 16.5 kbar. The PT constraints permitted to better define the pre-Alpine metamorphic scenario of the western Austroalpine sectors, as well as to better understand the influence of the pre-Alpine metamorphic inheritance on the forthcoming Alpine tectonic evolution.

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