scholarly journals Fluid-Present Partial Melting of Paleoproterozoic Okbang Amphibolite in the Yeongnam Massif, Korea

Lithosphere ◽  
2020 ◽  
Vol 2020 (1) ◽  
pp. 1-26
Author(s):  
Yuyoung Lee ◽  
Moonsup Cho
Keyword(s):  
Partial Melting ◽  
Shear Zones ◽  
Thin Layers ◽  
The North ◽  
Ductile Shear Zones ◽  
Ductile Shearing ◽  
Basaltic Melt ◽  
Yeongnam Massif ◽  
Migmatitic Gneiss ◽  
The Times

Abstract The waning stage of a long-lived collisional orogeny is commonly governed by an extensional regime in association with high-temperature metamorphism, anatexis, and magmatism. Such a late-orogenic process is well-recorded in the Okbang amphibolite, Yeongnam Massif, Korea, where thin layers or irregular patches of tonalitic leucosomes are widespread particularly in association with ductile shear zones. Various microstructures including interstitial felsic phases and former melt patches indicate that leucosomes are the product of partial melting. These leucosomes are aligned en echelon and contain large (up to ~2 cm) grains of peritectic hornblende, suggesting synkinematic fluid-present anatexis. The leucosomes are enriched in Na2O and Sr contents compared to the amphibolite but depleted in rare earth and high field-strength elements. P-T conditions of the anatexis were estimated at 4.6–5.2 kbar and 650–730°C, respectively, based on hornblende-plagioclase geothermobarometry. Sensitive high-resolution ion microprobe U-Pb analyses of zircon from an amphibolite and a leucosome sample yielded weighted mean 207Pb/206Pb ages of 1866±4 Ma and 1862±2 Ma, which are interpreted as the times for magmatic crystallization and subsequent anatexis of mafic protolith, respectively. The latter is consistent with the time of partial melting determined from a migmatitic gneiss and a biotite-sillimanite gneiss at 1861±4 Ma and 1860±9 Ma, respectively. The leucosomes are transected by an undeformed pegmatitic dyke dated at 1852±3 Ma; by this time, extensional ductile shearing has ceased. Initial εHft values of zircon from the amphibolite range from 4.2 to 6.0, suggesting juvenile derivation of basaltic melt from the mantle. In contrast, lower εHft values (–0.1 to 3.5) in leucosome zircons indicate a mixing of crust-derived melt. Taken together, the Okbang amphibolite has experienced synkinematic fluid-present melting during the waning stage of Paleoproterozoic hot orogenesis prevalent in the Yeongnam Massif as well as the North China Craton.

Strain-induced mineral growth in ductile shear zones and a preliminary study of ductile shearing in western Newfoundland

1988 ◽  
Vol 25 (12) ◽  
pp. 2118-2129 ◽  
Author(s):  
M. A. J. Piasecki
Keyword(s):  
Growth Characteristic ◽  
Shear Zones ◽  
The North ◽  
Mineral Growth ◽  
Inclined Surfaces ◽  
Ductile Shear Zones ◽  
Ductile Shearing ◽  
Preliminary Study ◽  
Ductile Shear ◽  
North And South

In the Fleur de Lys and the Central Gneiss terranes the presence of strain-induced mineral growth characteristic of ductile shear zones within zones of rocks with mylonitic fabrics indicates the existence of major belts of layer-parallel ductile shearing with complex evolutionary sequences. Kinematic markers in several of these shear belts indicate that shearing movements on initially probably gently inclined surfaces, directed not normal to the axial trend of the orogen but parallel to it, are tectonically important in western Newfoundland. The shear belts are in excess of 1 km thick, and one well-exposed example exhibits a pattern in which zones of the highest strain anastomose on the map scale.The base of the Fleur de Lys Supergroup is marked by one such major zone of shearing (décollement) in which kinematic indicators record movements directed to the north and to the south, before the regional attitude of the rocks was steepened. Along the Baie Verte Line, earlier north- and south-directed movements in the Fleur de Lys were succeeded by reverse movements towards the east, over the Dunnage Terrane.

A Discussion on global tectonics in Proterozoic times - Tectonics of the North Atlantic Proterozoic dyke swarm

1976 ◽  
Vol 280 (1298) ◽  
pp. 529-539 ◽  
Keyword(s):  
North Atlantic ◽  
Shear Zones ◽  
Dyke Swarm ◽  
Fracture System ◽  
West Greenland ◽  
The North ◽  
Ductile Shear Zones ◽  
The North Atlantic ◽  
Lateral Displacements ◽  
Global Tectonics

The Proterozoic North Atlantic dyke swarm occurs in Scotland, East and West Greenland, and Labrador, over an area of at least 250000 km2, and includes two dominant dyke sets which in West Greenland strike NNE-SSW, and ESE-WNW. The intrusive relations of the two sets, and their association with ductile shear zones and other lateral displacements of country rocks, show the dykes to represent a conjugate swarm emplaced along shear fractures, rather than along tensional openings. The mechanical behaviour of the Proterozoic lithosphere is considered in the context of the regional fracture system.

Structural analysis of ductile shear zones in the North Qinling Orogen and its implications for the evolution of the Proto-Tethys Ocean

2017 ◽  
Vol 52 ◽  
pp. 202-214 ◽  
Author(s):  
Shujuan Zhao ◽  
Sanzhong Li ◽  
Xiyao Li ◽  
Ian Somerville ◽  
Huahua Cao ◽  
...  
Keyword(s):  
Structural Analysis ◽  
Shear Zones ◽  
Qinling Orogen ◽  
The North ◽  
Ductile Shear Zones ◽  
North Qinling ◽  
Ductile Shear ◽  
Tethys Ocean

Transpression in an Archean greenstone belt, northern Minnesota

1988 ◽  
Vol 25 (7) ◽  
pp. 1060-1068 ◽  
Author(s):  
P. J. Hudleston ◽  
D. Schultz-Ela ◽  
D. L. Southwick
Keyword(s):  
Volcanic Rocks ◽  
Shear Bands ◽  
Shear Zones ◽  
The North ◽  
Strain Pattern ◽  
Ductile Shear Zones ◽  
Continuous Shear ◽  
Measured Strain ◽  
Archean Greenstone Belt ◽  
North And South

Weakly metamorphosed Archean sedimentary and volcanic rocks of the Vermilion district, northern Minnesota, occupy an east–west-trending belt between gneisses of the Vermilion granitic complex to the north and the Giants Range batholith to the south. All the measured strain, a foliation, and a mineral lineation in this belt are attributed to the "main" phase of deformation (D2). Foliation strikes parallel to the belt and dips steeply, and the mineral lineation plunges moderately to steeply east or west and is parallel to the maximum stretching direction, X, and subparallel to fold hinges. An earlier, possibly nappe-forming, event (D1) left little evidence of fabric in the Vermilion district.A number of features indicate that the D2 deformation involved a significant component of dextral strike-slip shear in addition to north–south compression. They include ductile shear zones with sigmoidal foliation patterns, shear bands, asymmetric pressure shadows, and the fact that the asymmetry of the F2 folds is predominantly Z. Other features are more simply explained by a deformation involving simple shear. The S2 cleavage is locally folded, and a new spaced cleavage developed in an orientation similar to that of the old cleavage away from the folds. We consider this the result of a process of continuous shear, with perturbations of flow resulting in folding of S2 and the development of a new foliation axial planar to the folds. The same type of perturbation can lead to the juxtaposition of zones of constrictional and flattening strains, a distinctive feature of the rocks of the Vermilion district otherwise hard to account for. The strain pattern requires a north–south component of shortening in addition to shear. The D2 deformation in the Vermilion district can therefore be characterized as one of transpression: oblique compression between two more rigid lithospheric blocks to the north and south.

Chapter 5 Paleoproterozoic (1.9–1.8 Ga), syn-orogenic magmatism and sedimentation in the Ljusdal lithotectonic unit, Svecokarelian orogen

2020 ◽  
Vol 50 (1) ◽  
pp. 131-153 ◽  
Author(s):  
Karin Högdahl ◽  
Stefan Bergman
Keyword(s):  
Variable Temperature ◽  
Volcanic Rocks ◽  
Shear Zones ◽  
Granulite Facies ◽  
Low Pressure ◽  
Metasedimentary Rocks ◽  
Ductile Shear Zones ◽  
Ductile Shearing ◽  
Pressure Medium ◽  
High Grade Metamorphism

AbstractDuctile shear zones with dextral transpressive deformation separate the Ljusdal lithotectonic unit from the neighbouring units (Bothnia–Skellefteå and Bergslagen) in the 2.0–1.8 Ga Svecokarelian orogen. Sedimentation steered by regional crustal extension at c. 1.86–1.83 Ga was sandwiched between two separate phases of ductile strain with crustal shortening and predominantly high-grade metamorphism with plutonic activity. Metamorphism occurred under low-pressure, medium- to high-temperature conditions that locally reached granulite facies. The earlier shortening event resulted in the accretion of outboard sedimentary and c. 1.89 Ga volcanic rocks (formed in back- or inter-arc basin and volcanic arc settings, respectively) to a continental margin. Fabric development (D1), the earlier phase of low-pressure and variable temperature metamorphism (M1) and the intrusion of a predominantly granitic to granodioritic batholith with rather high εNd values (the Ljusdal batholith) occurred along this active margin at 1.87–1.84 Ga. Thrusting with westerly vergence, regional folding and ductile shearing (D2–3), the later phase of low-pressure and variable temperature metamorphism (M2), and the subsequent minor shear-related intrusion of granite, again with relatively high εNd values, prevailed at 1.83–1.80 Ga. Mineral deposits include epithermal Au–Cu deposits hosted by supracrustal rocks, V–Fe–Ti mineralization in subordinate gabbro and norite bodies inside the Ljusdal batholith, and graphite in metasedimentary rocks.

Metamorphism and diachronous cooling in a contractional orogen: the Strandja Massif, NW Turkey

2011 ◽  
Vol 148 (4) ◽  
pp. 580-596 ◽  
Author(s):  
G. SUNAL ◽  
M. SATIR ◽  
B. A. NATAL'IN ◽  
G. TOPUZ ◽  
O. VONDERSCHMIDT
Keyword(s):  
Shear Zones ◽  
Structural Features ◽  
Amphibolite Facies ◽  
The South ◽  
The North ◽  
Ductile Shear Zones ◽  
Shear Sense ◽  
Nw Turkey ◽  
Northward Propagation ◽  
Peak Metamorphism

AbstractThe southern part of the Strandja Massif, northern Thrace, Turkey, comprises a basement of various gneisses, micaschists and rare amphibolite, and a cover of metaconglomerate and metasandstone, separated from each other by a pre-metamorphic unconformity. Metamorphic grade decreases from the epidote–amphibolite facies in the south to the albite–epidote–amphibolite/greenschist-facies transition in the north. Estimated P–T conditions are 485–530°C and 0.60–0.80 GPa in the epidote–amphibolite facies domain, and decrease towards the transitional domain between greenschist- and epidote–amphibolite facies. Rb–Sr muscovite ages range from 162.9 ± 1.6 Ma to 149.1 ± 2.1 Ma, and are significantly older (279–296 Ma) in the northernmost part of the study area. The Rb–Sr biotite ages decrease from 153.9 ± 1.5 Ma in the south to 134.4 ± 1.3 Ma in the north. These age values in conjunction with the attained temperatures suggest that the peak metamorphism occurred at around 160 Ma and cooling happened diachronously, and Rb–Sr muscovite ages were not reset during the metamorphism in the northernmost part. Structural features such as (i) consistent S-dipping foliation and SW to SE-plunging stretching lineation, (ii) top-to-the-N shear sense, and (iii) N-vergent ductile shear zones and brittle thrusts suggest a N-vergent compressional deformation coupled with exhumation. We tentatively ascribe this metamorphism and subsequent diachronous cooling to the northward propagation of a thrust slice. The compressional events in the Strandja Massif were most probably related to the coeval N-vergent subduction/collision system in the southerly lying Rhodope Massif.

Neoarchean tectonics: insight from the deformation of the Archean basement of North China Craton

2020 ◽  
Author(s):  
Yongjiang Liu ◽  
Jing Li ◽  
Weimin Li ◽  
Sanzhong Li ◽  
Liming Dai
Keyword(s):  
North China Craton ◽  
Tectonic Evolution ◽  
North China ◽  
Shear Zones ◽  
Banded Iron Formations ◽  
The North ◽  
Detailed Structural Analysis ◽  
Ductile Shear Zones ◽  
Ductile Shear ◽  
Iron Formations

<p>The controversy over the Archean tectonic regimes has lasted several decades focusing around horizontal and vertical tectonics, the two classical tectonic models for Archean times. Thus, more studies of the early crustal growth and tectonic evolution are requisite for better understanding geodynamic regimes in the early Precambrian. The North China Craton is one of the major Archean to Paleoproterozoic cratons in the world and oldest craton in China, which preserves a large amount of ancient basement and abundant structures showing the early earth tectonics.</p><p>In this study, we have carried out detailed structural analysis of two down-slip ductile shear zones which developed in eastern Anshan area and provided an example for revealing of Neoarchean vertical tectonics in the study area. There were also develop many structures of dome and keel style in the North China Craton, such as Qian ’an, Qingyuan areas.</p><p>Based on abundant structural evidence and previous studies, we infer that the vertical tectonics is still the dominant model for Neoarchean crust growth and tectonic evolution in Anshan area. The formation of dome and keel structure, and the deformation of the down-slip ductile shear zones may have resulted from the sagduction of the banded iron formations and synchronous Archean granite dome emplacement, supporting a vertical tectonic regime in Archean times.</p>

The timescale of the aseismic to seismic deformation in a cooling pluton: 40Ar-39Ar ages of the solid-state deformation in the Adamello (Southern Italian Alps)

2021 ◽  
Author(s):  
Silvia Mittempergher ◽  
Stefano Zanchetta ◽  
Federico Caldiroli ◽  
Andrea Bistacchi ◽  
Andrea Zanchi ◽  
...  
Keyword(s):  
Thermal Anomaly ◽  
Shear Zones ◽  
Wall Rock ◽  
Deformation Phase ◽  
Ductile Shear Zones ◽  
Ductile Shearing ◽  
Two Samples ◽  
Ductile Shear ◽  
Magma Pulses ◽  
Absolute Age

<p>The northern Adamello is crosscut by ductile shear zones and pseudotachylyte-bearing faults, both compatible with the same stress field, with ductile shear zones crosscut by brittle faults. These relations are coherent with the re-equilibration of the pluton-related thermal anomaly to temperatures typical of the base of the seismogenic continental crust (T = 250 – 300°). Our new <sup>40</sup>Ar-<sup>39</sup>Ar ages help to constrain the absolute age and duration of each deformation phase.</p><p>Samples included wall-rock biotite, bulk ultramylonites and pseudotchylytes. Before stepwise heating <sup>40</sup>Ar-<sup>39</sup>Ar measurements, samples were characterized by microstructural, geochemical and petrological analyses.</p><p>The wall-rock biotite is 33.4±0.1 Ma old, independently of grainsize. Mylonites feature complex age spectra between 28-31 Ma, including biotite and altered feldspar. Four pseudotachylyte matrices are clustered around 30-31.5 Ma, and two samples have 25-26 Ma ages.</p><p>Ductile shearing active 2 Ma after wall-rock emplacement indicates either low strain rates, or a long-lasting thermal anomaly, which might be due to high emplacement depth, and/or the progressive assemblage of adjacent plutons through small magma pulses. Seismogenic faulting overlaps with mylonitization around 31 Ma; younger pseudotachylyte ages may be due to late-stage reactivation.</p>

scholarly journals Evolution of the Palaeozoic basement of the North Patagonian Andes in the San Martín de los Andes area (Neuquén, Argentina): petrology, age and correlations

2018 ◽  
Vol 46 (1) ◽  
pp. 102 ◽  
Author(s):  
Samanta Serra-Varela ◽  
Pablo D. González ◽  
Raúl E. Giacosa ◽  
Nemesio Heredia ◽  
David Pedreira ◽  
...  
Keyword(s):  
Partial Melting ◽  
Melting Process ◽  
Shear Zones ◽  
Igneous Rocks ◽  
Low Grade ◽  
Metamorphic Evolution ◽  
Grade Metamorphism ◽  
The North ◽  
Plutonic Rocks ◽  
San Martín

In San Martín de los Andes area (Argentinian Patagonia) the Palaeozoic basement consists of metamorphic and plutonic rocks. The metamorphic rocks include strongly deformed schists, gneisses and migmatites. Their geochemical and petrographic characteristics suggest that the protholith could have been a sequence of pelites and greywackes. Detrital zircon analysis (U-Pb Q-ICP-LA-MS) yielded a maximum depositional age of 501±14 Ma (Series 3 Cambrian) for this sedimentary protolith. Metasedimentary rocks are affected by a regional foliation defined by the minerals of the metamorphic peak. This is a S2 foliation, since relics of a former foliation are present in some samples. This regional foliation is locally affected by open folds that develop an incipient crenulation cleavage (S3). The high-grade metamorphism includes partial melting processes, where the incipient segregates intrude parallel to the regional foliation and also cut it in presence of abundant melt. Zircons from anatectic granites formed during this partial melting process yielded a U-Pb Concordia age of 434.1±4.5 Ma (Llandovery-Wenlock, Silurian). The age of maximum sedimentation and the anatectic age constrain the metamorphic evolution of the basement into the lower Palaeozoic (between upper Cambrian and lower Silurian). The igneous rocks of the basement are granodiorites, tonalities, and some gabbros that cut the metamorphic basement and contain xenoliths and roof pendants from the country rocks. These plutonic rocks are affected by low-grade metamorphism, with the development of discrete, centimetric to hectometric, brittle-ductile shear zones. The age of these igneous rocks has been constrained through U-Pb zircons analysis, carried out by several authors between ca. 370 and 400 Ma (Devonian). The maximum sedimentation age for the protolith and its metamorphic evolution seems to be related to an early Palaeozoic orogenic event, probably the Patagonian Famatinian orogeny. In contrast, the Devonian igneous rocks of San Martín de los Andes could represent a Devonian magmatic arc, related to subduction processes developed at the beginning of the Gondwanan orogenic cycle, which culminates with the Gondwanan orogeny.

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