Metagabbronorite from DSDP hole 334: an example of high-temperature deformation and recrystallization near the Mid-Atlantic Ridge

1977 ◽  
Vol 14 (4) ◽  
pp. 886-898 ◽  
Author(s):  
Herwart Helmstaedt ◽  
John M. Allen
Keyword(s):  
Shear Zone ◽  
Regional Metamorphism ◽  
Shear Zones ◽  
Granulite Facies ◽  
High Temperature Deformation ◽  
Temperature Deformation ◽  
Mid Atlantic Ridge ◽  
Magmatic History ◽  
Highly Strained ◽  
Zoned Plagioclase

Five gabbros and three peridotites from hole 334 were examined for postmagmatic deformational changes and metamorphic recrystallization. The condition of the gabbros ranges from unaltered, to slightly altered and deformed, to highly deformed and metamorphosed, the latter type being confined to narrow shear zones. Petrofabric studies and electron microprobe analyses of mineral phases of a foliated metagabbronorite from such a shear zone suggest the following post-magmatic history: (1) strong plastic deformation and anhydrous synkinematic recrystallization of igneous plagioclase (An90−85), clinopyroxene, and orthopyroxene to the granulite facies mineral assemblage of strongly zoned plagioclase (An80−65), diopside, and orthopyroxene; relics of the igneous minerals survived as highly strained porphyroclasts; (2) synkinematic growth of amphibole consisting of a patchy intergrowth of tremolite and actinolitic hornblende; the amphibole fabric is compatible with strain estimates by the centre-point method; (3) mainly static hydrous alteration — growth of (?) vermiculite and sericitization.Penetrative deformation probably initiated shortly after intrusion and took place during cooling of the rocks from approximately 800 °C to 300 °C. The bulk of the strain in the metagabbronorite of the shear zone predates the serpentinization of the peridotites. Many textural and fabric features in alpine-type gabbro–peridotite complexes that are ascribed to regional metamorphism and deformation during orogenic emplacement may originate shortly after intrusion near or at an accreting plate margin.

scholarly journals Kinematics and Timing Constraints in a Transpressive Tectonic Regime: The Example of the Posada-Asinara Shear Zone (NE Sardinia, Italy)

Geosciences ◽  
2020 ◽  
Vol 10 (8) ◽  
pp. 288
Author(s):  
Rodolfo Carosi ◽  
Alessandro Petroccia ◽  
Salvatore Iaccarino ◽  
Matteo Simonetti ◽  
Antonio Langone ◽  
...  
Keyword(s):  
Shear Zone ◽  
Finite Strain ◽  
Pure Shear ◽  
Deformation Gradient ◽  
Shear Zones ◽  
Temperature Deformation ◽  
Timing Constraints ◽  
Important Variation ◽  
Deformation Phase ◽  
First Time

Detailed geological field mapping, integrated with meso- and microstructural investigations, kinematic of the flow and finite strain analyses, combined with geochronology, are fundamental tools to obtain information on the temperature–deformation–timing path of crystalline rocks and shear zone. The Posada-Asinara shear zone (PASZ) in northern Sardinia (Italy) is a steeply dipping km-thick transpressive shear zone. In the study area, located in the Baronie region (NE Sardinia), the presence of mylonites within the PASZ, affecting high- and medium-grade metamorphic rocks, provides an opportunity to quantify finite strain and kinematic vorticity. The main structures of the study area are controlled by a D2 deformation phase, linked to the PASZ activity, in which the strain is partitioned into folds and shear zone domains. Applying two independent vorticity methods, we detected an important variation in the percentage of pure shear and simple shear along the deformation gradient, that increases from south to north. We constrained, for the first time in this sector, the timing of the transpressive deformation by U–(Th)–Pb analysis on monazite. Results indicate that the shear zone has been active at ~325–300 Ma in a transpressive setting, in agreement with the ages of the other dextral transpressive shear zones in the southern Variscan belt.

Pre-Caledonian granulite and gabbro enclaves in the Western Gneiss Region, Norway: indications of incomplete transition at high pressure

2000 ◽  
Vol 137 (3) ◽  
pp. 235-255 ◽  
Author(s):  
M. KRABBENDAM ◽  
A. WAIN ◽  
T. B. ANDERSEN
Keyword(s):  
High Pressure ◽  
Shear Zones ◽  
Granulite Facies ◽  
Amphibolite Facies ◽  
Ultrahigh Pressure ◽  
Western Gneiss Region ◽  
Ultrahigh Pressure Metamorphism ◽  
Mountain Belts ◽  
Orogenic Cycle ◽  
Highly Strained

The Western Gneiss Region of Norway is a continental terrane that experienced Caledonian high-pressure and ultrahigh-pressure metamorphism. Most rocks in this terrane show either peak-Caledonian eclogite-facies assemblages or are highly strained and equilibrated under late-Caledonian amphibolite-facies conditions. However, three kilometre-size rock bodies (Flatraket, Ulvesund and Kråkenes) in Outer Nordfjord preserve Pre-Caledonian igneous and granulite-facies assemblages and structures. Where these assemblages are preserved, the rocks are consistently unaffected by Caledonian deformation. The three bodies experienced high-pressure conditions (20–23 kbar) but show only very localized (about 5%) eclogitization in felsic and mafic rocks, commonly related to shear zones. The preservation of Pre-Caledonian felsic and mafic igneous and granulite-facies assemblages in these bodies, therefore, indicates widespread (∼ 95%) metastability at pressures higher than other metastable domains in Norway. Late-Caledonian amphibolite-facies retrogression was limited. The degree of reaction is related to the protolith composition and the interaction of fluid and deformation during the orogenic cycle, whereby metastability is associated with a lack of deformation and lack of fluids, either as a catalyst or as a component in hydration reactions. The three bodies appear to have been far less reactive than the external gneisses in this region, even though they followed a similar pressure–temperature evolution. The extent of metastable behaviour has implications for the protolith of the Western Gneiss Region, for the density evolution of high-pressure terranes and hence for the geodynamic evolution of mountain belts.

scholarly journals Geological and geochemical implications of the genesis of the Qolqoleh orogenic gold mineralisation, Kurdistan Province (Iran)

Geologos ◽  
2015 ◽  
Vol 21 (1) ◽  
pp. 31-57 ◽  
Author(s):  
Batoul Taghipour ◽  
Farhad Ahmadnejad
Keyword(s):  
Shear Zone ◽  
Early Stage ◽  
Hanging Wall ◽  
Regional Metamorphism ◽  
Shear Zones ◽  
Northwestern Part ◽  
Type I ◽  
Two Phase ◽  
Mean Values ◽  
Calcite Veins

Abstract The Qolqoleh gold deposit is located in the northwestern part of the Sanandaj-Sirjan Zone (SSZ), within the NE-SW trending Qolqoleh shear zone. Oligocene granitoids, Cretaceous meta-limestones, schists and metavolcanics are the main lithological units. Chondrite-normalised REE patterns of the ore-hosting metavolcanics indicate REE enrichment relative to hanging wall (chlorite-sericite schist) and footwall (meta-limestone) rocks. The pattern also reflects an enrichment in LREE relative to HREE. It seems that the LREE enrichment is related to the circulation of SO42- and CO2-bearing fluids and regional metamorphism in the Qolqoleh shear zone. Both positive and negative Eu anomalies are observed in shear-zone metavolcanics. These anomalies are related to the degree of plagioclase alteration during gold mineralisation and hydrothermal alteration. In progressing from a metavolcanic protomylonite to an ultramylonite, significant changes occurred in the major/trace element and REE concentration. Utilising an Al-Fe-Ti isocon for the ore-hosting metavolcanics shows that Sc, Y, K, U, P, and M-HREE (except Eu) are relatively unchanged; S, As, Ag, Au, Ca, LOI, Rb and LREE are enriched, and Sr, Ba, Eu, Cr, Co and Ni decrease with an increasing degree of deformation. Based on geochemical features and comparison with other well-known shear zones in the world, the study area is best classified as an Isovolume-Gain (IVG) type shear zone and orogenic type gold mineralisation. Based on the number of phases observed at room temperature and their microthermometric behaviour, three fluid inclusion types have been recognised in quartz-sulphide and quartz-calcite veins: Type I monophase aqueous inclusions, Type II two-phase liquid-vapour (L-V) inclusions which are subdivided into two groups based on the homogenisation temperature (Th): a) L-V inclusions with Th from 205 to 255°C and melting temperature of last ice (Tm) from -3 to -9°C. b) L-V inclusions with higher Th from 335 to 385°C and Tm from -11 to -16°C. Type III three-phase carbonic-liquid inclusions (liquid water-liquid CO2-vapour CO2) with Th of 345-385°C. The mean values of the density of ore-forming fluids, pressure and depth of mineralisation have been calculated to be 0.79-0.96 gr/cm3, 2 kbar and 7 km, respectively. The δ18Owater and δD values of the gold-bearing quartz-sulphide veins vary from 7.2‰ to 8‰ and -40.24‰ to -35.28‰, respectively, which are indicative of an isotopically heavy crustal fluid and likely little involvement of meteoric fluid. The δ18Owater values of the quartz-calcite veins have a range of -5.31‰ to -3.35‰, and the δD values of -95.65‰ to -75.31‰, which are clearly lower than those of early-stage quartz-sulphide-gold veins, and are close to the meteoric water line. Based on comparisons of the D-O isotopic systematics, the Qolqoleh ore-mineralising fluids originated from metamorphic devolatilisation of Cretaceous volcano-sedimentary piles. Devolatilisation of these units occurred either synchronously with, or postdates, the development of penetrative (ductile) structures such as shear zones and during overprinting brittle deformation

High–pressure granulite–facies metamorphism and anatexis of deep continental crust: new insights from the Cenozoic Ailaoshan–Red River shear zone, SE Asia

2021 ◽  
Author(s):  
Haobo Wang ◽  
Shuyun Cao ◽  
Franz Neubauer ◽  
Junyu Li ◽  
Xuemei Cheng ◽  
...  
Keyword(s):  
Phase Diagram ◽  
Shear Zone ◽  
Thermal Evolution ◽  
Shear Zones ◽  
Granulite Facies ◽  
Red River ◽  
Crustal Anatexis ◽  
Early Oligocene ◽  
Facies Metamorphism ◽  
T Path

<p>Studies of crustal anatexis have given valuable insights into the evolution of metamorphism–deformation and the tectonic processes at convergent plate margins during orogeny. The transition of metatexite to diatexite migmatite records crucial information about the tectono–thermal evolution and rheology of the deep crust. Along the Ailao Shan–Red River shear zone, metatexite migmatites, diatexite migmatites and leucogranites are widely distributed within the upper amphibolite and granulite facies zones of the Diancang Shan metamorphic complex. The high–pressure granulite–facies metamorphism with mineral assemblage comprising garnet + kyanite + K–feldspar + plagioclase + biotite + quartz + melt is first recognized from the patch metatexite migmatites in the complex. Detailed petrographic evidence and phase diagram reveal that the migmatite underwent nearly isothermal decompression metamorphism, presenting a clockwise P–T path. The peak metamorphic P–T conditions are constrained by phase diagram at ca. 11 kbar and 810 °C, and the amount of melt generated during heating is up to 18 mol%. The extraction and segregation of melts are evidenced by the presence of leucosomes within migmatites and leucogranite dikes, which record the melt flow network through the crust. Zircons and monazites from migmatites record the ages of the melting episode that began at ca. 36 Ma and lasted to ca. 20 Ma. All these results are in accord with orogenic crust thickening accompanied by pervasive anatexis during the Later Eocene to the early Oligocene in the Ailao Shan–Red River shear zone. Combined with available data related to the other continental–exhumed shear zone, we propose that the crustal anatexis has an important effect on the thermal–state of deep–seated shear zones, is thus controlling the rheological behavior of the lithosphere and plays the essential role in the initial localizing of shearing in the lower crust.</p>

Striding-Athabasca mylonite zone: Complex Archean deep-crustal deformation in the East Athabasca mylonite triangle, northern Saskatchewan

1994 ◽  
Vol 31 (8) ◽  
pp. 1287-1300 ◽  
Author(s):  
Simon Hanmer ◽  
Randy Parrish ◽  
Michael Williams ◽  
Chris Kopf
Keyword(s):  
Flow Pattern ◽  
Shear Zone ◽  
Crustal Deformation ◽  
Principal Axis ◽  
Shear Zones ◽  
Granulite Facies ◽  
Strike Slip ◽  
Tectonic Zone ◽  
Early Proterozoic ◽  
Lower Deck

The geophysically defined Snowbird tectonic zone is manifested in northernmost Saskatchewan as a deep-crustal, multistage mylonitic structure, the East Athabasca mylonite triangle. The triangle, located at the northeastern apex of a stiff, crustal-scale "lozenge," is composed of mid-Archean annealed mylonites and late Archean ribbon mylonites, formed during two granulite facies events (850–1000 °C, 1.0 GPa). The flow pattern in the mylonites is geometrically and kinematically complex, and corresponds to that expected adjacent to the apex of a stiff elliptical volume subjected to subhorizontal regional extension parallel to its principal axis. The late Archean mylonites are divided into an upper structural deck, entirely occupied by a dip-slip shear zone, and an underlying lower deck. The latter is divided into two upright conjugate strike-slip shear zones, separated by a low-strain septum, which deformed by progressive coaxial flow. The flow pattern in the mid-Archean mylonites is compatible with that of the late Archean mylonites, and suggests that the crustal-scale lozenge influenced deformation since the mid-Archean. In the interval ca. 2.62–2.60 Ga, deformation in the upper and lower decks evolved from a granulite facies pervasive regime to a more localized amphibolite facies regime. With further cooling, deformation was localized within very narrow greenschist mylonitic faults at the lateral limits of the lower deck. By the late Archean, the East Athabasca mylonite triangle was part of a deep-crustal, intracontinental shear zone. This segment of the Snowbird tectonic zone was not the site of an Early Proterozoic suture or orogen.

870 Ma age of South Delhi Orogeny: A study on Geochronology of the granites and the metasediments, NW India.

2020 ◽  
Author(s):  
Subhash Singh ◽  
Tapas Kumar Biswal
Keyword(s):  
Interference Pattern ◽  
Boundary Migration ◽  
Shear Zones ◽  
High Temperature Deformation ◽  
Temperature Deformation ◽  
Subgrain Rotation ◽  
Ductile Shear Zones ◽  
Biotite Schist ◽  
Magmatic Fabric ◽  
Type 3

<p>South Delhi orogeny is constrained by correlating the deformational fabric with geochronology of the granites and metasediments around Beawar- Rupnagar-Babra, Rajasthan, NW India. The area consists of metaconglomerate, calcareous schist, mica schist and amphibolite. These were deformed by three stages of deformation(D<sub>1-3</sub>) and intruded by four types of granite plutons (G<sub>1-4</sub>). The D<sub>1</sub> deformation produced F<sub>1</sub>, reclined/recumbent folds with S<sub>1</sub> axial planar fabric in greenschist facies metamorphic condition. The D<sub>2</sub> deformation produced NE-SW trending F<sub>2</sub> folds coaxial with F<sub>1</sub>(type 3 interference pattern), crenulations and F<sub>2</sub>-axial parallel ductile shear zones.  The D<sub>3</sub> deformation produced NW-SE F<sub>3 </sub>folds, which superimposed on F<sub>1</sub> and F<sub>2</sub> to create type 1 and 2 interference pattern. Granites carry pervasive S<sub>1</sub> fabric. In G<sub>1-3</sub> granites, the S<sub>1</sub> is characterized by low temperature deformation fabric marked by bulging recrystallization of quartz. The G<sub>4</sub> granite (namely Sewariya granite) contains magmatic to submagmatic fabric and the S<sub>1</sub> fabric in it is a high temperature deformation fabric and lies parallel to magmatic fabric in the rock. Plagioclase is dynamically recrystallized by subgrain rotation and grain boundary migration and quartz shows chess board twinning. We interpret that the G<sub>4</sub> granite is syntectonic and G<sub>1-3 </sub>were pre-tectonic to D<sub>1</sub> deformation.  U-Pb data (SHRIMP method) of G<sub>1</sub>, G<sub>2 </sub>and G<sub>4 </sub>granites yield Concordia age calculated with <sup>206</sup>Pb/<sup>238</sup>U and <sup>207</sup>Pb/<sup>235</sup>U ratio at ~982 Ma, ~992 Ma and ~878 Ma respectively. Thus the South Delhi orogeny is constrained by the age of G<sub>4</sub> granite at ~ 878 Ma (~ 870 Ma).  The G<sub>1-3 </sub>granites are pre- Delhi orogeny and probably constrain the age of rifting of the Delhi basin. EPMA Th-U-total Pb monazite geochronology of the garnet-staurolite-quartz-feldspar-biotite schist from the basal conglomerate zone shows three distinct ages, ca. 1611 Ma, 864 Ma and 718 Ma.  Correlating with granite SHRIMP age, the ~ 864 Ma corresponds to Delhi metamorphism and D<sub>1</sub> deformation (~ 870 Ma). The event ca. 1611 Ga probably belongs to pre-Delhi age, which is observed in nearby pre-Delhi localities like Sandmata terrane.</p><p>Keywords: Deformational fabric, geochronology, metaconglomerate, granite and geochronology.</p>

Dynamic Deformation Response of G33 Steel under High Temperature

2015 ◽  
Vol 782 ◽  
pp. 61-70
Author(s):  
You Jing Zhang ◽  
Hong Nian Cai ◽  
Xing Wang Cheng ◽  
Shuang Zan Zhao
Keyword(s):  
High Temperature ◽  
Flow Stress ◽  
Strain Rate ◽  
Shear Bands ◽  
Shear Zones ◽  
Adiabatic Shear ◽  
High Temperature Deformation ◽  
Temperature Deformation ◽  
Strain Rates ◽  
Adiabatic Shear Bands

The high temperature deformation and fracture behavior of ultra-high strength G33 steel under high strain rate compression are investigated by means of a split Hopkinson p ressure bar. Impact tests are performed at strain rates of 1000/s and 2200/s and at temperatures ranging from 25°C to 700°C. The SEM and TEM techniques are also used to analyze the microstructure evolution of the adiabatic shear band (ASB) and fracture characteristics of the deformed specimens at high temperature. The experimental results indicate that the flow stress of G33 steel is significantly dependent on temperatures and strain rates. The flow stress of G33 steel increases with the increase of strain rates, but decreases with the increase of temperatures. The strain rate sensitivity is more pronounced at the low temperature of 25°C. In addition, G33 steel is more liable to fracture at high temperatures than at 25°C. Observations of microstructure show two well-developed symmetric parabolic adiabatic shear bands on the longitudinal cross-section of the cylindrical specimen deformed at the temperature of 700°C and at the strain rate of 2200/s. Within the ASB, the width of the fine equiaxed grain structure is about 7μm. The size of those equiaxed grains is approximately 100nm. The fracture analysis results indicate that the ASBs are the predominant deformation and the specimens fracture along adiabatic shear bands. The fracture surfaces of the deformed G33 steel specimens are characterized by two alternating zones: rough dimple zone and relatively smooth shear zone. Further observations reveal that smooth shear zones consist of severely sheared dimples.

The rapakivi granites of S Greenland—crustal melting in response to extensional tectonics and magmatic underplating

1992 ◽  
Vol 83 (1-2) ◽  
pp. 173-178 ◽  
Author(s):  
P. E. Brown ◽  
T. J. Dempster ◽  
T. N. Harrison ◽  
D. H. W. Hutton
Keyword(s):  
Regional Metamorphism ◽  
Three Dimensional ◽  
Shear Zones ◽  
Granulite Facies ◽  
Crustal Melting ◽  
Lithospheric Extension ◽  
Magmatic Underplating ◽  
Mantle Component ◽  
Juvenile Crust ◽  
Structural Levels

ABSTRACTEarly Proterozoic rapakivi intrusions in S Greenland occur as thick sheets which have ramp–flat geometry and were intruded along the median planes of active ductile extensional shear zones. These shear zones and their intrusions were linked via transfer zones in a major three-dimensional framework. At high structural levels (c. 6 km) the rapakivi intrusions developed thermal aureoles which overprint the regional assemblages, whereas at deeper levels in the regional structure they are contemporaneous with regional metamorphism. Thermobarometry on the regional and contact assemblages indicates low pressure granulite facies conditions (200–400 MPa, 650°-800°C) suggesting very high thermal gradients. The rapakivi suite and associated norites have low initial 87Sr/86Sr together with positive εNd values, indicating the involvement of predominantly young crust and/or mantle component in the generation of the igneous suite. It is considered that the voluminous norites are closely related to the mafic melts which underplated the juvenile crust to trigger the generation of the monzonitic rapakivi suite. Taken together, the data are consistent with a model of Proterozoic lithospheric extension, thinning of relatively juvenile continental crust and compression of mantle isotherms, resulting in high crustal heat flow, mafic underplating, and crustal melting with emplacement of magmas along a linked network of extensional shear zones.

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

2015 ◽  
Vol 265 ◽  
pp. 104-120 ◽  
Author(s):  
Lorraine Tual ◽  
Aránzazu Piñán-Llamas ◽  
Charlotte Möller
Keyword(s):  
High Temperature ◽  
Shear Zone ◽  
High Temperature Deformation ◽  
Temperature Deformation ◽  
Sveconorwegian Orogen ◽  
Basal Shear
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