Metamorphism of the Sierra de Maz and implications for the tectonic evolution of the MARA terrane

The Mesoproterozoic MARA terrane of western South America is a composite igneous-metamorphic complex that is important for Paleozoic paleogeographic reconstructions and the relative positions of Laurentia and Gondwana. The magmatic and detrital records of the MARA terrane are consistent with a Laurentian origin; however, the metamorphic and deformation records lack sufficient detail to constrain the correlation of units within the MARA terrane and the timing and mechanisms of accretion to the Gondwana margin. Combined regional mapping, metamorphic petrology, and garnet and monazite geochronology from the Sierra de Maz of northwest Argentina sug- gest that the region preserves four distinct litho-tectonic units of varying age and metamorphic conditions that are separated by middle- to lower-crustal ductile shear zones. The Zaino and Maz Complexes preserve Barrovian metamorphism and ages that are distinct from other units within the region. The Zaino and Maz Complexes both record metamorphism ca. 430–410 Ma and show no evidence of the regional Famatinian orogeny (ca. 490–455 Ma). In addition, the Maz Complex records an earlier granulite facies event at ca. 1.2 Ga. The Taco and Ramaditas Complexes, in contrast, experienced medium- and low-pressure upper amphibolite to granulite facies metamorphism, respectively, between ca. 470–460 Ma and were later deformed at ca. 440–420 Ma. The Maz shear zone that bounds the Zaino and Maz Complexes records sinistral oblique to sinistral deformation between ca. 430–410 Ma. The data suggest that at least some units in the MARA terrane were accreted by translation, and the Gondwana margin of northwest Argentina transitioned from a dominantly convergent margin to a highly oblique margin in the Silurian.

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Fracturing and crystal plastic behaviour of garnet under seismic stress in the dry lower continental crust (Musgrave Ranges, Central Australia)

Solid Earth ◽

10.5194/se-10-1635-2019 ◽

2019 ◽

Vol 10 (5) ◽

pp. 1635-1649 ◽

Cited By ~ 2

Author(s):

Friedrich Hawemann ◽

Neil Mancktelow ◽

Sebastian Wex ◽

Giorgio Pennacchioni ◽

Alfredo Camacho

Keyword(s):

Plastic Deformation ◽

Continental Crust ◽

High Stress ◽

Shear Zones ◽

Granulite Facies ◽

Lower Continental Crust ◽

Granulite Facies Metamorphism ◽

High Calcium ◽

Central Australia ◽

Lower Crustal

Abstract. Garnet is a high-strength mineral compared to other common minerals such as quartz and feldspar in the felsic crust. In felsic mylonites, garnet typically occurs as porphyroclasts that mostly evade crystal plastic deformation, except under relatively high-temperature conditions. The microstructure of granulite facies garnet in felsic lower-crustal rocks of the Musgrave Ranges (Central Australia) records both fracturing and crystal plastic deformation. Granulite facies metamorphism at ∼1200 Ma generally dehydrated the rocks and produced millimetre-sized garnets in peraluminous gneisses. A later ∼550 Ma overprint under sub-eclogitic conditions (600–700 ∘C, 1.1–1.3 GPa) developed mylonitic shear zones and abundant pseudotachylyte, coeval with the neocrystallization of fine-grained, high-calcium garnet. In the mylonites, granulite facies garnet porphyroclasts are enriched in calcium along rims and fractures. However, these rims are locally narrower than otherwise comparable rims along original grain boundaries, indicating the contemporaneous diffusion and fracturing of garnet. The fractured garnets exhibit internal crystal plastic deformation, which coincides with areas of enhanced diffusion, usually along zones of crystal lattice distortion and dislocation walls associated with subgrain rotation recrystallization. The fracturing of garnet under dry lower-crustal conditions, in an otherwise viscously flowing matrix, requires transient high differential stress, most likely related to seismic rupture, consistent with the coeval development of abundant pseudotachylyte. Highlights. Garnet is deformed by fracturing and crystal plasticity under dry lower-crustal conditions. Ca diffusion profiles indicate multiple generations of fracturing. Diffusion is promoted along zones of higher dislocation density. Fracturing indicates transient high-stress (seismic) events in the lower continental crust.

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Fracturing and crystal plastic behavior of garnet under seismic stress in the dry lower continental crust (Musgrave Ranges, Central Australia)

10.5194/se-2019-110 ◽

2019 ◽

Author(s):

Friedrich Hawemann ◽

Neil Mancktelow ◽

Sebastian Wex ◽

Giorgio Pennacchioni ◽

Alfredo Camacho

Keyword(s):

Plastic Deformation ◽

High Strength ◽

Calcium Content ◽

Shear Zones ◽

Granulite Facies ◽

Plastic Behavior ◽

Granulite Facies Metamorphism ◽

High Calcium ◽

Central Australia ◽

Lower Crustal

Abstract. Garnet is a high strength mineral compared to other common minerals such as quartz and feldspar in the felsic crust. In felsic mylonites, garnet typically occurs as porphyroclasts that mostly evade deformation, except under relatively high temperature conditions. The microstructure of granulite facies garnet in felsic lower-crustal rocks of the Musgrave Ranges (Central Australia) records both fracturing and crystal-plastic deformation. Granulite facies metamorphism at ~ 1200 Ma generally dehydrated the rocks and produced mm-sized garnets in peraluminous gneisses. A later ~ 550 Ma overprint under sub-eclogitic conditions (600–700 °C, 1.1–1.3 GPa) developed shear zones and with abundant pseudotachylyte, coeval with the neocrystallization of fine-grained, high-calcium garnet. The granulitic fractured garnet porphyroclasts in mylonites show high calcium content along rims and fractures. However, in certain cases, these rims are narrower than equivalent rims along original grain boundaries, indicating contemporaneous diffusion and fracturing of garnet. The fractured garnets exhibit internal crystal-plastic deformation, that coincide with areas of enhanced diffusion, usually along zones of crystal lattice distortions and dislocation walls and by subgrain rotation recrystallization. Fracturing of garnet under dry lower crustal conditions, in an otherwise viscously flowing matrix, requires transient high differential stress, most likely related to seismic rupture, consistent with the coeval development of abundant pseudotachylyte.

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Exhumation of high-grade terranes—a review

Canadian Journal of Earth Sciences ◽

10.1139/e92-063 ◽

1992 ◽

Vol 29 (4) ◽

pp. 737-745 ◽

Cited By ~ 7

Author(s):

Jacques Martignole

Keyword(s):

High Temperature ◽

Shear Zones ◽

Granulite Facies ◽

High Grade ◽

Granulite Facies Metamorphism ◽

Magmatic Underplating ◽

Show Evidence ◽

Juvenile Crust ◽

Uplift And Erosion ◽

Lower Crustal

High-grade (granulite-facies) terranes are brought to the surface by a combination of uplift and erosion (exhumation). The reported mechanisms and durations of exhumation are variable and depend partly on the mode of formation of a given high-grade terrane. In this paper, we consider the case of granulite-facies conditions that are attained (i) in juvenile crust, in the roots of magmatic arcs (e.g., Kohistan, Fiordland), (ii) around deep-seated high-temperature plutonic complexes, and (iii) in the lower parts of thickened continental crust. In the case of the roots of magmatic arcs, Phanerozoic examples suggest that they are exhumed along shallow-dipping contraction faults or shear zones that developed during continental obduction in a convergent tectonic regime. This process is not fundamentally different from processes leading to the exhumation of high-pressure (blueschist, eclogite) terranes. In contrast, deep-seated high-temperature plutonic complexes are thermostructural domes, analogous to the lower levels of core complexes, which may also have contributed to the uprise of high-grade terranes. Such domes should be sought for around anorthositic or mafic plutons, where their ascent may also have been favoured by continental extension. These modes of exhumation are compatible with a monocyclic evolution. However, many high-grade terranes show evidence of a polycyclic evolution and, in such cases, the nature of the thermal perturbation responsible for granulite-facies metamorphism is still debated. Thermal modelling based on heat conduction in collision orogens shows that granulites cannot form at mid-cristal levels, namely those exposed after isostatically driven denudation. Thus, magmatic underplating and crustal extension have been suggested as causes of steepened geotherms. Underplating (or intraplating) supplies the heat and thickens the crust from below. Postcollisional extension has also been considered as a mechanism providing a heat pulse emanating from the asthenosphere, probably after the "detachment" of a relatively cold thermal boundary layer. Finally, isolated crustal-scale intracratonic thrusting may favour the rise of intermediate to lower crustal wedges (e.g., the Kapuskasing wedge, uplifted prior to the trans-Hudson collision).

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Sr and Nd isotope systematics of polymetamorphic Archean gneisses from southern West Greenland and northern Labrador

Canadian Journal of Earth Sciences ◽

10.1139/e89-039 ◽

1989 ◽

Vol 26 (3) ◽

pp. 446-466 ◽

Cited By ~ 34

Author(s):

Kenneth D. Collerson ◽

Malcolm T. McCulloch ◽

Allan P. Nutman

Keyword(s):

Open System ◽

Shear Zones ◽

Granulite Facies ◽

Isotopic Data ◽

Nd Isotope ◽

Granulite Facies Metamorphism ◽

West Greenland ◽

System Behaviour ◽

Ductile Shear Zones ◽

Isotope Systematics

Sr and Nd isotopic data for middle to late Archean polymetamorphic felsic gneisses from localities in the Nuuk area, West Greenland, are compared and contrasted with new isotopic results for early Archean Amîtsoq gneisses and with data for isotopically reworked Kiyuktok gneisses from the Saglek area, Labrador. Sr isotopic data for individual suites of felsic gneisses record the time-integrated effect of variable Rb–Sr fractionation during prograde and retrograde events as well as the effect of source inhomogeneity.Contrasting petrologic and Sr–Nd isotopic characteristics are the result of differences in level of exposure, caused partially by juxtaposition of terranes of different metamorphic character by movement on ductile shear zones and post-shearing folding deformation. Sm–Nd systematics of felsic gneisses from Nordafar, Ikerasakitsup akornga, Tinissaq, and Kangimut sammisoq – Qasigianguit define a geologically meaningless ca. 3280 Ma Nd "isochron", which is the result of mixing of samples from unrelated suites and the effect of open-system behaviour. Gneisses lying on this pseudoisochron were variably affected by ca. 2800–2900 Ma prograde granulite-facies metamorphism and structurally controlled retrogression under amphibolite- to greenschist-facies conditions.The study shows that Sr–Nd isotope systematics of geologically identifiable units may be modified by open-system behaviour during prograde and retrograde metamorphism. Isotopic data from gneiss complexes metamorphosed under granulite-facies conditions may therefore yield equivocal information concerning isochron interpretation, significance of model ages, and estimates of crustal residence time.

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Evolution of brittle structures in plagioclase-rich rocks at high-grade metamorphic conditions – Linking laboratory results to field observations

10.5194/egusphere-egu2020-8367 ◽

2020 ◽

Author(s):

Sarah Incel ◽

Jörg Renner ◽

Bjørn Jamtveit

Keyword(s):

Structural Evolution ◽

Confining Pressure ◽

Shear Zones ◽

Ductile Shear Zones ◽

Laboratory Results ◽

Experimental Laboratory ◽

Eclogite Facies ◽

Ductile Shear ◽

Host Rocks ◽

Lower Crustal

Plagioclase-rich lower crustal granulites exposed on the Lofoten archipelago, N Norway, display pseudotachylytes, reflecting brittle deformation, as well as ductile shear zones, highlighting plastic deformation. Pristine pseudotachylytes often show no or very little difference in mineral assemblage to their host-rocks that exhibit limited, if any, metamorphic alteration. In contrast, host-rock volumes that developed ductile shear zones exhibit significant hydration towards amphibolite or eclogite-facies assemblages within and near the shear zones. We combine experimental laboratory results and observations from the field to characterize the structural evolution of brittle faults in plagioclase-rich rocks at lower crustal conditions. We performed a series of deformation experiments on intact granulite samples at 2.5 GPa confining pressure,&#160; a strain rate of 5&#215;10-5 s-1,&#160; temperatures of 700 and 900 &#176;C, and total strains of either ~7-8 % or ~33-36 %. Samples were either deformed &#8216;as-is&#8217;, i.e. natural samples without any treatment, or with ~2.5 wt.% H2O added. Striking similarities between the experimental and natural microstructures suggest that the transformation of precursory brittle structures into ductile shear zones at eclogite-facies conditions is most effective when hydrous fluids are available in excess.

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EVOLUÇÃO PETROLÓGICA E ESTRUTURAL DA PORÇÃO ORIENTAL DO ESTADO DE MINAS GERAIS E SUAS IMPLICAÇÕES GEOTECTÔNICAS

Revista Geonomos ◽

10.18285/geonomos.v1i1e2.236 ◽

1993 ◽

Cited By ~ 1

Author(s):

Antonio Gilberto Costa ◽

Carlos Alberto Rosière ◽

Lydia Maria Lobato ◽

Fernando V. Laureano

Keyword(s):

Shear Zones ◽

Granulite Facies ◽

Minas Gerais ◽

Magmatic Differentiation ◽

Metamorphic Belt ◽

Quartz Crystals ◽

Granulite Facies Metamorphism ◽

Minas Gerais State ◽

Positive Correlations ◽

Basic Rocks

A metamorphic terrain with high-grade rocks of the Atlantic Metamorphic Belt underlies the eastern part of Minas Gerais State, from south of the town of Manhuaçu to Caratinga. This terrain comprises peraluminous gneisses, igneous and meta-igneous rocks. Granulites occur as small nucleus and vary in composition between peraluminous and basic to intermediate, the latter represented by enderbitic mobilizate. Their formation, as well as that of migmatites of granitic composition, is considered to be related to mafic and ultramafic intrusions. In basic granulites, garnet-bearing mineral assemblages, with the development of corona textures, attest the effects of granulite facies metamorphism, although igneous assemblages and textures are still well preserved. Retrograde alteration assemblages are locally preserved. Despite of the diversity of metamorphic phenomena in this area, T and P calculations reveal consistent results. Temperature and pressure calculations were undertaken in basic granulites slightly affected by the retrograde process. Using Fe +²/Mg exchange between garnet and ortopyroxene as geothermometers and the exchange reaction: An +En = 2/3Pyr + 1/3Grs + Qz as geobarometers peak metamorphic temperatures in the range of 660 to 760°C, at 4,8 to 6,6 Kbar are obtained. Mineral, textural and geochemical evidences indicate that the metamorphic conditions have changed with time and suggest that the formation of the granulites is caused by the underplating of magmas, probably mantle-derived, at the base of the crust. Several rations between major, trace and rare earth elements have been employed. The basic rocks are similar in composition to tholeiites generated in within-plate tectonic settings. Positive correlations netween K2O and SiO2 and negative between MgO and SiO2 in fresh gabbro-noritic rocks and enderbites indicate magmatic differentiation. The geochemical character of altered basic rocks displays an unsystematic dispersion in correlations diagrams. This lack of correlation coupled with field and petrographic suggest the effects of a late metasomatic event on these rocks. This metasomatism comprises the dispersed development of charnockitic rocks with large K-feldspars and quartz crystals. Later dynamic processes gave place to subvertical shear zones with a well defined foliation.

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Age and tectonic evolution of Neoproterozoic ductile shear zones in southwestern madagascar, with implications for gondwana studies

Journal of African Earth Sciences ◽

10.1016/s0899-5362(01)80085-3 ◽

2001 ◽

Vol 33 (2) ◽

pp. XIX

Keyword(s):

Tectonic Evolution ◽

Shear Zones ◽

Ductile Shear Zones ◽

Ductile Shear

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Fluid inclusions in granulites and eclogites from the Bergen Arcs, Caledonides of W. Norway

Mineralogical Magazine ◽

10.1180/minmag.1990.054.375.02 ◽

1990 ◽

Vol 54 (375) ◽

pp. 145-158 ◽

Cited By ~ 43

Author(s):

T. Andersen ◽

H. Austrheim ◽

E. A. J. Burke

Keyword(s):

Fluid Inclusions ◽

Shear Zones ◽

Granulite Facies ◽

Mole Percent ◽

Molar Volumes ◽

Granulite Facies Metamorphism ◽

Laser Raman ◽

Granulite Metamorphism ◽

Facies Metamorphism ◽

Eclogite Facies

AbstractThe Grenvillian granulite-facies complex on Holsnøy island, Bergen Arcs, W. Norway, has been metamorphosed at eclogite-facies conditions during the Caledonian orogeny (ca. 425 Ma). The granulite-eclogite facies transition takes place along shear zones and fluid pathways. Mineral thermobarometry indicates PT conditions of 800–900°C and 8–10 kbar for the Proterozoic granulite facies metamorphism and 700–800°C and 16–19 kbar for the eclogite-forming event. Quartz in the granulite facies complex contains CO2 fluid inclusions with less than 2.5 mole percent N2; the molar volumes are compatible with the PT conditions of the Proterozoic granulite metamorphism. Quartz in pegmatitic quartz + omphacite and quartz + phengite/paragonite veins coeval with shear-zone eclogites contain N2 ± CO2 fluid inclusions. Combined laser Raman microanalysis and microthermometry show that the least disturbed inclusions have XCO2 = 0.1–0.3, and molar volumes less than 40 cm3/mole, which may agree with the PT conditions during Caledonian high-pressure metamorphism. Younger, low-density N2 and N2-H2O fluid inclusions are the results of decrepitation and redistribution of early inclusions during the retrograde PT evolution of the eclogites.

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