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

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.

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

Solid Earth ◽  
2019 ◽  
Vol 10 (5) ◽  
pp. 1635-1649 ◽  
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.

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

Geosphere ◽  
2021 ◽  
Author(s):  
Andrew Tholt ◽  
Sean R. Mulcahy ◽  
William C. McClelland ◽  
Sarah M. Roeske ◽  
Vinícius T. Meira ◽  
...  
Keyword(s):  
Tectonic Evolution ◽  
Shear Zones ◽  
Granulite Facies ◽  
Metamorphic Petrology ◽  
Granulite Facies Metamorphism ◽  
Northwest Argentina ◽  
Sufficient Detail ◽  
Ductile Shear Zones ◽  
Gondwana Margin ◽  
Lower Crustal

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.

Exhumation of high-grade terranes—a review

1992 ◽  
Vol 29 (4) ◽  
pp. 737-745 ◽  
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).

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

1993 ◽  
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.

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

1990 ◽  
Vol 54 (375) ◽  
pp. 145-158 ◽  
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.

The relationship between crustal magmatic underplating and granite genesis: an example from the Velay granite complex, Massif Central, France

1992 ◽  
Vol 83 (1-2) ◽  
pp. 235-245 ◽  
Author(s):  
B. J. Williamson ◽  
H. Downes ◽  
M. F. Thirlwall
Keyword(s):  
Country Rock ◽  
Granulite Facies ◽  
Massif Central ◽  
Granulite Facies Metamorphism ◽  
Magmatic Underplating ◽  
Granite Complex ◽  
Partial Melts ◽  
Crustal Xenoliths ◽  
Lower Crustal ◽  
The Relationship

ABSTRACTThe Velay granite pluton (Massif Central, France) is the youngest (304 ± 5 Ma) and largest (∼6,900 km2) of the major Massif Central monzogranites/granodiorites and was formed nearly 50 Ma after the cessation of Hercynian continental collision (Pin & Duthou 1990). It is a highly heterogeneous pluton consisting of I-type, high-Sr granites (Sr = 500-900 ppm) with low (+35 to +41) and high (-3 to -5), at its centre, grading into S-type and mixed I-S-type heterogeneous granites of more normal Sr content (100–420 ppm) and higher (+40 to +210) and lower (-3·8 to -7.3) at its margins.The metasedimentary lower crust of the Massif Central was underplated/intruded by mafic mantle-derived magmas between 360 Ma and 300 Ma. From 300-280 Ma (Downes et al. 1991) underplating led to partial melting and granulite facies metamorphism of the underplated material (represented by felsic and mafic meta-igneous lower crustal xenoliths, = –11 to +112, = +2·2 to 8·2, Downes et al 1990). The partial melts assimilated mainly schist but also felsic gneiss and older granite country rock material ( = +100 to +300, = - 5 to -9) to produce the heterogeneous granites. Plagioclase and biotite were accumulated at the base of the intrusion which was intruded to high levels to form the high-Sr granites.

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