Thermobarometry and geochronology of the Uvauk complex, a polymetamorphic Neoarchean and Paleoproterozoic segment of the Snowbird tectonic zone, Nunavut, Canada

2007 ◽  
Vol 44 (2) ◽  
pp. 245-266 ◽  
Author(s):  
A J Mills ◽  
R G Berman ◽  
W J Davis ◽  
S Tella ◽  
S Carr ◽  
...  
Keyword(s):  
High Pressure ◽  
Shear Zones ◽  
Granulite Facies ◽  
Crustal Thickening ◽  
High Grade ◽  
Retrograde Metamorphism ◽  
Tectonic Zone ◽  
History Of ◽  
Thermal Overprint ◽  
Temperature Time

The Uvauk complex is an ultramylonite-bounded, granulite-facies suite of anorthosite–gabbro that forms part of the Chesterfield Inlet segment of the Snowbird tectonic zone. Following initial anorthosite–gabbro magmatism at ca. 2.71 Ga and a cryptic 2.62–2.60 Ga event marked by zircon and monazite growth, the Uvauk complex experienced two high-grade tectonometamorphic events at 2.56–2.50 and 1.91–1.90 Ga. Similar to the 2.56–2.50 Ga development of other shear zones in the region, the upper-amphibolite-facies to granulite-facies, moderately high-pressure (8.4–11.0 kbar and 705–760 °C) (1 kbar = 100 MPA) M1 event is interpreted to have involved the structural emplacement of ca. 2.71 Ga Uvauk complex rocks on ca. 2.68 Ga tonalitic rocks to the south. Granulite-facies, high-pressure (11.2–14.7 kbar and 695–865 °C) M2 metamorphism, gabbroic magmatism, and mylonite development within the complex at ca. 1.9 Ga culminated with ~3.5 kbar decompression at high temperature. Clockwise pressure–temperature–time (P–T–t) paths reflect crustal thickening, thought to be related to the early accretionary history of the Trans-Hudson Orogen. A thermal overprint at ca. 1.85–1.75 Ga resulted in retrograde metamorphism (5.8–6.0 kbar and 625–695 °C) associated with post-tectonic granitoid plutonism.

The Pikwitonei Granulite Domain, Manitoba: a giant Neoarchean high-grade terrane in the northwest Superior ProvinceThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.N. Machado, T.E. Krogh, and W. Weber are deceased.

2011 ◽  
Vol 48 (2) ◽  
pp. 205-245 ◽  
Author(s):  
L. M. Heaman ◽  
Ch. O. Böhm ◽  
N. Machado ◽  
T. E. Krogh ◽  
W. Weber ◽  
...  
Keyword(s):  
Granulite Facies ◽  
Superior Province ◽  
High Grade ◽  
Metamorphic Zircon ◽  
Special Issue ◽  
Metamorphic History ◽  
Zircon Growth ◽  
The World ◽  
Northwestern Margin ◽  
History Of

The Pikwitonei Granulite Domain located at the northwestern margin of the Superior Province is one of the largest Neoarchean high-grade terranes in the world, with well-preserved granulite metamorphic assemblages preserved in a variety of lithologies, including enderbite, opdalite, charnockite, and mafic granulite. U–Pb geochronology has been attempted to unravel the protolith ages and metamorphic history of numerous lithologies at three main localities; Natawahunan Lake, Sipiwesk Lake, and Cauchon Lake. The U–Pb age results indicate that some of the layered enderbite gneisses are Mesoarchean (3.4–3.0 Ga) and the more massive enderbites are Neoarchean. The high-grade metamorphic history of the Pikwitonei Granulite Domain is complex and multistage with at least four episodes of metamorphic zircon growth identified: (1) 2716.1 ± 3.8 Ma, (2) 2694.6 ± 0.6 Ma, (3) 2679.6 ± 0.9 Ma, and (4) 2642.5 ± 0.9 Ma. Metamorphic zircon growth during episodes 2 and 3 are interpreted to be regional in extent, corresponding to M1 amphibolite- and M2 granulite-facies events, respectively, consistent with previous field observations. The youngest metamorphic episode at 2642.5 Ma is only recognized at southern Cauchon Lake, where it coincides with granite melt production and possible development of a major northeast-trending deformation zone. The timing and multistage metamorphic history recorded in the Pikwitonei Granulite Domain is similar to most Superior Province high-grade terranes and marks a fundamental break in Archean crustal evolution worldwide at the termination of prolific global Neoarchean greenstone belt formation.

Determining the significance of high-grade shear zones by using temperature-time paths, with examples from the Grenville orogen

Geology ◽  
1994 ◽  
Vol 22 (8) ◽  
pp. 743 ◽  
Author(s):  
Ben A. van der Pluijm ◽  
Klaus Mezger ◽  
Michael A. Cosca ◽  
Eric J. Essene
Keyword(s):  
Shear Zones ◽  
High Grade ◽  
Temperature Time

Macro- and microstructural analysis of the Zhujiafang ductile shear zone, Hengshan Complex: Tectonic nature and geodynamic implications of the evolution of Trans−North China orogen

2020 ◽  
Author(s):  
Lingchao He ◽  
Jian Zhang ◽  
Guochun Zhao ◽  
Changqing Yin ◽  
Jiahui Qian ◽  
...  
Keyword(s):  
Shear Zone ◽  
North China ◽  
Shear Zones ◽  
Crustal Thickening ◽  
Slip Systems ◽  
High Grade ◽  
Ductile Shear Zone ◽  
Crustal Rocks ◽  
Ductile Shear ◽  
Lower Crustal

In worldwide orogenic belts, crustal-scale ductile shear zones are important tectonic channels along which the orogenic root (i.e., high-grade metamorphic lower-crustal rocks) commonly experienced a relatively quick exhumation or uplift process. However, their tectonic nature and geodynamic processes are poorly constrained. In the Trans−North China orogen, the crustal-scale Zhujiafang ductile shear zone represents a major tectonic boundary separating the upper and lower crusts of the orogen. Its tectonic nature, structural features, and timing provide vital information into understanding this issue. Detailed field observations showed that the Zhujiafang ductile shear zone experienced polyphase deformation. Variable macro- and microscopic kinematic indicators are extensively preserved in the highly sheared tonalite-trondhjemite-granodiorite (TTG) and supracrustal rock assemblages and indicate an obvious dextral strike-slip and dip-slip sense of shear. Electron backscattered diffraction (EBSD) was utilized to further determine the crystallographic preferred orientation (CPO) of typical rock-forming minerals, including hornblende, quartz, and feldspar. EBSD results indicate that the hornblendes are characterized by (100) <001> and (110) <001> slip systems, whereas quartz grains are dominated by prism <a> and prism <c> slip systems, suggesting an approximate shear condition of 650−700 °C. This result is consistent with traditional thermobarometry pressure-temperature calculations implemented on the same mineral assemblages. Combined with previously reported metamorphic data in the Trans−North China orogen, we suggest that the Zhujiafang supracrustal rocks were initially buried down to ∼30 km depth, where high differential stress triggered the large-scale ductile shear between the upper and lower crusts. The high-grade lower-crustal rocks were consequently exhumed upwards along the shear zone, synchronous with extensive isothermal decompression metamorphism. The timing of peak collision-related crustal thickening was further constrained by the ca. 1930 Ma metamorphic zircon ages, whereas a subsequent exhumation event was manifested by ca. 1860 Ma syntectonic granitic veins and the available Ar-Ar ages of the region. The Zhujiafang ductile shear zone thus essentially record an integrated geodynamic process of initial collision, crustal thickening, and exhumation involved in formation of the Trans−North China orogen at 1.9−1.8 Ga.

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.

Cyclic brittle-ductile behaviour recorded in exhuming high-pressure continental units of the Northern Apennines.

2020 ◽  
Author(s):  
Francesco Giuntoli ◽  
Giulio Viola
Keyword(s):  
High Pressure ◽  
High Strain ◽  
Shear Zones ◽  
White Mica ◽  
Northern Apennines ◽  
Strain Partitioning ◽  
Coseismic Slip ◽  
Quartz Veins ◽  
Mylonitic Foliation ◽  
History Of

<p>Exhumation of subducted high-pressure units is favoured by relatively narrow, high-strain shear zones, where most metamorphic and deformational processes occur. Unfortunately, these are commonly overprinted and/or partly or fully obliterated along the exhumation path by younger fabrics or by metamorphic re-equilibration. Their identification and characterization are, therefore, of primary importance when aiming at reconstructing the deepest (and thus earliest) tectonometamorphic history of high-pressure crustal units.</p><p>The Northern Apennines (Italy) offer the opportunity to study a unique setting where continental units (Tuscan Metamorphic Units) were subducted to high-pressure conditions and then exhumed and juxtaposed against non-metamorphic units (Tuscan Nappe). We have studied a well exposed section in the Monticiano-Roccastrada Unit of the Mid Tuscan Ridge (MTR), where a mesoscopic (~20 m length and 5 m high) compressional duplex deforms the Palaeozoic-Triassic quartz-rich metasandstones, metaconglomerates and minor metapelites of the Monte Quoio - Montagnola Senese Unit with a top-to-the-NE sense of shear (Arenarie di Poggio al Carpino Formation; Casini et al., 2007).</p><p>Our approach is based on detailed fieldwork, microstructural and petrological investigations. Field observations reveal severe strain partitioning within the duplex between metapelite levels, corresponding to 10-50 cm thick high-strain zones, and metasandstone levels, which form relatively strain-free metric horses. Early generations of quartz veins are highly transposed (sheath folds occur) parallel to the metapelitic high-strain shear zones. Veins are composed of iso-oriented quartz, forming up to several centimetre long single-grain ribbons, Mg-carpholite (XMg~ 0.65) needles and K-white mica marking the stretching lineation. Carpholite in the transposed veins invariably defines the stretching direction of shear zones. These high-P veins coexist with a later generation of less deformed, oblique quartz veins. The mylonitic foliation in the metapelites is defined by quartz, chloritoid, pyrophyllite and K-white mica forming a stretching lineation coherent with the one visible in the veins. Geometrical, cross-cutting and petrographic relations suggest that there has occurred cyclic deformation between brittle and viscous conditions, with the veins forming broadly syn-mylonitic shearing. Thermodynamic modeling results suggest >0.8 GPa and ~350°C for the formation of both the high-pressure veins and the mylonitic foliation.</p><p>Shear zones were subsequently folded about the NNW-SSE axis of the regional antiform associated with the MTR. Later brittle overprinting is represented by quart-filled tension gashes and localized C’ planes, mostly within the more competent metasandstone levels, indicating top-to-the-SW reactivation. In summary, our results suggest a cyclic brittle-ductile behaviour occurring at high pressure conditions. This could potentially reflect the repeated alternation between aseismic creep (viscous) and coseismic slip (brittle) during the first stages of the exhumation history of this portion of the northern Apennines, from lower to middle crustal levels in a compressional top-to-the-NE setting. Dating of K-white mica is ongoing to constrain the geodynamic scenario of such shear zone.</p><p> </p><p>Casini, G., Decandia, F.A., Tavarnelli, E., 2007. Analysis of a mesoscopic duplex in SW Tuscany, Italy: implications for thrust system development during positive tectonic inversion. Geol. Soc. London, Spec. Publ. 272, 437–446.</p><p> </p>

scholarly journals Deeply subducted continental fragments – Part 1: Fracturing, dissolution–precipitation, and diffusion processes recorded by garnet textures of the central Sesia Zone (western Italian Alps)

Solid Earth ◽  
2018 ◽  
Vol 9 (1) ◽  
pp. 167-189 ◽  
Author(s):  
Francesco Giuntoli ◽  
Pierre Lanari ◽  
Martin Engi
Keyword(s):  
High Pressure ◽  
Diffusion Processes ◽  
Shear Zones ◽  
Granulite Facies ◽  
Thermodynamic Modelling ◽  
Italian Alps ◽  
Eclogite Facies ◽  
Compositional Patterns ◽  
And Diffusion ◽  
Insight Into

Abstract. Contiguous continental high-pressure terranes in orogens offer insight into deep recycling and transformation processes that occur in subduction zones. These remain poorly understood, and currently debated ideas need testing. The approach we chose is to investigate, in detail, the record in suitable rock samples that preserve textures and robust mineral assemblages that withstood overprinting during exhumation. We document complex garnet zoning in eclogitic mica schists from the Sesia Zone (western Italian Alps). These retain evidence of two orogenic cycles and provide detailed insight into resorption, growth, and diffusion processes induced by fluid pulses in high-pressure conditions. We analysed local textures and garnet compositional patterns, which turned out remarkably complex. By combining these with thermodynamic modelling, we could unravel and quantify repeated fluid–rock interaction processes. Garnet shows low-Ca porphyroclastic cores that were stable under (Permian) granulite facies conditions. The series of rims that surround these cores provide insight into the subsequent evolution: the first garnet rim that surrounds the pre-Alpine granulite facies core in one sample indicates that pre-Alpine amphibolite facies metamorphism followed the granulite facies event. In all samples documented, cores show lobate edges and preserve inner fractures, which are sealed by high-Ca garnet that reflects high-pressure Alpine conditions. These observations suggest that during early stages of subduction, before hydration of the granulites, brittle failure of garnet occurred, indicating high strain rates that may be due to seismic failure. Several Alpine rims show conspicuous textures indicative of interaction with hydrous fluid: (a) resorption-dominated textures produced lobate edges, at the expense of the outer part of the granulite core; (b) peninsulas and atoll garnet are the result of replacement reactions; and (c) spatially limited resorption and enhanced transport of elements due to the fluid phase are evident along brittle fractures and in their immediate proximity. Thermodynamic modelling shows that all of these Alpine rims formed under eclogite facies conditions. Structurally controlled samples allow these fluid–garnet interaction phenomena to be traced across a portion of the Sesia Zone, with a general decrease in fluid–garnet interaction observed towards the external, structurally lower parts of the terrane. Replacement of the Permian HT assemblages by hydrate-rich Alpine assemblages can reach nearly 100 % of the rock volume. Since we found no clear relationship between discrete deformation structures (e.g. shear zones) observed in the field and the fluid pulses that triggered the transformation to eclogite facies assemblages, we conclude that disperse fluid flow was responsible for the hydration.

Terranes and terrane boundaries in the Sudetes, northeast Bohemian Massif

1997 ◽  
Vol 134 (5) ◽  
pp. 717-725 ◽  
Author(s):  
Z. CYMERMAN ◽  
M. A. J. PIASECKI ◽  
R. SESTON
Keyword(s):  
High Pressure ◽  
Central Region ◽  
Bohemian Massif ◽  
Low Grade ◽  
High Grade ◽  
High Pressure Granulite ◽  
Facies Metamorphism ◽  
History Of ◽  
Ophiolitic Rocks ◽  
Southwestern Margin

In the Sudetes, seven distinct lithostratigraphic terranes exhibit a symmetric distribution. A central region of basinal/oceanic and ophiolitic rocks, the Central Sudetic terrane is bordered, respectively to the northwest and southeast, by the sialic Saxothuringian and Moldanubian terranes. These exhibit contrasting metasedimentary/metavolcanic successions and tectonic-metamorphic sequences, but both are characterized by Palaeozoic plutonism. These are in turn bordered (again respectively to the northwest and southeast) by the Lusatian and Moravian terranes, which are also sialic, but contain Cadomian granitoids and represent rifted and now widely separated fragments of Gondwana. Along the southwestern flank of the Sudetes, the Barrandian terrane, largely covered by younger sediments, extends to the southwestern margin of the Bohemian Massif. The Sowie Góry terrane forms a klippe of high grade gneisses tectonically emplaced on top of low-grade, sheared ophiolites of the Central Sudetic terrane. The Sowie Góry terrane exhibits a history of three distinct, probably multi-orogenic, regional metamorphic events: an early high-pressure granulite/eclogite metamorphism followed by medium- to low-pressure granulite, and in turn by amphibolite facies metamorphism. All the terrane boundaries are complex zones of ductile to brittle shearing, modified by later brittle movements. Some, such as the Leszczyniec shear zone, mark lines of old, pre-Variscan rift and suture zones, reactivated and overprinted during a series of Variscan ductile to brittle events of extensional shearing with related metamorphism and plutonism.

scholarly journals The Archaean gneiss complex of northern Labrador A review of current results, ideas and problems

1991 ◽  
Vol 39 ◽  
pp. 153-166
Author(s):  
D. Bridgwater ◽  
L. Schiøtte
Keyword(s):  
Major Part ◽  
Granulite Facies ◽  
High Grade ◽  
Late Archaean ◽  
West Greenland ◽  
Metamorphic History ◽  
Gneiss Complex ◽  
History Of ◽  
Augen Gneiss ◽  
Dominant Phase

1. The early Archaean rocks in northern Labrador can be subdivided into the ea. 3.78 Ga Nulliak supracrus­tal association, the migmatitic Uivak I gneisses, the dominant phase of which was emplaced at ea. 3.73 Ga, and the Uivak II augen gneiss. Inherited low-U rounded inclusions within igneous zircons in the Uivak I gneisses have ages between 3.73 and 3.86 Ga and are more likely to have been derived from a pre-existing high-grade metamorphic gneiss complex than from the Nulliak association. In the early Archaean there were probably several rapid cycles of sedimentary deposition and volcanism followed by emplacement of major plutons. Mid Archaean gneisses are more abundant in northern Labrador than previously realised. The late Archaean metamorphic history of these gneisses is different from the history of the early Archaean gneisses. Whereas an important part of the mid Archaean suite was emplaced in granulite facies and retrogressed at the time of granitoid veining at ea. 2.99 Ga, the major part of the early Archaean rocks were reworked under granulite facies conditions in a sequence of closely spaced events between 2. 7 and 2.8 Ga. The two groups of gneisses had different metamorphic histories until ea. 2.7 Ga, but late and post-tectonic granites of 2.5- 2. 7 Ga age cut across both. It is suggested that the terrane model in southern West Greenland can be extended to Labrador and that tectonic intercalation of early and mid Archaean gneisses took place around 2.7 Ga. Correlation between the Maggo gneisses around Hopedale, mid Archaean gneisses in northernmost Labrador and gneisses from the Akia terrane in West Greenland is suggested. Like the Malene supracrustals in West Greenland the Upernavik supracrustals in Labrador are composite associations, the youngest of which are thought to have been deposited around 2. 7 Ga.

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