Relative strength of mafic and felsic rocks during amphibolite facies metamorphism and deformation

Pearce, Mark A.; Wheeler, John; Prior, David J.

doi:10.1016/j.jsg.2011.01.002

Cited by 39 publications

(23 citation statements)

References 40 publications

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“…In the studied sample, amphibole, like plagioclase, shows no evidence of deformation via crystal plasticity, such as high intracrystalline misorientations, misorientation bands, subgrains. Instead, it appears to have deformed by fracturing and coupled dissolution–precipitation, as also suggested in other studies (Berger & Stünitz, ; Brodie & Rutter, ; Lafrance & Vernon, ; Nyman, Law, & Smelik, ; Pearce et al., ). Crystal plasticity is potentially a more effective deformation mechanism at higher temperatures (e.g., Skrotzki, ).…”

Section: Discussionsupporting

confidence: 78%

“…The CPO and shape‐preferred orientation of the amphibole can be acquired via different mechanisms: dissolution–precipitation creep (Bons & den Brok, ; Imon et al., ; Pearce et al., ), oriented grain growth and passive rotation after growth (Berger & Stünitz, ; Kanagawa, Shimano, & Hiroi, ), and/or diffusion creep (Getsinger & Hirth, ). In the studied sample, the CPO displayed by Amp2 is mostly inherited due to the pseudomorphic and topotaxial growth on Amp1.…”

Section: Discussionmentioning

confidence: 99%

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Replacement reactions and deformation by dissolution and precipitation processes in amphibolites

Giuntoli

Menegon

Warren

2018

Journal Metamorphic Geology

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The deformation of the middle to lower crust in collisional settings occurs via deformation mechanisms that vary with rock composition, fluid content, pressure, and temperature. These mechanisms are responsible for the accommodation of large tectonic transport distances during nappe stacking and exhumation. Here, we show that fracturing and fluid flow triggered coupled dissolution–precipitation and dissolution–precipitation creep processes, which were responsible for the formation of a mylonitic microstructure in amphibolites. This fabric is developed over a crustal thickness >500 m in the Lower Seve Nappe (Scandinavian Caledonides). Amphibolites display a mylonitic foliation that wraps around albite porphyroclasts appearing dark in panchromatic cathodoluminescence (CL). The albite porphyroclasts were dissected and fragmented by fractures preferentially developed along the (001) cleavage planes and display lobate edges with embayments and peninsular features. Two albite/oligoclase generations, bright in CL, resorbed and overgrew the porphyroclasts, sealing the fractures. Electron backscattered diffraction shows that the two albite/oligoclase generations grew both pseudomorphically and topotaxially at the expense of the albite porphyroclasts and epitaxially around them. These two albite/oligoclase generations also grew as neoblasts elongated parallel to the mylonitic foliation. The amphibole crystals experienced a similar microstructural evolution, as evidenced by corroded ferrohornblende cores surrounded by ferrotschermakite rims that preserve the same crystallographic orientation of the cores. Misorientation maps highlight how misorientations in amphibole are related to displacement along fractures perpendicular to its c‐axis. No crystal plasticity is observed in either mineral species. Plagioclase and amphibole display a crystallographic preferred orientation that is the result of topotaxial growth on parental grains and nucleation of new grains with a similar crystallographic orientation. Amphibole and plagioclase thermobarometry constrains the mylonitic foliation development to the epidote amphibolite facies (˜600°C, 0.75–0.97 GPa). Our results demonstrate that at middle to lower crustal levels, the presence of H2O‐rich fluid at grain boundaries facilitates replacement reactions by coupled dissolution–precipitation and favours deformation by dissolution–precipitation creep over dislocation creep in plagioclase and amphibole.

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Section: Discussionsupporting

confidence: 78%

Section: Discussionmentioning

confidence: 99%

Replacement reactions and deformation by dissolution and precipitation processes in amphibolites

Giuntoli

Menegon

Warren

2018

Journal Metamorphic Geology

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“…This is supported by experiments and natural examples, where the CPO of amphibole is weak when deformed by GBS accommodated by diffusion creep (e.g. Gardner, Piazolo, & Daczko, 2016;Getsinger, Hirth, St€ unitz, & Goergen, 2013;Pearce, Wheeler, & Prior, 2011). This is consistent with the absence of CPO in plagioclase and the weak CPO in clinozoisite (Figure 4f,g).…”

Section: Deformation In High-strain Zones: Meltpresent or Solid Stasupporting

confidence: 83%

The recognition of former melt flux through high‐strain zones

Stuart

Piazolo

Daczko

2018

Journal Metamorphic Geology

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High‐strain zones are potential pathways of melt migration through the crust. However, the identification of melt‐present high‐strain deformation is commonly limited to cases where the interpreted volume of melt “frozen” within the high‐strain zone is high (>10%). In this contribution, we examine high‐strain zones in the Pembroke Granulite, an otherwise low‐strain outcrop of volcanic arc lower crust exposed in Fiordland, New Zealand. These high‐strain zones display compositional layering, flaser‐shaped mineral grains, and closely spaced foliation planes indicative of high‐strain deformation. Asymmetric leucosome surrounding peritectic garnet grains suggest deformation was synchronous with minor amounts of in situ partial melting. High‐strain zones lack typical mylonite microstructures and instead display typical equilibrium microstructures, such as straight grain boundaries, 120° triple junctions, and subhedral grain shapes. We identify five key microstructures indicative of the former presence of melt within the high‐strain zones: (a) small dihedral angles of interstitial phases; (b) elongate interstitial grains; (c) small aggregates of quartz grains with xenomorphic plagioclase grains connected in three dimensions; (d) fine‐grained, K‐feldspar bearing, multiphase aggregates with or without augite rims; and (e) mm‐ to cm‐scale felsic dykelets. Preservation of key microstructures indicates that deformation ceased as conditions crossed the solidus, breaking the positive feedback loop between deformation and the presence of melt. We propose that microstructures indicative of the former presence of melt, such as the five identified above, may be used as a tool for recognising rocks formed during melt‐present high‐strain deformation where low (<5%) volumes of leucosome are “frozen” within the high‐strain zone.

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“…Instead, comparison of the LPO with the kinematics of the shear zone (see boxed inset in Figure 4) indicates slip could have occurred on (111) plane in the [110] direction (Burgers vector ¼ 0.7 nm) [Marshall and McLaren, 1977;Olsen and Kohlstedt, 1984]. Slip in the [110] direction has been inferred for recrystallized plagioclase in naturally deformed gabbros [Kanagawa et al, 2008] and mafic dikes [Pearce et al, 2011]. Irrespective of slip system, the pole figures show that there is a pronounced plagioclase LPO in the deformed monophase region, whereas the LPO in the other samples-including the directly adjacent deformed polyphase bands-is weak.…”

Section: Lattice-preferred Orientationmentioning

confidence: 99%

Influence of water on rheology and strain localization in the lower continental crust

Getsinger

Hirth

Stünitz

et al. 2013

Geochem Geophys Geosyst

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[1] We investigated deformation processes within a lower crustal shear zone exposed in gabbros from Arnïya, Norway. Over a distance of $1 m, the gabbro progresses from nominally undeformed to highly sheared where it is adjacent to a hydrous pegmatite. With increasing proximity to the pegmatite, there is a significant increase in the abundance of amphibole and zoisite (which form at the expense of pyroxene and calcic plagioclase) and a slight increase in the strength of plagioclase lattice-preferred orientation, but there is little change in recrystallized plagioclase grain size. Phase diagrams, the presence of hydrous reaction products, and deformation mechanism maps all indicate that the water activity (a H2O ) during deformation must have been high ($1) in the sheared gabbro compared with the nonhydrated, surrounding host gabbro. These observations indicate that fluid intrusion into mafic lower crust initiates syn-deformational, water-consuming reactions, creating a rheological contrast between wet and dry lithologies that promote strain localization. Thus, deformation of lower continental crust can be accommodated in highly localized zones of enhanced fluid infiltration. These results provide an example of how fluid weakens lower continental crust lithologies at high pressures and temperatures.

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Relative strength of mafic and felsic rocks during amphibolite facies metamorphism and deformation

Cited by 39 publications

References 40 publications

Replacement reactions and deformation by dissolution and precipitation processes in amphibolites

Replacement reactions and deformation by dissolution and precipitation processes in amphibolites

The recognition of former melt flux through high‐strain zones

Influence of water on rheology and strain localization in the lower continental crust

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