Evidence for deformation‐induced K‐feldspar replacement by myrmekite

Simpson, Carol; Wintsch, Robert P.

doi:10.1111/j.1525-1314.1989.tb00588.x

Cited by 302 publications

(169 citation statements)

References 64 publications

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“…Although this may represent a metamorphic replacement of boundaries. They are interpreted to represent new metamorphic minerals (neocrystals) that have grown to only a limited degree K-feldspar by plagioclase and quartz (Simpson & Wintsch, 1989), we interpret it to be an igneous texture recording resorption during within the old igneous texture (see reactions below). In summary, the low-strain samples display a combination of thermal variations in the Mary granite magma.…”

Section: G Eological Characteristics Of the Mary Granitementioning

confidence: 88%

Microstructural tectonometamorphic processes and the development of gneissic layering: a mechanism for metamorphic segregation

Williams

Melis

Kopf

et al. 2000

Journal Metamorphic Geology

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The Mary granite, in the East Athabasca mylonite triangle, northern Saskatchewan, provides an example and a model for the development of non-migmatitic gneissic texture. Gneissic compositional layering developed through the simultaneous evolution of three microdomains corresponding to original plagioclase, orthopyroxene and matrix in the igneous rocks. Plagioclase phenocrysts were progressively deformed and recrystallized, first into core and mantle structures, and ultimately into plagioclase-rich layers or ribbons. Garnet preferentially developed in the outer portions of recrystallized mantles, and, with further deformation, produced garnet-rich sub-layers within the plagioclase-rich gneissic domains. Orthopyroxene was replaced by clinopyroxene and garnet (and hornblende if sufficient water was present), which were, in turn, drawn into layers with new garnet growth along the boundaries. The igneous matrix evolved through a number of transient fabric stages involving S-C fabrics, S-C-C∞ fabrics, and ultramylonitic domains. In addition, quartz veins were emplaced and subsequently deformed into quartz-rich gneissic layers. Moderate to highly strained samples display extreme mineralogical (compositional) segregation, yet most domains can be directly related to the original igneous precursors. The Mary granite was emplaced at approximately 900°C and 1.0 GPa and was metamorphosed at approximately 750°C and 1.0 GPa. The igneous rocks crystallized in the medium-pressure granulite field (Opx-Pl) but were metamorphosed on cooling into the high-pressure (Grt-Cpx-Pl) granulite field. The compositional segregation resulted from a dynamic, mutually reinforcing interaction between deformation, metamorphic and igneous processes in the deep crust. The production of gneissic texture by processes such as these may be the inevitable result of isobaric cooling of igneous rocks within a tectonically active deep crust.

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Section: G Eological Characteristics Of the Mary Granitementioning

confidence: 88%

Microstructural tectonometamorphic processes and the development of gneissic layering: a mechanism for metamorphic segregation

Williams

Melis

Kopf

et al. 2000

Journal Metamorphic Geology

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“…Quartz microstructures include deformation bands and subgrains with equant recrystallized grains that exhibit a strong lattice-preferred orientation. Feldspar porphyroclasts contain deformation twins and some deformation-induced myrmekite [e.g., Simpson and Wintsch, 1989]. Collectively, quartz and feldspar microstructures suggest that deformation occurred at temperatures between 400°a nd 500°C [De Paor and Simpson, 1993;Stipp et al, 2002;Rosenberg and Stünitz, 2003].…”

Section: Ductile Deformation (D3 D4)mentioning

confidence: 99%

Regional deformation of the Sierra de San Luis, Argentina: Implications for the Paleozoic development of western Gondwana

Whitmeyer

Simpson

2004

Tectonics

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Metamorphic and ductile deformation fabrics within the Sierra de San Luis, central Argentina, provide evidence for the Early to Middle Paleozoic development of the paleo‐Pacific margin of Gondwana. Presumed Vendian‐aged metasedimentary rocks within both the Sierra de San Luis and the Sierras de Córdoba preserve early pressure solution cleavage development. Cambrian peak metamorphism in the Sierras de Córdoba and Ordovician peak metamorphism in the Sierra de San Luis indicate that juxtaposition of these two terranes did not occur prior to the Late Ordovician. Peak metamorphism and regional folding of the Sierra de San Luis metasedimentary rocks is equated with early stages of the development and shortening of the Famatinian magmatic arc along the western margin of Gondwana. Extensive, NNE trending zones of upper greenschist‐ to amphibolite‐facies ductile deformation within the Sierra de San Luis and along the western margin of the Sierra de Comechingones record the dextral oblique juxtaposition of the Sierra de San Luis terrane against the southwestern margin of the Sierras de Córdoba, coincident with, or closely followed by, the suture of the exotic Precordillera terrane to the western margin of Gondwana by the Early Devonian. Middle to Late Devonian restricted, lower greenschist‐grade reactivation of ductile faults may record the accretion of the Chilenia terrane, the final stage of Paleozoic convergent tectonism along this segment of western Gondwana.

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“…During deformation, however, the marginal parts of feldspars can recrystallize/neocrystallize, thus contributing to rock weakening. Reactions involving replacement of K-feldspar by myrmekite (Vernon et al 1983;Simpson and Wintsch 1989;Tsurumi et al 2003) are especially notable for producing fine-grained aggregates to be later transformed into foliae, contributing to rock deformation.…”

Section: Introductionmentioning

confidence: 99%

Protomylonite evolution potentially revealed by the 3D depiction and fractal analysis of chemical data from a feldspar

Słaby

Domonik

Śmigielski

et al. 2014

Contrib Mineral Petrol

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An alkali feldspar megacryst from a protomylonite has been studied using laser ablation-ICP-mass spectrometry combined with cathodoluminescence imaging, Raman spectroscopy, and electron probe microanalysis. The aim was to determine the original (magmatic) geochemical pattern of the crystal and the changes introduced by protomylonitization. Digital concentration-distribution models, derivative gradient models, and fractal statistics, e.g., Hurstexponent values are used in a novel way to reveal subtle changes in the trace-element composition of the feldspar. Formation of the crystal is reflected in a slightly chaotic trace-element (Ba, Sr, and Rb) distribution pattern that is more or less characterized by continuous development from a fairly homogeneous environment. Derivative gradient models demonstrate a microdomain pattern. Fractal statistics show that element behavior was changeable, with Ba and Sr always more persistent (continuing) and Rb always less persistent, with the latter showing a tendency to migrate. The variations in the Hurst exponent are, however, too large to be explained by magmatic differentiation alone. The observed element behavior may be explained by structural changes revealed by Raman spectroscopy and CL. In high-strain domains, T-O-T modes become stronger for Si-O-Al than Al-O-Al linkages. Increasing amounts of Al-O--Al defects are demonstrated by cathodoluminescence. Both may result from small-distance diffusion creep, making the crystal geochemical pattern slightly patchy. In turn, the marginal part of the megacryst has a mosaic of randomly orientated, newly crystallized K-feldspars. The re-growth is confirmed by trace-element distribution patterns and fractal statistics which identify an abrupt change in the transformation environment. The novel set of tools used in this study reveals a complicated history of megacryst formation and transformation that otherwise would be difficult to unravel and decipher.Communicated by J. Hoefs. Electronic supplementary materialThe online version of this article

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Evidence for deformation‐induced K‐feldspar replacement by myrmekite

Cited by 302 publications

References 64 publications

Microstructural tectonometamorphic processes and the development of gneissic layering: a mechanism for metamorphic segregation

Microstructural tectonometamorphic processes and the development of gneissic layering: a mechanism for metamorphic segregation

Regional deformation of the Sierra de San Luis, Argentina: Implications for the Paleozoic development of western Gondwana

Protomylonite evolution potentially revealed by the 3D depiction and fractal analysis of chemical data from a feldspar

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