Prograde reaction histories deduced from compositional zonation and mineral inclusions in garnet from the Gassetts schist, Vermont

Thompson, Alan Bruce; Tracy, Robert J.; Lyttle, P.T.; Thompson, James B.

doi:10.2475/ajs.277.9.1152

Cited by 112 publications

(37 citation statements)

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“…Assemblages developed in pelitic schists are particularly sensitive to changes in pressure and temperature. Furthermore, garnet porphyroblast inclusion assemblages in pelitic schists can sometimes be used to document metamorphic pressure/temperature time paths (e.g., Tracy et al 1976; Thompson et al 1977), and garnet zonation characteristics may provide direct evidence of the rocks' thermal history (e.g., Bohlen & Essene 1980;Tracy 1982).…”

Section: Introductionmentioning

confidence: 99%

Low pressure/high temperature metamorphism of Koettlitz Group schists, Taylor Valley and upper Ferrar Glacier area, South Victoria Land, Antarctica

Allibone

1992

New Zealand Journal of Geology and Geophysics

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Koettlitz Group metasediments of the Taylor Valley and upper Ferrar Glacier region contain rare horizons of garnetbearing and pelitic schist, together comprising 2% of the exposed outcrop. Pelitic schists containing the upper amphibolite facies assemblage quartz-sillimanite-cordieritebiotite-garnet-K-feldspar-relict andalusite are restricted to The Defile in the lower Taylor Valley. In the upper Taylor Valley, muscovite-K-feldspar-quartz-plagioclase-biotite assemblages occur adjacent to anatectic migmatites and quartz-K-feldspargarnet-biotite-plagioclase assemblages, with one sample containing sillimanite as inclusions in garnet At the confluence of the Darkowski and Ferrar Glaciers, quartz-biotiteplagioclase-garnet schists contain rare quartz-plagioclasecordierite leucosomes. Phase relations indicate peak metamorphic temperatures and pressures of 700 ± 50°C and 4-5 kbars throughout the Taylor Valley and upper Ferrar Glacier area. The presence of relict andalusite in sample 56816 suggests a low pressure prograde history, but the lack of relict pelitic schist assemblages preserved in garnet porphyroblasts prevents definition of a pressure/temperature trajectory.Geothermobarometric calculations based on sample 56816 from The Defile in the lower Taylor Valley indicate pressure/ temperature conditions in the range 4-6 kbars and 750 ± 50°C. Temperatures calculated using an estimated pressure of 4.5 kbars derived from phase relationships suggest peak metamorphic temperatures of 650-700°C in the upper Taylor Valley, Ferrar Glacier, and northern side of the Blue Glacier area.

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Section: Introductionmentioning

confidence: 99%

Low pressure/high temperature metamorphism of Koettlitz Group schists, Taylor Valley and upper Ferrar Glacier area, South Victoria Land, Antarctica

Allibone

1992

New Zealand Journal of Geology and Geophysics

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“…Such porphyroblasts may contain truncation planes, where one pattern of S, is truncated by another pattern ( Fig. 1; Thompson et al, 1977;Karabinos, 1984). Truncation planes apparently form by a relative change in the orientation of the porphyroblast with respect to the surrounding foliation, possibly with local partial solution of the porphyroblast, followed by new porphyroblast growth.…”

Section: Statement (6)-porphyroblasts In Pelitic Rocksmentioning

confidence: 99%

Porphyroblast rotation: *eppur si muove**?

Passchier

Trouw

Zwart

et al. 1992

Journal Metamorphic Geology

147

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In a number of recent papers, the theory has been postulated that porphyroblasts as a rule do not rotate with respect to geographical coordinates, and can be used to determine the original orientation of older foliations. Complex inclusion patterns in spiral garnets have even been used to advocate a new model of orogenesis, involving several alternating phases of horizontal shortening and extension. Critical assessment of the assumptions and data used to support the theory of irrotational porphyroblasts reveals numerous flaws. Millipede structures, used as proof for flow partitioning, can also form by other flow geometries. Evidence quoted to support irrotational behaviour of porphyroblasts is unsound. Porphyroblasts do occur in sets with a preferred orientation of the internal foliation trace, but these cannot be shown to represent original orientations. Microstructures which resemble truncation planes in spiral garnets are used as evidence that these structures developed by several phases of deformation and as proof for periodic extension and horizontal shortening in orogenesis. They can, however, also be explained by intermittent growth of a rotating porphyroblast during a single phase of deformation. Finally, porphyroblast sets in which orientation is a function of aspect ratio indicate that porphyroblast rotation with respect to kinematic axes does occur in at least some situations.

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“…for a time interval corresponding to the garnet growth (cf. Tracy et al, 1976;Thompson et al, 1977;Krogh, 1982;Spear and Selverstone, 1983). Such applications must be built upon spatial models for the garnet growth (e.g.…”

Section: Implications For P-t-time Interpretationsmentioning

confidence: 99%

Coronite and eclogite formation in olivine gabbro (Western Norway): reaction paths and garnet zoning

Mørk

1986

Mineral. mag.

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Region of Norway have been subjected to a high P-T regime and preserve evidence of the existence of several stages in the consequent reaction sequence. Incomplete reaction stages are characterized by corona structures between relict igneous phases and by frequent pseudomorphs after igneous olivine, plagioclase and augite. The increasing degree of reaction is recorded by successive increases of modal garnet, omphacite and phlogopite (+ orthopyroxene) to produce eclogite, often with excellent pseudomorphic preservation of the igneous fabric.Pseudomorphic replacement of olivine by orthopyroxene (in coronas and aggregates) and of plagioclase by local assemblages of garnet, spinel, and sodic plagioclase, is interpreted as a transient reaction stage with restricted and selective diffusion between the original mafic and felsic domains. Complete eclogitization is compatible with more extensive diffusion, especially of Na and AI (on a mm scale) leading to omphacite production by replacement of olivine, augite, and orthopyroxene. Concomitant reactions in the felsic domains lead to total replacement of the transient phases by garnet, with or without inclusions of minute omphacite grains.Strong Ca zoning of garnet in the coronites is interpreted as a relict growth zoning, attributed to local controls by diffusion and subreactions in the plagioclase host and to local garnet + plagioclase + spinel equilibria. In contrast, complete eclogitization is associated with diffusional homogenization of garnet by (Mg, Fe)Ca_ 1 and MgFe_ I exchange with omphacite_+phlogopite.

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Prograde reaction histories deduced from compositional zonation and mineral inclusions in garnet from the Gassetts schist, Vermont

Cited by 112 publications

References 0 publications

Low pressure/high temperature metamorphism of Koettlitz Group schists, Taylor Valley and upper Ferrar Glacier area, South Victoria Land, Antarctica

Low pressure/high temperature metamorphism of Koettlitz Group schists, Taylor Valley and upper Ferrar Glacier area, South Victoria Land, Antarctica

Porphyroblast rotation: *eppur si muove**?

Coronite and eclogite formation in olivine gabbro (Western Norway): reaction paths and garnet zoning

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Prograde reaction histories deduced from compositional zonation and mineral inclusions in garnet from the Gassetts schist, Vermont

Cited by 112 publications

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Low pressure/high temperature metamorphism of Koettlitz Group schists, Taylor Valley and upper Ferrar Glacier area, South Victoria Land, Antarctica

Low pressure/high temperature metamorphism of Koettlitz Group schists, Taylor Valley and upper Ferrar Glacier area, South Victoria Land, Antarctica

Porphyroblast rotation: eppur si muove*?

Coronite and eclogite formation in olivine gabbro (Western Norway): reaction paths and garnet zoning

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Porphyroblast rotation: *eppur si muove**?