Metamorphic differentiation in some Lewisian rocks of North West Scotland

Ghaly, Tharwat S.

doi:10.1007/bf00400126

Cited by 6 publications

(4 citation statements)

References 5 publications

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“…5d). This kind of banding is usually produced by metamorphic differentiation (solution + recrystallization + mechanical differentiation) during progressive deformation (e.g., Eskola, 1932;Ghaly, 1969), and/or typically by differentiation along cleavage planes formed in a previously foliated rock (e.g., Talbot and Hobbs, 1968;Gray, 1977), i.e. a rock containing a S 1 fabric.…”

Section: (S 1 )mentioning

confidence: 99%

Extensional flow produces recumbent folds in syn-orogenic granitoids (Padrón migmatitic dome, NW Iberian Massif)

2017

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Section: (S 1 )mentioning

confidence: 99%

Extensional flow produces recumbent folds in syn-orogenic granitoids (Padrón migmatitic dome, NW Iberian Massif)

2017

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Section: H E M I C a L C H A N G E S During V E I N Formation Own Ormentioning

confidence: 99%

“…The first method compares the bulk composition (obtained by whole-rock chemical analysis) of the presumed original rock to that of the differentiated system, i.e. vein+mafic selvedge (Turner, 1941;Ghaly, 1969;Chakraborty, 1977). 2. The second method compares the modal analyses of the presumed original rock with those of the differentiated system, to test for a material balance (e.g.…”

Section: H E M I C a L C H A N G E S During V E I N Formation Own Ormentioning

confidence: 99%

The subsolidus segregation of layer‐parallel quartz‐feldspar veins in greenschist to upper amphibolite facies metasediments

Sawyer

Robin

1986

Journal Metamorphic Geology

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Abstract. Layer-parallel (i.e. parallel to foliation or bedding) vein formation in the graywackes and pelites of the Quetico Metasedimentary Belt occurred during synchronous prograde metamorphism and regional (DJ compression. In a traverse across metasediments which change in metamorphic grade from greenschist to upper amphibolite (migmatite) facies, layer-parallel veins show the following trends:(1) an increase in thickness and internal cornplexity, the latter due to successive boudinage; (2) low-grade veins are parallel to planes of anisotropy due to the original sedimentary fabric of the host rocks, but at higher grades other sites are also used and (3) a systematic increase in plagioclase/quartz ratio in the veins towards higher grade, adjacent mafic selvedges first exhibit quartz depletion then, in the amphibolite facies, plagioclase depletion. Mineralogical zoning is often preserved in a single vein, older parts are more quartz-rich than younger.Mass balance calculations and whole-rock geochemistry based on veins, mafic selvedges and country rock are consistent with a closedsystem subsolidus segregation origin. The layerparallel veins are syntectonic, and migration of the mobile components required to form their mineralogy is a stress-induced mass transfer. The source of these components appears to be dominantly pressure solution of the same minerals in the host rocks, although metamorphic reactions may also have contributed. Veins nucleated first at those sites where initial sedimentary heterogeneites, such as fine-scale graded bedding, provided gradients of normal stress across grain boundaries, and hence of chemical potential, necessary to drive the subsolidus segregation process. The earliest veins are thus parallel to bedding. Later, nucleation of the veins could also occur along more randomly distributed sites within the metasediments, and these veins grew parallel to the schistosity rather than bedding, if the two were distinct. Once formed, the veins themselves, which are more competent than the surrounding rock, provide the stress heterogeneity required for their further growth. The increasing 'plagioclase/ quartz ratio in the veins may be due to a temperature dependent increase in plagioclase component mobility relative to quartz. Alternatively, the increasing transfer distances for silica, resulting from prior quartz depletion in the inner parts of the mafic selvedge, may increase the relative mobility of plagioclase component.

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“…More brittle minerals, such as feldspar and quartz, are rotated and segregate into layers between the shears. Ghaly (1969) described some Lewisian rocks from NW Scotland which showed a gradation from hornblende-feldspar schist to striped and banded schist and gneisses, through to hornblendic and felsic lenses and pods. He proposed that they had been formed by a process of metamorphic differentiation involving solution, recrystallization, and mechanical differentiation acting simultaneously or separately.…”

mentioning

confidence: 99%

Metamorphic differentiation; a mechanism indicated by zoned kyanite crystals in some rocks from the Lukmanier region, Switzerland

Bramwell

1985

Mineral. mag.

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ABSTRACT. Two samples of garnet kyanite-staurolite schist from Lukmanier, Switzerland, each contain two chemical varieties of kyanite which occur in texturally distinct areas of the rock. Type 1 form large idiomorphic crystals within an open crenulation cleavage S 3. They exhibit a systematic zonation of F%O3, with core values of 0.8 ~ decreasing to 0.3 ~ at the crystal margin. Type 2 form small, much less abundant crystals in areas between S 3 cleavage zones, and have a homogeneous distribution of 0.3 ~ Fe203 throughout the crystal.It is suggested that the first-nucleated crystals contain the highest core concentration of Fe203 and are the largest. A positive correlation between core Fe203 values and crystal size is interpreted as a nucleation and growth sequence. This indicates that the first crystals formed preferentially in S 3 (Type 1), with Type 2 crystals growing later outside the $3 zones.Concentration of kyanite in S 3 zones produces a distinct mineral banding in the rock. A mechanism for the development of metamorphic differentiation by preferred nucleation of kyanite in $3 is proposed.

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Metamorphic differentiation in some Lewisian rocks of North West Scotland

Cited by 6 publications

References 5 publications

Extensional flow produces recumbent folds in syn-orogenic granitoids (Padrón migmatitic dome, NW Iberian Massif)

Extensional flow produces recumbent folds in syn-orogenic granitoids (Padrón migmatitic dome, NW Iberian Massif)

The subsolidus segregation of layer‐parallel quartz‐feldspar veins in greenschist to upper amphibolite facies metasediments

Metamorphic differentiation; a mechanism indicated by zoned kyanite crystals in some rocks from the Lukmanier region, Switzerland

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