Migmatites in the Moines

Barr, David

doi:10.1007/978-1-4613-2347-1_7

Cited by 36 publications

(33 citation statements)

References 53 publications

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“…Although no quantitative metamorphic study has yet been undertaken, there is widespread petrographic evidence for thorough recrystallization within the amphibolite facies. These observations are consistent with a syn-D 2 age of emplacement for the granites, broadly synchronous with displacements along the Naver Thrust (Barr 1985;.…”

Section: Regional Settingsupporting

confidence: 86%

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U–Pb geochronology of deformed metagranites in central Sutherland, Scotland: evidence for widespread late Silurian metamorphism and ductile deformation of the Moine Supergroup during the Caledonian orogeny

Kinny

Strachan

Friend

et al. 2003

JGS

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Within the Caledonides of central Sutherland, Scotland, the Neoproterozoic metasedimentary rocks of the Moine Supergroup record NW-directed D 2 ductile thrusting and nappe assembly, accompanied by widespread tight-to-isoclinal folding and amphibolite-facies metamorphism. A series of metagranite sheets which were emplaced and penetratively deformed during D 2 have been dated using SHRIMP U-Pb geochronology. Zircon ages of 424 AE 8 Ma (Vagastie Bridge granite), 420 AE 6 Ma (Klibreck granite) and 429 AE 11 Ma (Strathnaver granite) are interpreted to date emplacement, and hence regional D 2 deformation, during mid-to late Silurian time. Titanite ages of 413 AE 3 Ma (Vagastie Bridge granite) and 416 AE 3 Ma (Klibreck granite) are thought to date post-metamorphic cooling through a blocking temperature of c. 550-500 8C. A mid-to late Silurian age for D 2 deformation supports published models that have viewed the internal ductile thrusts of this part of the orogen as part of the same kinematically linked system of forelandpropagating thrusts as the marginal Moine Thrust Zone. The new data contrast with previous interpretations that have viewed the dominant structures and metamorphic assemblages within the Moine Supergroup as having formed during the early to mid-Ordovician Grampian arc-continent orogeny. The mid-to late Silurian D 2 nappe stacking event in Sutherland is probably a result of the collision of Baltica with the Scottish segment of Laurentia.

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Section: Regional Settingsupporting

confidence: 86%

“…1b;Read 1931;Brown 1967Brown , 1971Barr 1985;). Some of the sheets analysed by Brown (1967) are peraluminous and may be of crustal derivation.…”

Section: Regional Settingmentioning

confidence: 99%

U–Pb geochronology of deformed metagranites in central Sutherland, Scotland: evidence for widespread late Silurian metamorphism and ductile deformation of the Moine Supergroup during the Caledonian orogeny

Kinny

Strachan

Friend

et al. 2003

JGS

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“…The amount of water available from dehydration of these rock compositions is likely to be very low (Lundgren, 1966) and substantial melting is unlikely to occur without access of external water (Vielzeuf & Holloway, 1985). The massive fabric of patch mesosomes is not conducive to such access, but dilatancy pumping of water into shear zones has been commonly proposed (Barr, 1985). The inferred contrast in stress regime between patch and diktyonitic migmatites may control or be a consequence of vapour-access and this may in turn control the extent of melting.…”

Section: Patch Versus Diktyonitic Migmatites and The Role Of Differenmentioning

confidence: 99%

“…Diktyonitic migmatites are characterized by low volume percentages of leucosome in the form of veins which dissect the host rock into isolated blocks; with increased leucosome percentage these are transitional to schollen migmatites (Mehnert, 1968;Ashworth, 1985) characterized by rafts of non-leucosome in leucosome. Both diktyonitic and schollen migmatites have been described as 'agmatitic' (for example, Barr, 1985) but the definition of agmatitic as 'breccia-like' (Ashworth, 1985) seems to imply a more brittle behaviour than is commonly seen in the host rock; the term 'agmatitic' will not be used in this paper.…”

Section: Introductionmentioning

confidence: 99%

Migmatite structures in the Central Gneiss Complex, Boca de Quadra, Alaska

Mclellan

1988

Journal Metamorphic Geology

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Abstract. Migmatite structures in the Coast Plutonic-Metamorphic Complex are well exposed in the inlet of Boca de Quadra, southeast Alaska. Two types of anatectic migmatites are present. Patch migmatites formed by in situ melting and subsequent crystallization of melt. Diktyonitic migmatites comprise a discontinuous veined network of leucocratic material, in which leucosomes enclose boudins of host rock. The margins of these boudins show the development of both melanosomes and shear band fabrics.Strain analysis of diktyonitic melanosomes indicates that these regions have undergone volume decreases of 2&27%. This volume decrease is attributed to melt extraction into the adjacent fracture-filling leucosomes. Thus, diktyonitic migmatites formed by shear-induced segregation of partial melt, whereas in patch migmatites the lack of shear stresses inhibited melt segregation. The variable structural style of anatectic migmatites in Boca de Quadra is not related to host-rock composition, but may be due to differences in the amount of differential stress during migmatization. These in turn may be controlled by host-rock strength andlor diachroneity of migmatization and deformation.Determination of volume changes during migmatization using strain analysis is potentially capable of discriminating intrusive and anatectic migmatites and consequently of documenting melt segregation and subsequent migration across crustal levels.

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“…On the other hand, Barr (1985) reported a chemical variance of migmatites and non migmatites in the Moines of northern Scotland. After applying statistic methods, he concluded that Si02, CaO and Na20 were enriched in migmatites, whereas A1203, Fe203 and K20 were depleted, and that these chemical differences be tween migmatites and non-migmatites corres pond to changes of mineral composition during migmatization such that the migmatites tend to be plagioclase-rich and the non-migmatites to be micaceous.…”

Section: Introductionmentioning

confidence: 99%

Anatexis and chemical evolution of pelitic rocks during metamorphism and migmatization in the Hidaka metamorphic belt, Hokkaido.

1989

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The southernmost Hidaka metamorphic belt consists mainly of cordierite tonalite intrusions and pelitic metamorphic rocks ranging from the greenschist to the granulite facies. Anatectic migmatites are common in the higher amphibolite and granulite facies zones. Compositional changes in major, rare earth elements and some other trace metals are so small that they are undetectable among the pelitic metamorphic rocks of zones A + B + C and D, but they are large enough to be detected in the higher am phibolite (zone D) to the granulite facies rocks (zone E). The enrichment of Fe, Mg, Na, Eu, and Sc, and the depletion of K, P, La, Ce, Nd, Cs and Rb are statistically significant in pelitic granulites, while heavy REEs are very variable. The chemical variation of pelitic granulite was derived from the accumulation of plagioclase +garnet. This suggests that more than 50-60% of the total volume of pelitic granulite was melted to produce a large amount of tonalitic magma, leaving pelitic granulite as a restite.Migmatites of the higher amphibolite facies are anatexites, and their K, P, Cs, Rb and light REE content is the same as that of lower grade metamorphic rocks. Migmatites of the higher amphibolite facies melted incipiently to segregate only a small amount of melt, and could not produce a large magmatic mass such as the cordierite tonalites.Cordierite tonalites are S-type granites, and their major elements, Cs, Rb and light REE concentra tions are similar to those of lower grade metamorphic rocks. The chemical variation of cordierite tonalites is explained by the extraction of plagioclase +garnet from a tonalitic magma and the variation of original sedimentary rocks. The small chemical difference between the cordierite tonalites and the lower grade metamorphic rocks suggests that the former was derived from a massive melting of meta pelites or that much of the restite is retained. The material migration among higher amphibolite facies rocks, pelitic granulites, migmatites and cordierite tonalites took place through mineral /melt interac tion in the lower crust.

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Migmatites in the Moines

Cited by 36 publications

References 53 publications

U–Pb geochronology of deformed metagranites in central Sutherland, Scotland: evidence for widespread late Silurian metamorphism and ductile deformation of the Moine Supergroup during the Caledonian orogeny

U–Pb geochronology of deformed metagranites in central Sutherland, Scotland: evidence for widespread late Silurian metamorphism and ductile deformation of the Moine Supergroup during the Caledonian orogeny

Migmatite structures in the Central Gneiss Complex, Boca de Quadra, Alaska

Anatexis and chemical evolution of pelitic rocks during metamorphism and migmatization in the Hidaka metamorphic belt, Hokkaido.

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