The Problems of the Rapakivi Granites

Backlund, Helge G.

doi:10.1086/624639

Cited by 25 publications

(3 citation statements)

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“…In connection with the evidence from sharp contacts, however, the phenomenon of rheomorphism, where mobility is imparted to country rock being granitized, must be borne in mind (see Backlund, 1937, Reynolds, 1937. Local movements may then lead to sharp contacts between the part mobilized and the immobilized rocks ; and in this way evidence for granitization, such as transition zones, may be obliterated.…”

Section: Discussionmentioning

confidence: 99%

On Granitization and Associated Processes

MacGregor

Wilson

1939

Geol. Mag.

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A review of the literature, and our own observations have led us to the conclusion that granitization, widely defined as the process by which solid rocks are converted to rocks of granitic character, has been an important process in the evolution of many subjacent masses of granite and allied rocks, such as granodiorite and diorite, in erogenic regions.Granitization of pre-existing rocks seems to involve two processes: (a) Metasomatism under the influence of permeating highly energized fluids—emanations—ahead of advancing magma; and (b) Mechanical penetration by magma. During the first process metasomatic exchanges take place between the minerals of the country-rock and its pore fluid: the reactions tend towards the achievement of equilibrium under the existing physico-chemical conditions. This metasomatism is selective, and promotes the convergence, in bulk and mineral composition, of rocks which were originally different, towards a common end-product. During the early stages conditions are essentially those of thermal metamorphism, but with rising energy level and increasing magmatic contributions the final conditions are dominantly “igneous”.The mechanical penetration of the altered rocks and their mixing with magma may follow the first process without a break. Alternatively, intrusion may occur before metasomatism is completed, or has even had appreciable effect. The mixing or intrusion stage is accompanied by reactions, which tend towards the attainment of equilibrium, under magmatic conditions, between the magma and the foreign material. In terms of bulk composition the products will be intermediate between that of the magma and that of the incorporated material.The metasomatism trends of the various examples that we have discussed are plotted together in Text-fig. 8. The diagram brings out the convergence of the curves from the normal sedimentary field to the rhyolite-diorite line, which is itself the locus of mixtures between metasomatized products and acid parental magmas.Granitic rocks may originate as the result of either or both these processes. That which predominates in any particular case depends on the supply of emanations and the local physico-chemical conditions. In the deeper parts of the granitic layer complete or selective fusion is to be expected, and the magmas so formed may rise and granitize the overlying rocks: firstly, by metasomatism, and at higher levels chiefly by mechanical penetration. At still higher levels, owing to loss of energy, rising magma is unable to cause extensive modification of the pre-existing rocks, and forms true intrusions.

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Section: Discussionmentioning

confidence: 99%

On Granitization and Associated Processes

MacGregor

Wilson

1939

Geol. Mag.

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“…The apparently contradictory field and petrographic features relating to these dikes might be reconciled by an interpretation that a continuance of more active emanations following perhaps passive metasomatic replacement caused the newly formed material to become rheomorphic and flow as a neomagma. In discussing the transformation of quartzite into granite, Backlund (1938) states, "this transformation was accompanied by increase of volume, the quartzites becoming rheomorphic with excellently developed fluidal textures, and thereby partly dynamic. "…”

Section: Rheomorphism and Neomagmasmentioning

confidence: 99%

Origin of Granites

Goodspeed

1948

Origin of Granite

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Field features and petrographic criteria for granitic rocks of magmatic origin are listed, and a brief discussion is presented concerning the mode of emplacement and the differentiation of intrusive masses.Transitional facies of granitic rocks of metamorphic origin are described in detail. These facies include replacement dikes, replacement breccias, and metamorphic migmatites. Interpretations of mechanisms of the advance of granitization including geochemical migrations are discussed, not only from the detailed evidence shown by miniature features, but also with regard to the relationship of metamorphic granitic rocks to regional metamorphism. Some aplites,_ pegmatites, and_ quartz veins are explained as retrogressive features following granitization. Granitic rocks which exhibit both crystalloblastic textures and marked flow structure are interpreted as examples of rheomorphism, and the term neomagma instead of magma is used for the material that flowed.The use of large lantern-slide (3J X 4J inches) thin sections has proved to be most helpful in the study of critical features of either magmatic or metamorphic granitic rocks. In these large sections, microtextures and microstructures can be observed much better than in an ordinary thin section, or a number of thin sections, and an integrated overall picture can be obtained.

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“…One recalls the vivid controversy between von Eckermann and Backlund before the second world war (cf. von ECKERMANN 1937 andBACKLUND 1938). Backlund went so far as to insist that molasse deposits were essential to the formation of rapakivi granite, which he considered to be the metasomatic equivalent of those sediments.…”

Section: Introductionmentioning

confidence: 99%

Bimodal magmatism and associated sedimentary facies with particular reference to the correlation between orogeny and regression

Ayrton

1987

Geol Rundsch

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ZusammenfassungZwischen Orogenese, Magmatismus und zugeh6rigen Sedimenten existiert ein enger Zusammenhang, der meist wenig verstanden ist und dringend eingehender Studien bedarf. Gewisse Sediment-Typen sind meist an einen ganz spezifischen, magmatischen Kontext gebunden. Dies ist insbesondere der Fall ftir die ozeanische Kruste, k6nnte aber auch fiir den bimodalen, kontinentalen magmatischen Zyklus mit der Bildung yon Granit zutreffen. Die grofG r~iumigen Regressionen und die damit verbundene Entstehung yon kontinentalen, detritischen Ablagerungen k6nnte die Antwort auf vertikale Mantel-Konvektion oder Plume-AktivitS.t sein. AbstractIt is stressed that a thorough investigation of the subtle and still poorly understood relationships between orogeny, magmatism and associated sediments is warranted. Certain types of sediments appear to be commonly linked to specific magmatic contexts. This is especially true for the oceanic crust, but it may be equally valid for bimodal continental magmatism with the development of granite. Grand scale regression with the formation of continental detrital deposits could represent the response to mantle upwelling and/or plume activity. R6sum6I1 est important de mieux comprendre les relations subtiles existant entre orogenhse, magmatisme et s6diments associ&. Certains types de s4diments sont syst4matiquement apparent& a un contexte magmatique sp4cifique. Ceci est particuli6rement 6vident pour la crofite oc4anique, mais pourrait ~tre 6galement valable dans le cadre du magmatisme continental bimodal avec d4veloppement de granite. Les r6gressions ~l grande &helle avec formation de s4di-ments d6tritiques continentaux pourraient repr4senter la cons4quence d'une activit6 accrue et/ou de mouvements convectifs du manteau.

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The Problems of the Rapakivi Granites

Cited by 25 publications

References 0 publications

On Granitization and Associated Processes

On Granitization and Associated Processes

Origin of Granites

Bimodal magmatism and associated sedimentary facies with particular reference to the correlation between orogeny and regression

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