On polygenetic myrmekite

Phillips, Evan R.

doi:10.1017/s0016756800033070

Cited by 50 publications

(24 citation statements)

References 18 publications

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“…The two main hypotheses for myrmekite development are those of replacement reactions (Becke, 1908) and solid-state exsolution (Schwantke, 1909). These ideas and variations thereon have been summarized and discussed in &tail by Sturt (1970), Widenfalk (1972), Smith (1974) and Phillips (1974;1980). In Schwantke's lattice diffusion model, a change in solid composition m u m in response to elastic strain gradients, but requires a relatively long time to accomplish (see also Bayly, 1987).…”

Section: Discussionmentioning

confidence: 99%

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Evidence for deformation‐induced K‐feldspar replacement by myrmekite

Simpson

Wintsch

1989

Journal Metamorphic Geology

302

156

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Several examples of deformation-induced myrmekite have been found in two amphibolite facies mylonites derived from granitic protoliths, namely a muscovite-poor S-C mylonite and a single foliation, muscovite-poor mylonitic gneiss. Back-scattered SEM and conventional optical microscopy show that in both rock types, syntectonic myrmekitic intergrowths of oligoclase and quartz formed on the two sides of &feldspar grains that faced the local inferred incremental shortening direction for the mylonite.Myrmekite does not occur on the two ends of the grain that faced the incremental stretching direction.The replacement of K-feldspar by plagioclase and quartz results in a volume decrease and is favoured on high normal stress sites around the grains. We suggest that the ambient temperature, pressure and chemical activities were such that the replacement reaction was favoured, but the addition of extra strain energy along the high-pressure sides of the grains localized the reaction at these sites. This energy could arise from elastic strain, or strain associated with tangled dislocations or twin boundaries. The relative roles of stress and strain energy concentrations in driving the replacement reaction are not known, but both were probably important.

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Section: Discussionmentioning

confidence: 99%

“…5a very well, when the reaction proceeds to the right. The Phillips (1980) modification of the Becke model requires the introduction of H,O into the reactions, so that muscovite forms at the expense of the existing K-feldspar . and plagioclase, and K,O is removed from the local environment.…”

Section: Discussionmentioning

confidence: 99%

Evidence for deformation‐induced K‐feldspar replacement by myrmekite

Simpson

Wintsch

1989

Journal Metamorphic Geology

302

156

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“…In this context, muscovite is expected to form together with myrmekite in a closed system, without implying the occurrence of K-metasomatism suggested by Phillips (1980).…”

Section: Metamorphic Myrmekite Coronasmentioning

confidence: 95%

Beauty and complexity of metamorphism: case studies from the frontal part of the Adria microplate

Cesare

Mazzoli

Sassi

et al. 2010

Rend. Fis. Acc. Lincei

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We review a selection of case studies that highlight the fascination and complexity of metamorphic processes and mechanisms, as revealed in rocks from the crystalline basement of the frontal part of the Adria microplate. The examples from the Austroalpine metamorphic rocks south of the Tauern Window, and from the crystalline basement lying more southwards underneath the Po plain, are presented for their petrologic importance and their relevance to other contexts worldwide. They mostly deal with important aspects of metamorphic processes such as kinetics of mineral reactions, thermobarometry in polymetamorphic rocks, fluid-rock interactions, and microstructures of contact metamorphism in previously metamorphosed rocks. Although they are reported primarily for their help in understanding nano-/micro-scale processes of metamorphism, these examples also have some implications for the geologic evolution of this sector of the Eastern Alps.

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“…For Phillips (1974Phillips ( , 1980, and Smith & Brown (1988), the myrmekite is formed by the exsolution of alkali feldspar accompanied of pressure-induced dissolution and reprecipitation in a subsolidus state with some participation of primary quartz and K-feldspar. Smith & Brown (1988) suggested that subsequent processes could partially or completely modify the myrmekite by recrystallizing it.…”

Section: Texture and Deformation Of Terra Preta And Canoas Granitoidsmentioning

confidence: 99%

Field and petrographic data of 1.90 to 1.88 Ga I- and A-type granitoids from the central region of the Amazonian Craton, NE Amazonas State, Brazil

Valério

Macambira

Souza

2012

RBG

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The SW Presidente Figueiredo district, which is located in the northeastern Amazonas State of the central Amazonian Craton, Brazil, consists of 1890 to 1898 Ma I-type granitoids (Terra Preta Granite, Água Branca Suite), A-type hornblende-bearing syenogranites (Canoas Syenogranite, Mapuera Suite), felsic to intermediate volcanic rocks (Iricoumé Group), and 1883 to 1889 Ma rapakivi granites (São Gabriel Granite, Mapuera Suite) and related rocks (quartz-gabbroanorthosite and diorite), in addition to Castanhal quartz-monzonite, mylonites, and hornfels. The quartz-diorite facies of Terra Preta Granite were formed by mingling processes between a synplutonic quartz-gabbro dike and a hornblende granodiorite. Partially assimilated globules of Canoas hornblende-bearing syenogranite and their clear contacts with Terra Preta hornblende granodiorite suggest that Canoas Syenogranite is slightly younger than Terra Preta Granite. Canoas Syenogranite xenoliths inside São Gabriel Granite show that the granite is younger than the Canoas Syenogranite. New geologic and petrographic evidence improve the petrological understanding of these rocks and suggest that, in addition to fractional crystallization, assimilation and magma mingling played a role, at least at the local scale, in the evolution and compositional variation of the plutons. Such evidence is found in Terra Preta Granite mingled quartz-diorite, felsic material associated with the Canoas Syenogranite and in the intermediate microgranular enclaves, which exhibit primary biotite in hornblende-bearing rocks, plagioclase dissolution, corrosion of feldspars rims, alkali feldspar mantles, second apatite generation, and high xenocrystal contents in intermediate enclaves formed from the fragmentation of mafic intrusions. Petrographic analyses show that a deformational event recorded in the western part of the study area (with progressive deformation from E to W) is dated between the 1.90 Ga postcollisional magmatism and the intrusions of the São Gabriel Granite and related mafic/intermediate rocks (intra-plate). However, it is extremely necessary to obtain absolute ages for this metamorphic event.Keywords: field data; petrography; I-and A-type granitoids; lithostratigraphy; NE Amazonas State. RESUMO Dados de campo e petrográficos de granitoides dos tipos I e A de 1,90 a 1,88 Ga da região central do Cráton amazônico, NE do Estado do Amazonas, Brasil. O SW do município de Presidente Figueiredo, localizado noEstado do Amazonas, Nordeste do Cráton Amazônico Central, Brasil, hospeda granitoides do tipo I de idade entre 1890 a 1898 Ma (Terra Preta Granito, Suíte Água Branca), hornblenda-sienogranitos do tipo A (Sienogranito Canoas da Suíte Mapuera), rochas vulcânicas ácidas à intermediárias (Grupo Iricoumé) e granitos rapakivi de idades entre 1883 a 1889 Ma (Granito São Gabriel da Suíte Mapuera), e rochas afins (quartzo-gabro-anortosito e diorito), além de quartzo-monzonito Castanhal, milonitos e hornfels. A fácies quartzo-diorito do granito Terra Preta foi formada por proces...

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On polygenetic myrmekite

Cited by 50 publications

References 18 publications

Evidence for deformation‐induced K‐feldspar replacement by myrmekite

Evidence for deformation‐induced K‐feldspar replacement by myrmekite

Beauty and complexity of metamorphism: case studies from the frontal part of the Adria microplate

Field and petrographic data of 1.90 to 1.88 Ga I- and A-type granitoids from the central region of the Amazonian Craton, NE Amazonas State, Brazil

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