Myrmekite associated with alkali feldspar megacrysts in felsic rocks from New South Wales

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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“…The production of myrmekite with muscovite during retrograde metamorphism (Phillips, Ransom & Vernon, 1972) is also elucidated by Fig. 6.…”

Section: Discussionmentioning

confidence: 98%

Evidence for deformation‐induced K‐feldspar replacement by myrmekite

Simpson

Wintsch

1989

Journal Metamorphic Geology

297

156

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“…These forms are closely related to myrmekite developed around plagioclase inclusions in subhedral alkali feldspar megacrysts (Fig. All of these examples come from massive, undeformed granitoids and they provide examples of myrmekite development which may be explained by the Schwantke model (Phillips, 1964;Phillips & Carr, 1973). 1C).…”

Section: Differences In Myrmekite Morphologymentioning

confidence: 92%

“…Phillips & Carr, 1973). The quartz stems protrude virtually to the grain boundary and pure albitic margins are absent.…”

Section: Differences In Myrmekite Morphologymentioning

confidence: 99%

“…In addition, although very fine albite rims may Myrmekite is shown by the outline of its quartz stems, non-myrmekitic plagioclase is usually multiply-twinned, mica flakes show closely spaced straight cleavages, granular quartz is clear and alkali feldspar is illustrated mainly by stippling of various intensities. Phillips & Carr, 1973). (A) Rim myrmekite with an outer margin of rim albite developed between plagioclase and perthitic alkali feldspar.…”

Section: Differences In Myrmekite Morphologymentioning

confidence: 99%

“…(A) Rim myrmekite with an outer margin of rim albite developed between plagioclase and perthitic alkali feldspar. Phillips & Carr, 1973). Phillips, 1964).…”

Section: Differences In Myrmekite Morphologymentioning

confidence: 99%

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On polygenetic myrmekite

Phillips

1980

Geol. Mag.

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A brief review of Becke's (1908) replacement and Schwantke's (1909) exsolution models for myrmekite genesis and further consideration of the morphology and spatial distribution of myrmekite as outlined by Phillips (1974) lead to the conclusion that both hypotheses have a place in explaining the origin of myrmekite. The Schwantke model is generally best applied, for example, to high-level undeformed massive granitoids and the Becke model to deformed metamorphic rocks. The latter hypothesis and a third model involving interaction between exsolution and replacement (Ashworth, 1972) may be used especially to explain the association of muscovite and myrmekite in rocks whose mineral assemblages have undergone retrograde modification, involving either redistribution of Na, Ca and K by local metasomatic reactions among the mineral grains, or metasomatic change of bulk rock composition.

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Intergrowths of Feldspars with Other Minerals

Smith

1974

Feldspar Minerals

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Myrmekite associated with alkali feldspar megacrysts in felsic rocks from New South Wales

Cited by 7 publications

References 21 publications

Evidence for deformation‐induced K‐feldspar replacement by myrmekite

Evidence for deformation‐induced K‐feldspar replacement by myrmekite

On polygenetic myrmekite

Intergrowths of Feldspars with Other Minerals

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