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Felsic gneisses, sillimanite schists, and amphibolites in Pelham Bay Park were subjected to three phases of deformation and metamorphism. During the third phase, compositionally distinct layers were produced within the amphibolites as a result of metasomatism. These layers are parallel to each other and range from a fraction of an inch to several feet in thickness and consist of the following assemblages: (a) plagioclase-biotite (muscovite-microcline) [plagioclase-biotite gneiss], (b) plagioclase (hornblende-magnetite-sphene) [plagioclase-rich layers], and (c) calcite-plagioclase (hornblende) [calcite-rich layers]. Plagioclase-biotite gneiss was produced by potash metasomatism of amphibolite (biotite replacing hornblende). Plagioclase-rich layers were produced by soda metasomatism of amphibolite (plagioclase replacing hornblende). Calcite-rich layers were produced by lime metasomatism of either feldspathic amphibolite or plagioclase-rich layers (calcite replacing plagioclase).

The metasomatically produced layers superficially resemble primary layering in para-amphibolites and also resemble secondary layering produced by metamorphic segregation operative during an early phase of deformation. Evidence for the metasomatic origin of these layers includes: (a) gradation over short distances, both across and along strike into amphibolite; (b) replacement textures; (c) mineral assemblages unlike those resulting from the metamorphism of common sedimentary and volcanic rocks; and (d) the presence of relict foliations and skialiths.

The metasomatically produced layers generally occur either in the hinge zones of F3 folds or at the ends of amphibolite layers separated by boudinage on the limbs of F3 folds. These were areas of relatively low pressure toward which fluids rich in potash, soda, and lime migrated, causing metasomatic replacement of amphibolites.

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