Abstract
Metagabbros from two widely separated areas in the Adirondacks show development of coronas. In the Southern Adirondacks, these are cored by olivine which is enclosed in a shell of orthopyroxene that is partially, or completely, rimmed by symplectites consisting of clinopyroxene and spinel. Compositions of the corona phases have been determined by electron probe and are consistent with a mechanism involving three partial reactions, thus:
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(a)
Olivine=Orthopyroxene+(Mg, Fe)++.
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(b)
Plagioclase+(Mg, Fe)+++Ca++=Clinopyroxene+Spinel+Na+.
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(c)
Plagioclase+(Mg, Fe)+++Na+=Spinel+more sodic plagioclase+Ca++.
Reaction (a) occurs in the inner shell of the corona adjacent to olivine; reaction (b) in the outer shell; and (c) in the surrounding plagioclase, giving rise to the spinel clouding which is characteristic of the plagioclase in these rocks. Alumina and silica remain relatively immobile. These reactions, when balanced, can be generalized to account for the aluminous nature of the pyroxenes and for changing plagioclase composition. Summed together, the partial reactions are equivalent to:
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(d)
Olivine + Anorthite = Aluminous orthopyroxene + Aluminous Clinopyroxene + Spinel (Kushiro and Yoder, 1966).
In the Adirondack Highlands, coronas between olivine and plagioclase commonly have an outer shell of garnet replacing the clinopyroxene/spinel shell. The origin of the garnet can also be explained in terms of three partial reactions:
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(e)
Orthopyroxene+Ca++=Clinopyroxene+(Mg, Fe)++.
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(f)
Clinopyroxene+Spinel+Plagioclase+(Mg, Fe)++=Garnet+Ca+++Na+.
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(g)
Plagioclase+(Mg, Fe)+++Na+=Spinel + more sodic plagioclase+Ca++.
These occur in the inner and outer corona shell and the surrounding plagioclase, respectively, and involve the products of reactions (a)-(d). Alumina and silica are again relatively immobile. Balanced, and generalized to account for aluminous pyroxenes and variable An content of plagioclase, they are equivalent to:
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(h)
Orthopyroxene+Anorthite+Spinel=Garnet (Green and Ringwood, 1967).
Amphibole coronas about opaque oxides in rocks of both areas are the result of oxide/plagioclase reactions with addition of magnesium from coexisting olivine.
Based on published experimental data, pressure and temperature at the time of corona formation were on the order of 8 kb and 800° C for the garnet bearing coronas, with somewhat lower pressures indicated for the clinopyroxene/spinel coronas.
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Published by permission of the Director, New York State Museum and Science Service, Journal Series No. 144.
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Whitney, P.R., McLelland, J.M. Origin of coronas in metagabbros of the Adirondack mts., N. Y.. Contr. Mineral. and Petrol. 39, 81–98 (1973). https://doi.org/10.1007/BF00374247
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DOI: https://doi.org/10.1007/BF00374247