The sillimanite-potash feldspar isograd in Western Maine,U.S.A. |
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| Authors: | Bernard W Evans Charles V Guidotti |
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| Institution: | (1) Department of Geology and Geophysics, University of California, Berkeley, California, U.S.A.;(2) Department of Geology, University of California, Davis, California, U.S.A. |
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| Abstract: | The techniques of electron probe microanalysis and x-ray diffractometry have been utilized in a study of the sillimanite-potassium
feldspar isograd in western Maine. The isograd reaction is theoretically a discontinuous one, calling for the nearly instantaneous
loss of muscovite and crystallization of sillimanite and orthoclase, with a small contribution of albite from the pre-existing
plagioclase. In fact, muscovite coexists with orthoclase, sillimanite, and plagioclase for a distance of at least seven miles
from the isograd (marked by the initial coexistence of orthoclase and sillimanite). In this assemblage, muscovite has an extremely
narrow range of composition, about an average of Ms93.5Pg6.5. A possible explanation for the divariant character of the isograd reaction is that, during dehydration, PH2O slowly increased from initial values less than Ptotal + rock strength, under conditions of low permeability, the actual value of PH2O being controlled by a buffer assemblage and local conditions of P and T. An alternative explanation postulates the flattening
of thermal gradients following the onset of fractional melting.
The isograd reaction is dependent in only a minor way upon the anorthite content of the plagioclase. Below the isograd, a
continuous reaction takes place leading to a diminution in paragonite content of muscovite stable in the presence of quartz.
It is possible that this reaction leads to the nearly ubiquitous normal zoning of the plagioclase.
Changes in the composition of biotite at the isograd are not conspicuous, and can be satisfactorily explained by the release
of Mg, Fe, and Ti impurity from the muscovite, and a continuous reaction between ilmenite, quartz, and muscovite. Garnets
are not abundant and are high in Mn, both facts probably due to the low pressure of metamorphism, The presence of garnet probably
relates to the Mn content of the rock, and seems to be independent of the Mg/Fe ratio of the biotite. The garnets are zoned
with respect to Mn and Mg, but often Mn is enriched and Mg depleted in the marginal zone. The Mg/Fe ratio of the biotite varies
twofold depending on the presence or absence of pyrrhotite. The transition: microcline → orthoclase depends upon the amount
of dissolved albite; the polymorph is orthoclase in the pelitic schists but microcline in the calc-silicate rocks which are
much lower in sodium. The plagioclases are of “low” structural type, although
is slightly greater than many other “low” plagioclases. A correlation of d(002) of muscovite and paragonite solid solution
for the range 0 to 20 % paragonite is given. An appreciable positive volume of mixing for the binary system muscovite-paragonite
is indicated. |
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