Dear Javier,
Though not zoisites, the sector zoning
in some megacrysts of alkali felspars in
a granite of southern California might
be relevant. These observations are by
F.W.Dickson of the Mackay School of
Mines, University of Nevada at Reno*.
Those megacrysts that are orthoclase
display at least three distinct
hour-glass sectors, defined both by
chemical concentration varying in
potassium, sodium, & barium, and by
mineral inclusions varying in kind and
amount. Many display, in addition,
oscillatory zoning of orthoclase &
hyalophane. Of note is that the
interfacial angles between the top &
side pinacoid can be as great as 94
degrees.
Their field relations indicate to Prof
Dickson a syntectonic growth by
metasomatic replacement that was clearly
disequilibrium. [Though not an expert,
it isn't clear to me why the
syntectonic, sector-zoned crystals
listed thus far in this thread are not
believed to have grown in thermodyamic
equilibrium.]
Most Respectfully,
Bruce Bathurst
*Dickson, F.W. 1996. Porphyroblasts of
barium-zoned K-feldspar and quartz,
Papoose Flat, Inyo Mountains,
California, genesis and exploration
implications. p.909-924 in Geology and
Ore Deposits of the American Cordillera,
eds. A.R. Coyner & P.L. Fahey,
Geological Society of Nevada, Reno.
Frank Dickson <[log in to unmask]>
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