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BB29

Abstract

The four phenocrystic phases in this thin section all display disequilibrium textures. Tabular plagioclase pheoncrysts are oscillatorily-zoned at the core and surrounded by as many as three distinct, cloudy, inclusion-rich rims. Quartz phenocrysts are rounded by resorbtion; thick coronas of calcite and hematite pepper, though do not entirely replace, their rims. Relict biotite phenocrysts, pseudomorphed by an opaque phase, are recognized more readily by the consistant rectancular and hexagonal shape than by the rare inclusion of the residual 'host'. These grains, frequently clustered together, have thick black rims and are sometimes infilled with a calcite-like mineral. This calcite-like mineral, frequently found replacing the biotite and as part of the quartz phenocryst coronas, is also observed pseudomorphing a tabular-shaped mineral found both as a phenocryst and part of the groundmass. The calcite seems to be replacing rectangular serpentine phenocrysts, presumably, pyroxene pseudomorphs themselves. The trachytic groundmass consists predominantly of microlites of plagioclase, prismatic opaques, glass, and the periodic rectangular calcite pseudomorph. Granular hematite veins cross-cut the thin section.

Note

The author has given permission for this work to be deposited in the Digital Archive of Colorado College.

Administrative Notes

The author has given permission for this work to be deposited in the Digital Archive of Colorado College.

Copyright
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Publisher
None
PID
coccc:19401