INTERPRETATION OF PLAGIOCLASE ZONATION IN CALCIC PELITIC SCHISTS AND THE EFFECTS ON THERMOBAROMETRY, SOUTH STRAFFORD, VERMONT
THOMAS MENARD
Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4
FRANK S. SPEAR
Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, New York 12180, U.S.A.
Abstract
Metamorphic plagioclase in calcic pelitic schists from South Strafford, Vermont, grew during heating of the assemblage garnet + biotite ±
chlorite + plagioclase + epidote ± calcite + quartz + muscovite + graphite + fluid during garnet growth with chlorite in the assemblage, or during
garnet consumption after chlorite was removed from the assemblage. These grains have a variety of patterns of compositional zoning. The simplest
pattern is continuous concentric zoning, which records the sequence of plagioclase compositions produced in the rock during progressive
metamorphism. The most strongly zoned single grain found is zoned from An20 to An70 from core to rim. Sodic plagioclase was consumed during
growth of calcic plagioclase, and the locations of growth and consumption can be affected by partitioning of deformational strain in a rock. As a result
of discontinuous growth of a particular grain, plagioclase can have discontinuous zoning or patchy zoning. Zoning patterns in plagioclase can also
reflect the character of the matrix overgrown: smoothly zoned plagioclase overgrew relatively homogeneous matrix, whereas plagioclase with
complex zoning patterns overgrew crenulated muscovite or other inhomogeneities. The peristerite gap is displayed as alternating lamellae of
two plagioclase compositions, as a simple gap in the zoning of concentrically zoned grains, and as portions of grains that are nearly unzoned with
compositions near An18. This variety of zoning features and the complexity of possible zoning patterns make it imperative to document carefully
any correlation of mineral compositions. Petrologically unreasonable interpretations can potentially lead to wild errors in thermobarometric estimates.