isograd

Definitions

noun geology a line, either on a map or the surface of the Earth, joining points at which metamorphism reached similar pressures and temperatures; actually the intersection of a surface and that of the Earth

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In the metamorphosed Thunderhead Sandstone it was found that at the staurolite isograd, the boundary between the garnet and staurolite zones, the mineral chlorite disappears from the rocks and muscovite decreases sharply, whereas staurolite appears and biotite becomes more abundant.

Ken Miller in Cleveland: WEBCAST ARCHIVE URLS - The Panda's Thumb 2006
As can be readily seen in figure 1, whereas seven of the samples averaged around 30 Po radiohalos each, the two samples straddling the staurolite isograd contained 177 and 147 Po radiohalos respectively.

Ken Miller in Cleveland: WEBCAST ARCHIVE URLS - The Panda's Thumb 2006
However, where the mineral reaction around the staurolite isograd has produced a lot of hot water, large numbers of Po radiohalos have formed.

Ken Miller in Cleveland: WEBCAST ARCHIVE URLS - The Panda's Thumb 2006