METAMORPHIC GRADES AND ZONES

WHEN ROCK OF A PARTICULAR COMPOSITION IS METAMORPHOSED A SERIES OF MINERAL ASSEMBLAGES DEVELOP DEPENDING ON P&T.

FOR EXAMPLE, IF A SHALE IS METAMORPHOSED BY INSTRUSION OF A GRANITE:

UNMETAMORPHOSED SHALE
CHLORITE SLATE
BIOTITE-GARNET SCHIST
STAUROLITE-KYANITE SCHIST
SILIMANITE SCHIST
(GRANITE - HEAT SOURCE)

THE MINERALS THAT CHARACTERIZE THESE ASSEMBLAGES ARE KNOWN AS INDEX MINERALS & THEIR FIRST APPEARANCE DEFINES A METAMORPHIC ISOGRAD (I.E. A CONTOUR OF EQUAL PT).

THE REGION BETWEEN THE ISOGRADS IS A METAMORPHIC ZONE.

[ Kinds of metamorphic rocks | Metamorphic Facies ]

© 1997, W.K. Fletcher
Department of Earth and Ocean Sciences The University of British Columbia
http://www.science.ubc.ca/geol100/fletcher/met_grade.html