Imperial College Rock Library

Glossary: Albite Epidote Hornfels Facies

    Abite epidote hornfels facies is a low-grade metamorphic facies associated with contact metamorphism at low pressures and temperatures and often represents the outer portions of a thermal aureole. Albite epidote hornfels facies metamorphism occurs at temperatures of approximately 250 to 500 degrees celsius and at low (upper crust) pressures.
 
Changes in mineralogy depends very much on protolith. The presence of epidote, albite and chlorite is characteristic. In pelites biotite and andalusite can occur as porphyroblasts, often rich in inclusions, however, these minerals are not restricted to the albite epidote hornfels facies. Andalusite forms at the expense of muscovite. Typical assemblages for different protoliths include:
 
mafic - actinolite + epidote + chlorite +/- biotite +/- talc +/- albite.
 
pelites - quartz + albite + epidote + chlorite + muscovite + biotite +/- andalusite.
 
quartz-feldspathic - microcline + albite + quartz +/- biotite +/- muscovite.
 
calc-silicate - highly variable: calcite + epidote + tremolite.
 
Like other contact metamorphic rocks, albite epidote hornfels facies rocks can contain a wide range of minerals due to hydrothermal alteration to a skarn. Albite epidote hornfels facies rocks trend towards equigranular fine-grained hornfels with granoblastic textures, however, porphyroblasts do occur.

Related Terms

metamorphic grade, metamorphic facies, zeolite facies, prehnite pumpellyite facies, greenschist facies, blueschist facies, amphibolite facies, granulite facies, eclogite facies, sanidinite facies, albite epidote hornfels facies, pyroxene hornfels facies, hornblende hornfels facies, thermal metamorphism, thermal aureole, granoblastic texture, porphyroblast