hornblende–hornfels facies

Quick Reference

A set of metamorphic mineral assemblages produced by contact metamorphism of a wide range of starting rock types under the same metamorphic conditions and typically characterized by the development of the mineral assemblage hornblende–plagioclase in rocks of basic igneous composition. Other rocks of contrasting composition, e.g. shales and limestones, would each develop their own specific mineral assemblage, even though they are all being metamorphosed under the same conditions. The variation of mineral assemblage with starting rock composition reflects a particular range of pressure, temperature, and P(H2O) conditions. Experimental studies of mineral P–T stability fields indicate that the facies represents the low-pressure, moderate-temperature conditions met close to igneous intrusions.

Subjects: Earth Sciences and Geography.

Reference entries