Experimental studies of greenschist and amphibolite facies metamorphism of basic rocks

The transition from the greenschist to the amphibolite metamorphic facies is essentially defined on the basis of changes in the mineral assemblages found in basic rocks. In broad terms, the change is from a chlorite - actinolite - albite - epidote - quartz - bearing assemblage to one dominated by hornblende and plagioclase. The reactions involved in the transition are complex, and are manifested in cryptic compositional variation in co- existing phases as much as in actual phase changes.