Blueschist
Blueschist or Glaukophanschiefer are often bluish-colored rocks that have suffered a metamorphosis at relatively low temperatures and high pressures. Such conditions are found primarily in subduction zones. The bluish color is caused by the mineral glaucophane, an amphibole, which is not always available, so not every blue shale is actually bluish. The blueschist are so characteristic of rocks that have undergone a metamorphosis that by them a metamorphic facies is named, called the blueschist facies.
Blueschist facies
Source rock for blueschist are basalts and basaltic rocks with similar composition. These rocks are metamorphosis at about 200-400 ° C and 6-12 kilobars. The specific metamorphic conditions lead to the formation of an association of minerals, their occurrence can be used to determine the metamorphic facies.
The eponymous blue amphiboles are stable over a wide pressure-temperature range which can extend into the field of the greenschist facies. The occurrence of these minerals is therefore not a reliable indication of the blueschist facies. However, If they occur also with the bluish lawsonite, with aragonite and / or jadeitic pyroxene, this is a sure sign of the blueschist facies. Other special features include the absence of biotite, andalusite and sillimanite and of feldspars except albite.
As with all metamorphic facies formation of the characteristic minerals is dependent on the composition of the parent rock. In metabasites form in addition glaucophane and lawsonite the minerals chlorite, titanite, epidote, phengite, paragonite and omphacite. In metagreywackes quartz and jadeite prevail next lawsonite, phengite, glaucophane and chlorite. Typical of carbonate rocks (marble) is the occurrence of aragonite, while in metapelites phengite, paragonite, Carpholit, chlorite and quartz are characteristic.