Very low grade metamorphism of basic rocks: definition, mineral assemblages, and p–T conditions

The subgreenschist facies, representing a part of the fi eld of very low grade metamorphism, is characterized by pressures lower than those of the glaucophane-schist facies and temperatures lower than the low-temperature limit of the greenschist facies (up to ca. 300 °C and ca. 4–5 kbar). The subgreenschist facies involves three various metabasite subfacies (prehnite–pumpellyite facies, prehnite–actinolite facies, and pumpellyite–actinolite facies) defined by specific and diagnostic mineral assemblages of prehnite + pumpellyite, prehnite + actinolite, pumpellyite + actinolite, and also, according to certain authors, laumontite. The very low grade metamorphic rocks have been found over large areas of the Earth’s continental and oceanic crust and it is expected that these rocks develop in a variety of tectonic settings. The Earth’s crust, subduction zones, continental orogenic, arcs, and ridges are the most frequent geological environment where these transformations occur. In the Western Carpathians, the very low grade metamorphic rocks have been found in the Tatric, Hronic, and Gemeric superunits.