Blueschist- to eclogite-facies rocks: from HP to UHP

Abstract

Impressed by the beauty of a metamorphic rock from the Saualpe, Austria, Rene-Just Hauy (1743–1822) created the name “eclogite” (the chosen rock), which he derived from the Greek word eκλογή (choice). The name was used for the first time in 1822 in the second edition of Hauy’s “Traite de mineralogie”. However, the first “scientific” description dates back to the second half of the 18th century, when Horace-Benedict de Saussure (1740–1799) mentioned a “beautiful rock that is not described yet” from the Rhone Valley in his famous book “Voyages dans les Alpes”. Eclogite is a metabasic rock dominated by the minerals omphacite and garnet, and in the 20th century eclogite was recognized as a high-pressure (HP) metamorphic rock. Concurrently, it was discovered that metabasic blueschist, dominated by blue sodium-rich amphiboles, represented another HP metamorphic rock formed at lower metamorphic grade than eclogite. Then, in the mid nineteen-eighties, metamorphic coesite (and somewhat later even diamond) was discovered in eclogite-facies rocks, indicating pressures even higher than had been considered possible up to that time. The term “ultrahigh-pressure (UHP) metamorphism” was born. The term describes metamorphism at depths corresponding to pressure–temperature conditions exceeding the lower limit of the coesite stability field. The significance of UHP metamorphism is most striking for rocks that initially formed in the crust and were then subducted to UHP conditions, but our understanding of mantle processes has been enriched through study of mantle-derived garnet peridotites at UHP conditions as well. These findings generated numerous new research projects with enormous implications for petrology, geodynamics, global tectonics, seismology, and geochemical recycling. This special issue contains a selection of contributions which were presented at the 11th International Eclogite Conference (IEC), in accordance with the theme “Blueschist-to eclogite-facies rocks: from HP to UHP”. The meeting took place near Rio San Juan …