The sedimentary record of ultrahigh-pressure metamorphism: a perspective review

Tracing ultrahigh-pressure (UHP) metamorphism of crustal rocks through the geological record is a key for understanding the evolution of plate tectonics on Earth due to the linkage with deep subduction processes. Until recently, UHP research was almost exclusively based on the investigation of crystalline rocks, but findings of coesite and diamond inclusions in detrital mineral grains demonstrate that the sedimentary record archives mineralogical evidence for UHP metamorphism. We here review previous attempts to link sediments to UHP sources and the recent findings of detrital UHP garnet, and thoroughly discuss the new approach in the search for UHP metamorphism. The indicative UHP minerals were identified by Raman spectroscopy and include monomineralic coesite and bimineralic coesite + quartz inclusions in detrital garnets from the Scandinavian Caledonides of Norway, the D'Entrecasteaux Metamorphic Complex of Papua New Guinea, and the Central European Variscides of Germany, as well as diamond inclusions in the latter. Garnet chemistry and inclusion assemblages are used to gain information about the origin of these mineral grains and to discriminate different UHP sources. Presumably, the value of information will increase in future studies by considering other detrital containers of UHP minerals such as the ultrastable heavy minerals zircon, rutile, and tourmaline, for which also a range of single-grain provenance tools exist. Abundant monomineralic coesite inclusions in detrital minerals allow for investigating coesite preservation factors and potentially elastic thermobarometry in the coesite stability field. Altogether, the method allows for (i) screening large regions systematically for the presence of UHP rocks, (ii) studying the exhumation history of UHP terranes, and (iii) monitoring the former existence of UHP terranes at the Earth's surface.