ULTRA HIGH PRESSURE METAMORPHISM 533 ULTRA HIGH PRESSURE METAMORPHISM H-J Massonne, Universitaăt Stuttgart, Stuttgart, Germany ò 2005, Elsevier Ltd All Rights Reserved Introduction Ultra high pressure (UHP) metamorphic rocks of common basic to felsic nature are defined by the occurrence of coesite, a silica polymorph that is denser than quartz According to several experimental studies, the transition from quartz to coesite at 600 C requires a pressure (P) of around 27 kbar, a temperature (T) of conditions that occur on Earth at depths close to 100 km Coesite in nature was detected first in rocks affected by impact metamorphism, but a coesite-bearing rock that had been subjected to regional metamorphism was described in 1983 by Chopin Based on the coesite–quartz transition pressure and temperature curve, this rock, which was from the Dora Maira Massif of the Western Alps, must have been buried at depths of about 100 km or more Transition curves of SiO2 polymorphs show a moderate pressure (P) and temperature (T) slope, dP/dT, of only 10 bar C (Figure 1) Prior to discovery of the Dora Maira Massif rock, it was thought that metamorphic rocks (excepting ultrabasites) that were formed during orogenic events, and now exposed at Earth’s surface, represent, in general, a fossil record of pressure and temperature conditions only of Earth’s crust, equivalent to maximum depths of 70 km and pressures up to $18 kbar Eclogites (basalts that have been metamorphosed under high pressure) were believed to have formed, depending on temperature, at pressures between 12 and 16 kbar, corresponding to the jadeite content of omphacite, the mineral that characterizes eclogites However, these estimates are only justified when omphacite co-exists with plagioclase and quartz (but plagioclase is often only a retrograde product in eclogites, due to the breakdown of omphacite) Rare jadeite occurrences in felsic rocks with plagioclase supported the view of metamorphic pressures not exceeding 18 kbar for crustal rocks In geodynamic models of subduction of oceanic crust under oceanic or continental plates, it was assumed that the return of subducted material was possible only for shallower regimes of the collision wedge Such subducted material then became, for instance, part of an accretionary wedge complex Within the framework of such a scenario, the discovery of coesite in regional metamorphic rocks was sensational and a real turning point in scientific thinking about deep burial and subsequent exhumation of crustal rocks Soon after Chopin’s report in 1983, it became evident that UHP rocks are more widespread than had been thought Coesite was recognized in rocks of the Norwegian Caledonides, the Dabie Shan in China, and orogenic regions elsewhere Moreover, in 1990, microdiamonds, another indicator mineral for UHP metamorphism, were reported by Sobolev and Shatsky in marbles and gneisses from the Kokchetav Massif, Kazakhstan These diamonds provided evidence for burial of crustal rocks to depths of at least 130 km Identification of UHP Rocks The discovery of UHP rocks at Earth’s surface at a relatively late date in geological science history can be attributed to the nature of the processes involved Retrogression of rocks during exhumation can result Figure Pressure and temperature stability of various mineral phases that are of relevance to UHP metamorphism Except for the transition curves below 30 kbar, the experimental error is even higher than is expressed by the thickness of the lines The clinoenstatite orthoenstatite transition refers to a composition with 10 mol% of ferrosilite component