538 ULTRA HIGH PRESSURE METAMORPHISM Figure Scanning electron microscope image of a polyphase diamond bearing inclusion in garnet, interpreted as trapped sili ceous fluid or melt before crystallization The example is a quart zofeldspathic rock from the Saidenbach reservoir, central Saxonian Erzgebirge Arrows point to rational mica garnet inter faces qz, quartz; par, paragonite; phl, phlogopite; ap, apatite ‘megamelange’, including medium-pressure rocks, may best describe the geological situation Nevertheless, two types of UHP rocks can be distinguished in terms of maximum pressure and temperature conditions One type has experienced pressure and temperature conditions similar to those of UHP rocks of the Western Alps (Tmax $750 C) The other type, characterized by abundant microdiamond inclusions in various host minerals, was metamorphosed at pressures as high as 70 kbar At these high pressures, temperatures exceeded 1000 C, probably resulting in partial melting Polyphase aggregates (Figure 6), commonly containing microdiamonds, are enclosed in garnet and other phases These aggregates serve as evidence for siliceous fluids or melts The silica-rich material of these inclusions was probably trapped in growing minerals during exhumation of the (partially molten) rocks at decreasing temperatures This happened at 530 Ma, as deduced from zircon dating, also indicating a fast uplift of the rock in the range of centimetres per year temperature conditions close to 700 C and 23– 30 kbar A common feature of these eclogites is the replacement of omphacite by amphibole porphyroblasts due to the influx of hydrous fluids, probably close to the pressure and temperature climax In the younger UHP event, occurring about 340 Ma, felsic, basic, and ultrabasic rocks were involved Peak temperature conditions can exceed 1000 C–1200 C and 80 kbar are indicated by a diamondiferous quartzofeldspathic rock from the Saxonian Erzgebirge; this rock has polyphase inclusions in garnet (Figure 6), similar to rocks from the Kokchetav Massif The anatectic evolution of the Erzgebirge rock is inferred to be similar to that of diamondiferous rocks from the Kokchetav Massif, in addition to the fast exhumation The diamondiferous rock from the Erzgebirge forms lenses up to km long, like other UHP rocks in the Variscides The surrounding gneisses display a medium to high pressure signature, but also a clear peak pressure contrast to the UHP rocks Himalaya UHP rocks in the Himalaya are very rare, but their geodynamic context is much clearer compared to other UHP regions The protoliths of coesite-bearing eclogites and surrounding metasediments in the north-western Himalaya were part of the continental margin of India that was subducted beneath Asia The prograde metamorphism during this event followed a geothermal gradient of 6 C km reaching peak pressure and temperature conditions at depths between 90 and 120 km about 50 Ma ago Amphibole blastesis at the expense of clinopyroxene is common The subsequent exhumation of the UHP rocks is characterized by moderate cooling at uplift rates in the range of centimetres per year, slowing down to millimetres per year for the past 40 million years Mid- and West-European Variscides Mechanisms An abundance of ascertained and potential UHP rocks have been detected in several crystalline complexes of the Variscides, including those later involved in the Alpine Orogeny However, due to a significant fragmentation of specific major units by a late orogenic event, a former, possibly extended, coherency of UHP terranes has been broadly lost Nevertheless, HP to UHP metamorphism cannot be related to a single event For instance, in the Bohemian Massif, representing the north-eastern section of the Variscides, two major HP–UHP events can be discriminated from abundant age data The earlier event, around 395 Ma, led to eclogites, probably former oceanic crust, which experienced maximum pressure and The known UHP rocks at Earth’s surface can be subdivided into two groups One group suffered from metamorphism along geothermal gradients of about 7 C km 1, reaching maximum pressures generally below 40 kbar (Figure 7) Frequently observed peak pressure and temperature conditions are 30 kbar at 650–700 C Moderate cooling and uplift rates in the range of centimetre per year characterize the exhumation of this group of UHP rocks, commonly starting with the influx of hydrous fluids (for instance, into eclogites) The subduction of oceanic crust under continental crust, including the exhumation within a subduction channel, best explains the characteristics of these UHP rocks However,