Ultrahigh pressure metamorphism is the term which covers the highest pressures of metamorphism recorded in reasonably large bodies of rock at the Earth's surface – those pressures being in the region of 3 GPa or higher, and representing burial depths of 100 km or more. The existence of such rocks presents challenges in terms of the burial process (since buoyant continental material records such pressures) and exhumation (erosion alone being inadequate to uncover such deep rocks). The editors claim ‘It is now recognized that the formation and exhumation of these ultrahigh-pressure – or UHP – rocks is an inherent and fundamental dynamic aspect of collisional orogenesis that affects a panoply of Earth processes including, but not limited to, exchange of material between the crust and mantle, the generation and collapse of mountain belts, the formation of continental crust, and the influence of UHP rock formation and exhumation on tectonic plate motions’. This collection of ten papers presents a snapshot of recent work on this topic: the works are based primarily on detailed and sometimes wide-ranging field, petrographic and geochronological studies. Inevitably, they do not all advance the fundamental issues highlighted by the editors, but they certainly illustrate the reality of ongoing problems concerning UHP regions.

For example: the first six papers all involve geochronology. There is no doubt that this is essential for unravelling the mechanisms and rates of UHP-related processes. There has been in the past decade or so an explosion in the use of U–Pb on zircon as a means of dating. Spot analyses allow age patterns within individual grains to be determined; ages obtained by conventional mass spectrometry methods can be very precise; cathodoluminescence and REE analyses from zircons provide extra information on their genesis which can be tied to U–Pb ages. There is a down side though: as an accessory phase it is far from clear petrographically how a zircon in a rock relates to the other rock-forming minerals which can be reliably linked to UHP episodes. A great deal of work has gone into correlating zircon characteristics with grade. For example, Gilotti et al. use flat heavy REE profiles in zircon rims to deduce that those rims grew at eclogite facies. In contrast Leach et al. use multiple features of zircon rims but are not able to distinguish peak and retrograde rim growth. Other work uses different dating systems and may give rise to different deductions, illustrated in particular by several papers on the Chinese Dabie–Sulu UHP orogen. Webb et al. use Ar–Ar dating to infer that the Sulu region was unroofed before Dabie; Leach et al. however have zircon ages suggesting Sulu UHP was, at 230 Ma, 10 Ma younger than Dabie. The non-specialist reader would do well to be aware of the strengths and weaknesses of different dating techniques when evaluating such conflicts.

Other papers in this volume cover refinements or revisions of PT estimates in various regions. UHP metamorphism was first recognized through the discovery of coesite and diamond which have such simple phase relationships to their low pressure polymorphs that there can be no doubt what their occurrence is telling us. Nowadays, though, investigation has passed via more and more complex PT grids to exploring subtle but significant aspects of mineral texture and chemistry. The paper by Kikuchi et al. uses OH content in diopside from Kokchetav to infer pressures of 5 GPa. Overarching insights into geodynamics are not emphasized in this volume, though the improving understanding of individual UHP regions will underpin such advances. The paper by Tsujimori et al. on Guatemalan eclogites does, though, conclude that basalts can convert to eclogites at temperatures as low as 300°C in cold subduction zones. This took me by surprise and it is this sort of claim which can enlighten future studies of subduction zone geodynamics.

In summary, then, this volume represents a snapshot of work on UHP metamorphism in the early-mid 2000s: it is informative both in regard of the evolution of individual regions and in relation to the techniques available to drive the subject forward.