Abstract Ultrahigh pressure metamorphism (UHPM) is an important type of orogenic metamorphism that over recent years has been increasingly recognised as a characteristic, though poorly preserved, feature of many Phanerozoic plate collision zones. UHPM can be defined as “a type of metamorphism that occurs at very high lithostaticpressures within the eclogite facies but above the stability field of quartz”.

Abstract In the Western Alps, two tectonic units have unquestionably experienced ultrahigh pressure metamorphism (UHPM): the continental Brossasco-Isasca Unit of the southern Dora-Maira Massif, in which coesite was first reported by Chopin (1984) , and the ocean-derived Lago di Cignana Unit of the Piemonte zone, in which coesite was first reported by Reinecke (1991) . In both units the UHPM recrystallisation, acquired during the early stages of the Alpine orogeny, is largely obliterated by a late Alpine greenschist facies retrogression, more pervasive in the felsic lithologies.

Abstract Eskola (1921) drew attention to some of the aesthetically impressive eclogites and garnet peridotites that outcrop in the coastal region of west Norway between Bergen and Trondheim. These occurrences lie within the so-called Western Gneiss Region (WGR), the lowest exposed structural level in the southern Scandinavian Caledonides. The WGR is now recognised as a composite tectono-metamorphic terrane that mostly comprises Proterozoic autochthonous to para-autochthonous basement rocks with minor late Proterozoic cover belonging to the leading edge of the Baltic Plate, along with infolds of the main, outboard-derived Caledonian allochthon. Much of this composite edifice experienced short-lived deep level subduction beneath the Laurentian Plate during the Scandian phase of the Caledonian orogeny. Several more recent papers, including those by Andersen et al , (1991) ; Carswell et al. (2003a) ; Cuthbert et al. (1983 , 2000 ); Cuthbert & Carswell (1990) ; Dewey et al. (1993) ; Griffin et al. (1985) ; Krogh & Carswell (1995) ; Smith (1995) , have considered the stabilisation and exhumation of eclogites and other cofacial high pressure (HP) and ultrahigh pressure (UHP) rocks in this region, within the context of the tectono-metamorphic development of this segment of the Scandinavian Caledonides.

Abstract The recognition of abundant microdiamonds included along with coesite in primary rock-forming minerals and zircons of metamorphic rocks from the Kokchetav massif, Northern Kazakhstan ( Sobolev & Shatsky, 1990 ; Shatsky et al. , 1991 ; Sobolev et al. , 1991 ) indicates that crustal segments reached pressures of the order of at least 40 kbar (4 GPa), implying their subduction to depths greater than 100 km. The Kokchetav massif became internationally recognised as the type locality of diamondiferous metamorphic rocks; the petrological study of diamondiferous ultrahigh pressure (UHP) rocks provides a unique insight into the formation of diamond in crustal rocks at very high pressures. Along with the finding of coesite ( Chopin, 1984 ) the discovery of diamond in supracrustal rocks has drastically changed the current ideas concerning the limits of UHP metamorphism of supracrustal rocks. The specific features and significance of such unique ultrahigh pressure metamorphism have been extensively discussed in different works ( e.g. Coleman & Wang, 1995 ) and in numerous subsequent publications ( Ernst & Liou, 2000 ). However, some workers preferred a hypothesis of a metastable diamond growth at low P–T parameters ( Ekimova et al. , 1994 ). The most recent collection of papers devoted specifically to petrotectonic characteristics of the Kokchetav massif is published as a special issue of island Arc ( Liou & Banno, 2000 ). The significance of metamorphic processes at the origin of a new type of diamond has been extensively discussed ( e.g. Haggerty, 1999 ). it is important to note that the Kumdy-Kol microdiamond deposit covers only a small portion of a more than 200 square km large area in which diamondiferous rocks are distributed in the Kokchetav massif ( Shatsky et al. , 1991 ; Dobretsov et al. , 1999a ). The proven microdiamond reserves of this deposit exceed 3 billion carats ( e.g. Haggerty, 1999 ), making it an absolutely unique phenomenon worldwide. Apart from the Kokchetav massif, occurrences of microdiamonds in other UHP metamorphic terranes elsewhere are less well documented because of the need of bulk extraction and the lack of an unambiguous confirmation of microdiamond in situ ( Xu et al. , 1992 ; Dobrzhinetskaya et al. , 1995 ). Another microdiamond locality in gneisses, confirmed by direct observations of thin sections, is from Erzgebirge, Germany, which has been suggested to be similar to the type locality of the Kokchetav massif ( Massonne, 1999 ; Stöckhert et al. , 2001 ).

Abstract The Qinling-Tongbai-Hong’an-Dabie Shan area is an about 2000 km long Triassic Indosinian orogenic belt produced by the collision between the Sino-Korean and the Yangtze cratons ( Fig. 1 ). Its eastern extension, the Sulu area, occupies the southeastern side of the Shandong Peninsula, and is considered to be displaced about 500 km by the NE-SW trending left lateral Tan-Lu Fault after the Mesozoic ( Fig. 1 ). Among these areas, most of the UHP rocks were found from Hong’an, Dabie Shan and Sulu areas, suggesting that these areas represent the most extensive UHP metamorphic belt in the world. Their UHP peak is dated around 220-230 Ma ( e.g. Ames et al. , 1993 , 1996 ; Li et al. , 1993 ; Hacker & Wang, 1995 ; Hacker et al. , 1996 ; Rowley et al. , 1997 ) and these UHP terrains are considered to be formed chiefly by attempted north-directed subduction of the Yangtze craton or a microcontinent beneath the Sino-Korean craton ( e.g. Hacker et al. , 1996 ).

Abstract Although the occurrence of eclogites and garnet peridotites in the Bohemian Massif has been known for more than a century, evidence for ultrahigh pressure metamorphism (UHPM) by indicator minerals has been reported only very recently (diamond: Massonne, 1999 ; coesite: Massonne, 2001a ). In contrast, although eclogites were recognised in the Tso Morari area by Berthelsen (1953) , the first real petrological investigation of eclogites in the NW Himalaya followed their discovery in Pakistan in the 1980’s (Ghazanfar & Chaudhry, 1986, 1987). The finding of coesite soon after, in both Pakistan and India (O’Brien et al., 1999, 2001; Sachan et al., 2001) indicates UHP metamorphic conditions for these rocks. The timing of detection can, of course, be no criterium for treating both areas in one chapter. Rather it seems to be that both areas are very contrasting, which is certainly true in regard of the outcrop situation. In the well-mapped Bohemian Massif, natural exposures in deep valleys or as cliffs or crags at higher levels are rare and are only supplemented by a few quarries. In the poorly mapped NW Himalaya, the majestic and steep mountains provide excellent outcrops although they are less accessible and cover an enormous area. Further contrasts could also be listed, such that at first glance both areas addressed here seem to be perfect opposites. However, in the subsequent section we will outline the many common features of the HP and UHP areas of the Bohemian Massif and the NW Himalaya within a larger geographical framework. After presenting some detailed petrographic and geochronological information on key areas in both orogenic sections, we will try to interpret these in terms of a continent-continent collision model accounting for the different states of both the Bohemian Massif and NW Himalaya in terms of orogenic development.

Abstract Most of, if not all, the evidence for the attainment of UHP conditions by metamorphic rocks is a mineralogical one. Some minerals by their nature, composition, texture or reactions, may be specific of such conditions and are briefly reviewed here. As pressure, an intensive parameter, has direct influence on volume, this mineralogical survey is organised on a structural basis: we emphasised the more or less efficient packing of atoms into crystal structures. Then chemical, petrological or phase stability data are presented, in the hope of offering a slightly different perspective from earlier reviews by Smith (1988) , Chopin & Sobolev (1995) , Liou et al. (1998) , Carswell & Zhang (1999) and Zhang & Liou (2000) . The reader is explicitly referred to the latter three in addition to this review, in order to have the most complete coverage.

Abstract Thermobarometry of HP/UHP rocks is of vital importance for the understanding of tectonic and rock forming processes and large-scale vertical transport of matter at great depths within the Earth. In recent years new sets of experiments, the extraction of internally consistent thermodynamic data set, together with elaborated composition-activity models for critical minerals have taken the art of estimation of metamorphic conditions several important steps forward. In this contribution we would like to present methods we regard as important for basic and ultrabasic compositions, and address problems of vital interest in modern geothermobarometric evaluations of HP/UHP rocks.

Abstract When nucleation and growth of minerals in rocks occur, the system as a whole approaches a lower energy state. At given temperature, pressure and composition, the stable state corresponds to the minimum value of the rock Gibbs function. However, a description in term of equilibrium is not appropriate to understand the genesis of spatially ordered dispositions of minerals, non-equilibrium minerals morphology, zonation, and all features preceding equilibrium. Very commonly the kinetics of the transformation is frozen in, and disequilibrium textures beautifully show up. These situations must be described in terms of flow of components driven by the gradients of chemical potentials. For instance when nucleation occurs, the components' chemical potential differences are determined by the local associations of minerals in different parts of the rock ( Fisher, 1973 , 1977 ). The relative rates of intergranular diffusion and of mineral growth and dissolution determine the steepness of these gradients ( Fig. 1 ).

Abstract In the last decade, the considerable amount of ultrahigh pressure terrains found around the world (Liou et al., 1998) and recent findings of mineral inclusions suggesting equilibration pressures up to more than 20 GPa (e.g. Stachel et al., 2000) have led to an increasing interest in petrological tools to be used to unravel the P-T evolution at extreme conditions. The expected resolution of such tools should provide a key for understanding tectonic processes responsible for the dynamics of UHP terrains.

Abstract Ultrahigh pressure (UHP) rocks are extraordinary rocks that recorded extreme metamorphic conditions. Their study has been an exciting part of geosciences and in the last few years has emerged as a distinct discipline. Only few UHP terranes have been recognised worldwide so far and this only over the last ca. 20 years. They occur within major continental collision belts, predominantly in Eurasia and Africa (see e.g. Ernst & Liou, 2000 ; Liou, 2000 ; Carswell & Compagnoni, 2003 for a general review). Even though UHP rocks have been subject to intense studies in the last two decades, many questions regarding their formation and evolution remain open. Geochronology can assist this search for information in providing the absolute time of their formation and several other parameters regarding their evolution.

Abstract After more than a decade of research, the concept of subduction of light continental rocks into the denser upper mantle has become a well-established fact. Primary evidence for this phenomenon is found in the similarity of lithologic successions of UHP metamorphic terranes with stratigraphic sequences observed in upper continental rocks. Interlayered quartzite, marble, mica schist, and paragneiss resemble sedimentary sequences of sandstone, limestone, shale and graywacke deposited along continental margins. Concordant layers of eclogite in quartzite, marble, schist, and biotite gneiss suggest basaltic sills or lava flows. More importantly, based on chemical and isotopic analyses, eclogites from many UHP metamorphic terranes, regardless of their occurrence types, can be proven to have a continental affinity, hence a part of continental crust.

Abstract In this paper we examine the mechanical and penological interactions during formation and exhumation of ultrahigh pressure (UHP) terrains with the intention of producing a general three-dimensional dynamic model of continental subduction. We consider aspects of ancient and modern continental subduction to provide boundary conditions, rheological constraints and characteristic scales of time and space for the dynamic model in the generation and evolution of UHP during continental collision. The UHP and HP assemblages of the Western Gneiss of Norway provide rheological, geometric, and geochronological information for the modelling, while the active obliquely convergent plate boundary of central New Zealand serves as a modern analogue of the collision-subduction transition. The geodynamic models of oblique convergence, conditioned by these observations, provide orogen-wide velocity and strain rates and identify the characteristic length scales of strain partitioning within oblique subduction. The petro-structural evolution of individual packets within the oblique orogen are examined within a Lagrangian mixing model currently being developed that allows us to relate the large scale dynamic model to observations made at the level of an outcrop.

Abstract Garnet peridotite forms a large portion of the upper mantle but also occurs not uncommonly in association with crustal rocks in collisional mountain belts. The examples described here are all of the latest type, with the emphasis placed upon texture and metamorphism. All these examples are from the Central Alps and from the Eastern Alps, i.e., areas that have not been dealt with in previous chapters.

Abstract Fluid-rock interaction occurs at all steps of metamorphic evolution. Most metamorphic reactions, in fact, involve volatiles - aqueous for hydroxyl-bearing minerals, carbonic for carbonates. The determination of the P–T conditions of trapping and of the precise chemical composition of these fluids has always been a major problem of metamorphic petrology. Many methods are possible, mainly based on experimental or indirect approaches ( e.g. thermodynamic modelling, experimental mineral reactions, stable isotopes etc.). Fluid inclusion studies ( e.g. Roedder, 1984 ) represent a possible direct way to study fluids trapped at high P and T .

Abstract This is the first volume in this series dealing with a petrological subject and contains the contributions of the lectures given at the 5th School of the European Mineralogical Union (EMU) on “Ultrahigh Pressure Metamorphism” held in Budapest from 21 to 25 July 2003. The topic of UHPM was selected because this extreme type of metamorphism, initially considered as a petrographic oddity by the geologic community, has now become recognised as a normal feature of continental plate collisional orogens and important to understanding just how deep the upper part of the continental lithosphere can subduct. We note that this School took place just twenty years from the first report by Christian Chopin of coesite in exposed orogenic metamorphic rocks of the continental crust. The lectures given at this school benefited by the scientific results of the research promoted by the ILP Task Groups III-6 and III-8, active on UHPM from 1994 to 1998 and 1999 to 2004, respectively, and published in a number of monographs and special issues of international journals. It is our strong belief that this petrologic topic should be recognised to be of paramount importance in the education of students and young researchers in Earth Science.