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.