J. metamorphic Geol., 2000, 18, 121

PREFACEGarnet peridotites and ultrahigh-pressure mineralsJ . G. L IOU 1 AND D. A. CARSWELL 2

1Department of Geological and Environmental Sciences, Stanford University, Stanford, CA-94305, USA2Environmental and Geological Sciences, University of Sheffield, Dainton Building, Brookhill, Sheffield S3 7HF, UK

The discovery of unambiguous ultrahigh-pressure 4. Petrogeneses and exhumation of UHP eclogites andthe enclosing gneisses.(UHP) minerals such as microdiamond and coesite in

supracrustal rocks is revolutionizing our understanding This workshop was followed by an AGU SpecialSession on UHP Metamorphism and Geodynamics inof continental collision zones and the dynamics

attending subduction of continental lithosphere. Collision-Type Orogenic Belts at San Francisco onDecember 8–9, 1998. Most of the workshop partici-Garnet-bearing ultramafic rocks are minor but wide-

spread as an important component in UHP terranes, pants also attended the AGU meeting, and presented38 papers in the two oral sessions and one postersuch as the Western Gneiss Region, Norway, the

Bohemian Massif of central Europe, the Dabie-Sulu session.Many of the papers presented in both the StanfordUHP terrane of east-central China, and Indonesia.

These ultramafic rocks may have been derived from Workshop and the AGU Special Session have beensubmitted for publication in several special issues.different tectonic settings and subjected to coeval UHP

metamorphism with their enclosing supracrustal rocks. These are: (1) garnet peridotites in this issue of theJournal of Metamorphic Geology ; (2) other HP/UHPThey may represent: (1) peridotite derived from

subducted lithospheric mantle; (2) peridotites from the Rocks in L ithos; (3) the Kokchetav Massif in IslandArcs; and (4) a final report for ILP Task Group III-6mantle wedge above a subduction zone that sank and

were incorporated in subducted continental crust; or in International Geology Review.The seven papers in this issue are examples of some(3) products of subduction-zone metamorphism of

peridotite previously emplaced into the crust. of the exciting findings and research projects describedby participants at these meetings which served toTherefore, garnet peridotites provide important win-

dows into petrotectonic processes operational within expedite scientific exchanges among geoscientists fromvarious countries. Our thanks and appreciation go tothe subcontinental mantle or the mantle wedge above

the subduction slab. the many conference participants, particularly to theauthors of the papers in this special issue and to thoseThe task group ‘Ultrahigh-Pressure Metamorphism

and Geodynamics in Collision-type Orogenic Belts’ who offered their time and expertise to review themanuscripts.was established in 1993 as Project III-6 of the

International Lithosphere Program (ILP). Since then, Most of the papers in this thematic set are concernedwith key aspects of the petrological, geochemical andstudy of UHP metamorphic rocks in terms of their

genesis and exhumation, and implications for the role P–T evolution of garnet peridotite bodies, and withthe dynamics of crust/mantle interaction in differentof fluid in geochemical cycling and descending slab/

mantle wedge interactions have attracted widespread continental plate collision zones. In addition, thestartling discovery of relict majoritic garnet withininternational attention. An international workshop on

UHP metamorphism and exhumation organized by Norwegian garnet peridotite signals that some of thesebodies retain vestiges of previous, precollisional histor-J. G. Liou and W. G. Ernst, was held at Stanford on

December 3–5, 1998; this represented the fifth work- ies, and thus provides tantalizing insights into deepmantle diapiric processes. The dramatic discovery ofshop for the ILP Task Group III-6. Ninety-seven

participants from around the world attended the majoritic garnet and as reported at these meetings,other possible mineral indicators such as clinoenstatiteworkshop; 43 oral and 38 poster papers were presented

in five different sessions, which were grouped in four challenge our understanding of deep mantle processes.Finally one paper in this thematic set, though notmajor themes:

1. Petrogenesis and tectonics of UHP terranes in east- specific to garnet peridotites, provides a valuableupdated calibration of the mineral thermometer mostcentral China;

2. Petrogenesis and tectonics of the Kokchetav Massif, widely applied to evaluate the equilibration tempera-tures of both garnet peridotites and associated eclogites.northern Kazakhstan, and Western Gneiss Region,

Norway;3. Study of UHP garnet peridotites as a new windowinto mantle processes; and

121© Blackwell Science Inc., 0263-4929/00/$15.00Journal of Metamorphic Geology, Volume 18, Number 2, 2000