Introduction to geodynamics for high- and ultrahigh-pressure metamoprhism

Since the first workshop on ultrahigh-pressure (UHP-metamorphism at Stanford in 1994 and the special issue of The Island Arc 'Ultrahigh-Pressure Metamorphism and Tectonics' published in December 1995, many symposia and special sessions specifically with regard to the UHP metamorphic terrane have been held. While we are still wondering how exhumation of UHP rocks from mantle depths to the surface takes place, the finding of possible records from the mantle transition zone at 300-400 km depths is astonishing. The study of the UHP regime has expanded to include input from mineral physics, experimental geochemistry and kinetics in addition to the petrochemical and tectonic study of a variety of HP-UHP rocks. It was with this theme that the second workshop for the task group III-6 'Ultrahigh-Pressure Metamorphism and Geodynamics in Collision-type Orogenic Belts' of the International Lithosphere Program was held during the International Geological Congress in Beijing, 1996. The Symposium 8-9 'Dynamic Metamorphic Rocks and High- and Ultrahigh-Pressure Metamorphism' (Cong Bolin and J.G. Liou conveners) had more than 25 presentations in two oral sessions and 70 papers in one poster session. This second special issue of The Island Arc includes nine papers from this symposium and a few related contributions to the geodynamics of HP-UHP metamorphism and tectonics. It is our hope that The Island Arc will continue to publish a special issue on this increasingly recognized subject that is essential to our understanding of continental collision, mantle dynamics and geochemical + fluid cycles.