Abstract

An historical account provides a background to the recent discoveries of coesite and diamond. Descriptions of the general geologic and tectonic setting for ultrahigh pressure metamorphism (UHPM) areas in the Western Gneiss Belt of Norway; Kockchetav area, Kazakhstan; Dabie Shan, Eastern China; Western Alps and the Bohemian massif, Eastern Europe, indicate a similarity in tectonic setting. It is shown that the size of these areas is large and that they consist of coherent lower crustal material previously consolidated as part of older continental fragments. The protoliths of these rocks is usually characterized as old, dry, and cold supracrustal rocks. These older crustal rocks often appear to have undergone anatexis prior to being metamorphosed as part of a continent-continent collision. Special conditions are required to exhume these rocks which have reached depths in excess of 100 km. Numerous tectonic schemes have been proposed, but no single mechanism can be applied to all of the known occurrences of UHPM. Future studies will enhance our sketchy knowledge of the tectonic situation that allows UHPM rocks to be formed and then rapidly exhumed so as to be preserved within crustal exposures of metamorphic rocks.

Introduction

The recent discoveries of coesite and diamond in metamorphic rocks that are part of the Earth's upper crust has drastically changed our ideas concerning the limits of metamorphism. Formation of these ultrahigh pressure minerals requires pressures in excess of that expected within the crust of the Earth.