Minerals that require formation at very high pressures, including diamond, recently have been identified in crustal rocks from ancient orogenic belts associated with continental collision. The presence of these minerals implies that these rocks were subducted to depths of 100 to 200 kilometers (or perhaps even deeper), and then exhumed rapidly and brought near the surface. Two recent studies may imply an even faster exhumation than has been appreciated.
O'Brien et al. have discovered coesite, a high-pressure phase of silica (quartz is the stable phase at Earth's surface), in rocks from the Himalayan orogen in Pakistan. The presence of coesite and other minerals in fairly young rocks from the Himalayas implies rapid movement (approaching 1 centimeter per year) from depths close to 100 kilometers. This finding may force changes in models of the evolution of this part of the Himalayas. Stöckhert et al. examined metamorphic microdiamonds (stable at depths exceeding about 130 kilometers) discovered in central Europe. They argue that the diamonds crystallized from trapped fluids in minerals as the rocks cooled at high pressure within the diamond stability field and were then rapidly transported to regions of lower pressures. -- BH
Geology 29, 435; 391 (2001).
