Introduction
Many aspects of metamorphic petrology had a delayed response to the plate tectonic revolution. The concept of isostasy was so powerful that the possibility of deep subduction of continental material (100s of km) was not considered seriously in the early years. A major exception was the interpretation of blueschists. W.G. Ernst (1963) in field and experimental studies of the Franciscan Terrane of California determined that glaucophane was stable only at low temperatures and “high” pressures, implying 10s of km depth. This discovery led him to surmise correctly that the rocks had to go down fast and back up equally fast and he tied the implied dynamics to the new concept of subduction (e.g. Ernst, 1971). This connection immediately solved the blue-schist conundrum – explaining how sediments could be subducted so quickly and coldly that glaucophane could become stable and return of this material to the surface in mélanges like the Franciscan Formation so quickly that glaucophane could be preserved. Nevertheless, there was a great resistance to acceptance that rocks could be transported to even these depths where such mineral assemblages could grow under equilibrium conditions and then be returned to the surface preserving those assemblages. As a consequence, the faulty concept of tectonic overpressure was created and continues to be appealed to today (see Green, 2005). Moreover, the possibility of much greater subduction of such rocks – or even exhumation of mantle rocks sufficiently fast that “memory” of very high pressure events could be preserved – apparently was little considered. There were exceptions but even eclogites and garnet peridotites were generally considered to represent depths of considerably less than 100 km (see the extensive review by Spalla et al., 2010, and references therein).
The discovery of coesite (Chopin, 1984; Smith, 1984) and diamond (Sobelev and Shatsky, 1990) in continental collision terranes ushered in the new discipline of Ultra-High-Pressure Metamorphism (UHPM). Nevertheless, these rocks were considered to be rare outliers that probably represented depths just beyond the onset of stability of these pressure indicators. Indeed, discovery of microstructural evidence of exhumation from hundreds of kilometers (Dobrzhinetskaya et al.,1996; van Rohrmund and Drury, 1998; Bozhilov et al., 1999) once again evoked disbelief. This Ultra-High-Pressure saga continues to evolve with numerous observations of coesite and diamond across the globe, including evidence of the former presence of stishovite in pelitic gneiss (Liu et al., 2007b) that requires subduction of shale to more than 350km and their return to the surface carrying evidence of the voyage.
The latest turn of events in this field is verification of phases and microstructures indicating depths greater than 300 km in the mantle portion of ophiolites (Yang et al., 2007; Dobrzhinetskaya et al., 2009). Thus, to a degree never imagined until very recently, we know that rocks have been exhumed from more than 300km from parts of both the downwelling and upwelling arms of mantle convection. This paper briefly recapitulates a few of these discoveries, evaluates their implications, and looks toward the future.