Fluid-rock interactions during high-pressure and ultrahigh-pressure metamorphism

Fluid inclusions and stable-isotope data of MP and UHP terrains fi om China anti the European Alps are used to evaluate: fluid-rock interactions during subduction anti continental collision. The choice of the European Alps and China for a comparative study is compelling, because the rocks of the Alps exemplify subduction of a cold (mature) oceanic lithosphere, whereas those of China illustrate the responses of continental crust to subduction under relatively. higher temperatures representative of the base of a thickened continental wedge. Our study demonstrates that there is no difference between the fluid-rock behavior of continental and oceanic crust during subduction anti metamorphism. In both regions, there is evidence for the presence of a grain-boundary fluid phase during HP metamorphism. Recognition of grain- to hand specimen-scale chemical and isotopic heterogeneity attests to local buffering by mineral assemblages of fluid compositions. Assuming a one-dimensional fluid flow perpendicular to rock-layer contacts, we estimated fluid fluxes of 15 to 465 cm(3)/cm(2) to as low as 1.6 cm(3)/cm(2) in China and Alpine rocks. respectively. The values from China are to be considered as mat;ima because, in most cases, sampling was at a reconnaissance scale and does not permit evaluation of disequilibrium on a scale of one centimeter. An important finding, characteristic of the entire range of conditions represented by these rocks, is the retention of recognizable vestiges of pre-subduction fluid-rock interactions, be they a geothermal system with a surface-exposure length scale on the order of 100 kilometers in continental rocks of China, or relies of high- and low-temperature hydrothermal alteration features in oceanic material of the Western Alps. A key issue arising from these results concerns the plausibility of transferring a slab component to the source of arc-magma genesis in a low fluid/rock regime. Possible scenarios are discussed in light of several recent innovative experiments that have dispelled some of the speculation obscuring our understanding of fluid-melt-rock interactions during HP and UHP metamorphism.