Low grade metamorphism of mafic rocks

Through most of this past century, metamorphic petrologists in the United States have paid their greatest attention to high grade rocks, especially those which constitute the core zones of exhumed, mountain belts. The pioneering studies of the 50's through the 80's, those which applied the principles of thermodynamics to metamorphic rocks, focused almost exclusively on high temperature systems, for which equilibrium processes could be demonstrated. By the 1980's, metamorphic petrologists had developed the methodologies for deciphering the thermal and baric histories of mountain belts through the study of high grade rocks. Of course, low grade metamorphic rocks ‐ here defined as those which form at pressures and temperatures up to and including the greenschist facies ‐ had been well known and described as well, initially through the efforts of Alpine and Circum‐Pacific geologists who recognized that they constituted an integral and contiguous portion of mountain belts, and that they underlay large portions of accreted terranes, many of oceanic origins. But until the mid 80's, much of the effort in studying low grade rocks ‐ for a comprehensive review of the literature to that point see Frey (1987) ‐ had been concentrated on mudstones, volcanoclastic rocks, and associated lithologies common to continental mountain belts and arcs. In the mid 80's, results of the Deep Sea Drilling Project (DSDP) rather dramatically mitigated a shift in the study of low grade metamorphic rocks.