REGIONAL METAMORPHISM IN THE MANHATTAN PRONG, NY AND CT

The metamorphic record provides a key element for understanding the development of mountain belts and for tectonic processes in general. Here we describe regional-scale high-pressure granulite assemblages of early-Taconian age in the Manhattan Prong, rocks reflecting the most extreme P T conditions recorded in the Appalachian belt. These granulites were heavily overprinted by retrograde assemblages, so that their discovery depended on extensive petrographic work; they raise questions about "hidden " high-grade histories in other areas where garnets have been thermally homogenized. Traditional petrographic study of metamorphic rocks has been largely supplanted by electron-microprobe analysis over the past 25 years. It is commonly assumed that reasonably consistent P-T results from "representative " rocks will faithfully correspond to peak metamorphic conditions. Retrograde metamorphic affects are "avoided " by use of core mineral compositions and by rocks with high biotite:garnet ratios (e.g., Spear, 1991). However, the Manhattan Prong illustrates how microprobe analysis in the absence of extensive petrographic work can lead to dramatically mistaken conclusions