Regionally metamorphosed sedimentary rocks are the products of a complex series of events that typically includes deposition, burial and diagenesis, deep burial as part of a tectonic collision, deformation and heating under pressure, metamorphism in response to changes in temperature and pressure inluding dehydration, chemical reactions, the growth of minerals, and textural changes, then decompression and cooling due to erosion and possibly tectonic unroofing. The Dutchess County, NY rocks shown in this activity are metamorphosed shales and sandstones that were deposited early in the Paleozoic Era (Cambrian-Odovician) and metamorphosed in the late Ordovician by a collisional event that included significant thrust faulting (see Stanley and Radcliffe, 1985, as can be observed on the geologic map. Our examination of Duchess County samples yielded considerable information about their geologic history, some of which are recounted here:

• In spite of this complex P-T history, the metamorphosed shales of Dutchess County have mineral assemblages that are largley consistent with chemical equilibrium at metamorphic grades that increase from west to east.
• This conclusion is supported by the observation that most samples consist of 3-phase assemblages on J.B. Thompson AFM diagrams, in addition to the projection minerals Ms and Qz and mobile H₂O, satifying the mineralogical phase rule.
• Additional evidence for equilibrium is that nearby rocks of differing chemical composition and mineralogy have mineral assemblages that are consistent with the same P-T conditions.
• Based on available chemical analyses, there is no consistent variation of the bulk chemical compositions of these rocks as a function of metamorphic grade, consistent with isochemical metamorphism (except for H₂O).
• The mineral assemblages can be used to identify equilibrium conditions based on the results of laboratory experiments. Based on the Spear-Cheney KFMASH petrogenetic grid, conditions range from roughly 500-700°C at pressures from 0.6-0.8 GPa, consistent with a a Barrovian, medium-P/T, regional metamorphic belt.
• A few samples have too many minerals, violating the mineralogical phase rule, and possibly capturing incomplete reactions. Further study of these and similar samples may reveal details of their P-T path history.
• Chlorite appears in a number of samples, indicating rehydration and reaction at temperatures below those consistent with equilibrium among the other minerals present. The limited amount of retrograde chlorite indicates little H₂O return to these rocks following the thermal peak of metamorphism.

This activity is built upon a series of rock samples and thin sections collected by Donna Whitney and William Peck, and desribed in a short field guide (Whitney and Peck, 2004). Donna and William have not only shared their samples, but they have also provided additional information, photos, advice, and feedback on some drafts of this activity. Clémentine Hamelin selected, assembled, photographed, and mailed Donna's samples. Ann Pufall photographed most of the thin sections. And none of this would have been possible without the scientific work of many previous geologists who have studied the Dutchess County rocks. References to some of this work are listed on the next page.