2.1 Activity Overview

Owing to faulting and erosion, very few medium P/T regional metamorphic terrains in the world contain a consistent and complete sequence of rocks recording a continuous series of metamorphic conditions from very low to very high grade. However, there are a few places in the world that do, and metamorphic petrologists have studied these places intensively, particularly those that have one rock type, metamorphosed shale, that occurs in all metamorphic zones. These medium P/T terrains are called Barrovian sequences, after George Barrow, who described such a sequence in Scotland more than 100 years ago (Barrow, 1893).

We can study Barrovian sequences to 'see' in the rocks a series of metamorphic reactions and how they result in the appearance and disappearance of minerals as a function of temperature and pressure changes. A complete Barrovian sequence would consist of the shale protolith and a series of metamorphic zones, from the first appearance of metamorphic minerals up to granulite facies or even melting conditions.

One of the most complete Barrovian sequences in the world is in Dutchess County, NY, and it contains the protolith shale and a metamorphic series from very low to very high grade. The metamorphic rocks of Dutchess County, NY, USA, were studied in detail by Robert Balk (1936) and Tom Barth (1936), and have been the subject of many subsequent studies and field trips (see the Dutchess County References page at the end of this activity).

Images of and information about Dutchess County metamorphic rocks are presented here along with a series of quesions to help readers focus on the petrology of these exemplary rocks. This activity begins with an examination of images of hand samples and thin sections of metamorphosed shales from Dutchess County. You are then asked to use your observations of these rocks to complete the following tasks.

1. Identify the minerals present in each rock from a set of possible minerals.
2. Based on the minerals present, place each sample in a metamorphic zone on the geologic map.
3. Identify the possible T-P conditions of metamorphism for the samples based on the observed mineral assemblage and a model petrogenetic grid.
4. Answer several other questions about the samples and constraints they place on the metamorphic history of Dutchess County.

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