The Polymetamorphic Sequence in the Paleozoic Rocks of Northern Vermont: A New Approach Using Metamorphic Veins as Petrologic and Structural Markers

Citation

Anderson, James Rodney (1977) The Polymetamorphic Sequence in the Paleozoic Rocks of Northern Vermont: A New Approach Using Metamorphic Veins as Petrologic and Structural Markers. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/smdr-at29. https://resolver.caltech.edu/CaltechETD:etd-11012006-140440

Abstract

The sequence of metamorphic events that have affected the lower and middle Paleozoic rocks of northern Vermont has been defined in this study. The use of metamorphic veins as structural and petrologic markers has helped to establish the correspondence between deformational and mineral growth features. The effects of the major metamorphic events have been followed through the area along several traverses, the longest of which is 50 miles in length. The study combines detailed analysis of the sequence of structural elements with electron microprobe and petrographic analysis of the mineral growth in the metamorphic veins and host rock.

Much of the history of the chemical path of the vein and host rock systems is preserved in zoned grains and grains included in other minerals. The zoning trends in certain key minerals such as plagioclase, amphibole, muscovite, and coexisting calcite and ankerite indicate that the grade of each rock system was changing with time. The path of the systems can be followed in considerable detail. Not only are the effects of changing grade during a single event typically preserved, but also the effects of several superimposed mineral growth events can be preserved in a sample.

Five major metamorphic events and several minor events have occurred in northern Vermont. The two oldest events affected only the Cambrian and Ordovician rocks; these events are designated Oa and Ob. Event Oa involved widspread biotite grade mineral growth and the formation of a secondary bedding schistosity. Ob is the most prominent of the pre-Silurian events and is Late Ordovician in age. It produced high grade mineral growth in the Green Mountains and in the Worcester Mountains; in the latter area staurolite-kyanite grade assemblages occur. There was also widespread formation of small scale isoclinal folds with east-west axial trends during Ob. Major east-west trending folds of the same generation occur locally.

Three events affected both the pre-Silurian rocks and the Silurian and Devonian rocks. These events are Middle to Late Devonian in age. Da is the oldest of the three and produced biotite to garnet grade or higher mineral growth over the entire study area. Da was also a significant deformational event, responsible for the formation of major and minor north-south trending folds. Event Db mainly involved deformation, but biotite grade mineral growth occurred in rocks where the axial plane foliation associated with Db was developed. Folds formed during Db have north-south trends and are most prominent in the eastern part of the area. The last major event, Dc, was a period of mineral growth without associated deformation and was widespread in extent. The highest grade mineral growth of Dc, up to staurolite-andalusite and sillimanite grades, has a close spatial relationship to the intrusive bodies of the New Hampshire plutonic series. In areas far from such plutons, this mineral growth is biotite grade or absent.

Metamorphic veins are associated with each of the major events except Db. The veins are discontinuous and have a general chemical correspondence to mineral growth of the same generation in the adjacent host rock. Differences in the relative timing of growth in the veins and host rocks occur in some instances, so that the correspondence is fairly complicated. The veins appear to have formed during the metamorphic events from material derived locally in the host rocks.

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