Date of Award
Master of Science (MS)
Mineralogical, textural and chemical changes of ultramafic rocks in response to regional deformation and metamorphism are, at best, imperfectly known (Miyashiro, 1973, p. 30). In Vermont, which has an extremely prominent and well-exposed belt of ultramafics (fig. 1), investigation of these rocks has largely been directed toward such processes as serpentinization, steatitization, and the formation of metasomatic zones at the contacts with country rocks. With few exceptions, there is a lack of detailed descriptions of regional metamorphic textures, mineralogy, and structures developed in the Vermont ultramafic rocks. It is the main purpose of this thesis to describe the mineralogical and textural changes that accompany regional metamorphism and deformation in the large ultramafic body at East Dover, Vermont. Serpentinization processes or the effects of hydrothermal alteration are not dealt with in detail, although some observations are made on these topics. Two field seasons (1973-1974) were spent in an area approximately 6 x 3 km in and around East Dover, Vermont. When existing geologic maps of the area (Skehan, 1961; Vermont State Geologic map, 1961) were found to lack sufficient detail for these studies, field mapping was initiated to help correlate the petrology and structure of the body. Indeed, as mapping continued it became evident that complete analysis of the structural complexities of the body was far beyond the scope of this research. However, mapping of different rock types, measurement and description of the most prominent-foliations, and a preliminary analysis of folding in the body were carried out to provide basic structural data. Structures in the country rocks at or near the contacts were studied to gain a better idea of the relationships of the ultramafic body to the country rocks. A preliminary comparison of structural elements in the ultramafic and country rocks was also attempted. Only cursory petrographic examination of the country rocks was undertaken, mainly to determine the metamorphic grade and general rock types that surround the ultramafic body. The textures and mineralogy of the ultramafic rocks were studied in detail to provide information essential to interpreting the metamorphic and deformational history of the body. In this regard, previous petrographic work on these rocks was found to be inadequate and inaccurate.
Concluding Remarks: (pp.98-99)
In summary, the following can be said about the petrology of the East Dover ultramafic rocks: 1. The distribution of olivine textural and chemical variations, and chrome spinel textural. variations can be attributed to differing intensities of recrystallization. 2. Serpentinization is uniform over large areas of the ultramafic bodies but must be described as randomly developed with respect to the margins. 3. The only pyroxene now observed is metamorphic in origin (but may be pseudomorphic). 4. The distribution of magnetite is not simply related to the development of serpentine, as a simple serpentinization process would imply. It may, rather, be a function of removal of the iron component from olivine during recrystallization, or differing partial pressures of oxygen during serpentinization. 5. A stage of tremolite development (uralitization) occurred at a different time than the development of T2 olivine and diopside.
Hoffman, Mark Allen, "A Study of Some Petrologic and Structural Aspects of the East Dover Ultramafic Bodies, South Central Vermont" (1975). Geology Theses and Dissertations. 58.