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A Study of Some Petrologic and Structural Aspects of the East Dover Ultramafic Bodies, South Central Vermont
Full text linkINTRODUCTION (pp.1-4)
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
Structural Studies in the Moretown and Cram Hill Units near Ludlow, Vermont
Full text linkThe geology of the eastern limb of the Green Mountain Anticlinorium consists of a series of Paleozoic metasedimentary rocks with lithologic boundaries arranged in a remarkably straight trend approximately parallel to the axis of the Green Mountains. Published reports of the area, consisting largely of reconnaissance mapping, have treated this complex series of polyphase deformed rocks as an essentially upright autocthonous sedimentary sequence. Boundaries between rock units have, for the most part, been assumed to be primary in origin, as have various structural elements within the rock units. More recent work in selected areas within the Ludlow Quadrangle has revealed problems in the application of stratigraphic techniques toward field mapping in these medium grade metamorphic rocks. One of the main problems is the consistent misidentification of demonstrably secondary mesostructures as primary sedimentary features by early workers. Many rocks in the area have been shown to contain secondary structures which mimic such primary features as bedding and conglomeratic pebbles (Gregg and Nisbet, in preparation) and which have in fact been mistaken for these structures in the past. In addition to problems arising from incorrect interpretation of mesostructures, other problems have resulted from the failure of some workers to distinguish various fold groups on the basis of overprinting relationships, This investigation deals with a subarea within the Ludlow quadrangle where detailed structural mapping was performed by the author from 1971 to 1974. The central feature of the area is the ultramafic zone consisting dominantly of serpentinized ultramafic rock masses up to 1 km long with minor zones of talc carbonate rocks around the boundaries, This ultramafic zone is situated along the boundary of two rock units, the Moretown member and: the Cram Hill member of the Missisquoi Formation. Although these units have been called members of the same formation by early workers, the author has found a number of striking contrasts in the deformational features of the units. For example, the Cram Hill phyllites have been involved in only two phases of deformation, while the Moretown gneisses contain structures from at least one earlier deformational phase. The Cram Hill phyllites contain layering which may be of sedimentary origin and which is moderately deformed in most cases. The Moretown member has been severely deformed, and all trace of initial layering is obliterated. In addition to these contrasts the author has shown a discordancy between early layering and the contact between the units which is probably of tectonic origin. Because. of these contrasts the author considers that there is little basis for classifying the various rock types by the Formational and Member designations, and that future structural work in the Central Vermont area will result in the abandonment of the stratigraphic nomenclature now being applied. During field work and thin section examination the author observed a number of tabular garnets in the rocks of the Moretown member. In most cases the garnets were formed during the earliest deformational event and were deformed into tabular shape by later deformation. The deformation, however, was not the typical flattening assumed by most workers, but a slicing process in which segments of garnets from an initially equidimensional crystal are sheared parallel to rock layering. A number of examples of partially sliced crystal sections were observed on mesoscopic and microscopic scales
Structural Studies in the Mafic and Ultramafic Rocks of the Lewis Hills, Western Newfoundland
Full text linkTable of contents:CHAPTER I. INTRODUCTIONCHAPTER II. REGIONAL GEOLOGYCHAPTER III. THE LEWIS HILLS COMPARED TO THE NORTHERN AREAS OF THE BAY OF ISLAND COMPLEXCHAPTER IV. PETROGRAPHYCHAPTER V. STRUCTURAL GEOLOGYCHAPTER VI. SUMMARY, CONCLUSIONS, AND SPECULATIONSBIBLIOGRAPHYAPPENDIX I. CONTOURED STEREOGRAPHIC PROJECTIONS OF FOLIATIONS AND LINEATIONS IN THE HINES POND ARE
Petrography, metamorphism, and geochemistry of the Bermeja Complex and related rocks in southwestern Puerto Rico and their significance in the evolution of the eastern Greater Antillian Island Arc
Full text linkChapter I. Introduction and previous work 1 Chapter II. Geological setting and the petrography of the igneous and metamorphic rocks from southwestern Puerto Rico 6 Chapter III. Metamorphism in southwestern Puerto Rico and relations to the Eastern Greater Antilles 46 Chapter IV. Geochemistry of the Bermeja complex 84 Chapter V. Secular compositional changes of the volcanic rocks in Puerto Rico and other islands in eastern West Indies and the significances of the Bermeja complex 179 Appendix A-E: Petrographic data of the analyzed samples of the Bermeja complex 222 References cited 23
The Investigation of Snowfall Rate Using Optical Techniques
Full text linkExperiments involving the attenuation of light by falling snow, or hydrometeors in general, lend themselves to the study of at least two problems of meteorological interest. The first problem is that of visibility in adverse weather conditions, and the second is that of finding a better method to measure precipitation. From the late 1940\u27s through the late 1960\u27s, various investigators have attempted to apply forward light scattering theory in the geometric optics range (α ≥ 200, where α = (2π_r)/λ, λ=wavelength and r = radius)* to quantitatively describe the attenuation of a light beam, and thereby visibility changes due to hydrometeors
Petrology of the Nemeiben Lake Ultramafic and Associated Nickel-Sulphide Deposits
Full text linkThe Nemeiben Lake ultramafic body is located in the center of Saskatchewan, Canada, within the Churchill Province of the Canadian Shield. The ultramafic rocks consist of serpentinites, partly serpentinized and uralitized pyroxenite, and unaltered pyroxenite. Associated sulphide mineralization is of disseminated, net texture, and fracture filling types. The ore minerals present are pyrrhotite, pentlandite, chalcopyrite, pyrite, marcasite, violarite, bravoite and native copper. The sulphides are considered to be initially of magmatic origin formed from a sulphide melt separated at a late stage during crystallization of the ultramafic rocks. Subsequent serpentinization has locally redistributed the ores. Serpentinization in the Nemeiben Lake ultramafic rocks was accompanied by introduction-of water with no apparent changes of SiO2 and MgO contents. A minor removal of CaO and possible addition of sulfur during serpentinization is the only apparent chemical change. From investigation of co-existing clinopyroxenes, orthopyroxene and olivine and the distribution of Mg and Fe between pyroxenes, it appears that the Nemeiben Lake ultramafic body crystallized at high temperature between 1100-1200 ºC and in a pressure range between 5-9 Kb
ORIGIN OF THE MOUNT MERINO CHERT AND SHALE, MIDDLE ORDOVICIAN, EASTERN NEW YORK STATE
Full text linkMount Merino Chert and Shale, Middle Ordovician, is one of the most siliceous units of the Taconic sequence (eastern New York and western Vermont); it is composed of interbedded shale, siliceous hale, argillite and chert. Non-clastic quartz — aggregates of quartz having a.mosaic or felted texture — predominates in all beds, except `shale. All siliceous beds are finely laminated; most laminae are distinguished from adjacent laminae by the texture of the quartz groundmass, and the amount of clastics, carbonates, chlorite and sulphides. Statistical comparison of the textures of the quartz aggregates which occur with the other mineral components suggests that the components of each lamina represent a stable mineralogic assemblage; the assemblages probably formed during silica precipitation and early diagenesis. These assemblages are compositionally-consistent with experimental data regarding the formation of authigenic minerals in the presence of colloidal silica. Mount Merino rocks comprise a minor.part of the Giddings Brook slice of the Taconic allochthon; the Mount Merino fauna is the youngest of the Giddings Brook slice rock sequence. Mount Merino rocks also occur as boulders and blocks in Forbes Hill Conglomerate, an autochthonous wildflysch-like terrain underlying the Giddings Brook slice. Petrographic aspects of Mount Merino rocks indicate a starved depositional environment distant from an extensive land area. The predominant source of silica for the rocks was probably vulcanism (the Ammonoosuc volcanics) which became relatively intense during Mount Merino time, just preceding emplacement of the Taconic allochthon