Masters Theses

Date of Award

5-2001

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Theodore C. Labotka

Committee Members

Lawrence Taylor, Harry McSween, Jr.

Abstract

Detailed quantitative and qualitative x-ray maps of garnets from the Cullowhee Gneiss, Cullowhee, North Carolina indicate a complex metamorphic history. The Cullowhee Gneiss is a migmatitic quartzo-feldspathic gneiss with boudins of amphibolite and quartz. Thin-sections from the same outcrop have the same major mineral assemblage which includes biotite + muscovite + plagioclase + quartz + sillimanite + kyanite + garnet. Three garnets, with diameters that range from 7mm to 1.4 cm, taken from the same outcrop, contain inclusions of biotite + rutile + ilmenite + quartz + muscovite + K-feldspar + plagioclase + apatite + monazite and sillimanite needles in the rims. These inclusions indicate that at one time the metamorphic conditions were at the second sillimanite isograd and above the muscovite breakdown point. Garnet zoning patterns are the result of both diffusion and growth processes and indicate that early garnet compositions were altered along the rims, cracks, and large fractures during a later period of metamorphism. The fracturing of the garnet created asymmetry in one zoning profile, an atoll effect in another sample that is most visible in the grossular content of the garnet. End-member zoning profiles of the garnets have an increase in grossular, almandine, and spessartine content, towards the core, while the pyrope profile decreases towards the core. In garnets that have the atoll of high calcium, there is a break in all the profiles and it is harder to distinguish the shape of the original profiles. Flattened profiles, towards the center of the garnet, indicate that diffusion processes had a dominant influence on the original zoning profiles. The larger garnet has an euhedral core encased by an anhedral overgrowth of garnet. The overgrowth contains very few inclusions, but has fluid inclusion trails. There is a great contrast between the An content of plagioclase inclusions at the cores (An35-40), the high calcium ring (atoll) (An40-66) outside the core, and the An content in the matrix (An23). These contrasts reflect the differences in the P-T conditions during different metamorphic events. Thermobarometry indicates an early high -pressure (~ 7.5 kbars) high - temperature (~ 620°C) period within the garnet's core. At this stage in the garnet's history, diffusion processes were dominant in determining the type of zoning profile. Depressurization resulted in the crossing into the sillimanite field. During this stage, growth zoning dominated the chemical zoning patterns of the garnet and an overgrowth of garnet had plagioclase with lower anorthite content, a pressure drop (to 5 kbars) and a decrease in temperature. The final period of metamorphism produced rim conditions with An23 and pressures (~ 4 kbars) and temperatures (475 °C). P-T paths calculated with the GIBBS program by Spear and Kohn, (1997) are clockwise, as is typical of overthrust terranes. They indicate that there was an overprint of a high-pressure metamorphic event by a high temperature event that went above the muscovite breakdown curve. There was decompression and cooling during the retrograde path, and finally an increase in pressure with a decrease in temperature at the rim of the garnet. The gametes of the Cullowhee Gneiss recorded the polymetamorphic history of the Eastern Blue Ridge Province. This history spans the Grenville, Taconian, Acadian, and Alleghanian, orogenies.

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