Masters Theses

Date of Award

5-1995

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Theodore C. Labotka

Committee Members

Robert D. Hatcher, Jr., Harry Y. McSween

Abstract

Migmatitic biotite gneisses and schists, near Cullowhee, North Carolina, within the eastern Blue Ridge province, exhibit textures and assemblages that indicate a complex polymetamorphic history that is apparently different from that of adjacent, nonmigmatitic, schistose rocks. Mapping by this study, along with previous work, has divided these rocks into two tectonostratigraphic units called the Cullowhee gneiss and the Otto Formation. This division is based on field and petrographic differences, along with thermobarometric calculations and P-T-X diagrams that further suggest the Cullowhee gneiss was subjected to an early high-pressure, high-temperature metamorphic event before the common metamorphic event apparently experienced by all field rocks. The findings presented in this study suggest that the Cullowhee gneiss may be part of a belt of high-grade rocks extending from the SW that has apparently similar polymetamorphic histories.

Rocks within the Sylva South and eastern Greens Creek quadrangles are structurally complex, with the occurrence of at least four fold generations and two major eastern Blue Ridge faults, the Hayesville and the Soque River fault. The contact separating the Cullowhee gneiss and the Otto Formation is interpreted to be an annealed fault contact. Minor shear and mylonitic zones are common throughout both rock units. Garnet inclusion fabrics and phases indicate synkinematic, prograde growth. These fabrics record the early S1 foliation, related to the D1 regional event, and the transposition of this surface to the dominant S2 foliation, related to the D2 regional event, as planar and S-shaped inclusion fabrics respectively.

Pelitic rocks of the field area commonly contain the mineral assemblage qtz + pi + bt + grt + ms ± St ± ky ± sil. Accessory minerals, occurring as garnet inclusions and within the matrix, include ± ilm ± hem ± rt ± ap ± crn. Minerals observed within the Cullowhee gneiss that suggest an earlier, higher-grade metamorphism includes K-spar, corundum, and rutile. Ruble, found as inclusions within garnet porphyroblasts only within the Cullowhee gneiss, suggest a higher pressure metamorphism than that indicated by ilmenite and titanhematite inclusions observed within the Otto Formation. This is based on a calculated rutile + muscovite + garnet stability field with pressure in excess of 7.3 kbars. Sillimanite occurs in two forms within the field area: as prismatic, needle-like grains between plagioclase and quartz and as intergrowths in garnet porphyroblasts that formed after garnet growth was complete. Sillimanite intergrowths are arranged tangential to the garnet cores and become pinched near the garnet rims suggesting nucleation during a late-stage deformational event, probably the same event that formed shear and mylonitic zones. Kyanite is highly resorbed within both rock units and is obviously a relict phase of an early prograde event. Staurolite is common within both field units and its texture IV suggests that both units may have been subjected to a common, late staurolite-grade event.

Chemical zoning profiles of garnet porphyroblasts within both field units exhibit relaxed chemical profiles indicating low to moderate degrees of diffusional homogenization. Prominent near-rim zoning, in the outer 0.2 mm, is the product of reaction zoning during retrogression. Garnet chemical profiles from previous studies, and this study, exhibit significant difference in chemical profiles between these two terrains. Garnet cores of the Cullowhee gneiss are enriched in Ca and show depletion approaching the rim and increase Ca content at the rim. Other garnet samples exhibit similar rim enrichment in Ca and Fe. Mn exhibits mild increases near the garnet rims as Mg decreases slightly at the garnet rim. Garnets of the Otto Formation have lower Ca content and higher Mn content within the core in comparison to the Cullowhee gneiss. Garnets of the Otto Formation also show near rim reaction as Ca content increase at the near rim.

Thermobarometric calculations reflect peak metamorphic conditions of 750 ± 20°C and 7.3 ± 1 kbar for the Cullowhee gneiss and 530 ± 20°C estimated at 4 ± 1 kbars for the Otto Formation. Other thermobarometric data are suspect because of extensive re-equilibration. T-X diagrams were calculated using the program GIBBS based on reaction zoning observed at the garnet rim. P-T paths deduced from these T-X diagrams suggest near isobaric cooling from 5.5 kbar and 640°C for garnets from the Otto Formation. P-T paths calculated from garnets of the Cullowhee gneiss exhibit decompression with cooling from higher P-T conditions in excess of approximately 8 kbar and 720°C. P-T paths from both field units and Great Smoky group rocks, west of the Hayesville fault, exhibit converging P-T arrays at approximately 5.4 kbars and 570°C.

The apparent polymetamorphic history of the Cullowhee gneiss in contrast to the lower pressure, single prograde event observed within the Otto Formation, reflects different tectonic evolutions. Garnets of the Cullowhee gneiss may be a relict of an earlier metamorphic event, suggesting that the gneiss is possibly Proterozoic in age. Alternatively, the converging of P-T arrays may reflect differences in the timing of accretion to the North American craton, in which the Cullowhee gneiss was metamorphosed prior to accretion followed shortly by accretion and subsequent metamorphism of cover-sequence rocks including the Otto Formation.

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