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Paper No. 3
Presentation Time: 9:00 AM-6:30 PM

GARNET GROWTH IN RESPONSE TO MYLONITIZATION, GRANDFATHER MOUNTAIN WINDOW, BLUE RIDGE PROVINCE, WESTERN NORTH CAROLINA


FRUSHOUR, Anthony M. and ABBOTT Jr, Richard N., Department of Geology, Appalachian State University, Boone, NC 28608, frushouram@appstate.edu

The Grandfather Mountain Window in western North Carolina exposes the lowest structural level in the Blue Ridge Province. Rocks in the window constitute a Late Proterozoic basement-cover sequence. The basement consists mainly of Blowing Rock Gneiss (sic, porphyroblastic schist) and Wilson Creek Gneiss, both overlain unconformably by the Grandfather Mountain Formation. All of these rocks have been pervasively overprinted by greenschist facies metamorphism. The typical greenschist mineral assemblage involves combinations of chlorite, muscovite, biotite, actinolite, epidote, calcite, quartz, albite and K-feldspar. Garnet discovered in basement rock calls into question the metamorphic grade. The average garnet(core-rim) is

(Fe1.63-1.71Mn0.64-0.77Ca0.52-0.37Mg0.10-0.12)Al1.98-1.96Si3.06-3.04O12; the average biotite is

(K0.96Na0.06Ca0.02)(Fe1.73Mg0.87Mn0.02Ti0.04Al0.23)(Si2.83Al1.17)O10(OH)2; the average muscovite is

(K1.03Na0.02Ca0.02)(Al1.57Fe0.26Mg0.16Ti0.01)(Si3.31Al0.69)O10(OH)2. Thermometry involving Fe-Mn-Mg components in these minerals gives 580°C (+/- 90°C) to 590°C (+/- 90°C) from 1 to 4 kbars, respectively. There are at least four explanations for garnet in these rocks: (1) Garnet may have been stabilized in the greenschist facies by non-AFM components (esp. Mn), but the compositions are not unusual for metamorphic garnet, biotite and muscovite, and the calculated temperatures are too high for greenschist facies. (2) The garnet may be relict from an earlier episode of regional metamorphism, but this would be unusual as the only such evidence. (3) The garnet may be relict from earlier contact metamorphism, but the garnet is not spatially related to otherwise common metamorphosed (greenschist facies) mafic dikes. Finally, and most likely, (4) the garnet is a product of heating during mylonitization. Pressure shadows around the garnet indicate growth during mylonitization.

Session No. 332--Booth# 30

Metamorphic Petrology (Posters)
Wednesday, 30 October 2013: 9:00 AM-6:30 PM
Hall D (Colorado Convention Center)
Geological Society of America Abstracts with Programs. Vol. 45, No. 7, p.765
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