Petrogenesis of Garnet-bearing Rocks in the Grandfather Mountain Window, Blue Ridge Province, Western North Carolina

Abstract:

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
(Fe_1.63-1.71Mn_0.64-0.77Ca_0.52-0.37Mg_0.10-0.12)Al_1.98-1.96Si_3.06-3.04O₁₂; the average biotite is (K_0.96Na_0.06Ca_0.02)(Fe_1.73Mg_0.87Mn_0.02Ti_0.04Al_0.23)(Si_2.83Al_1.17)O₁₀(OH)₂; the average muscovite is (K_1.03Na_0.02Ca_0.02)(Al_1.57Fe_0.26Mg_0.16Ti_0.01)(Si_3.31Al_0.69)O₁₀(OH)₂. Thermometry involving Fe-Mn-Mg components in these minerals gives 766°C (+/- 91°C) at 13.6 kbars (+/- 1.4 kbar), 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 earlier contact metamorphism, but the garnet is not spatially related to otherwise common metamorphosed (greenschist facies) mafic dikes. (3) The garnet is a product of heating during mylonitization. Finally, and most likely, (4) the garnet may be relict from an earlier episode of regional metamorphism. Samples of porphyroblastic schist and greenstone from the same outcrop give low temperature, greenschist facies conditions.