43 Appalachian Petrology: A Session to Honor the Career of Dr. Robert J. Tracy

Saturday, 21 March 2020
1:30 PM - 5:30 PM
Lake Fairfax

P-T CONDITIONS AND CHEMISTRY OF GEDRITE - ANTHOPHYLLITE - GARNET SCHISTS FROM THE LITTLE PINE GARNET MINE, NC: IMPLICATIONS FOR THE TECTONIC EVOLUTION OF THE WESTERN BLUE RIDGE

Paper number: 43-4
Program: Theme Sessions
Time: 2:15 PM – 2:35 PM

Authors

Abstract

The Little Pine Garnet Mine (LPGM) is located between the Mars Hill and Grenville basement terranes in the Western Blue Ridge province of the Southern Appalachians, and is considered part of the garnet zone of the Taconic orogeny (e.g. Hatcher & Merschat, 2005). It exposes anthophyllite (Anth) – gedrite (Gd) - chlorite (Chl) – biotite (Bt) ± garnet (Gt) ± staurolite (St) ± ilmenite (Ilm) schists (group I), Gt – Bt – Gd - quartz gneisses (group II), and talc (Tc) - Chl – coarse grained Gt ± Anth ± Gd ± Ilm schists (group III), interbedded with quartzofeldspathic mica gneisses. Both the fine (groups I and II) and coarse grained garnets (group III) are almandine –rich (Xalm = 0.59 – 0.67; Xprp = 0.22 – 0.3; Xgrs = 0.05 – 0.09), although the fine – grained variety is weakly zoned with higher pyrope in core. Gedrite has Aliv of 1.41-1.5 for cores and 1.6-1.74 for rims.

Textural and mineral chemical relations suggest that in group I, Gd + St + Bt + Gt were replaced by Anth + sillimanite, and Chl + ilmenite during retrogression. In group III, Ged + Gt + Tc were replaced by Chl + Ilm. Conventional thermobarometry, coupled with petrogenetic grid considerations suggest that peak P-T conditions for LPGM were 480 - 530°C, 5.5 – 6 bar, characteristic of the sillimanite zone. Geochemical analysis reveals that groups I & III were E-MORBs affected by hydrothermal alteration, whereas group II rocks were likely shales. None of the rocks analyzed have any subduction zone signatures. Isocon diagrams show that the LPGM schists were depleted in Si, Ca, Na, Sr, Ba, Ni, V, and Sc, but enriched in Mg, Fe, La, and Ce relative to E-MORBs. These results suggest that the E-MORBs were affected by hydrothermal alteration on the seafloor, which in turn suggests the existence of a small ocean basin between the Mars Hill terrane and Laurentia.

Geological Society of America Abstracts with Programs. Vol. 52, No. 2

doi: 10.1130/abs/2020SE-344274

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