GEOCHEMICAL AND THEMOBAROMETRIC CONSTRAINTS ON THE PERRY HALL AND TECTONICALLY ASSOCIATED GNEISSES FROM THE BALTIMORE AREA PIEDMONT, MARYLAND

In the central Appalachian Piedmont Province, Early Ordovician to Late Devonian tectonomagmatic and collisional processes resulted in accretion of continental-margin sediments, oceanic crust, and arc-stage plutons and volcanic rocks. The exposed rocks in the Baltimore piedmont represent some combination of magmatism associated with differing tectonic settings and continual accretion of microcontinents and magmatic arcs to eastern Laurentia. This study presents major and trace element data representing five of these arc related bodies as well as detailed petrographic analysis and thermobarometric modeling to constrain the metamorphic conditions for the Perry Hall Gneiss. Spider diagrams normalized to a primordial mantle mostly reflect influences from the upper crust with minor element concentrations similar to the average continental crust. Granite tectonic discrimination plots (Pearce et al., 1984) of Nb vs Y or Rb vs Y + Nb show all samples in the VAG (volcanic arc granite field). Plots of Ta vs Yb also support most samples in the VAG sector with three samples on the border between VAG and syn-collisional fields. The Perry Hall pluton (461 ± 5 Ma by Horton et al., 2010) is a medium grained, well foliated (N35°E, 70-75°NW) garnet-bearing granitic gneiss. Field relations from the Perry Hall Gneiss and the mapped correlative Franklinville Gneiss do not show evidence of partial melting, indicating that these rocks remained below the wet solidus for granite. P-T estimation on the assemblage hornblende (ferro-pargasite), plagioclase (An ≈42%), garnet (Sps ≈11%, Pyp ≈10%, Alm ≈64%, Grs ≈13%, And ≈2%), quartz, titanite and ilmenite with minor biotite, epidote and trace alkali feldspar was made using several methods. P-T estimates using classical Grt-Hbl, Grt-Ilm, Grt-Pl exchange thermometers and the Grt-Pl-Hbl-Qtz barometer yielded 560-620°C and 4-7 kbar. Pseuodosections in the system MnNCKFMASHTO constructed using Theriak-Domino further define the P-T space on the basis of phase stability and yield PT estimates consistent with exchange thermometry.

References:

Pearce et al. Journal of petrology 25, no. 4 (1984)

Horton et al. In From Rodinia to Pangea: The Lithotectonic Record of the Appalachian region. Geological Society of America Inc, (2010)