GEOTHERMOMETRY AND GEOBAROMETRY OF THE LOWER PELITIC SCHIST AT NORTHWEST BRANCH PARK, MONTGOMERY COUNTY, MARYLAND

This study examines reaction relationships between garnets (almandine to pyrope- (Mg,Ca,Mn,Fe)3(Al,Fe)2(Si,Al)3O12) and biotites (biotite-phlogopite-annite- K(Mg,Fe)3(AlSi3O10)(OH)2) in metapelitic rocks in order to obtain P-T estimates and reconstruct metamorphic conditions. Coarse-grained garnet biotite metapelitic schists were sampled from an exposure at Northwest Branch Park in Montgomery County, Maryland. Mineral assemblage and location identify these samples as part of the Lower Pelitic Schist of the Loch Raven Formation with an age ranging between the Upper Cambrian to Lower Ordovician. The mineral assemblage includes biotite + oligoclase + muscovite + quartz + garnet + staurolite and kyanite with minor retrograde chlorite after biotite. Preliminary work has concentrated on identifying mineral compositions and textural relationships through hand sample and thin section examination as well as using the SEM-EDS and electron microprobe. Garnets are primarily almandine with a pyrope component ranging from four to six percent in the cores. The garnets show zoning of Fe- and Mg-rich cores with increases in Mn and Al towards the rims. Garnet-biotite contacts are characterized by reaction zones showing a decrease in Mg and an increase in Fe in the garnet and an increase in Mg and a decrease in Fe in the biotite. Compositional data on each of the salient minerals will be used to model temperature and pressure of metamorphism using the GASP geobarometer and Fe-Mg exchange reaction geothermometers. Element maps and qualitative analyses from the SEM-EDS will be used to direct probe traverses and point analyses on the microprobe, which will better constrain the P-T estimates. These geobarometry and geothermometry estimates will be compared to existing models of metamorphic conditions present during the Appalachian and Alleghenian orogenys to relate the rocks to these two major tectonic events.